Download Introduction What Is a Seed? Development of the Seed

Seeds
Introductory article
Article Contents
Michael Black, King’s College, London, UK
. Introduction
In the seed-bearing plants (angiosperms and gymnosperms) the embryo-containing seeds
develop from the fertilized ovule. The structural and physiological features that are laid
down during development confer upon the seed properties that account for its greatly
successful role in the life cycle of the dominant plants on Earth.
. What Is a Seed?
. Development of the Seed
. Maturation and Desiccation Tolerance
. Dormancy
. The Success of Seeds
Introduction
. Seeds and Biotechnology
Sexual reproduction in the gymnosperms and angiosperms
results in the formation of seeds from the fertilized ovules.
The properties that make the seed uniquely suitable as the
starting point of the new generation of plant arise during
seed development. The structure of the seed, its storage
reserves and, in most cases, its ability to remain alive in the
almost dry state for long periods of time, enable it to fulfil
its critical role in plants’ survival and dispersal.
What Is a Seed?
A seed contains an embryo plant consisting of a relatively
simple axis (radicle/hypocotyl) to which the cotyledons
(seed leaves) are attached. In the Monocotyledons and
Dicotyledons classes of flowering plant (angiosperms)
there are respectively one and two cotyledons, and in some
gymnosperms, for example the conifers, there are several
(Figure 1). In many angiosperms the embryo in the mature
seed is surrounded by the endosperm (e.g. castor bean), or
this tissue may be more developed laterally, as in the
cereals. The endosperm is absent from other mature
angiosperm seeds (peas, beans) or is reduced to a few cell
layers (lettuce, cucumber, tomato, shepherd’s purse).
Included around the embryo of some angiosperm seeds
(e.g. beet) is a tissue of a different developmental origin
from the endosperm, the perisperm. The embryo of
gymnosperm seeds is surrounded by yet another type of
tissue, coming from the megagametophyte. The periphery
of the seed is the seed coat or testa: in some angiosperm
dispersal units that are commonly called seeds, tissues
derived from the fruit may be present (the pericarp),
external to the testa (e.g. in cereal grains).
Seeds of different species vary enormously in size and the
dimensions of the different parts – embryo, endosperm,
testa – vary accordingly (Table 1; Figure 1).
Development of the Seed
The seed forms from the ovule, after the egg cell (ovum)
within has been fertilized by a male gamete from the
germinated pollen grain. The resulting zygote carrying the
diploid chromosome complement, half maternal and half
paternal, develops into the embryo. In angiosperms, the
division of the zygote into two cells establishes a polarity.
Subsequently, the ordered sequence of cell divisions and
differentiation by the ‘apical’ cell results in the gradual
formation of the embryo (embryogenesis) (Figure 2).
During this process, the embryonic axis first begins to
differentiate and then the cotyledon(s) appear(s). The
‘basal’ cell from the first zygotic division becomes the
multicellular, filamentous suspensor that pushes the
incipient embryo into the developing semiliquid, nutritive
endosperm. The endosperm of the angiosperm seed is a
product of fusion between a second male gamete and two
polar (endosperm) nuclei in the embryo sac of the ovule
and is therefore triploid, containing two maternal genomes
and one paternal genome. In some species the endosperm
continues to develop and becomes prominent in the mature
seed (e.g. cereals, castor bean), but in others it may become
only a few cells thick (e.g. lettuce, tomato) or even be totally
resorbed by the growing embryo and not survive into the
mature seed (e.g. pea). The perisperm in angiosperms
develops from the maternal diploid nucleus, in which the
embryo sac is embedded. The developed megagametophytic tissue surrounding the embryo in the gymnosperm seed
is haploid. The testa in all types of seed originates from the
boundary layers of the ovule, the integuments, and is
diploid maternal tissue. The genetic composition of seeds is
therefore relatively complex, in all cases consisting of cells
of three different genetic complements.
Once the embryo, endosperm, perisperm (where present)
and megagametophyte are well developed, the food
reserves are deposited in a particular tissue according to
the species (Table 1). Deposition is so abundant in cereal
endosperm that the cells are killed because of disruption of
the contents.
Apart from the ontogeny of the tissue surrounding the
embryo, angiosperm and gymnosperm seeds have another
important difference. The ovules in angiosperms are
enclosed within an ovary and as the ovule develops into a
seed the ovary wall becomes the fruit tissues (the term
‘angiosperm’ is derived from Greek words meaning
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
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Seeds
Embryo
Coat (pericarp and testa)
Hypocotyl– Cotyledons
radicle axis
Endosperm
Scutellum
Coleoptile
Plumule
Plumule
(Seed cut
open)
Seed coat
Embryo
Radicle
Micropyle
Hilum
Coleorhiza
0.5 cm
(a)
1 cm
Maize
(b)
Front view
of whole seed
Broad bean
2 Cotyledons
Seed coat
Seed coat
Cotyledon
Endosperm
Cotyledon
Endosperm
Shoot apex
Embryo
Hypocotyl–
radicle axis
1 mm
(c)
Radicle/
hypocotyl
Radicle apex
1 cm
Onion
(d)
Hypocotyl–
radicle axis
Shoot
apex
Radicle
apex
Castor bean
Seed coat
Shoot apical
meristem
Cotyledons (several)
Seed coat
Apical meristem
Embryo
Hypocotyl–radicle
Embryo
axis
Cotyledons
Apical meristem
of radicle
Radicle apical
meristem
Female (mega)
gametophyte
500 µm
Hypocotyl–
radicle axis
100 µm
Root cap
Endosperm
(e)
Pine
(f)
Shepherd’s purse
Figure 1 Structure of different seeds (from various sources).
‘encased seed’). In many angiosperm species the fruit
tissues remain relatively thin, become dry and adhere
closely to the seed, e.g. in cereal grains, lettuce, dandelion
and sycamore. Such ‘seeds’ therefore are botanically fruits.
The ovule is situated directly upon the scale of the cone in
most gymnosperms, not within an ovary (‘gymnosperm’
means ‘naked seed’). The fruits of angiosperms may play
an important role in seed dispersal, for example by being an
attractive food for animals or by attaching to them, or by
being modified to aid carriage by wind. In many
gymnosperms seed dispersal is encouraged by the wing-
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like outgrowth of the seed coat, which catches the wind.
Modifications of the testa or pericarp (fruit tissues) in
angiosperms may also aid wind dispersal, for example the
hairs on willow herb seeds and the wings of sycamore.
Numerous mutants affecting embryo and endosperm
development have been generated, for example in maize,
Arabidopsis and barley, from which genes involved in these
processes can be identified. Several genes active at specific
stages have been isolated and sequenced. At present,
however, functionality can be linked with only a few genes,
for example the one determining polarity of movement of
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
Seeds
Table 1 Size and storage tissue of some seeds
Seed a
Approx. size (g)
Major storage tissue
Orchids
Tobacco
Poppy
Lettuce
Wheat
Barley
Pine
Maize
Pea
Beans
Coconut
5 10 –6
10 –4
5 10 –3
10 –3
5 10 –2
5 10 –2
8 10 –2
0.4
0.5
0.2–1
4–600
b
a
b
Cotyledons
Cotyledons
Cotyledons
Endosperm
Endosperm
Megagametophyte
Endosperm
Cotyledons
Cotyledons
Endosperm (liquid and solid )
Dry seed.
Orchid seeds have virtually no reserves.
the plant hormone, auxin (MONOPTEROS): in most
cases the gene products are not characterized. It is clear,
nevertheless, that seed development involves the sequential
expression of genes for embryogenesis and endosperm
development, storage reserve synthesis and other processes
(Figure 2).
Besides the genes, other factors are involved in the
regulation of seed development. The hormone abscisic acid
(ABA) participates in the control of embryo growth in
angiosperms, especially in preventing the embryo from
passing into the germination phase while it is in the fruit,
and it is also implicated in the promotion of storage protein
accumulation. The osmotic status of the developing seed is
important and this, too, contributes to the regulation of
embryo growth and reserve deposition. Good evidence for
the role of ABA is provided by mutants, for example those
in maize and Arabidopsis, in which ABA synthesis is
impeded or the sensitivity to the hormone is much reduced
or absent. In these cases, growth of the embryo continues
without a rest period and it germinates within the fruit
(vivipary). This effect can also be obtained in some species
(e.g. maize) by treating developing seeds with a chemical
inhibitor (fluridone) that blocks the synthesis of ABA. The
hormone is also involved in other processes in developing
seeds; these are discussed below.
Maturation and Desiccation Tolerance
The end of seed development in most species is marked by
maturation, during which the seeds lose water, perhaps
drying down to as little as 5% water content, on a fresh
weight basis. The seeds become quiescent. The ability to
withstand dehydration (desiccation tolerance) is a rare
property of living cells and requires the operation of special
mechanisms to protect cell constituents from a potentially
fatal experience. Experimental drying of seeds during the
early phases of their development kills them and they only
become resistant to dehydration at about the time they
reach maximum size and dry weight. It is not clear how
tolerance is induced but a signal may be the initial loss of a
small proportion of water, which then triggers protective
mechanisms.
Two factors are thought to be involved in the protection
of cell membranes against the damage that would
otherwise occur as water is removed. Through their
hydroxyl groups, the carbohydrate sucrose and, possibly,
oligosaccharides such as raffinose bind to the phospholipids of the membranes, replacing the bound water
molecules that are present in the normal hydrated state.
This ‘water replacement’ stabilizes the membrane against
disruption under the increasing dehydration and preserves
the vitality of the cells of the seed tissues. These
carbohydrates are also thought to play an important role
because of sucrose vitrification, i.e. the formation of a glass
as water is lost. Glass formation contributes to the
maintenance of cell structure and helps to inhibit
potentially damaging chemical reactions within the cells
by interfering with diffusion processes. A water replacement effect might also be achieved by special proteins, the
LEAs (late embryogenesis abundant), which are produced
in embryos in the later stages of embryogenesis, some
induced by ABA. Because of their unique amino acid
composition, these proteins are strongly hydrophilic and
might serve to maintain aqueous domains associated with
cell membranes. The proteins may also have other roles, for
example binding certain ions or having enzymic activity
(protein kinases). The LEAs certainly have an important
part to play in plant responses to water stress, in vegetative
tissues (e.g. leaves) as well as seeds.
Not all species of seeds tolerate desiccation, however,
and some are killed by dehydration even when they are
fully mature. Such types dry relatively little during their
maturation and can survive only for comparatively short
times after their release from the parent plant. Seeds of
many tropical (e.g. mangroves, lychee) and temperate (e.g.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
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Seeds
Genes: 1
2
3
4
Seed coat
Proembryo
A
B
Zygote 2-cell
C
B
En
Ax
E
C
S
Ax
2/4-cell
8-cell
Heart
embryo
16-cell
Globular
embryo
S
Fully formed
seed
Embryo-specific genes
Genes for early embryogenesis 1, 2, 3, 4
Genes for LEA proteins
Desiccation tolerance (
Embryo dry weight (mg) (
)
Decrease in protein
mRNA synthesis
)
Endosperm genes
Genes for constitutive proteins and enzymes
Genes for reserve synthesis enzymes
Storage reserve accumulation
Cell division
Cell expansion
Onset of dormancy
Loss of water
Quiescence
Maturation
Fertilization
Age of developing seed
Figure 2 Events in seed development.
Top row: Embryogenesis (the example chosen is Arabidopsis, but the major developmental changes are typical of a dicotyledonous seed). The time
course is indicated by the large red arrow. A, apical cell; B, basal cell; Ax, embryo axis; C, cotyledon(s); E, embryo; En, endosperm; S, suspensor.
Genes: The ones named have been identified from studies on many mutants of Arabidopsis. Several others have been described.
1. GNOM regulates the position of the first zygotic division. In the mutant this division is symmetrical.
2. MONOPTEROS regulates cell divisions in the very early (pro) embryo. In the mutant, there are 16 cells in what is normally the 8-cell stage. The
development of the basal part of the embryo (giving the axis) is abnormal.
3. FACKEL regulates differentiation in the central zone of the globular-stage embryo. In the mutant the hypocotyl fails to develop.
4. GURKE regulates development in the apical region. The mutant has no apical meristem and the cotyledon primordia do not form.
horse-chestnut, sycamore) trees are in this category.
Several kinds of mangrove undergo no drying and, without
passing into a quiescent state, germinate while still on the
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parent plant – another example of vivipary. Those seeds
that do tolerate desiccation can, of course, be stored in a
dry state, in most cases for many years: such seeds are said
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net
Seeds
to have orthodox storage behaviour. Desiccation-intolerant seeds are difficult to store: they are described as being
recalcitrant.
Dormancy
Seeds of many species undergo a physiological change
during their development (Figure 2) that renders them
incapable of germinating after maturity, even when
rehydrated, unless certain conditions are fulfilled: such
seeds are said to be dormant. There is good evidence that
this physiological state is induced on the parent plant by
ABA, though other factors may also be involved.
Dormancy does not occur in seeds of ABA-deficient or
ABA-insensitive mutants of Arabidopsis or in seeds of
some species treated during their development with
fluridone, an inhibitor of ABA biosynthesis.
To become germinable, dormant seeds must experience
at least one of several conditions. In many cases dormancy
is slowly lost by the dry seed, over several months or years,
a change known as after-ripening. Dormancy of the
hydrated seed (i.e. wetted in the soil) may be broken by
one of the following, the effectiveness depending upon the
species:
1. low temperature (1–58C approx.) for several weeks;
2. light – generally for a few minutes, sunlight being most
active;
3. fluctuating temperature, e.g. a day/night fluctuation of
1–108C; and
4. various chemicals, nitrate probably being the most
important in nature.
These requirements operate to determine when and where
the seed germinates, which is critically important if the new
seedling is to receive the best start in life: dormancy is thus a
device to secure the distribution of germination in time and
space. Dormant seeds that require a low-temperature
treatment cannot germinate immediately after dispersal
from the parent plant in, say, spring or summer, but will do
so only after experiencing the cold of winter. The
requirement for alternating temperatures acts to prevent
germination in the soil beneath dense vegetation because
the latter dampens the day/night temperature fluctuation.
The seeds therefore are stimulated to germinate only when
there is little or no vegetation cover, where the new
seedlings will suffer minimum competition from already
established plants. Dormancy of light-requiring seeds will
be removed only if seeds are at or near the soil surface
where light can penetrate, a mechanism that deters
germination at a depth. This is especially important for
small seeds, whose reserves are insufficient to support
enough growth to carry the seedling leaves into the light to
begin photosynthesis. The light-detection system in
dormant seeds (phytochrome) is activated mainly by light
relatively rich in red wavelengths, but is antagonized by
far-red light. Since red light, but not far red, is absorbed by
chlorophyll, the light passing through green leaves does not
break dormancy. This effect serves to restrict germination
to open spaces, such as light gaps in forests (fallen trees) or
in springtime before deciduous leaves have appeared, i.e. in
situations where there is no competition from other plants.
Another important aspect of dormancy is that it can, in
some cases, be imposed upon a mature, hydrated seed
experiencing stressful conditions such as a temperature
that is too high or too low, or potentially desiccating,
bright sunlight. The seed is forced into dormancy
(secondary dormancy), which blocks germination until
more favourable conditions arise.
Though the significance of seed dormancy is best
appreciated in an ecological context, it is also important
in agriculture and industry. For example, the presence of
dormancy in cereals (wheat, barley) causes delayed,
nonuniform germination that is undesirable in agriculture,
horticulture and in industries such as malting. On the other
hand, the absence of dormancy can be dangerous, for
example in cereals, because mature seeds on the ear wetted
by rain before harvest might germinate, causing crop
spoilage.
The Success of Seeds
Their development in the relatively protected situation on
the parent plant and their subsequent dispersal, sometimes
over long distances, are two reasons for the successful role
of seeds in the plant life cycle, but other features are also
significant in this respect. The first is that during its
development the seed has consigned to it relatively massive
quantities of stored food reserves – protein, starch or oils –
deposited in different regions (Table 1). In the pea or bean,
for example, the bulky cotyledons are packed with protein
and starch; in cereals (e.g. wheat, maize, rice) the
endosperm is similarly loaded, while in castor bean this
tissue is rich in oil and protein. These reserves are utilized
by the germinated seed to provide the basic molecules
(sucrose, amino acids) for the support of growth and
establishment of the seedling until it becomes photoautotrophic (able to make its own food by photosynthesis). It is
because of these reserves that seeds are exploited as food
for human and other animal life. The second important
feature is the relatively tough, resistant testa that protects
the embryo against physical damage or microbial attack:
this is an important factor aiding seed longevity, especially
in natural conditions in the soil. The third important
property of seeds, in most but not all species, is their very
low water content. In many cases, seeds dry to 5–10%
water content as they mature on the parent plant, or after
they have been released from the fruit. The very low water
content causes a virtual cessation of metabolic activity, and
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Seeds
Table 2 Biotechnology of seeds
Type of transformation
Example
Modification of storage protein
Altered gluten of wheat (nutrition and nonallergenic)
Altered protein of rice (nutrition)
Elevated methionine content (nutrition)
High oleic acid oil (industrial use)
Altered lauric acid oil (industrial use)
Epoxidized fatty acids in oil (industrial use)
Xylanase
Phytase
β-Glucanase
Antibodies
Vaccines
Enkephalin (neuropeptide)
Hirudin (anticoagulant)
Avidin (diagnostic reagent)
Fatty acid composition of oil
Industrial enzymes produced in seeds
Pharmaceuticals and medicinals
All the above transformations have been achieved in seeds but are not necessarily yet in use for production
purposes.
seeds can survive in this state often for very many years.
These characteristics – their development while retained
on the parent plant, their dispersal, the abundant food
reserves, the protective seed coat, the longevity while
almost dry and the positional and temporal detection
systems of dormancy – contribute to the success of the seed
during evolution (seed-bearing plants appeared over 300
million years ago) and the establishment of seed plants as
the dominant vegetation types.
Seeds and Biotechnology
Seeds are profoundly important because they provide food
for humans (70% of human food is consumed as seeds or
seed products), feed for animals and substances for
industrial use. Their composition, nevertheless, may not
be ideal: for example, the protein may be deficient in
nutritionally important amino acids, or the fatty acids of
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the oil may be inferior for a particular industrial purpose.
By genetic engineering techniques it is now possible to alter
seed composition to suit our needs and requirements. Also,
because seeds accumulate relatively massive quantities of
material, they can be used as reservoirs for substances
made by genetically engineered parent plants, such as
various medicinals, vitamins, industrial enzymes, etc.
Seeds will feature very prominently in the agrobiotechnology of the future (Table 2).
Further Reading
Baskin CC and Baskin JM (1998) Seeds: Ecology, Biogeography, and
Evolution of Dormancy and Germination. San Diego: Academic Press.
Bewley JD and Black M (1994) Seeds: Physiology of Development and
Germination, 2nd edn. New York: Plenum Press.
Larkins BA and Vasil IK (eds) (1997) Cellular and Molecular Biology of
Plant Seed Development. Dordrecht: Kluwer Academic.
Kigel J and Galili G (eds) (1995) Seed Development and Germination.
New York: Marcel Dekker.
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