PLS 622 Plant Physiology I, Monday, September 11, 2006
Section II: Embryo and seed development:
Lecture VIII: Fruit and Seed development:
IMPORTANT TOPICS TO BE COVERED:
Fruit development:
- We will learn about the four different developmental phases fruit progress through from the time
following fertilization of the ovules in the ovary until the carpel(s) (in most cases) ripen.
- We will learn about the the hormonal messages (at least those we have discovered to date) that
act as cues for each of the four developmental phases.
- The different morphological classes of "fruit" and what distinguishes them will be examined.
Seed development:
- The different types of reserves stored in developing seeds will be discussed.
- The four cardinal classifications of protein based on solubility will be listed.
- We will examine the mode of and site of accumulation of the 3 major stored reserves in
developing seeds.
- The two fundamental divisions of seeds, based on their ability to withstand desiccation and
remain alive in the dehydrated state, will be introduced.
Fruit development:
Upon fertilization in many angiosperms, the carpel(s) of the gynoecium usually develop
into a fruit enclosing the fertilized zygotes in an environment conducive to their development into
mature seeds. The fruit can also aid seed dispersal and, in some plants, need not develop from
the gynoecium. In so called pseudocarpic fruit, receptacle bracts, the floral tube, or enlarged
inflorescence axes can contribute to the “fruit”.
Esau (1977) classifies fruit into many different types based on whether the fruit is dry or
fleshy, dehiscent or indehiscent, true or false. This classification was tabulated by Mauseth
(1988) in 12 groups to which I have added a 13th (number 8 in the table) and excerpted below as
Table 1.
When the ovary of the gynoecium develops into a fruit the ovary wall is termed a
pericarp. The ovary of tomato is such a fruit with two, three or four carpels (depending on the
species and variety of tomato) fusing to form it. The number of cells recruited from the L3 layer
(Fig. 1) in the shoot apical meristem into the inflorescence meristem determines the size of the
floral meristem and the number of carpels it is to produce. It is the L3 layer that gives rise to the
Peripheral
Zone
(PZ)
Central
Zone
(CZ)
Rib
Zone
(RZ)
L1
L2
Tunica
L3
Corpus
The arrangement of the SAM
in arabidopsis with the L1 (epidermis)
and underlying L2 layers comprising
the Tunica and the L3 layer comprising
the Corpus. These layers in the meristem
may or may not comprise symplastic domains
i.e. be connected to each other via
plasmodesmata. The cells within a layer
(L1, L2, L3) are clonally related (derived
from the same cell) and so share primary
plasmodesmatal connections except in special
circumstances.
Figure 1: The three zones of the shoot apical meristem (SAM).
1
preponderance of the pericarp tissue and this may be the reason why the L3 layer dictates the
final carpel number in tomato. The argument runs that, genetically, the cells of the L3 layer will
control how great a sink the developing fruit is and therefore, how much assimilate will be
potentially available to the fruit and enclosed seeds.
The fruit of tomato consists of two to four carpels that fuse and develop into a pericarp.
Developmentally, the carpel can be compared to a leaf that has curled around upon itself and
houses the ovules. The cells of the carpel of tomato most resemble those of the palisade layer of
the leaf and maintain their chloroplasts; functional chlorophyll, and photosynthetic capacity well
into fruit development. Although it is not substantial, the fruit photosynthesis does contribute to
photoassimilate required for fruit development. The carpel expresses many of the genes that are
typically associated with leaves, albeit, only a sub-set of so-called “leaf specific” genes and often
at stages of development divergent from that of the leaf.
Table 1: A classification of fruit types from
Mauseth (1988) Plant Anatomy.The
Benjamin/Cummings Publishing Co., Inc.
Menlo Park, California, USA.
Developing from a compound gynoecium with
several carpels
7. A capsule may open in one of many ways:
spitting along the fusion lines or sutures (Iris,
lily, poppy).
8. A silique develops from two carpels that
enclose a false partition that holds the seeds
following dehiscence (arabidopsis).
Dry Fruits
Indehiscent Fruits
Developing from a single carpel
1. An achene is simple and small, contains
only one seed. Fruit wall thin, papery with
seed loose inside (sunflower).
2. A caryopsis has attributes of an achene but
the pericarp and testa of the single seed
become fused (wheat, corn).
3. A samera is a one-seeded fruit with a winglike extension (maple).
Fleshy Fruits
9. A berry (bacca) is a fleshy fruit in which the
endocarp, mesocarp, and epicarp are easily
distinguishable and are soft (grape, tomato).
10. A drupe has the endocarp especially thick
and hard (peach, cherry, almond).
11. A pome has a papery, not a stony
endocarp (apple, pear).
12. A pepo has a hard exocarp forming a rind
(pumpkin, squash).
Developing from a compound gynoecium with
several carpels.
4. A nut commences from an ovary with
several carpels, all but one of which
degenerate leaving one carpel and one seed.
The pericarp is sclerenchymatous (walnut).
False Fruits
Dehiscent Fruits
13. A cypsela is similar to an achene except
that it develops from an inferior ovary and
therefore contains extra tissues around the true
pericarp (dandelion).
Developing from a single carpel
Schizocarpic Fruits
5. A follicle is a podlike fruit that splits open on
the ventral side (columbine).
6. A legume breaks open on both the ventral
and the dorsal sides (beans, peas).
Fruits that develop from a compound ovary but
then break up into individual mericarps
(achenes) when mature (parsley).
There are at least four (arguably 5) discernable periods or phases of fruit growth,
excluding processes occurring prior to pollination and fertilization. Those over which there can be
no debate include: 1) fruit set; 2) cell division, 3) cell enlargement (both isotropic and
anisotropic cell growth) and; 4) maturation (ripening). The controversial 5th phase is senescence.
Phase I: Fruit set: Although little is known about the control of ovary development into fruit, it is
known to usually be dependent on successful pollination or pollination and fertilization of ovules
within the ovary. Some signal is released between the time of pollination to the time of
fertilization, perhaps throughout, that avoids ovary abortion and sets the stage for the second
phase of fruit development, cell division. This positive signal may be gibberellic acid (GA) a.k.a.
gibberellin, for this hormone is released by the pollen, the pollen tube and the fertilized ovule.
2
Exogenously supplied GA can promote parthenocarpic fruit production in tomato. A
parthenocarpic fruit is one that develops to maturity; a) without pollination (banana); b) without
fertilization (some orchids); or c) with pollination and fertilization but subsequent abortion of the
ovules (grapes). The lack of maintained, fertilized ovules results in a fruit that, at maturity is
devoid of seeds. However, the GAs produced by parthenocarpic fruit are not the same set as
those produced by normally fertilized fruit. *Additionally, applied GA stimulates auxin (a second
plant hormone) production by the ovary, and auxin too could participate in signaling the ovary to
avoid abortion. It is possible that parthenocarpy is a direct result of improper synthesis of auxin
both temporally and spatially within the ovary. This suggests that the synthesis of GA and auxin
during ovary development must be highly regulated in order to coordinate both fruit set and the
commencement of cell division…the next phase of fruit development.
When there is little GA produced in the ovary due to a lack of pollination/fertilization, the
shoot apex appears to inhibit expansion of the ovary. This may be due to basipetal transport of
auxin (IAA) from the apex of the shoot to the ovary and this may trigger ABA production which
inhibits ovary growth. Yet, did we not just see that GA-stimulated auxin production in the fruit
assists fruit set (* above)? The current wisdom is that the response of the ovary to auxin and GA
changes upon fertilization, possibly due to increased sensitivity of the ovule to auxin. So, while
prior to fertilization auxin from the shoot enhances ABA production and inhibits the ovary from
proceeding further in development, after fertilization, GA-stimulated auxin production in the fruit
assists in maintaining the fruit on the plant (avoids abscission). In any case, for most plants
producing fruit, should pollination/fertilization NOT occur, the ovary will senesce and abscise from
the plant.
Phase II: Cell Division: In normal fruit development it is the developing seed that controls the
rate of cell division in the surrounding tissue. Phase II lasts between 7 to 10 days during which
cell division occurs throughout the fruit. Mitotic activity is high initially in the whole pericarp, but
more so in the outer relative to the inner pericarp. Additionally, cell division is initially higher in the
integumentary layers of the developing seeds than in the embryo. The developing vascular trace
in the placenta also exhibits high cell division activity early during phase II. At the middle of phase
II, mitotic activity is restricted to the outer pericarp, developing seeds, vascular tissue, and
placenta proximal to the developing seeds. At the end of Phase II, cell division remains the same
in the pericarp as it was in the middle portion of Phase II, Cell division in the placenta is also
restricted to the outer layer of cells from which the locular tissue arises. The vascular tissue and
the developing embryo also exhibit high mitotic activity.
There is a general observation that the number of fertilized ovules within a fruit controls
the initial rate of cell division in the ovary. This is exemplified by the observation that if seeds are
produced on one side of a fruit but not on the other, the fruit becomes lop-sided with the larger
lobe containing the developing seeds. An additional correlation perhaps explaining the first is that
the amounts of cytokinin (the third plant hormone known to be involved in fruit development
[ABA above is still only suspected to play a role to date]) present in the fruit tissue surrounding
the developing seeds is correlated with the amount of cytokinin present in the seeds. It appears
as though the fruit acquires cytokinin from the developing seeds. Yet, the seeds themselves do
not produce cytokinin but transport it in. Hence, the more seeds, the more cytokinin present in
them that may leach out into the surrounding tissue leading to more cell division and larger
potential fruit. Alternatively, high cytokinin concentration in the seeds drives the production of a
positive regulator of cell division in the tissues surrounding the seed which is released into the
tissue. Additionally, prolonged fruit development in the next phase, cell enlargement, is also
dependent to a certain extent on the presence of seeds in the ovary. However, this is not the
complete story because part of the control of final fruit size is the number of cells recruited to the
ovary prior to fertilization. Additionally, the extent of cell enlargement also affects final fruit size.
The latter parameter is largely controlled by phytohormonal levels present in the fruit during
Phase III. It is important to attempt to differentiate between factors that control fruit size
genetically versus those that do so hormonally although these two controls are difficult to
disentangle.
3
Phase III: Cell expansion: This stage increases the fruit far beyond its size during the previous
two stages through the controlled expansion of the cells formed in the previous stage. The
primary hormonal stimulant driving fruit cell expansion is auxin and the hormone peaks twice
during Phase III. The first peak indicates the onset of phase III and auxin is most prominent in the
developing seeds. However, in parthenocarpic fruit, exogenously supplied auxin is not sufficient
to replace developing seeds as a stimulant of cell expansion. This has lead to the belief that it is
the seeds’ ability to act as intense sinks that permit access to photoassimilate that is also used for
fruit development. Alternatively, it is possible that high seed auxin concentrations promote the
production of another stimulatory molecule within the seed that diffuses or is transported into the
surrounding tissue and stimulates cell expansion. Because seeds are not present in
parthenocarpic fruit, applied auxin would not stimulate the production of the second compound
and would therefore be insufficient to stimulate fruit expansion. This scenario is further supported
by the observation that the tomato mutant diageotripica (dgt) (exhibiting attributes of auxindeficiencies in its growth) produces normal fruit.
The second peak of auxin coincides with the end of the fruit expansion phase
occurring at the period of maximum embryo expansive growth. Again, auxin levels are high in the
seed but low in the fruit and is absent in parthenocarpic fruit. This peak is associated with the cell
expansion occurring in the embryo since the fruit cells are at their final size.
A second plant hormone that may play a role in fruit development is gibberellic acid
(GA). As with auxin there are two peaks of GA occurring during fruit development one during
Phase II the second during Phase III. In Phase II, GA peaks at the onset of cell division. In Phase
III GA peaks, possibly due to auxin stimulation of GA production, during cell expansion, at
maximal fruit growth. However, parthenocarpic fruit exhibit an exaggerated first GA peak in the
absence of seeds that are the main sites of auxin concentration in the fruit (see above).
Both the rate of fruit development and final fruit size are controlled by cell number and the
sink strength of the cells comprising the fruit. The affect of cell number may be a function of a
greater cumulative sink produced by many versus few cells. This is apparent when the timing of
fruit set on a truss is altered. Normally fruit set occurs basipetally and the first fruit to set are those
proximal to the main axis of the plant and these fruit contain more cells than do fruit that set more
distally along the truss. If fruit are prevented from setting proximally until after more distal fruit
have set, the more distal fruit have a greater rate of development despite having fewer cells. In
addition to metabolic control of fruit development, there are indications that phytosterols are also
responsible for early events in fruit development. Injection of an inhibitor of hydroxy-methyl CoA
reductase (HMGR) results in arrested fruit development. However, whether phytosterols
themselves are responsible for the observed effects or whether it is a lack of the cytokinin and
gibberellic acid produced from the mevalonic acid common to both pathways is unknown.
One noticeable morphological trait becoming obvious during fruit expansion is an
increase in the width of the peduncle (stem of an inflorescence of flowers) and the pedicle
(attachment of the flower [and now the fruit] to the plant). This is accompanied by an increase in
the amount of the vasculature in these organs but this is not easily observed. Can you think of
why such an increase may be necessary for the expanding fruit?
Phase IV: Maturation (Ripening): There are two different types of fruit based on their
physiological behavior during the maturation (ripening) phase. Some fruit show a considerable
burst of respiratory activity at the onset of ripening. These fruit are said to be climacteric,
evolving considerable CO2 and the plant hormone ethylene (the fourth plant hormone known to
be involved in fruit development) during this phase. Climacteric fruit include apple, banana and
tomato.
Other fruit do not undergo a climacteric burst of respiration (CO2 production) and
ethylene evolution. The fruit of strawberry, citrus and pineapple belong to this class. Ethylene has
long been known to be a potent stimulant of maturation in climacteric fruit. Many genes whose
products are implicated directly in fruit ripening are positively up-regulated by ethylene. These
ripening associated processes include: 1) partial cell-wall disassembly resulting in decreased
fruit firmness; 2) a change in the composition of fruit volatile substances resulting in altered
fruit aroma and flavor; 3) a change in pigmentation resulting in altered fruit color and; 4) an
4
Crease
Cereal
seed
Aleurone layer
Seed coat
Endosperm
Vascular bundle
Funiculus-chalazal region
Embryo
Maize
seed
Aleurone
layer
Starchy
(Floury)
endosperm
Funiculus
Scutellum
Horny
endosperm
Pea
seed
Cotyledons
Radicle
Residual
endosperm
Funiculus
Pea pod vasculature
Redrawn from Bewley and Black 1985.
Seeds: Physiology of Development and Germination
Figure 2: Some anatomical features of mono- and di-cot seeds.
increase in fruit respiratory activity. Ethylene biosynthesis itself is controlled by ethylene in
climacteric fruit in a positive feedback loop known as “auto-catalytic” ethylene production.
This biosynthetic pathway is known as System II to distinguish it from the non-auto-catalytic
biosynthetic pathway operating in vegetative tissue (System I). In tomato the dominant mutation
Never-ripe (Nr) is defective in an ethylene receptor and produces fruit that stay green and firm.
5
One of the ethylene-inducible enzymes responsible for partial cell wall disassembly during
ripening-induced fruit softening (polygalacturaonse) has been antisensed to produce commercial
varieties of tomatos that maintain a long shelf life (Flavor saver). In arabidopsis there are at least
5 receptors for ethylene, all of which are negative regulators of the ethylene response. In the
absence of ethylene, these receptors send signals that actively repress responses to ethylene.
When ethylene is present, the receptors cease to send their message and the result is a
response to ethylene. See Anthony Bleecker's excellent review on this subject in Trends in
Plant Science, 4: (7): 269-274.
The physiological significance of climacteric fruit ripening is not understood. However, it
has provided a convenient means of controlling fruit ripening by decreasing the amount of and
sensitivity to ethylene. Thus, a highly perishable crop such as bananas can be picked prior to
maturity, and shipped under conditions that minimize ethylene production/sensitivity, and then
artificially ripened at the point of sale by exposing fruit to ethylene.
Seed development:
Our knowledge of the commencement of seed development (embryogeny and storage
tissue formation) has been discussed by Dr. Perry over the course of the last few lectures. I will
continue this discussion on the latter stages of development focussing on the synthesis and
deposition of reserves to fill the cells of the storage tissues and the phases of seed maturation.
Basic anatomy: Deposition of reserves in the storage tissue of seeds is the hallmark of every
seed known, providing material for energy and early growth. The assimilate for storage reserve
synthesis is translocated from the mother plant into the sink cells of the seed. The nature of the
reserves laid down as well as the tissue in which they are laid down delimits major seed
characteristics as well as morphologically distinguishing features used to classify plants. One of
the most fundamental differences is related to whether the embryo develops one or two
cotyledons (or many cotyledons as in the Pinaceaea). This trait has permitted a sharp
differentiation between two classes of Angiosperm, the monocotyledones (one cotyledon) and
dicotyledones (two cotyledons). The single cotyledon of the monocots (the scutellum Fig. 2,
4A) is usually of a secretory and absorptive nature, never exiting the seed proper, even after
germination is complete. It abscises from the seedling and is shed along with the exhausted
endosperm and testa upon the completion of seedling establishment. It rarely contains
substantial amounts of stored reserves since it is usually associated with endospermic seeds
(seeds in which the major storage organ is an endosperm). Dicotyledonous embryos can be
found in endospermic and non-endospermic seeds. Many dicots remain endospermic but some
(e.g. Pea) absorb the endosperm during development, redistributing the reserves within the
cotyledons (Fig. 2). Endospermic seeds may retain live endosperm cells while in other
endospermic seeds the endosperm is dead at maturity. In the latter instance, a thin layer of live,
non-storage, secretory cells often surrounds the endosperm along the periphery of the seed,
inside the testa. This is known as the aleurone layer and aids in digesting the endosperm to
provide the embryo with the nutrients stored in the dead endosperm (Fig. 2).
Reserve deposition in storage tissue: There are usually only two of three possible major storage
reserves deposited in seeds as well as several minor reserves. The major storage reserves are:
1) Protein; 2) Lipid and; 3) Complex Carbohydrates (Starch, Fructans, diverse
Hemicelluloses). Storage protein is found ubiquitously in seeds as one form of major storage
reserve comprised of nitrogen and carbon along with either polysaccharides (usually starch) or
lipid, never both in major quantities, as the second major reserve. So, some seeds house protein
and primarily polysaccharides, while others house protein and primarily lipids. These major
reserves are extensively hydrolysed only after the completion of germination and the seed
depends initially on a minor reserve of soluble carbohydrate (sucrose, and often the raffinose
family oligosaccharides (RFO)) for initial metabolism and structural molecules prior to radicle
protrusion. Additionally, essential macro- and micro-nutrients are sequestered in the seed, usually
in a complex with hexaphosphorylated myo-inositol named phytin or phytic acid located as an
aggregate embedded in storage protein in the protein storage vacuole/protein body. How are
these reserves deposited in the seed?
6
Storage Protein Synthesis and Deposition
Monocots
Blebed ER
Burst ER
membrane bounded
unbounded
protein body
protein body
(wheat)
(maize)
Endoplasmic
Reticulum
Endoplasmic
Reticulum
Dicots
3’ end
Ribosome
receptor
Vacuole
Ribosomal
complex
Signal
peptidase
removes
signal
cotranslationally
AU
G
mRNA
5’ end
Signal
peptide
on elongating
protein
Signal
receptor
Golgi
Stacks
Endoplasmic
Reticulum
Start
codon
Figure 3: Synthesis and deposition of storage proteins in mono- and di-cots.
Storage protein deposition: During seed development, massive (relatively speaking) amounts of
specialized proteins are synthesized for use as a source of nitrogen, amino acids, and energy
during seed germination and subsequent seedling establishment. There are many families of
storage proteins that can be synthesized and stored in the same seed. They are often
oligomers of several different polypeptides and frequently exhibit different physical
characteristics most notably regarding solubility. Hence, seeds can have storage proteins that
freely dissolve in water (albumins), those that dissolve in buffers of high ionic strength but not
7
A.
coleoptile
leaves
Scutellum
the single cotyledon
radicle
coleorhiza
Details of the monocot embryo.
Lipid deposition in oleosomes
B.
Lipid accumulation
between the ER
ER
membranes
Oleosomes
Figure 4: A) Details of the embryo of many monocots (maize is featured). B) A schematic of the
deposition of lipid in oleosomes.
water (globulins), those dissolving in aqueous (70-90% v/v) alcohol (prolamins) and those
requiring dilute buffers of either high or low pH to solubilize (glutelins).
8
The storage proteins are all synthesized on the rough endoplasmic reticulum (ER) and
are deposited in the lumen of the rough ER. The storage proteins are placed in the lumen by
virtue of an amino-terminal signal peptide that directs the nascent protein through the ER
membrane while the protein is still being translated from the storage protein mRNA in the
cytoplasm. The signal peptide binds with a special docking protein in the ER membrane and is
cleaved off the nascent protein co-translationally. Upon deposition of the full-length protein into
the ER lumen, the ribosomal complex that has orchestrated the translation disassembles from the
mRNA and disassociates from the ER (Fig. 3). The storage protein aggregates within the ER
lumen and is moved to a peripheral terminus that then has various fates depending on the class
of plant (Monocotyledones vs Dicotyledones).
Monocotyledonous storage protein deposition: In monocots, the protein body may be simply an
aggregate of the storage proteins that have stretched the ER until it ruptured, releasing the
aggregate into the cytoplasm unbounded by any membrane (e.g. wheat, Fig. 3). Alternatively, the
storage proteins can accumulate until the ER terminus eventually blebs off and forms a lipid
bilayer surrounding the protein in the protein body (a unit membrane) (Fig. 3).
Dicotyledonous storage protein deposition: The storage protein aggregated in a terminus of the
ER in dicots accumulates until the ER terminus blebs off from the ER. However, the ER vesicle
containing the storage protein is then transported to the Golgi apparatus where it fuses,
releasing the storage protein into the Golgi. Considerable processing of the storage protein can
occur in the ER and the Golgi usually in the form of glycosylation, partial or complete assembly of
oligomeric polypeptides, the formation of disulfide bonds, and/or the cleavage of cotranslated
subunits. The protein is passed from the proximal stacks to the distal in the cisternae until it is
packaged in a Golgi vesicle and transported to the vacuole. The vacuole of the dicots during seed
development can either remain as a single, central vacuole or, more commonly, become highly
convoluted, pinching off small vacuoles that are filled with storage protein and phytin. Regardless,
they are referred to as protein storage vacuoles to reflect their origin and not as protein bodies
(Fig. 3).
Storage lipid formation and deposition: Many, but not all, seeds deposit lipid for use as an energy
and carbon source during germination and seedling establishment. Lipid is synthesized in the
seed from assimilate sucrose delivered to the seed from the mother plant. It is converted via the
pentose phosphate pathway-, glycolytic-, and the triglyceride biosynthetic pathway-enzymes
located in the cytosol and proplastids into three primary free fatty acid precursors of the large
diversity of lipids stored in seeds (Palmitate (16:0), Stearate (18:0), and Oleate (18:1)). From the
proplastids, these precursors are transferred to the endoplasmic reticulum where they undergo a
myriad of alterations (elongation, addition of double bonds, hydroxylation, addition to glycerol to
form triglycerides, and temporary phosphorylation) before being bulked and packaged in
membrane vesicles formed from the ER, the oleosomes (lipid bodies). There is a variety of
oleosome forms, all arising from deposition of lipid in the ER but in different manners. The
oleosome can remain attached to the ER though thin membrane channels, or bud off forming an
oleosome bounded by a single layer of ER membrane (a half unit membrane) that either does or
does not have an extension of ER membrane attached to it (Fig. 4).
Storage polysaccharide formation and deposition: Polysaccharides constitute a third form of
major storage compound. Usually, seeds storing carbohydrate do not also store major amounts of
lipid. By far the most common form of polysaccharide used by seeds as a storage reserve is
starch. Starch is formed in proplastids by the daily deposition of amylose (unbranched chains of
α-1-4 linked glucose) or amylopectin (chains of α-1-4 linked glucose with branches of α-1-4 linked
glucose chains attached to the main chain through α-1-6 links). The proplastid, termed the
amyloplast, is filled with a single grain or numerous grains of starch.
In other seeds, the major polysaccharide deposited is fructan, galactomannan, or
xyloglucan. These reserves, except for fructan, are deposited outside the cell as a secondarily
thickened cell wall. In some seeds such as tomato, the secondarily thickened cell wall of the
9
endosperm does not affect the cells themselves while in other species, e.g. fenugreek, the
secondary thickened cell wall eventually occludes the cytoplasm and kills the cell.
Phytin synthesis and deposition: Little is known about phytin synthesis or deposition. Phytin is a
water-insoluble accumulation of a mixed salt of magnesium, calcium, and potassium ionically
bound to myo-inositol hexaphosphoric acid (phytic acid). It can also contain iron, manganese, and
copper, and sodium salts. Current evidence suggests that phytin is synthesized in the ER and,
upon sufficient accumulation, an ER vesicle buds from the ER and fuses with the protein body or
protein storage vacuole and deposits the phytin in the globoid (the aggregate of phytin
embedded in the storage protein).
Soluble carbohydrate (oligosaccharide) synthesis and deposition: The non-reducing sugar
sucrose is usually found in considerable quantities in mature seeds. The non-reducing nature of
sucrose along with its versatility, being quickly converted into energy and/or carbon skeletons for
use in germination and/or seedling establishment, enables those seeds that do desiccate to
maintain a quick energy source and do so without incurring non-enzymatic reactions of cellular
contents with the reducing end of a sugar. These so-called Amadori and Maillard reactions (nonenzymatic reactions between proteins and carbohydrates) result in deleterious denaturing of
essential cellular components that would severely damage if not destroy the seed should they
occur (they are responsible for the browning of food during cooking). Sucrose, when present in
large quantities, during the removal of water tends to crystallize out of solution which constitutes a
quandary for seeds undergoing maturation drying. How to keep the sucrose from crystallizing as
it becomes more concentrated due to water loss? Typically, the inclusion of some of the raffinose
series oligosaccharides (sucrose with one or more galactose moieties added to them) provides
sufficient disruption of the sucrose crystalline structure to prevent crystallization and instead leads
to an amorphous, highly viscose solution or glass. It is thought that those seeds that have the
ability of losing most of their water and yet maintain life (anhydrobiosis) do so by substituting
sucrose and raffinose for water in providing a shell of hydrogen bonding substances around
membranes and proteins. This serves to preserve most of the intracellular structural organization
until such time as water is reintroduced by imbibition.
Maturation desiccation: A fundamental difference between the seeds of two classes of plants is
exhibited at this stage of seed maturation. Those species whose seeds naturally undergo drastic
water loss upon maturing are referred to as orthodox seeds and represent the vast majority of
the angiosperms. Those species whose seeds do not undergo desiccation upon reaching
maturity are designated as recalcitrant. Recalcitrant seeds that are dried do not survive. There
have been further classifications based also on the ability of the seeds to survive cold
temperatures but these are less clear than the orthodox and recalcitrant designations.
For orthodox seeds maturation desiccation is necessary for the seed to subsequently
complete germination normally. Seeds that do not undergo maturation drying typically stay in the
maturation phase of reserve synthesis and deposition and fail to complete germination. Hence,
desiccation is considered as a physiological and genetic switch from maturation to germination
phases of a seed's development. Recalcitrant seeds do not undergo sever maturation desiccation
and yet are capable of making the switch from the maturation to the germination programs. It has
been suggested that recalcitrant seeds do decrease in water content slightly upon maturation and
that this is sufficient to trigger the switch to the germinative mode.
It is important to realize that most forms of major storage reserve exist in the seed as
large polymers of insoluble material (ergastic substances). This allows the accumulation of
sufficient stored reserves to provide the embryo with food and energy to establish and become
autotrophic without incurring the huge negative osmotic potential that would be associated with
such huge amounts of reserves if they were osmotically active (i.e. soluble and unpolymerized).
The hydrolysis of these reserves to make them available for utilization will therefore, require the
utmost control so that the osmotic potential of the cell does not become too great, resulting in
excessive turgor pressure leading to cell rupture and death.
10