The Plant Cell, Vol. 18, 1542–1557, July 2006, www.plantcell.org ª 2006 American Society of Plant Biologists
HISTORICAL PERSPECTIVE ESSAY
Osmogenetics: Aristotle to Arabidopsis
WATER: THE PRINCIPLE OF NATURE
IT MUST BE OSMOSIS!
The historical beginning of understanding the importance of
water to life started with the Ionic Philosophers (from the Ionic
Sea), who began to think that physiology (referring to nature,
from the ancient Greek physis ¼ nature; logos ¼ talk) could
explain life better than theology (referring to god, from the
ancient Greek theos ¼ god; logos ¼ talk). Among these, Talete
(624–547 BC) is credited with the observation that the ‘‘principle
of nature is water,’’ and all life begins with water. In this essay,
we explain how water moves through plants and is used for
growth by recounting the history of ideas, observations, and
experimentation that form our present basis of understanding of
plant water homeostasis. We briefly summarize the history of
approaches and discoveries that have identified the growing
collection of genes that control altered growth and/or altered
water use properties of plants during osmotic mediated
stresses, with the exception of freezing stress. These genes,
which have often been identified by forward genetic mutation
screens, impact one or more of several mechanisms controlling
water absorption, movement, and use. A better understanding
of the basis of water homeostasis should be a useful tool in the
arsenal of molecular geneticists, and increased awareness of
the utility of genetics to understand adaptation to osmotic stress
should be a great benefit to physiologists.
The first systematic and definitive quantitative analyses of
water absorption and movement within plants were reported
by Stephen Hales (1727). Hales was greatly influenced by the
work of William Harvey (1628), who first described blood
circulation, and also John Ray, who started examining water
movement in plants in the 1660s. By simple wet and dry weight
measurements, he was able to determine that most of the
water absorbed by plants was lost to the air (transpired, or
perspired as it was then called). These experiments revealed
that waste water was somehow expelled from plants and
corrected the mistaken view long held from the vitalist perspective. Not only were plants composed mostly of water as had
been proposed earlier by Jan Baptista Van Helmont (1577–1644)
(Weevers and Went, 1949), they were also big expellers of water.
Hales made several other critically important and novel observations (the highly prestigious honor in plant biology is aptly named
the Stephen Hales Prize). He realized that some physical force
was responsible for the movement of water in plants. Although he
never brought together the precise physical relationships of
absorption of water by roots and movement through stems and
transpiration from leaves, he laid the conceptual foundation for
these relationships. Even more importantly, he demonstrated the
value of accurate quantitative results for the purpose of interpretation, a lesson learned again from Mendel’s simple measurements that led to a correct understanding of heritable traits
(reviewed in Lander and Weinberg, 2000).
In the early 19th century, Nicolas Théodore de Saussure made
a critical discovery that minerals dissolved in water were not
absorbed by plants in the same proportion as water was absorbed. Furthermore, different minerals were not absorbed in
the same proportions that they existed in the external water
supply (de Saussure, 1804). This was the beginning of our understanding of the idea of differential permeability. Many investigators struggled to explain this phenomenon on the basis of
physical and chemical knowledge at that time, but no clear
explanations resulted. A breakthrough came finally with the
work of Henri Joakim Dutrochet (1837) who coined the term
osmosis. Dutrochet understood that water was attracted from a
solution with a low concentration of solutes, such as sugar or
salt, to a solution with a higher concentration (this is essentially
osmosis as it was eventually defined in a formal physical
chemical manner as we shall explain later). Although he constructed an osmometer and demonstrated that it could generate
an osmotic pressure (a tremendously important concept that we
also will examine in more detail later), he did not fully understand
the consequential role of differential permeability of the osmotic
HOW DOES YOUR GARDEN GROW? THE FIRST IDEAS
Following Talete’s doctrine, the Greeks from the time of Aristotle
(384–322 BC) and Theophrastus (372–287 BC) believed that
plants absorbed their nutrients from the soil and, much like
animals, converted them into body mass by some vital principle
that separated living from nonliving things. Part of this ‘‘vital’’
view of plants was that they absorbed only nutrients that were
used for growth, as there were no observable waste products
that were expelled from plants. This is a crucial incorrect
view that later had far-reaching implications for understanding the growth processes of plants. Centuries later, a seldomremembered Italian physician, Andrea Cesalpino (1519–1603),
made the earliest attempt to explain absorption of water by
plants from a purely physical perspective. Following the earlier
suggestions of Claudius Galen (129–200), Cesalpino proposed
that water uptake in seeds and plant tissues occurred by a
process of imbibition through the microcapillarity of their internal
structure (reviewed in Meidner, 1983). John Ray (1627–1705)
and Marcello Malpighi (1628–1694) first considered the possibility that capillary action brought water from the roots through
the walls of the vascular cells to the leaves.
July 2006
1543
HISTORICAL PERSPECTIVE ESSAY
membrane. However, at this point, water absorption and movement in plants was placed on a firm physical-chemical foundation, and the vital force was thereafter fully abandoned.
Moritz Traube (1867), using information provided by the work
of Thomas Graham (1862), was apparently the first investigator
to prepare artificial membranes and demonstrate that they
were capable of discriminating between water molecules and
small solute molecules dissolved therein. This was a significant
improvement in Dutrochet’s concept of osmosis and was later
supported by Wilhelm Pfeffer’s accurate measurements of
osmotic pressure using membranes described by Traube
(Pfeffer, 1877). However, the osmosis concept could be put
into a proper biological context only because of the understanding of the structure of cells that was advanced greatly in
1846 by Hugo von Mohl, who described the protoplasm as a
separate entity from its surrounding wall (reviewed in Weevers
and Went, 1949). Thus, it was possible for Pfeffer to present
the important idea that the osmotic pressure generated by the
concentration of solutes in cellular (protoplasmic) water was
able to create a pushing force on the inner walls of plant cells
(which he actually called turgor pressure) that could cause their
volumes to expand. Pfeffer also believed that a thin film of
heterogeneous material was deposited on the inner surface of
cell walls that provided a semipermeable barrier by which
osmosis could take place. He called this thin film the plasma
membrane. Thus, the work of Dutrochet (and those that
expanded on his ideas about osmosis) represents a true
milestone that placed our knowledge of how plants are able to
use water to grow on a firm scientific basis. Using dissolved
solutes to cause the attraction of water into cells by osmosis
across a semipermeable membrane and subsequently creating a hydrostatic pressure (turgor), plant cells could become
bigger. Unfortunately, Pfeffer and other investigators of that
time did not believe that the plasma membrane was anything
other than a mechanical-physical structure that allowed the
passage of solutes based on size, as described by Hugo de
Vries (1877). The work of Waymouth Reid (1890) showed that
the plasma membrane in some places adhered to the cell wall
and in others moved away from the cell wall during osmotic
withdrawal of water from cells (reviewed in Kramer and Boyer,
1995). Much later, this was found to result from special
structural features of the plasma membrane and cell wall
called Hectian strands (Oparka, 1994). de Vries called this
phenomenon plasmolysis, and it was then realized that the
plasma membrane behaved as an active vital filter for specific
solutes. This remains today the central biological concept
by which plant cell enlargement by absorption of water is
explained.
THE FREE ENERGY CONCEPT: WHERE BIOLOGY
MEETS MATHEMATHICS
To understand the history of how the osmotic theory of water
absorption and subsequent cell growth became mathematically
formalized, it is essential to begin by explaining the physical
chemical basis for Dutrochet’s theory of osmosis. We will present only the basic essential formulas that are needed to see how
these concepts evolved.
Most of our formal treatment of the descriptions of water
movement in plants originates from one common concept: molecules, including water, contain energy that can be dissipated in
various forms (Slatyer, 1967; Salisbury and Ross, 1992). Part of
the energy dissipated and transferred between a system and its
surroundings may generate work, the amount of which depends
on the number of molecules exchanging energy. This work may
be used for many purposes, including moving water into and
through the plant. Willard Gibbs (1931) was the first to formally
define the amount of energy available for work, including work
required to drive biological functions, which he termed the
change in free energy (G) as a function of the changes in entropy
(S), enthalpy (H), and temperature (T) of a system (Equation 1):
DG ¼ DH2TDS
ð1Þ
Park Nobel (1991) elegantly framed the application of this
equation in the context of plant physiology, explaining that any
biological function or process in plants involving free energy
changes that occur spontaneously proceeds toward decreasing
free energy. In this respect, the movement of water into and
throughout plants may be predicted based on the DG. A negative DG value is indicative of a spontaneous process; by contrast, a positive DG indicates the minimum amount of work (free
energy) input required for a process to occur.
Considering that the Gibbs free energy of a system is the sum
of all of its components, it is possible to assign an amount of free
energy to each component of an aqueous solution containing
solutes on a molar basis. The chemical potential (mj) of a species
j is therefore defined as the amount of free energy (available to
do work) that one mole of that substance possesses (Slatyer,
1967). Many factors will affect the chemical potential of a substance, including pressure, particle charge, gravity, and other
physical factors. The chemical potential of any substance mj is
mathematically defined in Equation 2, which introduces the term
‘‘activity’’ of a substance (aj). In describing the chemical
potential of a substance, it is proper in thermodynamics to refer
to activity instead of concentration because intermolecular
interactions increase as concentration increases, affecting free
energy content (aj ¼ gjc, where g, the activity coefficient is ,1;
c ¼ concentration). This important concept was originally introduced by Noyes and Bray (1911) and further developed by
Brønsted (1920), who was the first to link formally the activity (a)
of a component to its chemical potential;
o
mj ¼ mj 1RT ln aj 1 VjP 1 zj FE 1 mj gh;
ð2Þ
where R ¼ gas constant; T ¼ temperature on the absolute
scale; aj ¼ activity of j; Vj ¼ partial molal volume of species j;
1544
The Plant Cell
HISTORICAL PERSPECTIVE ESSAY
P ¼ pressure (in excess of atmospheric pressure); zj ¼ charge
number of species j; F ¼ Faraday’s constant; E ¼ electrical
potential; mj ¼ mass per mole; g ¼ gravitational acceleration; h ¼
height; and mjo ¼ the chemical potential for a reference state,
which is obtained when j ¼ 1 (therefore, RT ln aj ¼ 0), VjP ¼ 0 (i.e.,
the pressure in excess of atmospheric pressure ¼ 0), zjFE ¼ 0 (i.e.,
the species j is uncharged or the electrical potential is 0), and
mjgh ¼ 0 (i.e., there is no gravitational component). When all these
conditions exist, mj ¼ mjo. The presence of a term for a standard
state (mjo) is important because, as we will see later, in most
applications of chemical potential, we refer to differences in
chemical potentials rather than to the actual values. Therefore,
chemical potentials must be expressed relative to a reference
level.
This equation and its derived forms became the basis for
predictions of the direction and extent of movement of water in
aqueous solutions into, throughout, and out of plants.
FROM CHEMICAL POTENTIAL TO WATER POTENTIAL: THE
MATHEMATICS OF OSMOSIS
In the late 19th century, the concept of Gibbs free energy was
further developed to describe the properties and behavior of
water in growing plants by applying the formal mathematical
treatment of the chemical potential of aqueous solutions to the
ideas of osmosis developed earlier by Dutrochet (1837), Traube
(1867), Pfeffer (1877), and others.
In 1886, Francois Marie Raoult, using as a foundation Henry’s
gas law, demonstrated that the vapor pressure of mixed liquids
is dependent on the vapor pressures of the individual liquids and
the mole fraction of each present. Raoult’s law allows us to
calculate the vapor pressure of any solution, since the partial
vapor pressure above the solution is related to the escaping
tendency (or chemical potential) of each component. At equilibrium, the chemical potential of a component i (mi) is the same
in the solution and in the vapor phase.
Raoult’s law established the first link between the chemical
potential of a solvent (water) and the solutes dissolved in it.
Following Raoult’s law, in the same year (1887), Jacobus
Henricus van’t Hoff (Nobel Prize in Chemistry in 1901), based
on experimental data provided by Pfeffer (1877), developed an
equation that linked the osmotic pressure to solute concentration (osmotic pressure is defined as the pressure exerted by the
flow of water through a semipermeable membrane separating
two solutions with different concentrations of solutes) (Nobel,
1991; Salisbury and Ross, 1992). van’t Hoff’s equation (Equation
3) became the first mathematical formalization of the concept of
osmosis previously presented by Dutrochet and further developed by Traube (1867), Pfeffer (1877), and others.
pV ¼ ns RT,
ð3Þ
where p ¼ osmotic pressure (MPa), V ¼ solvent volume (m3),
ns ¼ mole of solute; R ¼ gas constant, and T ¼ temperature in K.
The shift from activity to solute concentration made these
concepts more amenable to application in biology and certainly
friendlier to nonmathematicians. In addition, both the van’t Hoff
and Raoult equations provided the theoretical basis on which
many instruments were developed and used to measure and
calculate osmotic pressures of solutions, including intracellular
solutions. Thus, a pivotal role of solutes was formally established
in affecting both water movement and turgor in a compartmentalized environment confined by a semipermeable membrane,
such as the living cell.
As a natural consequence of the van’t Hoff observations,
much work between 1920 and 1950 was focused on defining
relationships between osmotic pressure and the ability of plants
to acquire water from a dry soil environment or one with a high
concentration of solutes (salinity) (Kramer, 1950). It was clear to
most scientists, however, that this approach had limitations.
Finally, in the 1950s and 1960s, the concept of water potential
Cw was developed and proposed for standard use by Slatyer
and Taylor (1960).
By combining the chemical potential (Equation 2) and the
van’t Hoff (Equation 3) relationships, it is possible to define the
chemical potential for water, which can be expressed using a
quantity proportional to mw 2 mwo. This quantity will be called
from here on water potential (Cw) (Equation 4):
o
Cw ¼ ðmw 2mw Þ=Vw ¼ P 2 p 1 rw gh;
ð4Þ
where all symbols are as for Equation 2: rw ¼ density of water
and Cw, the water potential, is a quantity proportional to Dmw.
Here, the term mwo refers to the chemical potential of pure water
at atmospheric pressure. In addition, the chemical potential has
been divided by the partial molal volume of water (Vw) to obtain
more convenient units of pressure (see Nobel, 1991; Kramer and
Boyer, 1995).
Numerous expressions of Cw have been proposed (Nobel,
1991; Salisbury and Ross, 1992; Taiz and Zeiger, 1998) after its
original derivation (Slatyer and Taylor, 1960) from the chemical
potential formula. Among these, a familiar one for most plant
physiologists and practitioners is the following (Equation 5):
Cw ¼ Cp 1 Cp 1 Cm ;
ð5Þ
where Cp ¼ osmotic potential, Cp ¼ pressure potential, and
Cm ¼ matrix potential. The osmotic and pressure potentials
represent the effects of solutes and pressure, respectively. The
matrix potential refers to the effects of liquid–solid interactions
on the chemical potential of water. The contribution of a
gravitational component Cg to Cw is 0.0098 MPa m21. Therefore, this term is important when describing vertical movement
of water in tall trees, whereas it is otherwise generally neglected.
The concept of Cw provided a new basis on which to predict,
within certain conditions, how water will move from one place to
another. Equation 5 made it possible to standardize measurements (having pure water as a reference term), so that comparisons between data collected by different investigators using
July 2006
1545
HISTORICAL PERSPECTIVE ESSAY
different species and environments could be made (including
comparisons of the degrees of stress to which plants are exposed). Therefore, the fundamental accomplishment of transition from m to Cw was to establish an easy method to compare
measurements of m by relating all measurements to m of pure
water. Despite contrasting opinions on the soundness of the Cw
concept and its applications (Kramer, 1988; Passioura, 1988),
Equation 5 may be considered the most basic and useful formula for plant biologists to consider when examining the water
relations of plants under most conditions.
OSMOSIS, TENSION, AND TRANSPIRATION: THE SOIL,
PLANT, AND ATMOSPHERE CONTINUUM
Once the Cw concept was established, two fundamental
physical-chemical principles of plant water movement emerged:
(1) water moves within a system in the direction toward the
lowest free energy state (lowest/most negative Cw) to establish
Cw equilibrium; (2) the components of Cw are therefore the
driving forces for direction and degree (amount) of water movement. It was finally realized that plants need to be in favorable
Cw balance with two components of the plant environment: the
soil and the air. The relative ability of different plants to tolerate
environments with diminished water availability could be accurately determined by measuring the Cw of each part of a system
consisting of the soil, plant, and atmosphere continuum (SPAC),
a term that was first used by Huber (1924).
In nontranspiring plants, the DCw of Equation 5 will depend on
the solute concentration of the root cells and the Cw of the
external soil solution, which will be dominated by either solute
concentration (saline conditions) or the matrix component of Cw
(drying soils). In fact, the effect of soil dehydration on its Cw can
be complex depending on the soil type. Shull (1916), founder of
the journal Plant Physiology, determined that water can be
absorbed from some soil types more easily than from others.
The mechanism of water movement in slowly or nontranspiring
plants was originally described by Renner (1912), who referred
to it as active absorption. Under these conditions, roots behave
essentially as osmometers since solutes that accumulate in the
root cells decrease the Cw below that of the external nutrient
solution in the soil. This will allow water that diffuses into the cell
wall structure of the roots to move along the Cw gradient on its
way to the vascular system, passing through at least one plasma
membrane at the endodermis, where selectivity of solute (especially ion) absorption takes place (Boyer, 1985). Ion accumulation
into the xylem of non- or low-transpiring plants may generate a
positive hydrostatic pressure called root pressure, a term coined
by von Sachs (1882), that is usually visible as guttation or
exudation from the stem of detopped plants. Although movement
of water to the shoot probably passes both along a cytoplasmic
route through several serial plasma membranes (cell-to-cell path)
and an apoplastic path, by 1966 it was accepted that the majority
of water that flows to the shoot after passing the endodermis is
apoplastic (Weatherley, 1966). More recently, the exact path of all
water movement under various conditions has been debated
(Steudle, 2001), especially after the discovery of membrane water
channels in animals (Preston et al., 1992), later found by Maarten
Chrispeels and colleagues in plants (Maurel et al., 1993; Maggio
and Joly, 1995). Although they do not alter the direction or final
amount of water that moves to establish Cw equilibrium (Steudle
and Henzler, 1995), water channels may substantially influence
the rate of water flow (flux) by affecting the Rj component of the
following relationship (Equation 6):
J jVw Aj ¼ DC j=R j ;
ð6Þ
where J jVw Aj ¼ volume of water across component j in unit time
(Aj, for example, could be the root surface area), DC ¼ water
potential drop across component j, and R j ¼ the resistance of
component j (Nobel, 1991).
Water channels may contribute to the rate of water movement
j
) as much as or more than the Cw gradient (DCjw). Net
(JVw
water movement into the root system per se can be accomplished by a simple osmotic adjustment of root cells to produce
a cell Cw that is lower than the Cw of moderately dehydrated or
saline soil.
Water movement into roots of highly transpiring plants, known
as passive absorption, is controlled by other important factors
and events taking place at the leaf surface. Alphonse de
Candolle (1832) coined the term ‘‘stomata’’ to describe the small
pores in the leaf surface. von Sachs (1882) later pointed out that
transpiration was controlled by some ‘‘living feature’’ of the leaf,
restraining or allowing water to evaporate into the air. Francis
Darwin, using a newly developed porometer, then found that
these crucial living features of leaves were the stomata that are
composed of the two guard cells and subsidiary cells (Darwin
and Pertz, 1911). Darwin, expanding the work of several others
(such as Von Mohl and Malpighi), even realized that the stomatal
pores might be controlled by the gain and loss of turgor of the
guard cells through the in and out movement of unidentified
solutes.
In 1804, de Saussure first concluded that plants absorb CO2
from the atmosphere. Jean Baptiste Boussingault, in 1864,
measured O2 and CO2 exchange from leaves, and Sachs finally
determined that plants are able to absorb CO2 through stomata
to make sugar using solar energy. Therefore, it was finally understood that to produce food by photosynthesis, plants needed to
open stomata to acquire the necessary CO2, which Alexander
von Humboldt and others had speculated as early as the 1790s
(Kramer and Boyer, 1995). Therefore, plants that are actively
photosynthesizing with open stomata have high rates of loss of
water from the leaf to the atmosphere that dominates the Cw
gradient by producing a tension (negative pressure) in the xylem.
Tension will be transmitted through the xylem column of water
(held together by the cohesive properties of water) to the roots.
This will further reduce the root Cw, causing a continuous and
more rapid water absorption by the plant. The idea that tension
is created by the force of transpiration and actually causes the
movement of water upward from the roots throughout the plant
1546
The Plant Cell
HISTORICAL PERSPECTIVE ESSAY
was first put forward by Eugen Askenasy (Weevers and Went,
1949). Dixon and Joly (1895) substantially promoted the tension
cohesion theory, and it is now well recognized (Steudle, 2001).
Strong tension on xylem water sometimes even breaks the
cohesion of water molecules interrupting water flow by causing
cavitation (an air bubble in the xylem water column).
In actively transpiring plants, the force of water evaporation
from the leaf is the major factor controlling the speed and
amount of the uptake of water by the root system, which is an
important aspect of dehydration (drought) stress because it is
the major cause of depletion of soil water. This is true because of
the basic thermodynamic properties of water vapor versus liquid
water. In formal terms, the driving force for movement of water
vapor (the state of water as it leaves the plant) is the difference in
vapor pressure or vapor concentration (C). If we consider the
leaf-air component of the SPAC, which includes the water vapor
movement within intercellular spaces and out of the stomata,
and relative resistances at the leaf-air boundary, the transpiration rate (T) can be described by the following relationship
(Equation 7):
T ¼ ðCleaf 2 Cair Þ=ðrleaf 1 rair Þ;
ð7Þ
where Cleaf ¼ vapor concentration at the evaporating surface
inside the leaf, Cair ¼ vapor concentration in the bulk air, rair ¼
resistance of the air boundary layer to water vapor diffusion, and
rleaf ¼ diffusive resistance of all the paths for vapor diffusion.
Equation 7 states that the rate of transpiration is proportional
to the water vapor concentration difference. If we express the
Cw in the air according to the formula C ¼ (RT/Vw) ln(% relative
humidity/100) 1 rwgh, we may calculate that the Cw of air at
25C and 50% relative humidity is 295.1 MPa (Nobel, 1991). It is
evident that the DCw leaf-air is much larger than the DCw soil-root,
and without tension from transpiring leaves, DCw soil-root is the
result of the difference in Cw of root cells and their surrounding
soil, which is normally in the range of 0.1 to 2.0 MPa. Thus, the
Cw concept made clear that the thermodynamic forces causing
water to leave the plant through stomata are much larger than
those causing water to be absorbed by roots in the absence of
transpiration.
It is apparent that water loss through the stomata would be
reduced by passive closure when there is the loss of turgor of
the guard cells due to leaf wilting. However, by the late 19th
century, Hugo de Vries understood that closing stomata actively
can occur before the leaf cell turgor is lost (wilting) by specific
sensing of the environment (such as light) and subsequent
signaling that results in solute movement out of the guard cells,
specifically causing their turgor loss and pore closure before leaf
wilting (Kramer and Boyer, 1995). These observations clearly
established active stomatal closing to increase the resistance
(R j ) to water vapor flux as one of the most important responses
to a disruption of the SPAC. Thus, it became clear that it is the
water potential (Cw), not only of the soil and the plant but also of
the air that forms a Cw gradient continuum (SPAC), that controls
both water loss to the air and water uptake from the soil by
plants when stomata are open and the leaf/air R j is low.
Therefore, it must be emphasized that the Cw status of the entire
continuum (and not just the Cw status of only one part of the
continuum) controls direction and amount of water movement.
For instance, as early as 1914, Lyman Briggs and Homer L.
Shantz demonstrated that wilting was associated with a sort of
balance and/or exchange between the atmosphere, plant, and
soil water contents. We now understand that wilting (net loss of
water from cells) would occur in soil with low Cw because the
small R j (R j remains small until stomata begin to close, when R j
becomes larger) and the large DCwj at the plant/air continuum
would result in a faster rate of water loss out of the plant than the
rate of uptake into the plant at the soil/root/shoot continuum,
where there is a relatively large Rj and smaller DCwj. This
situation worsens when the DCwj at the soil/root diminishes as
the falling Cw of drying or salinizing soil becomes closer to the
Cw of the root, while the DCwj of the plant/air would remain very
large (Equation 6). The plant could rely only on the increased R j
caused by stomatal closure to prevent death by dehydration,
thus making stomatal closure the most critical response to soil
conditions with low Cw caused by dehydration or salinization.
After gaining an understanding of the Cw concept and the
SPAC, a firm thermodynamic (osmotic) basis for the uptake of
water by plants was established and the stage was set to use
this concept to establish the role of water relations in growth. We
should always keep in mind that the water participating in cell
enlargement (growth) is ,1% of the total water removed from
the soil by the plant. The vast majority of absorbed water is lost
via transpiration. The large amount of water lost in transpiration
forced recognition of the central dilemma that plants face to
acquire both the water and CO2 needed for growth, especially in
water limiting environments (Raschke and Hedrich, 1985). Diffusion of CO2 into the leaf cells occurs almost exclusively through
the stomata because leaf surfaces are practically sealed to gas
exchange by cuticular and epicuticular barriers. Evolution has
never produced a differentially permeable membrane that can
discriminate between H2O and CO2; therefore, stomatal pore
size became the mechanism by which the loss of water by transpiration and gain of CO2 is optimized. The ratio of dry weight
(mainly from CO2) gained (growth) to water lost is called water
use efficiency (WUE) and is a fundamental factor controlling the
maximum size (growth) of plants that are exposed to environments with limited water availabilities. WUE was first described
by Bierhuizen and Slatyer (1965) and varies considerably
between species as clearly demonstrated first in the data of
Briggs and Shantz (1914) and later by many others (de Wit, 1958;
Passioura, 1977; Farquhar and Sharkey, 1982). Genetic control
of optimizing the amount of CO2 gained per amount of water lost
is dramatically illustrated by the description of C4 photosynthesis in 1966 by Hatch and Slack (see Salisbury and Ross, 1992)
and in the extreme example of C4 crassulacean acid metabolism plants that have a WUE 3 to 5 times greater than other
plants. Although it has been assumed that WUE is genetically
very complex and varies little within a species, alleles of
July 2006
1547
HISTORICAL PERSPECTIVE ESSAY
individual loci that control WUE have recently been identified
(Masle et al., 2005). Evidently, natural selection has provided
mechanisms to use water as it passes out of the plant through
transpiration, not for maximum but for optimal growth that
assures survival.
WATER POTENTIAL AND PLANT CELL ENLARGEMENT:
IT’S NOT JUST TURGOR
The osmotically driven growth process was first formalized by
Lockhart (1965) and later by Green et al. (1971), who established
a first order relationship between turgor and growth according to
Equation 8:
E ¼ mðP 2 YÞ,
ð8Þ
where E ¼ expansion growth, m ¼ cell extensibility, P ¼ cellular
turgor pressure, and Y ¼ threshold turgor pressure required for
cell expansion.
Based on this equation, expansive growth is described as
occurring only when the Cw of the plant cell is lower than its
immediate outside water source (the apoplast), which is thermodynamically connected to both the soil (roots) and the air
(leaves). The inside of the plant cell wall is then under turgor
pressure, which is created by the difference between the
osmotic pressure of the cell and the water potential of apoplasm.
When the wall loosens, it will yield to the turgor pressure,
allowing cell volume expansion with water entering the cell. This
process of cell enlargement also affects cell division, since cells
enlarge until a certain threshold before starting division (Cleland,
1971; Taiz, 1984; Cosgrove, 1987; Beemster et al., 2005). The
above relationship also indicates that any water status factor
that will reduce turgor pressure, such as the lowering of the soil
Cw by dehydration or salinity or increasing the H2O vapor pressure gradient of the air (low humidity and high temperature), will
also reduce growth by requiring lower cell Cw to allow water to
move into cells. Many studies published after the Lockart equation established the existence of a linear relationship between
leaf (or root) cell growth and turgor (Passioura and Fry, 1992;
Turner, 1997).
Considerable research that was based on the work of Charles
Darwin has been aimed at understanding the growth mechanism
underlying the Lockhart equation through studies of auxininduced cell growth (Woodward and Bartel, 2005). Eventually, a
well-established model of turgor-driven growth was developed
in the 1960s that explained cell enlargement as the result of
expansion of cells whose walls were loosened by an acidification of the apoplast through proton extrusion. This became
known as the acid growth theory (Rayle and Cleland, 1970; Taiz,
1984). The validity of this theory has not been seriously
questioned, but the underlying basis of the acid growth phenomenon and the cell wall properties involved in expansion have
not been clearly established. However, these ideas were significantly advanced in 1992 with the discovery by Daniel Cosgrove
and colleagues of cell wall proteins called expansins that mediate acid growth (McQueen-Mason et al., 1992; reviewed in
Cosgrove et al., 2002).
Meyer and Boyer (1981) reported the important observation
that a reduced growth rate resulting from loss of turgor was
maintained in plant tissues even after the adaptive response had
restored turgor to levels equal to or higher than those existing
before a lowered external Cw resulted in growth retardation. This
discovery was independently confirmed by other labs (Matsuda
and Riazi, 1981; Bressan et al., 1982, 1990), and although these
observations made it clear that growth after osmotic stress is
not turgor limited, they did not explain or advance the understanding of the basis of growth control beyond the acid theory.
These results did, however, eventually force a new perspective
on growth process control. Zhu and Boyer (1992) introduced a
new conceptual framework for the relationship between cell
water status and growth that is called the growth-induced water
potential gradient theory. This perspective implicitly placed the
growth process itself, through cell wall metabolism, as the inducer of a lower turgor pressure. As such, the turgor and thereby
the water status of the cell is controlled by the growth
process(s). In other words, water status (including turgor) is
not in control of (or does not constrain) growth, but rather the
opposite is true. Essentially, turgor became understood to be
required but not sufficient for growth. This concept, which is still
poorly understood by many plant scientists, cannot be overemphasized because the previous view largely neglected the
need to control other biological processes that are coordinately
regulated during growth (Figure 1), such as membrane extension, wall synthesis, osmotic readjustment, cytoskeleton development, energy and protein production, and many other cellular
processes, including cell division. This becomes clearer when
the relationship between cell size and cell division is considered.
As recently summarized by Beemster et al. (2005), cell division
and enlargement processes are tightly coordinated, and both
contribute significantly to overall growth. Bressan et al. (1990)
demonstrated that after adaptation to osmotic stress, the relationship between cell division and enlargement is altered and
is not controlled by the turgor or osmotic status of the cell, indicating that there must be proactive genetic control of the
integrated growth process that can be reset after osmotic stress
adaptation, resulting in slower growth with equal or higher turgor
levels. Thus, the wall loosening process, subsequent cell expansion, and many other cell processes involved in growth must be
under the control of a still elusive, master regulatory system of
cell growth and division. This system may involve both abscisic
acid (ABA) and ethylene to distinguish between dehydration and
saline-based osmotic stresses (Ruggiero et al., 2004).
ON THE GENETICS ROAD: ARISTOTLE
MEETS ARABIDOPSIS
As early as 1910, the German physiologist George Klebs pointed
out that environmental effects (such as salinity and drought) on
1548
The Plant Cell
HISTORICAL PERSPECTIVE ESSAY
Figure 1. Physical (Osmo) Path of Water through Plants from Soil into Root Tissues, through Xylem Vessels, and Finally Out of Leaves via Stomata into
the Atmosphere.
Critical (genetic) control points that mediate plant responses to osmotic stresses are indicated, including reduced stomatal conductance, reduced
shoot meristem activity, unchanged or increased root meristem activity, reduced water uptake, and increased osmolyte accumulation. Many other
responses occur but are not shown for simplicity. Left: plant before osmotic (dehydration or salinity) stress. Right: plant after a period of stress and
recovery (adaptation/acclimation). Drawing by Matilde Paino D’Urzo.
the growth (phenotype) of plants acted through physiological/
metabolic processes that were limited by genetic potentials that
differed between species. Yet, for several decades, little effort
was made to understand the genetic bases of water use control.
One of the first clear indications that knowing the bases of
genetic potentials could dramatically increase our understanding of water homeostasis appeared when a few tomato mutants
with highly compromised abilities to maintain a SPAC favorable
for growth and survival were discovered. These mutants were
also found to be impaired in the ability to close the stomata during osmotic (dehydration) stress as a result of an ABA
accumulation deficiency (Tal and Imber, 1971). These ABAdeficient mutants provided evidence that an increase in ABA
levels after osmotic stress (e.g., Pierce and Raschke, 1980;
Saab et al., 1990) is causatively related to osmotic stress
tolerance, validating a clear genetic basis for osmotic stress
tolerance. Epstein (1985) summarized experiments based on
crossing plants with different salt tolerances that established a
genetic basis for the adaptive tolerance to osmotic stress.
Eventually this adaptability was concluded to be a multigenetic
trait that defied simple genetic introgression. Later, studies with
rice confirmed the multigenic basis of tolerance responses but
also indicated that substantial tolerance may be mediated by
just a few genes (Gorham et al., 1997). Evidence that single
genes could affect tolerance to osmotic stress continued to
appear sporadically from the 1960s through the 1980s. Soybean
mutants with increased tolerance to osmotic stress were
reported in 1969, and tobacco and fern mutants were found in
the 1980s (see Zhu, 2000). Also, Handa et al. (1983) reported
that plant cells not only could adapt to osmotic stress but that
long-term exposure to a low osmotic potential environment
leads to a gradual selection for genetically stable cell lines
July 2006
1549
HISTORICAL PERSPECTIVE ESSAY
(mutants) with increased tolerance. Later experiments, discussed in more detail below, revealed how the monogenic and
polygenic nature of a single trait can be reconciled.
Although the discovery of mutants that are tolerant to ABA
and osmotic stress exposed the dramatic dependency of stress
tolerance on single genes, these mutants were not available in
genetically tractable species that would allow molecular genetic
approaches to be used to identify DNA sequences of the genes.
Therefore, even as the genetic basis of growth control in response to osmotic stresses continued to become more evident,
plant water relations research before 1990 remained focused on
the physics of water movement, and most studies only correlated physiological responses with osmotic stress. However,
many responses to stress are not adaptive. Even detailed studies
of species with different tolerances, including genomic scale
microarray gene expression studies (Maggio et al., 2003), will
not easily yield the mechanisms of adaptation without a mutational loss- or gain-of-function approach. Sung et al. (2003)
pointed out that the plethora of descriptive studies of cold
acclimation (adaptation) did not lead to a clear way to understand or manipulate cold tolerance. Numerous studies of these
responses also have not led to a clear way to manipulate the
ability to adjust to a disrupted SPAC that causes net water loss,
just as the case with cold acclimation. However, gradually investigators began to recognize the importance of earlier genetic
work and how a genetic approach could reveal which responses
to an unfavorable osmotic environment cause adaptation.
Eventually, in the 1980s and 1990s, mutants with altered
osmotic stress tolerance began to be reported from studies using
Arabidopsis, a highly tractable molecular genetic model system
(Saleki et al., 1993; Finkelstein and Gibson, 2002). Using mutational
studies in Arabidopsis, Koornneef et al. (2004) discovered the
crucial first genetic connection between altered osmotic stress
sensitivity and altered sensitivity to ABA (see Alonso-Blanco
and Koornneef, 2000). It then became of great interest to
identify mutant alleles of genes that altered ABA responses or
sensitivities.
In the early 1990s, mutants in Arabidopsis became available
that were altered in ABA sensitivity, osmotic sensitivity, or both.
However, the identification of the genes responsible for the
altered phenotypes, although technically quite possible, at that
time could be pursued only by map-based cloning technology.
This was then quite a challenging undertaking. Because of this
difficulty, to identify genes that control osmotic or ABA sensitivity, an indirect approach was used that involved the identification of genes whose transcripts accumulated in response to
osmotic stress or ABA. The reasoning behind this approach was
that genes whose transcript accumulation was controlled by
ABA or osmotic stress would be involved in the phenotype of
stress adaptability. Because ABA plays an important role in
many plant developmental processes (e.g., seed dormancy and
germination, leaf abscission, and senescence), the mechanism
of ABA-mediated regulation of gene expression gained the early
attention of numerous researchers, many of whom were not
necessarily interested primarily in osmotic stress (Rock and
Quatrano, 1995).
By the 1990s, it was known that the transcript levels of
hundreds of genes, which probably participate in many phenotypes, were influenced by ABA (Rock, 2000). The promoters of
genes whose expression was controlled by ABA were examined
by several research groups to determine the regulatory ciselement sequences responsible for ABA control. Work by Ralph
Quatrano on the Emb-1 gene from wheat yielded the first
revelation of an ABA control element that was called the
ABA-responsive element (ABRE) (Marcotte et al., 1988). The
ABRE element binds to members of the basic leucine zipper–
type transcription factor family also called AREB and ABF that
are homologs of the ABI15 gene. Michael Thomashow, Kazuo
Shinozaki, and others led separate exhaustive characterizations
of genes that are either cold regulated and were found to contain
a cold-responsive cis-element that was designated the C-repeat
(CRT) element (Baker et al., 1994) or genes that are dehydration
responsive and contain a dehydration-responsive (DRE) element
(Yamaguchi-Shinozaki and Shinozaki 1994). These turned out to
be the same cis-elements and are now referred to as the CRT/
DRE element (Yamaguchi-Shinozaki and Shinozaki, 2005).
Thomashow and colleagues made the historic breakthrough
that identified the family of transcription factors that bind to the
CRT/DRE elements and named them CBF (for C-repeat binding
factor), followed shortly by Shinozaki and colleagues, who
named the same gene family DREB (for dehydration response
element binding protein) (Stockinger et al., 1997; Liu et al.,
1998). Although these family members correspond to identical
loci in Arabidopsis, the separate names have persisted, resulting
in some confusion; therefore, it is worthwhile to indicate here the
matching pairs: CBF1 ¼ DREB1b; CBF2 ¼ DREB1c; CBF3 ¼
DREB1a. These are all members of the Apetala-2/ethylene
response element binding protein family, originally found to control flower development and ethylene responses (Riechmann
and Meyerowitz, 1998). Some osmotic stress–controlled genes
contain both ABRE and CRT/DRE elements, and others contain
either or none of the sequences. Members of the CBF/DREB1
family of transcription factors are inducible, and their stress
induction is mediated by the ICE1 family of HLH-type transcription factors (Chinnusamy et al., 2003).
After these discoveries of ABRE and CRT/DRE elements,
extensive work pioneered by David Ho and others revealed that
ABA activation through ABRE elements can be enhanced by the
action of a coupling element, CE1 (reviewed in Ingram and
Bartels, 1996). Eventually, additional cis-elements became
known to be involved in ABA responsiveness, including RY/
Sph elements and MYB and MYC recognition elements
(Finkelstein et al., 2002; Yamaguchi-Shinozaki and Shinozaki,
2005). The identification of these cis-elements eventually
allowed the discovery of the DNA binding proteins (transcription
factors) that recognize and bind to them and thereby mediate
expression of ABA-controlled genes. Transcription factors
corresponding to these additional types of cis-elements include
1550
The Plant Cell
HISTORICAL PERSPECTIVE ESSAY
the B3 domain proteins that bind to RY/Sph elements (ABI3) and
MYB- and MYC basic helix-loop-helix–type transcription factors
that bind to MYB and MYC recognition elements, respectively.
Other transcription activators and repressors have been identified that mediate osmotically controlled gene expression, such
as the NAC, HDBZIP, and AZF protein families, and the
transcription cascade system has been extensively reviewed
and modeled (Himmelbach et al., 2003; Yamaguchi-Shinozaki
and Shinozaki, 2005).
Before transcription factors can bind to and activate (or
repress) their target genes, they need to be either transcribed
themselves in a transcription cascade, or if they are at the
beginning of the cascade, they need to be activated. A common
activation mechanism is phosphorylation mediated by a protein
kinase. Several protein kinases have been implicated in osmotic
stress responses, especially following the early discovery of the
major role for the mitogen-activated protein kinase (MAPK)
cascade that controls osmotic adjustment in yeast and that
terminates in transcription factor activation (Hasegawa et al.,
2000). In fact, several members of the plant MAPK group,
including stress-induced map kinase, stress-activated map
kinase, salicyclic acid–induced protein kinase, wound-induced
protein kinase (Jonak et al., 2002), and other kinases, are known
to be activated by osmotic stresses (Zhu, 2002). Extensive
studies of these kinases by several labs led to the discovery that
at least one MAPK plays a functional role in osmotic stress
tolerance in plants by controlling gene expression (Teige et al.,
2004). Other protein kinases have been connected to osmotic
adaptation, such as the calcium-dependent protein kinases and
the SNF1 (SnRK2) family through the discovery of the fava bean
ABA-activated protein kinase that was shown to dramatically
control stomata function in fava bean (Li et al., 2000) and in
Arabidopsis where it is called Open Stomata-1 (Mustilli et al.,
2002). All 10 members of the SnRK2 family genes are now
known to be activated by osmotic stress or ABA (Boudsocq and
Laurière, 2005).
After the discoveries of the promoter control elements and
their corresponding transcription factors and possible activation
by kinase and transcription factor cascades, additional work by
many researchers allowed osmotic-induced signal systems to
be organized into a complex of both ABA-dependent and ABAindependent pathways, and these now have been extensively
reviewed (Ingram and Bartels, 1996; Rock, 2000; Chinnusamy
et al., 2004; Yamaguchi-Shinozaki and Shinozaki, 2005).
ABA: THE CENTRAL SIGNAL MOLECULE CONTROLLING
WATER HOMEOSTASIS
Since the 1960s a number of mutants that display either
precocious seed germination or a wilty phenotype have been
described (see Xiong and Zhu, 2003). Although several of these
were known to be deficient in ABA biosynthesis (e.g., the flacca
mutant that clearly linked ABA to stomatal control), the ABA
biosynthetic pathway remained uncertain for decades. One of
the most persistent pursuers of this pathway has been Jan
Zeevaart, who contributed extensively to our understanding of
the biochemistry of ABA metabolism and participated in the
discovery of several genes encoding pathway components (see
Xiong and Zhu, 2003). The DNA sequence of any mutant locus
causing an ABA deficiency phenotype was not known until 1996
when the sequence of the zeaxanthin epoxidase locus was
reported (Marin et al., 1996). This was followed by the identification of the 9-cis epoxycarotenoid dioxygenase, short-chain
alcohol dehydrogenase/reductase, ABA aldehyde oxidase, and
molybdenum cofactor sulfurylase loci (see Xiong and Zhu, 2003)
by 2002. Extensive studies of gain and loss of function of these
genes have revealed that all of them are involved in water
homeostasis and have confirmed the central role of ABA in
adaptation to osmotic stresses.
The important discovery that all of the ABA biosynthetic
pathway genes are themselves induced not only by osmotic
stress but also by ABA has implied the existence of a feedforward control system capable of amplifying the stress-induced
ABA signal (Xiong and Zhu, 2003). Evidence exists that several
plant hormones participate in such feed-forward induction of
their own synthesis beginning with the very early observation by
Hans Kende, Shang Fa Yang, and others of the autocatalytic
nature of ethylene (Yang and Hoffman, 1984). Although both
osmotic stress and ABA affect transcription of all of the key
genes encoding proteins in the ABA biosynthetic pathway, the
major components, including the transcription factors that
mediate these ABA pathway genes, remain elusive. A recent
report, however, indicates that the transcription factor high
osmotic sensitive (HOS10) may control transcription of ABA
biosynthesis genes. An important challenge for the ability to
modify stress tolerance will be the elucidation of the control
mechanism(s) of the ABA biosynthetic pathway (Chinnusamy
et al., 2004).
Besides the important role of ABA in mediating osmotically
induced gene expression, ABA also has direct effects on water
homeostasis that do not require altered gene expression.
Paramount among these are the effects of ABA on direct stomatal function. ABA mediation of stomatal behavior is presently
the most studied physiological process that controls water use
by plants (Kuhn and Schroeder, 2003; Fan et al., 2004; Riera
et al., 2005). Again, this is due in large measure to Arabidopsis
molecular genetic studies. Guard cell turgor, which was shown
to involve K1 accumulation as early as 1905 by Archibald
B. Macallum, and substantiated by Fischer and Hsiao (1968), is
now understood to be due primarily to uptake of K1 that occurs
down the electrochemical potential gradient across the plasma
membrane. The counter ion is typically Cl2 or malate22, but NO32
also may serve to balance the K1. Membrane depolarization
results in passive efflux of ions and other solutes, including sucrose, causing turgor loss and stomatal closure. The functioning
of many transport proteins in stomatal opening and closure have
now been described, sometimes in great detail, and have been
reviewed extensively (Fan et al., 2004; Chaerle et al., 2005).
July 2006
1551
HISTORICAL PERSPECTIVE ESSAY
ABA is perceived in the guard cells both intercellularly and
extracellularly through an unidentified sensor(s) (Fan et al.,
2004). The extreme importance of ABA perception likely means
that functionally overlapping receptors exist and thereby preclude their easy identification by mutation screening. However,
recently, a leucine-rich repeat receptor-like protein kinase
(RPK1) was determined to be involved in early ABA signaling,
perhaps even in perception (Osakabe et al., 2005). Although
ABA has been known for many decades to be the principal
mediator of stomatal closure in response to osmotic stress, it is
now known to do so through several signal pathways that can be
generally divided into Ca21-independent and Ca21-dependent
systems (Kuhn and Schroeder, 2003; Fan et al., 2004; Chaerle
et al., 2005; Riera et al., 2005). Because Ca21 is involved in the
mediation of several signal pathways, the mechanism of its
specificity has been an important goal of many investigators.
Elegant experiments performed by a number of researchers,
including Alistair Heatherington, Michael Blatt, Gethyn Allen, and
Julian Schroeder, for the first time identified Ca21 level oscillations as a manner by which Ca21 specifically mediates stomatal
behavior (reviewed in Blatt, 2000). Ca21-dependent stomatal
behavior involves reactive oxygen species (ROS), inositol polyphosphate, and nitrous oxide cyclic nucleotide pathways. Ca21independent signaling includes a heterotrimeric G-protein
cascade that involves phosphatidic acid and ROS. Other signal
components have been discovered and include sphingosine,
ascorbate, inositol-1,4,5-trisphosphate, cyclic ADP-ribose, and
[H1] (Blatt, 2000). In some instances, the enzymes involved in
the production of the signaling intermediates have been identified (Fan et al., 2004; Chaerle et al., 2005). Using primarily
genetic approaches, a complex signal system that controls
stomatal behavior in response to osmotic stresses has been
modeled (Chaerle et al., 2005).
Although valuable information about genes that control water
homeostasis was being obtained from the study of ABA biosynthesis and response mutants, by the late 1990s, no genes had
been identified by directly screening for hypersensitive growth
responses to osmotic stress. Even though Arabidopsis became
the most tractable plant genetic system and as such, the
obvious choice for a genetic model to screen directly for loss of
osmotic tolerance, it is not particularly tolerant of either salinity
or dehydration stress. Arabidopsis would thereby seem unsuitable for loss-of-function genetic screening (there is very little
tolerance to lose and thereby detect in screens). However, some
important observations pointed the way to the use of Arabidopsis for loss-of-function screening for osmotic sensitivity. (1)
Previous studies determined that tolerance to low Cw could be
controlled by a few or even single genes (Zhu, 2000). (2) Several
reports indicated that essentially all plants (not just halophytes
and xerophytes) possess genes that contribute significantly (can
be screened) for loss of the ability to adapt to low Cw (Bressan
et al., 1990). (3) Indeed, as we have pointed out, a number of
mutants in Arabidopsis were isolated that exhibited changed
tolerance to NaCl, although these studies did not lead to the iden-
tification of the mutated genes (Alonso-Blanco and Koornneef,
2000; Finkelstein and Gibson, 2002). (4) Mutants screened for
insensitivity of germination to ABA were found to be altered in
dehydration tolerance also (Koornneef et al., 2004). Armed with
all of this information, Jian-Kang Zhu and coworkers began in
the late 1990s to isolate Arabidopsis mutants by directly screening for altered growth responses to NaCl-mediated osmotic
stress and then to identify the genes that control adaptation to
low Cw (Verslues et al., 2006).
THE SOS PATHWAY: CONNECTING Ca21 SIGNALING
WITH ION TRANSPORT PHYSIOLOGY
Forward genetic screening for loss of osmotic stress tolerance in
Arabidopsis resulted in the discovery of the Salt Overly Sensitive3 (SOS3) gene in 1998 (Liu and Zhu, 1998). Later, discovery
of other SOS pathway genes revealed part of the molecular
basis of the long-observed amelioration effect of Ca21 on NaCl
stress. The Ca21-dependent SOS pathway transduces salt
stress–mediated signaling to activate the plasma membrane
Na1/H1 antiporter (SOS1) that is required for efflux of Na1
during the gradual net accumulation of NaCl for osmotic
adjustment (Zhu, 2003). The discovery of SOS1 by Shi et al.
(2000) and the vacuolar Na1/H1 antiporter family by Jerry Fink
and others (see Pardo et al., 2006) revealed the molecular
genetic basis of the well-established detoxification/osmotic
balance role of cytoplasmic efflux and vacuolar compartmentation of Na1, which had been based on the multiphasic concept
of transport of ions presented several decades ago in the classic
work of Epstein (1985). The SOS pathway is functionally
analogous to the calcineurin pathway that controls ion homeostasis in yeast, where the calcineurin B subunit binds Ca21 and
activates the phosphatase calcineurin A (Bohnert and Bressan,
2002). The Ca21 binding protein SOS3 is activated by a Ca21
transient and then recruits to the plasma membrane and
activates SOS2, which, instead of a phosphatase, is a Ser/Thr
kinase (Knight et al., 1997). The SOS3-SOS2 protein kinase
complex then phosphorylates SOS1, resulting in activation of
Na1/H1 antiporter activity. The SOS3-SOS2 complex also
activates SOS1 expression, perhaps through processes that
are responsible for SOS1 mRNA stability. ABA may participate in
the SOS pathway through the ABA-Insensitive2 (ABI2) protein,
which is a PP2C phosphatase that interacts with SOS2.
SOS3 family Ca21 Binding Protein5 and its interacting kinase,
Protein Kinase SOS2 Family3, are components of a regulatory
circuit that negatively controls ABA signaling through ABI1 and
ABI2 (Chinnusamy et al., 2004). This regulatory circuit possibly
controls transport systems that function to lower cytosolic Ca21
after ABA-induced Ca21 channel gating. It is necessary to also
lower cytosolic Ca21 levels to produce Ca21 oscillations that
activate the SOS pathway and other signaling required for salt
and osmotic adaptation (Chinnusamy et al., 2004). Mutants in the
SOS pathway (sos1, sos2, and sos3) also exhibit K1 deficiency,
indicating that SOS signaling has a positive regulatory effect on
1552
The Plant Cell
HISTORICAL PERSPECTIVE ESSAY
K1 acquisition, which has been long understood to be inhibited by
excess NaCl. However, SOS1 does not posses innate K1
transport capacity. Other direct or indirect outputs of the SOS
and SOS-like signal pathways that function in Na1 and K1
homeostasis include the vacuolar cation/H1 antiporter NHX and
perhaps HKT1 and AKT1 (Rus et al., 2001; Chinnusamy et al.,
2005). SOS1 may also be a Na1 sensor (Zhu, 2003).
THE FUTURE
The Regulatory Small RNAs and the Dark Genome
Although noncoding RNAs (rRNA and tRNA) have been recognized
for many decades, genetic phenomena that eventually led to the
general regulatory functions of RNAs did not surface until the
discovery of genetic cosuppression in plants in 1990 by Rich
Jorgensen (Napoli et al., 1990) and RNA interference in Caenorhabditis elegans (Fire et al., 1998). The connection to regulatory
small RNAs was made when small interfering RNAs (siRNAs) were
found in plants that are undergoing posttranscriptional gene
silencing (Hamilton and Baulcombe, 1999). In a separate line of
work, microRNAs (miRNAs) were discovered in C. elegans mutants
defective in developmental timing (Lee et al., 1993; Wightman et al.,
1993). In eukaryotes, it is now recognized that there is an ocean of
small RNAs ranging between 20 and 30 nucleotides in size that
regulate gene expression at many levels from transcription to
translation (Lu et al., 2005). Although we know that these RNAs are
encoded extensively in plant genomes, we remain very much in the
dark about how they function in relation to the classic protein
encoding genome. The small RNAs are classified as either miRNAs
or siRNAs based on their biogenesis (Carrington and Ambros, 2003;
Baulcombe, 2004). miRNAs and siRNAs are ultimately generated
by the Dicer endonucleases from transcripts forming stem-loop
structures and long double-stranded RNAs, respectively. They are
incorporated into the Argonaute-containing RNA-induced silencing
complex and guide the complex to complementary mRNAs to
cause mRNA cleavage or translational inhibition (Carrington and
Ambros, 2003). Additionally, siRNAs can also guide epigenetic
inheritance through a nuclear chromatin remodeling complex to
cause histone and/or DNA methylation, leading to transcriptional
gene silencing (Baulcombe, 2004). Because of their small and
noncoding nature, the small RNAs have evaded detection by
traditional and microarray-based transcriptional profiling as well as
by all but a few genetic screens. The role of miRNAs and
endogenous siRNAs in plant and animal development is now well
established (Carrington and Ambros, 2003). Evidence that small
RNAs are involved in osmotic stress responses was reported by
Sunkar and Zhu (2004). Thus far, evidence indicates that some
miRNAs and siRNAs are inducible by osmotic stresses, and
they play critical roles in removing negative determinants of stress
(e.g., osmolyte catabolizing genes and ROS generating genes) to
achieve tolerance (Borsani et al., 2005; X. Hu and J.-K. Zhu,
unpublished data). Certain miRNAs and siRNAs may also be
downregulated by stress to relieve their suppression on positive
determinants of stress tolerance. These new regulators and other
chromatin level regulation (i.e., DNA and histone modifications) hold
enormous potential for unraveling the complex network of stress
responses to uncover key loci that have alleles that actually cause
stress tolerance.
Mendelization of Polygenic Traits
An important lesson being relearned from Arabidopsis genomicsscale research is that, depending on the germplasm pool being
studied, a trait can be either polygenic or monogenic. From the
work of geneticists like J. Doebley and S. Tanksley, we know that
complex traits such as grain and fruit yield have been largely
imparted to crops through genetic manipulation involving specific
alleles of only a few genes (see Richards, 1996; Doebley et al.,
1997; Peng et al., 1999; Lippman and Tanksley, 2001). An example
involving flowering time in Arabidopsis provides a clear illustration
of this concept. Scores of genes in Arabidopsis have now been
shown to affect flowering time (Sung and Amasino, 2005).
However, classical genetic experiments have indicated that
ecotype differences in flowering time can be controlled by specific
alleles of only a few loci. F1 plants of the C24 3 Columbia cross, for
instance, exhibit dramatically late flowering. Subsequent mapbased cloning of FRIGIDA (FRI) and FLOWERING LOCUS C (FLC)
revealed that FLC was an impaired locus in C24 and that the FRI
locus in Columbia was null. FLC is a suppressor of flowering, and
FRI is required for expression of FLC. The combination of these
particular alleles of only these two loci results in a dramatically
altered phenotype of a trait that is clearly polygenic.
The flowering time story teaches that the solution to the
problem of dramatically altering traits that appear to be polygenic is to identify appropriate alleles of key loci. As more
functions are assigned to Arabidopsis genes, it appears that
many, if not most traits, are actually polygenic, but many are also
observed to be dramatically affected by specific alleles at a few
important loci. This is because alleles of key loci for many traits,
like flowering time, are divergent enough within the commonly
used natural ecotype gene pools of Arabidopsis to exhibit
qualitative phenotypic effects. The transfer of a highly effective
(qualitative trait) allele into a genome that contains several loci,
each contributing a small effect to that trait (thereby defined as a
quantitative trait controlled by quantitative trait loci), has been
referred to as the Mendelization of the quantative trait (AlonsoBlanco and Koornneef, 2000). The limited allelic diversity at
important loci is recognized by far-sighted plant breeder/
geneticists such as Gurdev Khush (Brar and Khush, 1997),
who expanded the breeding gene pool of rice, and Don Duvick,
who demonstrated that after alleles of key loci that control yield
were introduced to maize by domestication, the narrow genetic
base of modern maize has hardly allowed any advance in
genetic potential for grain yield in over seven decades of
breeding (Duvick and Cassman, 1999). Modern breeders have
been left to work with only the minor (quantitative) loci that
control environment effects on yield. Interestingly, Yamasaki
July 2006
1553
HISTORICAL PERSPECTIVE ESSAY
et al. (2005) have recently reported that alleles of loci that have or
have not been fixed by domestication or crop improvement can
be identified by gene sequence diversity analyses. New unexploited loci controlling agronomic traits that show low allelic
diversity in elite cultivars and thus do not contribute much to
agronomic performance could still have useful allelic diversity in
land races and wild relatives of maize where specific alleles have
not become fixed by selection bottlenecking.
The practical goal of plant molecular geneticists is the same
as that of the plant breeding: to improve crop performance. To
that end, the strategies of both groups are, in reality, remarkably
similar: to find and transfer superior alleles controlling agronomic traits. Only the tactics differ. In the quest for stress tolerant crops, molecular geneticists need to identify as many genes
as possible that affect different parts of the water homeostasis
control systems, including epigenetic and regulatory RNA control components. Physiologists can play an important role in
exploring the specific physiological functions of genes and gene
products in water homeostasis using the genetic material provided by model systems. This would greatly help to identify the
best combinations of key loci by discovering which genes affect
different control points (see Figure 1).
An important perspective on crop yield, promoted in particular
by John Passioura, emphasizes the amount of harvestable
product that can be produced per unit of water used (reviewed in
Richards, 1996). This can be greatly affected by several traits,
such as overall plant size, root growth and architecture, leaf and
canopy architecture, flowering time and development of fruit,
seed growth and development, and other agronomic traits
(Richards, 1996; Maggio et al., 2003). In fact, reducing the
anthesis/silking interval in maize flowering has been shown to
significantly improve yield under drought conditions (Bolaños
and Edmeades, 1993). Although most molecular geneticists do
not now focus on these traits, the next generation molecular
genetics approach toward improvement of osmotic stress tolerance will undoubtedly involve the development of special
forward genetic screens that are designed to identify genes
controlling these kinds of morphological and phenological traits.
This should be followed by the search for, or creation of,
superior alleles of key genes, first by the use of new Arabidopsis
relatives model systems (Inan et al., 2004) and eventually by the
use of exotic (tolerant) germplasm (secondary or tertiary gene
pools) of important crop species. The approach of using genes
identified by model systems that has been expanded to wild
relatives, combined with either direct transformation or zerodistance marker assisted breeding (Rus et al., 2005), should
allow minimization of impaired function or undesirable pleiotropic effects that may result from the direct use of taxonomically
more distant alleles found in Arabidopsis relatives model
systems or other model systems.
The long road from Aristotle to Arabidopsis may be leading us
back to the farmer’s field, which allowed a large human population and the resulting development of modern human cultures
and their eventual adoption of scientific agriculture. Science has
shown that there are five basic requirements for growing crops:
sunlight, CO2, essential minerals, water, and favorable temperatures. Where we grow our crops, we have available plenty of
sunlight, appropriate, although not always optimum, temperatures, and CO2. We can also provide essential minerals. We do
not have enough water of high enough quality (in particular, low
salinity) at the right locations. We know how to engineer (transport
and purify) enough quality water at the needed locations, but we
don’t have enough energy to do it. Without that energy, we must
now learn how to engineer plants to produce more food with less
water of less quality or we will not meet the growing food
demands of our expanding population.
ACKNOWLEDGMENTS
We thank Jose Pardo, Stefania Grillo, Giancarlo Barbieri, and Brian
Larkins for their generous hospitality during the writing of the manuscript
and Rebekah Fagan for infinite patience with the countless versions she
helped to edit. We apologize to the many investigators whose work was
not mentioned only because of space limitations.
Albino Maggio
Department of Agricultural Engineering
and Agronomy
University of Naples Federico II
Portici, Italy 80055
Jian-Kang Zhu
Institute for Integrative Genome Biology, Botany,
and Plant Sciences
University of California
Riverside, CA 92521
Paul M. Hasegawa
Center for Plant Environmental Stress Physiology
Department of Horticulture and
Landscape Architecture
Purdue University
West Lafayette, IN 47907
Ray A. Bressan
Center for Plant Environmental Stress Physiology
Department of Horticulture and
Landscape Architecture
Purdue University
West Lafayette, IN 47907
REFERENCES
Alonso-Blanco, C., and Koornneef, M. (2000). Naturally occurring
variation in Arabidopsis: An underexploited resource for plant genetics. Trends Plant Sci. 5, 22–29.
1554
The Plant Cell
HISTORICAL PERSPECTIVE ESSAY
Baker, S.S., Wilhelm, K.S., and Thomashow, M.F. (1994). The 50region of Arabidopsis thaliana cor15a has cis-acting elements that
confer cold-, drought- and ABA-regulated gene expression. Plant
Mol. Biol. 24, 701–713.
Baulcombe, D. (2004). RNA silencing in plants. Nature 431, 356–363.
Beemster, G.T., Mironov, V., and Inze, D. (2005). Tuning the cell-cycle
engine for improved plant performance. Curr. Opin. Biotechnol. 16,
142–146.
Bierhuizen, J.F., and Slatyer, R.O. (1965). Effect of atmospheric
concentration of water vapour and CO2 in determining transpirationphotosynthesis relationships of cotton leaves. Agric. For. Meteorol. 2,
259–270.
Blatt, R.M. (2000). Cellular signaling and volume control in stomatal
movements in plants. Annu. Rev. Cell Dev. Biol. 16, 221–241.
Bohnert, H.J., and Bressan, R.A. (2002). Abiotic stresses, plant
reactions and new approaches towards understanding stress tolerance. In Abiotic Stresses and Stress Tolerance, J. Nösberger, H.H.
Geiger, and P.C. Struik, eds (Cambridge, MA: CABI Publishing Series,
Oxford University Press), pp. 81–100.
Bolaños, J., and Edmeades, G.O. (1993). Eight cycles of selection for
drought tolerance in lowland tropical maize. II. Responses in reproductive behaviour. Field Crops Res. 31, 253–268.
Borsani, O., Zhu, J., Verslues, P., and Zhu, J.K. (2005). Endogenous
siRNAs derived from a pair of natural cis-antisense transcripts
regulate salt tolerance in Arabidopsis. Cell 123, 1279–1291.
Boudsocq, M., and Laurière, C. (2005). Osmotic signalling in plants.
Multiple pathways mediated by emerging kinase families. Plant
Physiol. 138, 1185–1194.
Boyer, J.S. (1985). Water transport. Annu. Rev. Plant Physiol. 36,
473–516.
Brar, D.S., and Khush, G.S. (1997). Alien introgression in rice. Plant
Mol. Biol. 35, 35–47.
Bressan, R.A., Handa, A.K., Handa, S., and Hasegawa, P.M. (1982).
Growth and water relations of cultured tomato cells after adjustment
to low external water potentials. Plant Physiol. 70, 1303–1309.
Bressan, R.A., Nelson, D.E., Iraki, N.M., La Rosa, C.P., Singh, N.K.,
Hasegawa, P.M., and Carpita, N.C. (1990). Reduced cell expansion
and changes in cell walls of plant cells adapted to NaCl. In Environmental Injury to Plants, F. Katterman, ed (San Diego, CA: Academic
Press), pp 137–171.
Briggs, L.J., and Shantz, H.L. (1914). Relative water requirements of
plants. J. Agric. Res. 3, 1–63.
Brønsted, J.N. (1920). Studies on solubility. I. The solubility of salts in
salt solutions. J. Am. Chem. Soc. 42, 761–786.
Carrington, J.C., and Ambros, V. (2003). Role of microRNAs in plant
and animal development. Science 301, 336–338.
Chaerle, L., Saibo, N., and Van Der Straeten, D. (2005). Tuning the
pores: Towards engineering plants for improved water use efficiency.
Trends Biotechnol. 23, 308–315.
Chinnusamy, V., Ohta, M., Kanrar, S., Lee, B.-h., Hong, X., Agarwal,
M., and Zhu, J.-K. (2003). ICE1, a master regulator of cold induced
transcriptome and freezing tolerance in Arabidopsis. Genes Dev. 17,
1043–1054.
Chinnusamy, V., Schumaker, K., and Zhu, J.K. (2004). Molecular
genetic perspectives on cross-talk and specificity in abiotic stress
signalling in plants. J. Exp. Bot. 55, 225–236.
Chinnusamy, V., Zhu, J., and Zhu, J.-K. (2005). Gene regulation during
cold acclimation in plants. Physiol. Plant. 126, 52–61.
Cleland, R.E. (1971). Cell wall extension. Annu. Rev. Plant Physiol. 22,
197–222.
Cosgrove, D.J. (1987). Wall relaxation and the driving forces for cell
expansive growth. Plant Physiol. 84, 561–564.
Cosgrove, D.J., Li, L.C., Cho, H.-T., Hoffman-Benning, S., Moore,
R.C., and Blecker, D. (2002). The growing world of expansins. Plant
Cell Physiol. 43, 1436–1444.
Darwin, F., and Pertz, D.F.M. (1911). On a new method of estimating
the aperture of stomata. Proc. R. Soc. Lond. B84, 136–154.
de Candolle, A.P. (1832). Physiologie Végétale. (Paris: Bechet Jeune).
de Saussure, N.T. (1804). Recherches Chimiques sur la Végétation.
(Paris: Madam Huzard).
de Vries, H. (1877). Untersuchungen über die Mechanische Ursachen
der Zellstreckung. (Leipzig, Germany: W. Engelmann).
de Wit, C.T. (1958). Transpiration and Crop Yield, No. 64. (Wageningen,
The Netherlands: Institute of Biological and Chemical Research on
Field Crops and Herbage).
Dixon, H.H., and Joly, J. (1895). The path of transpiration current. Ann.
Bot. (Lond.) 9, 416–419.
Doebley, J., Stec, A., and Hubbard, L. (1997). The evolution of apical
dominance in maize. Nature 386, 485–488.
Dutrochet, H.J. (1837). Memoires Pour Servir a l’Histoire Anatomique et
Physiologie des Végétaux et des Animaux. (Paris: JB Baillière et Fils).
Duvick, D.N., and Cassman, K.G. (1999). Post-green revolution trends
in yield potential of temperate maize in the North-Central United
States. Crop Sci. 39, 1622–1630.
Epstein, E. (1985). Salt-tolerant crops – Origins, development, and
prospects of the concept. Plant Soil 89, 187–198.
Fan, L.M., Zhao, Z.X., and Assmann, S.M. (2004). Guard cells: A
dynamic signaling model. Curr. Opin. Plant Biol. 7, 537–546.
Farquhar, G.D., and Sharkey, T.D. (1982). Stomatal conductance and
photosynthesis. Annu. Rev. Plant Physiol. 33, 317–345.
Finkelstein, R.R., Gampala, S.S.L., and Rock, C.D. (2002). ABA
signaling in seeds and seedlings. Plant Cell 13, S15–S45.
Finkelstein, R.R., and Gibson, S.I. (2002). ABA and sugar interactions
regulating development: ÔCross-talkÕ or Ôvoices in a crowdÕ? Curr.
Opin. Plant Biol. 5, 26–32.
Fire, A., Xu, S., Montgomery, M.K., Kostas, S.A., Driver, S.E., and
Mello, C.C. (1998). Potent and specific genetic interference by doublestranded RNA in Caenorhabditis elegans. Nature 391, 806–811.
Fischer, R.A., and Hsiao, T.C. (1968). Stomatal opening in isolated
epidermal strips of Vicia faba. II. Responses to KC1 concentration and
the role of potassium absorption. Plant Physiol. 43, 1953–1958.
Gibbs, J.W. (1931). The Collected Works, Vol. 1. (New York: Longmans,
Green and Co.).
Gorham, J., Bridges, J., Dubcovsky, J., Dvorak, J., Hollington, P.A.,
Luo, M.C., and Khan, J.A. (1997). Genetic analysis and physiology of
a trait for enhanced K1/Na1 discrimination in wheat. New Phytol. 137,
109–116.
Graham, T. (1862). Liquid diffusion applied to analysis. Philos. Trans. R.
Soc. Lond. B Biol. Sci. 151, 183–224.
Green, P.B., Erickson, R.O., and Buggy, J. (1971). Metabolic and
physical control of cell elongation rate. In vivo studies of Nitella. Plant
Physiol. 47, 423–430.
Hales, S. (1727). Vegetable Staticks. (London: W. & J. Innys and
T. Woodward). (Reprinted by Scientific Book Guild, London).
Hamilton, A.J., and Baulcombe, D.C. (1999). A species of small
antisense RNA in post-transcriptional gene silencing in plants.
Science 286, 950–952.
July 2006
1555
HISTORICAL PERSPECTIVE ESSAY
Handa, A.K., Bressan, R.A., Handa, S., and Hasegawa, P.M. (1983).
Clonal variation for tolerance to polyethylene glycol-induced waterstress in cultured tomato cells. Plant Physiol. 72, 645–653.
Hasegawa, P.M., Bressan, R.A., Zhu, J.-K., and Bohnert, H.J. (2000).
Plant cellular and molecular responses to high salinity. Annu. Rev.
Plant Physiol. Plant Mol. Biol. 51, 463–499.
Himmelbach, A., Yang, Y., and Grill, E. (2003). Relay and control of
abscisic acid signalling. Curr. Opin. Plant Biol. 6, 470–479.
Huber, B. (1924). Die beurteilung des wasserhaushaltes der pflanze.
Jahr. Wiss. Bot. 64, 1–120.
Inan, G., et al. (2004). Salt cress. A halophyte and cryophyte Arabidopsis
relative model system and its applicability to molecular genetic analyses of growth and development of extremophiles. Plant Physiol. 135,
1718–1737.
Ingram, J., and Bartels, D. (1996). The molecular basis of dehydration
tolerance in plants. Annu. Rev. Plant Physiol. Plant Mol. Biol. 47,
377–403.
Jonak, C., Ökrész, L., Bögre, L., and Hirt, H. (2002). Complexity, cross
talk and integration of plant MAP kinase signaling. Curr. Opin. Plant
Biol. 5, 415–424.
Knight, H., Trewavas, A.J., and Knight, M.R. (1997). Calcium signalling in Arabidopsis thaliana responding to drought and salinity. Plant J.
12, 1067–1078.
Koornneef, M., Alonso-Blanco, C., and Vreugdenhil, D. (2004). Naturally occurring genetic variation in Arabidopsis thaliana. Annu. Rev.
Plant Biol. 55, 141–172.
Kramer, P.J. (1950). Effects of wilting on the subsequent intake of water
by plants. Am. J. Bot. 37, 280–284.
Kramer, P.J. (1988). Changing concepts regarding plant water relations.
Plant Cell Environ. 11, 565–568.
Kramer, P.J., and Boyer, J.S. (1995). Water relations of plants and soil.
(San Diego, CA: Academic Press).
Kuhn, J.M., and Schroeder, J.I. (2003). Impacts of altered RNA metabolism on abscisic acid signaling. Curr. Opin. Plant Biol. 6, 463–469.
Lander, E.S., and Weinberg, R.A. (2000). Genomics: Journey to the
center of biology. Science 287, 1777–1782.
Lee, R.C., Feinbaum, R.L., and Ambros, V. (1993). The C. elegans
heterochronic gene lin-4 encodes small RNAs with antisense complementarity tol in-14. Cell 75, 843–854.
Li, J., Wang, X.Q., Watson, M.B., and Assmann, S.M. (2000). Regulation of abscisic acid-induced stomatal closure and anion channels
by guard cell AAPK kinase. Science 287, 300–303.
Lippman, Z., and Tanksley, S.D. (2001). Dissecting the genetic pathway to extreme fruit size in tomato using a cross between the smallfruited wild species Lycopersicon pimpinellifolium and L. esculentum
var. giant heirloom. Genetics 158, 413–422.
Liu, J., and Zhu, J.-K. (1998). A calcium sensor homolog required for
plant salt tolerance. Science 280, 1943–1945.
Liu, Q., Kasuga, M., Sakuma, Y., Abe, H., Miura, S., YamaguchiShinozaki, K., and Shinozaki, K. (1998). Two transcription factors,
DREB1 and DREB2, with an EREBP/AP2 DNA binding domain
separate two cellular signal transduction pathways in drought- and
low-temperature-responsive gene expression, respectively, in Arabidopsis. Plant Cell 10, 1391–1406.
Lockhart, J.A. (1965). An analysis of irreversible plant cell elongation.
J. Theor. Biol. 8, 264–275.
Lu, C., Tej, S.S., Luo, S., Haudenschild, C.D., Meyers, B.C., and
Green, P.J. (2005). Elucidation of the small RNA component of the
transcriptome. Science 309, 1567–1569.
Maggio, A., and Joly, R.J. (1995). Effects of mercuric chloride on root
hydraulic conductivity of tomato root systems: Evidence for a channel
mediated water pathway. Plant Physiol. 109, 331–335.
Maggio, A., Joly, R.J., Hasegawa, P.M., and Bressan, R.A. (2003).
Can the quest for drought tolerant crops avoid Arabidopsis any
longer? In Crop Production Under Saline Environments: Global and
Integrative Perspectives, S.S. Goyal, S.K. Sharma, and D.W. Rains,
eds (Binghamton, NY: Food Products Press, The Howorth Press), pp.
99–129.
Marcotte, W.R.J., Bayley, C.C., and Quatrano, R.S. (1988). Regulation
of a wheat promoter by abscisic acid in rice protoplasts. Nature 335,
454–457.
Marin, E., Nussaume, L., Quesada, A., Gonneau, M., Sotta, B.,
Hugueney, P., Frey, A., and Marion-Poll, A. (1996). Molecular
identification of zeaxanthin epoxidase of Nicotiana plumbaginifolia, a
gene involved in abscisic acid biosynthesis and corresponding to the
ABA locus of Arabidopsis thaliana. EMBO J. 15, 2331–2342.
Masle, J., Glmore, S.R., and Farquhar, G.D. (2005). The ERECTA
gene regulates plant transpiration efficiency in Arabidopsis. Nature
436, 866–870.
Matsuda, K., and Riazi, A. (1981). Stress induced osmotic adjustment
in growing regions of barley leaves. Plant Physiol. 68, 571–576.
Maurel, C., Reizer, J., Schroeder, J.I., and Chrispeels, M.J. (1993).
The vacuolar membrane protein g-TIP creates water specific channels
in Xenopus oocytes. EMBO J. 12, 2241–2247.
McQueen-Mason, S., Durachko, D.M., and Cosgrove, D.J. (1992).
Two endogenous proteins that induce cell wall extension in plants.
Plant Cell 4, 1425–1433.
Meidner, H. (1983). Our understanding of plant water relations. J. Exp.
Bot. 34, 1606–1618.
Meyer, R.F., and Boyer, J.S. (1981). Osmoregulation, solute distribution, and growth in soybean seedlings having low water potentials.
Planta 151, 482–489.
Mustilli, A.C., Merlot, S., Vavasseur, A., Fenzi, F., and Giraudat, J.
(2002). Arabidopsis OST1 protein kinase mediates the regulation of
stomatal aperture by abscisic acid and acts upstream of reactive
oxygen species production. Plant Cell 14, 3089–3099.
Napoli, C., Lemieux, C., and Jorgensen, R. (1990). Introduction of a
chimeric chalcone synthase gene into petunia results in reversible cosuppression of homologous genes in trans. Plant Cell 2, 279–289.
Nobel, P.S. (1991). Physicochemical and Environmental Plant Physiology. (San Diego, CA: Academic Press).
Noyes, A.A., and Bray, W.C. (1911). The effect of salts on the solubility
of other salts. I. J. Am. Chem. Soc. 33, 1643–1649.
Oparka, K.J. (1994). Plasmolysis: New insights into an old process.
New Phytol. 126, 571–591.
Osakabe, Y., Maruyama, K., Seki, M., Satou, M., Shinozaki, K., and
Yamaguchi-Shinozaki, K. (2005). Leucine-Rich Repeat ReceptorLike Kinase1 is a key membrane-bound regulator of abscisic acid
early signaling in arabidopsis. Plant Cell 17, 1105–1119.
Pardo, J.M., Cubero, B., Leidi, E.O., and Quintero, F.J. (2006). Alcali
cation exchangers: Roles in cellular homeostasis and stress tolerance. J. Exp. Bot. 57, 1181–1199.
Passioura, J.B. (1977). Grain yield, harvest index, and water use of
wheat. J. Aust. Inst. Agric. Sci. 43, 117–120.
Passioura, J.B. (1988). Response to Dr. P. J. Kramer article ÔChanging
concepts regarding plant water relationsÕ, Volume 11, Number 7, pp.
565–568. Plant Cell Environ. 11, 569–571.
1556
The Plant Cell
HISTORICAL PERSPECTIVE ESSAY
Passioura, J.B., and Fry, S.C. (1992). Turgor and cell expansion:
Beyond the Lockhart equation. Aust. J. Plant Physiol. 19, 565–576.
Peng, J., et al. (1999). ÔGreen revolutionÕ genes encode mutant
gibberellin response modulators. Nature 400, 256–261.
Pfeffer, W.F.P. (1877). Osmotische Untersuchunger. Studien zur Zell
Mechanik. (Leipzig, Germany: W. Engelmann). (English translation by
G.R. Kepner and E.J. Stadelmann, 1985: Osmotic Investigations:
Studies on Cell Mechanics, Van Nostrand Reinhold, New York).
Pierce, M.L., and Raschke, K. (1980). Correlation between loss of
turgor and accumulation of abscisic acid in detached leaves. Planta
148, 174–182.
Preston, G.M., Carrol, T.P., Guggino, W.B., and Agre, P. (1992).
Appearance of water channels in Xenopus oocytes expressing red
cell CHIP28 protein. Science 265, 385–387.
Raschke, K., and Hedrich, R. (1985). Simultaneous and independent
effects of abscisic acid on stomata and the photosynthetic apparatus
in whole leaves. Planta 163, 105–118.
Rayle, D.L., and Cleland, R.E. (1970). Enhancement of wall loosening
and elongation by acid solutions. Plant Physiol. 46, 250–253.
Reid, E.W. (1890). Osmosis experiments with living and dead membranes. J. Physiol. (Lond.) 11, 312–351.
Riechmann, J.L., and Meyerowitz, E.M. (1998). The AP2/EREBP family
of plant transcription factors. Biol. Chem. 379, 633–646.
Renner, O. (1912). Versuche zur mechanik der wasserversorgung. Über
wurzeltätigkeitt. Ber. Dtsch. Bot. Ges. 30, 642–648.
Richards, R.A. (1996). Defining selection criteria to improve yield under
drought. In Drought Tolerance in Higher Plants: Genetical, Physiological and Molecular Biological Analysis, E. Belhassen, ed (Dordrecht,
The Netherlands: Kluwer Academic Publishers), pp. 79–88. (Reprinted
from Plant Growth Regulation, Vol. 20, No. 2).
Riera, M., Valon, C., Fenzi, F., Giraudat, J., and Leung, J. (2005). The
genetics of adaptive responses to drought stress: Abscisic aciddependent and abscisic acid-independent signalling components.
Physiol. Plant. 123, 111–119.
Rock, C., and Quatrano, R.S. (1995). The role of hormones during seed
development. In Plant Hormones: Physiology, Biochemistry and
Molecular Biology, P. Davies, ed (Dordrecht, The Netherlands: Kluwer
Academic Publishers), pp. 671–697.
Rock, C.D. (2000). Pathways to ABA-regulated gene expression. New
Phytol. 148, 357–396.
Ruggiero, B., Koiwa, H., Manabe, Y., Quist, T.M., Inan, G., Saccardo,
F., Joly, R.J., Hasegawa, P.M., Bressan, R.A., and Maggio, A.
(2004). Uncoupling the effects of ABA on plant growth and water
relations: Analysis of sto1/nced3, ABA deficient salt stress tolerant
mutant in Arabidopsis. Plant Physiol. 136, 3134–3147.
Rus, A., Yokoi, S., Sharkhuu, A., Reddy, M., Lee, B.-H., Matsumoto,
T.K., Koiwa, H., Zhu, J.-K., Bressan, R.A., and Hasegawa, P.M.
(2001). AtHKT1 is a salt tolerance determinant that controls Na(1)
entry into plant roots. Proc. Natl. Acad. Sci. USA 98, 14150–14155.
Rus, A.M., Bressan, R.A., and Hasegawa, P.M. (2005). Unraveling salt
tolerance in crops. Nat. Genet. 37, 1029–1030.
Saab, I.N., Sharp, R.E., Pritchard, J., and Voetberg, G.S. (1990).
Increased endogenous abscisic acid maintains primary root growth
and inhibits shoot growth of maize seedlings at low water potentials.
Plant Physiol. 93, 1329–1336.
Saleki, R., Young, P., and Lefebvre, D.D. (1993). Mutants of
Arabidopsis thaliana capable of germination under saline conditions.
Plant Physiol. 101, 839–845.
Salisbury, F.B., and Ross, C.W. (1992). Plant Physiology, 4th ed.
(Belmont, CA: Wadsworth Publishing).
Shi, H., Ishitani, M., Kim, C., and Zhu, J.-K. (2000). The Arabidopsis
thaliana salt tolerance gene SOS1 encodes a putative Na1/H1
antiporter. Proc. Natl. Acad. Sci. USA 97, 6896–6901.
Shull, C.A. (1916). Measurement of the surface forces in soil. Bot. Gaz.
62, 1–31.
Slatyer, R.O. (1967). Plant-Water Relationships. (New York: Academic
Press).
Slatyer, R.O., and Taylor, S.A. (1960). Terminology in plant- and soilwater relations. Nature 187, 922–924.
Steudle, E. (2001). The cohesion-tension mechanisms and the acquisition of water by plant roots. Annu. Rev. Plant Physiol. Plant Mol. Biol.
52, 847–875.
Steudle, E., and Henzler, T. (1995). Water channels in plants: Do
basic concepts of water transport change? J. Exp. Bot. 46, 1067–
1076.
Stockinger, E.J., Gilmour, S.J., and Thomashow, M.F. (1997).
Arabidopsis thaliana CBF1 encodes an AP2 domain-containing
transcriptional activator that binds to the C-repeat/DRE, a cis-acting
DNA regulatory element that stimulates transcription in response to
low temperature and water deficit. Proc. Natl. Acad. Sci. USA 94,
1035–1040.
Sung, D.Y., Kaplan, F., Lee, K.J., and Guy, C.L. (2003). Acquired
tolerance to temperature extremes. Trends Plant Sci. 8, 179–187.
Sung, S., and Amasino, R.M. (2005). Remembering winter: Towards a
molecular understanding of vernalization. Annu. Rev. Plant Biol. 56,
491–508.
Sunkar, R., and Zhu, J.-K. (2004). Novel and stress regulated
miRNAs and other small RNAs from Arabidopsis. Plant Cell 16,
2001–2019.
Taiz, L. (1984). Plant cell expansion: Regulation of cell wall mechanical
properties. Annu. Rev. Plant Physiol. 35, 585–657.
Taiz, L., and Zeiger, E. (1998). Plant Physiology. (Sunderland, MA:
Sinauer Associates).
Tal, M., and Imber, D. (1971). Abnormal stomatal behavior and
hormonal imbalance in flacca, a wilty mutant of tomato. III. Hormonal
effects on the water status in the plant. Plant Physiol. 47, 849–850.
Teige, M., Scheikl, E., Eulgem, T., Dóczi, R., Ichimura, K., Shinozaki,
K., Dangl, J.L., and Hirt, H. (2004). The MKK2 pathway
mediates cold and salt stress signaling in Arabidopsis. Mol. Cell 15,
141–152.
Traube, M. (1867). Experimente zur Theorie der Zellbildung und
Endosmose. Archiv. Anat. Physiol. wiss Medicin 87, 165.
Turner, N.C. (1997). Further progress in crop water relations. Adv.
Agron. 58, 293–338.
Verslues, P.E., Agarwal, M., Katiyar-Agarwal, S., Zhu, J., and Zhu,
J.-K. (2006). Methods and concepts in quantifying resistance to
drought, salt and freezing, abiotic stresses that affect plant water
status. Plant J. 45, 523–539.
von Sachs, J. (1882). Lectures on the Physiology of Plants, 2nd ed.
(English translation by Ward, Oxford University Press, UK).
Weatherley, P.E. (1966). The state and movement of water in the leaf.
Symp. Soc. Exp. Biol. 19, 157–184.
Weevers, T., and Went, F.W. (1949). Fifty Years of Plant Physiology.
(Waltham, MA: The Chronica Botanica Co.).
Wightman, B., Ha, I., and Ruvkun, G. (1993). Posttranscriptional
regulation of the heterochronic gene lin-14 by lin-4 mediates temporal
pattern formation in C. elegans. Cell 75, 855–862.
July 2006
1557
HISTORICAL PERSPECTIVE ESSAY
Woodward, A.W., and Bartel, B. (2005). Auxin: Regulation, action, and
interaction. Ann. Bot. (Lond.) 95, 707–735.
Xiong, L., and Zhu, J.-K. (2003). Regulation of abscisic acid biosynthesis. Plant Physiol. 133, 29–36.
Yamaguchi-Shinozaki, K., and Shinozaki, K. (1994). A novel cisacting element in an Arabidopsis gene is involved in responsiveness
to drought, low-temperature, or high-salt stress. Plant Cell 6,
251–264.
Yamaguchi-Shinozaki, K., and Shinozaki, K. (2005). Organization of
cis-acting regulatory elements in osmotic- and cold-stress-responsive
promoters. Trends Plant Sci. 10, 88–94.
Yamasaki, M., Tenaillon, M.I., Bi, I.V., Schroeder, S.G., SanchezVilleda, H., Doebley, J.F., Gaut, B.S., and McMullen, M.D. (2005). A
large-scale screen for artificial selection in maize identifies candidate
agronomic loci for domestication and crop improvement. Plant Cell
17, 2859–2872.
Yang, S.F., and Hoffman, N.E. (1984). Ethylene biosynthesis
and its regulation in higher plants. Annu. Rev. Plant Physiol. 35,
155–189.
Zhu, G.L., and Boyer, J.S. (1992). Enlargement in Chara studied with a
turgor clamp: Growth rate is not determined by turgor. Plant Physiol.
100, 2071–2080.
Zhu, J. (2000). Genetic analysis of plant salt tolerance using Arabidopsis
thaliana. Plant Physiol. 124, 941–948.
Zhu, J.-K. (2002). Salt and drought stress signal transduction in plants.
Annu. Rev. Plant Biol. 53, 247–273.
Zhu, J.-K. (2003). Regulation of ion homeostasis under salt stress. Curr.
Opin. Plant Biol. 6, 441–445.
Osmogenetics: Aristotle to Arabidopsis
Albino Maggio, Jian-Kang Zhu, Paul M. Hasegawa and Ray A. Bressan
Plant Cell 2006;18;1542-1557
DOI 10.1105/tpc.105.040501
This information is current as of June 17, 2017
References
This article cites 117 articles, 42 of which can be accessed free at:
/content/18/7/1542.full.html#ref-list-1
Permissions
https://www.copyright.com/ccc/openurl.do?sid=pd_hw1532298X&issn=1532298X&WT.mc_id=pd_hw1532298X
eTOCs
Sign up for eTOCs at:
http://www.plantcell.org/cgi/alerts/ctmain
CiteTrack Alerts
Sign up for CiteTrack Alerts at:
http://www.plantcell.org/cgi/alerts/ctmain
Subscription Information
Subscription Information for The Plant Cell and Plant Physiology is available at:
http://www.aspb.org/publications/subscriptions.cfm
© American Society of Plant Biologists
ADVANCING THE SCIENCE OF PLANT BIOLOGY