Plant Nutrition 1: Membrane
energetics and transport, potassium
nutrition and sodium toxicity
• Introduction to plant nutrition
• Energizing the membrane
• H+-ATPases and H+-PPases
• Potassium
• Uptake, transport and homeostasis
• Sodium
• Toxicity, transport and tolerance
© 2014 American Society of Plant Biologists
Plant nutrition: Introduction
Plants are ~70 to
>90% water by
weight
N
Nitrogen
P Phosphorus
K
Potassium
CO2,
photosynthesis
7% Other,
from soil
42% Carbon
7% Hydrogen
93% of plant
dry mass is
composed of
C, O and H
44% Oxygen
H2O water
Ca Calcium
Mg Magnesium
S Sulfur
Si Silicon
Cl Chlorine
Other
These elements are
obtained mainly from soil,
are often referred to as
mineral nutrients, and are
the subject of the topic
Plant Nutrition
© 2014 American Society of Plant Biologists
Nutrient uptake, assimilation and
utilization involve many processes
Nutrient
acquisition
efficiency
Root
exudates
Nutrient usage
efficiency
X
Root system
architecture
Intercellular
transport
efficiency
P
P
Symbioses
NH3
R-X
Assimilation and
remobilization
efficiency
Transporters
and pumps
Regulatory and
homeostatic
networks
N N
Rhizosphere
microbiota
© 2014 American Society of Plant Biologists
Nutrients are concentrated in the
plant relative to the environment
Energy is expended to assimilate nutrients
against a steep concentration gradient
Soil abundance
(ranges or typical
values)
Cell
[K+]o
0.1 – 1 mM
[K+]i
50 - 100 mM
[H2PO4-]o
[HPO42-]o
< 1 μM
[H2PO4-]i
[HPO42-]i
5 - 10 mM
The driving force of
the nutrient’s
chemical gradient is
outwards
[NO3-]o
<100 μM –
>1 mM
[NO3-]i
10 mM
[NH4+]o
<100 μM –
>1 mM
[NH4+]i
~1 mM
© 2014 American Society of Plant Biologists
The electrochemical gradient is
important for ion transport
The electrochemical gradient
defines the energetic demands for
transport, and integrates the
electrical and concentration
gradients
[K+]o
0.1 – 1 mM
[K+]i
50 - 100 mM
The cell’s electrical gradient
drives anions OUT and cations IN
[H2PO4-]o
[HPO42-]o
< 1 μM
[H2PO4-]i
[HPO42-]i
5 - 10 mM
[NO3-]o
<100 μM –
>1 mM
[NO3-]i
10 mM
[NH4+]o
<100 μM –
>1 mM
[NH4+]i
~1 mM
Em = ~ -150 mV
© 2014 American Society of Plant Biologists
Transport can be down or against an
electrochemical gradient
Down an electrochemical gradient
(Diffusion or facilitated diffusion)
Against an electrochemical gradient
(Active transport)
Symport
Antiport
OUT
IN
Through
membrane
Through
channel
Through
carrier
ATP
ADP + Pi
Primary active transport:
Directly coupled to ATP
hydrolysis
Secondary active
transport: Indirectly
coupled to ATP
hydrolysis
© 2014 American Society of Plant Biologists
Solutes cross membranes through
different types of transporters
ATP
ADP
•
•
•
•
X
Pumps are often drawn
like lollipops, with a large
cytoplasmic catalytic
domain
X H+
Channels:
are protein-formed holes in the membrane
can be open or closed
move one type of solute at a time
do not provide an energy source for the
movement; solutes can only move down
their electrochemical gradient
•
move solutes against a
chemical or charge
gradient
couple transport to
hydrolysis of ATP or
pyrophosphate
X
Coupled transporters
are often drawn as
circles with arrows
indicating the direction
of flow for each ion
Carriers /
Coupled Transporters
Pumps:
•
H+
X
X
Channels are often
drawn as two adjacent
ovals (or a cross-section
of a doughnut)
•
•
•
•
are membrane proteins
can be active or inactive
can move more than one solute at a time
the driver (usually H+ in plants) moves down its
electrochemical gradient, which provides the energy
for the co-transported solute’s transport
© 2014 American Society of Plant Biologists
Energizing the membrane: Plant H+ATPases and VH+-PPases
The plasma-membrane
H+-ATPase uses energy
from ATP to pump
protons out of the cell
The vacuolar-type proton
pumps transport protons into
the lumen of endomembrane
compartments (e.g., vacuole)
The VH+-ATPase
is a multimeric
protein complex
The H+-PPase uses energy
stored in pyrophosphate
Sze, H., Li, X. and Palmgren, M.G. (1999). Energization of plant cell membranes by H +-pumping ATPases: Regulation and biosynthesis. Plant Cell. 11: 677-689.
© 2014 American Society of Plant Biologists
The PM H+-ATPase is a “master
enzyme” and “powerhouse”
ATP
H+
+++++
pH ~ 5 - 6
----ADP
~ -150 mV
pH ~ 7.5
Channels
Cations Anions
Antiporters
Symporters
H+
Uniporters
By pumping protons out
of the cell, PM H+ATPases produce electric
and pH gradients
The electrochemical
gradient produced by the
PM H+-ATPase drives other
transport processes
H+
See Palmgren, M.G. (2001). Plant plasma membrane H+-ATPases: Powerhouses for nutrient uptake. Annu. Rev. Plant Physiol. Plant Mol.
Biol. 52: 817-845; Figure adapted from Michelet, B. and Boutry, M. (1995). The plasma membrane H+-ATPase. Plant Physiol. 108: 1-6.
© 2014 American Society of Plant Biologists
Vacuolar pumps pump protons into
the vacuole and endocompartments
H+
pH 5 - 6
pH 7.5
H+
PPi
Protons are pumped into the
vacuole by:
• Vacuolar H+-ATPases (VH+ATPases) and
• Vacuolar pyrophosphatases
(H+-PPases)
H+
H+
pH 3 - 6
ATP
ADP + Pi
2 x Pi
H+
Em = ~ -30 mV
Sze, H., Li, X. and Palmgren, M.G. (1999). Energization of plant cell membranes by H +-pumping ATPases: Regulation and biosynthesis. Plant Cell. 11: 677-689.
Isayenkov, S., Isner, J.C. and Maathuis, F.J.M. (2010). Vacuolar ion channels: Roles in plant nutrition and signalling. FEBS letters. 584: 1982-1988.
© 2014 American Society of Plant Biologists
K+ and Na+ - “The twins”. So alike
yet so different
Potassium NaCl
deficiency toxicity
Sodium (Na) and potassium (K):
• Same column of the periodic table
• Both have a single electron in the
outer shell so form monovalent
cations
• Both are very abundant elements
K
And yet, potassium is an
essential nutrient, and
sodium frequently is toxic
FAO
© 2014 American Society of Plant Biologists
Potassium uptake, transport and
homeostasis
Potassium is an essential macronutrient
Enhances
fertility
Promotes stress
tolerance
Regulates
enzyme activities
Strengthens
cell walls
Stimulates
photosynthate
translocation
Maintains turgor
and reduces wilting
Regulates
stomatal
conductance,
photosynthesis
and transpiration
Symptoms of
potassium deficiency
Maintains ionic
homeostasis
[K+] in soil = ~0.1 – 1 mM
[K+] in plant cell
cytoplasm = ~100 mM
See Wang, M., Zheng, Q., Shen, Q. and Guo, S. (2013). The critical role of potassium in plant stress response. Intl. J. Mol. Sci. 14: 7370-7390; Sin Chee Tham /Photo; Purdue extension; Onsemeliot.
© 2014 American Society of Plant Biologists
Potassium is an essential plant
nutrient
K+ is a counter ion for
negatively charged molecules
including DNA and proteins
K+ moves in and
out of the vacuole
through specific
transporters
K+ uptake
involves high
and low affinity
transporters
K+ is a cofactor for
some enzymes
As the major cation in
the vacuole, K+
contributes to cell
expansion and
movement, including
that of guard cells
Reprinted from Maathuis, F.J.M. (2009). Physiological functions of mineral macronutrients. Curr. Opin. Plant Biol. 12: 250-258 with permission from Elsevier.
© 2014 American Society of Plant Biologists
Potassium homeostasis: Responses
to low K+ availability
Low K
Membrane
hyperpolarization
Hormonal changes
(auxin, ethylene)
Calcium
signaling
Direct effects
Indirect effects
Transcriptional
induction of HAK5
K+ channel
More efficient
uptake through
K+ channels
Enhanced root
growth and
gravitropic
responses
K+ uptake
Adapted from Chérel, I., Lefoulon, C., Boeglin, M. and Sentenac, H. (2013). Molecular mechanisms
involved in plant adaptation to low K+ availability. J. Exp. Bot. 65: 833-848.
© 2014 American Society of Plant Biologists
K+ mobilization is critical for K+
homeostasis
As K+ becomes
limiting, it becomes
preferentially allocated
to the cytosol
Cytosol
Vac.
K+ can be remobilized
from less essential tissues
into prioritized tissues
such as growing and
photosynthetic tissues
Prioritized
NonPrioritized
Adapted from Amtmann, A., and Leigh, R. (2010). Ion homeostasis. In Abiotic Stress Adaptation in Plants: Physiological, Molecular and Genomic
Foundation, A. Pareek, S.K. Sopory, H.J. Bohnert and Govindjee (eds) (Dordrecht, The Netherlands: Springer), pp. 245 – 262.
© 2014 American Society of Plant Biologists
Sodium toxicity, transport and
tolerance
To demonstrate his (fake)
madness, Odysseus plowed
salt into his field
You can’t take salt
out of soil easily;
once it is there it
stays there
Colum, P. (1918). The Adventures of Odysseus and the Tale of Troy. Project Gutenberg; USDA, USDA; FAO
© 2014 American Society of Plant Biologists
How can we address the problems
caused by soil salinization?
Avoid adding to the
problem by better
management of
fragile soil systems
Areas of concern
Salicornia europaea
Identify halophytes
that can be used as
food or energy crops
Chenopodium
quinoa
Arthrocnemum
macrostachyum
Identify
responses to salt
stress in saltsensitive species
(glycophytes)
Thinopyrum
ponticum
Study salt-tolerant relatives
of crop plants
Munns, R., James, R.A., Xu, B., Athman, A., Conn, S.J., Jordans, C.,
Byrt, C.S., Hare, R.A., Tyerman, S.D., Tester, M., Plett, D. and
Gilliham, M. (2012). Wheat grain yield on saline soils is improved by
an ancestral Na+ transporter gene. Nat Biotech. 30: 360-364.
CSIRO; The State of Victoria; Maurice Chédel; Marco Schmidt
Learn about salt
tolerance from
naturally salttolerant species
(halophytes)
Introduce salinity-tolerance
traits into crop plants through
breeding and engineering
Geng, Y., Wu, R., Wee, C.W., Xie, F., Wei, X., Chan, P.M.Y.,
Tham, C., Duan, L. and Dinneny, J.R. (2013). A spatio-temporal
understanding of growth regulation during the salt stress response
in Arabidopsis. Plant Cell. 25: 2132-2154.
© 2014 American Society of Plant Biologists
Plant species have a broad range of
salinity tolerances
Saltbush (Atriplex amnicola)
is a halophyte that can
tolerate very salty soil
Q. Can we identify and
exploit the mechanistic
basis of increased
salinity tolerance?
A. YES!
Arabidopsis
and rice are
quite sensitive
Reprinted by permission of Annual Reviews from Munns, R. and Tester, M. (2008). Mechanisms of salinity tolerance. Annu. Rev. Plant Biol. 59: 651-681.
© 2014 American Society of Plant Biologists
Mechanisms of sodium toxicity and
tolerance
SALINITY STRESS
Ionic stress:
K+ deficiency /
excess Na+ influx
Inhibition of:
enzyme activity,
protein synthesis,
photosynthesis
Leaf senescence
Osmotic stress
Oxidative
stress
Detoxification
strategies
Ion homeostasis:
Na+ extrusion,
Na+ exclusion,
Na+ compartmentation
Inhibition of:
water uptake,
growth,
photosynthesis
Osmotic
adjustment:
Accumulation of
solutes
Adapted from Horie, T., Karahara, I. and Katsuhara, M. (2012). Salinity tolerance mechanisms in glycophytes: An overview with the central focus on rice plants. Rice. 5: 11; see also Munns, R. and Tester, M. (2008). Mechanisms of salinity
tolerance. Annu. Rev. Plant Biol. 59: 651-681 and Shabala, S. and Pottosin, I. (2014). Regulation of potassium transport in plants under hostile conditions: implications for abiotic and biotic stress tolerance. Physiol. Plant. 151: 257-279.
© 2014 American Society of Plant Biologists
General sodium tolerance strategy:
Keep sodium out of cytosol & shoot
1. Keep Na+ from
entering plant / cells
OUT
Na+
2. Pump out any
Na+ that leaks in
IN
Na+
4. Extrude Na+
via salt glands
“OUT”
K+
Na+
3. Compartmentation
of Na+ in vacuole
Na+
5. Accumulate K+
to maintain a high
ratio of K+ to Na+
Compatible
solutes
6. Synthesize
compatible solutes for
osmotic balance
7. Prevent Na+
from moving into
the shoot and
leaves
© 2014 American Society of Plant Biologists
Breeding and engineering for salt
tolerance
Salt tolerance can
be attributed to
three nonexclusive
mechanisms
Salinity
tolerance can
be enhanced by
breeding or
engineering
Reprinted from Roy, S.J., Negrão, S. and Tester, M. (2014). Salt resistant crop plants. Curr. Opin. Biotech. 26: 115-124.
© 2014 American Society of Plant Biologists
Wheat yield on saline soils improved
by an ancestral Na+ transporter gene
A pair of genes derived from a
relative of wheat confers enhanced
salinity tolerance
Because these species are
closely related, the genes
can be introduced into
cultivated wheat without
using GM methods
Tetraploid
pasta wheat
Hexaploid
bread wheat
Durum wheat carrying salttolerance genes
Huang, S., Spielmeyer, W., Lagudah, E.S. and Munns, R. (2008). Comparative mapping of HKT genes in wheat, barley, and rice, key determinants
of Na+ transport, and salt tolerance. J. Exp. Bot. 59: 927-937 by permission of Oxford University Press; Credit: Dr Richard James, CSIRO
© 2014 American Society of Plant Biologists
Interaction between K+ nutrition and
Na+ toxicity
Cytosol
Vac.
K+ / Na+ ratio
Plants must
coordinate the
actions of K+ and Na+
transporters to
maintain a high ratio
of K+ to Na+ in
prioritized tissues
Prioritized
NonPrioritized
K+ / Na+ ratio
Adapted from Amtmann, A., and Leigh, R. (2010). Ion homeostasis. In Abiotic Stress Adaptation in Plants: Physiological, Molecular and Genomic
Foundation, A. Pareek, S.K. Sopory, H.J. Bohnert and Govindjee (eds) (Dordrecht, The Netherlands: Springer), pp. 245 – 262.
© 2014 American Society of Plant Biologists
Summary and ongoing research
• Nutrient uptake is extremely energetically demanding
• Proton motive force generated by proton pumps is
essential for nutrient uptake
• Dozens of membrane transporters are involved in
uptake, allocation and homeostasis of mineral nutrients
• Most plants require a high cytosolic ratio of K+ to Na+
• Plants require large amounts of potassium for optimal
growth
NO3-
NO3-
K+
K+
• Sodium toxicity is a real and growing problem
• The mechanisms of sodium tolerance are being
identified and exploited for plant breeding
PO43PO43-
PO43-
© 2014 American Society of Plant Biologists