Embryogenesis(Plants)

Embryogenesis

In plants, the term embryogenesis covers development from the time of fertilization until

dormancy occurs. The basic body plan of the sporophyte is established during embryogenesis;

however, this plan is reiterated and elaborated after dormancy is broken. The major challenges of

embryogenesis are

1. To establish the basic body plan. Radial patterning produces three tissue systems, and axial

patterning establishes the apical-basal (shoot-root) axis.

2. To set aside meristematic tissue for postembryonic elaboration of the body structure (leaves,

roots, flowers, etc.).

3. To establish an accessible food reserve for the germinating embryo until it becomes

autotrophic.

Embryogenesis is similar in all angiosperms in terms of the establishment of the basic body plan

(Steeves and Sussex 1989) (see Figure 20.15). There are differences in pattern elaboration,

however, including differences in the precision of cell division patterns, the extent of endosperm

development, cotyledon development, and the extent of shoot meristem development (Esau 1977;

Johri et al. 1992).

The basic body plan of the angiosperm laid down during embryogenesis also begins with an

asymmetrical* cell division, giving rise to a terminal cell and a basal cell (Figure 20.15). The

terminal cell gives rise to the embryo proper. The basal cell forms closest to the micropyle and

gives rise to the suspensor. The hypophysis is found at the interface between the suspensor and

the embryo proper. In many species it gives rise to some of the root cells. (The suspensor cells

divide to form a filamentous or spherical organ that degenerates later in embryogenesis.) In both

gymnosperms and angiosperms, the suspensor orients the absorptive surface of the embryo

toward its food source; in angiosperms, it also appears to serve as a nutrient conduit for the

developing embryo. Culturing isolated embryos of scarlet runner beans with and without the

suspensor has demonstrated the need for a suspensor through the heart stage in dicots

Maternal effect genes play a key role in establishing embryonic pattern in animals . The role of extrazygotic genes in plant embryogenesis is less clear, and the question is

complicated by at least three potential sources of influence: sporophytic tissue, gametophytic

tissue, and the polyploid endosperm. All of these tissues are in close association with the

egg/zygote (Ray 1998). Endosperm development could also be affected by maternal genes.

Sporophytic and gametophytic maternal effect genes have been identified in Arabidopsis, and it is

probable that the endosperm genome influences the zygote as well. The first maternal effect gene

identified, SHORT INTEGUMENTS 1 (SIN1), must be expressed in the sporophyte for normal

embryonic development (Ray et al. 1996). Two transcription factors (FBP7 and FBP11) are

needed in the petunia sporophyte for normal endosperm development (Columbo et al. 1997). A

female gametophytic maternal effect gene, MEDEA (after Euripides' Medea, who killed her own

children), has protein domains similar to those of a Drosophila maternal effect gene

(Grossniklaus et al. 1998). Curiously, MEDEA is in the Polycomb gene group ,

whose products alter chromatin, directly or indirectly, and affect transcription. MEDEA affects an

imprinted gene ) that is expressed by the female gametophyte and by maternally

inherited alleles in the zygote, but not by paternally inherited alleles (Vielle-Calzada et al. 1999).

How significant maternal effect genes are in establishing the sporophyte body plan is still an

unanswered question.

Radial and axial patterns develop as cell division and differentiation continue (Figure 20.18; see

also Bowman 1994 for detailed light micrographs of Arabidopsis embryogenesis). The cells of

the embryo proper divide in transverse and longitudinal planes to form a globular stage embryo

with several tiers of cells. Superficially, this stage bears some resemblance to cleavage in

animals, but the nuclear/cytoplasmic ratio does not necessarily increase. The emerging shape of

the embryo depends on regulation of the planes of cell division and expansion, since the cells are

not able to move and reshape the embryo. Cell division planes in the outer layer of cells become

restricted, and this layer, called the protoderm, becomes distinct. Radial patterning emerges at

the globular stage as the three tissue systems (dermal, ground, and vascular) of the plant are

initiated. The dermal tissue (epidermis) will form from the protoderm and contribute to the outer

protective layers of the plant. Ground tissue (cortex and pith) forms from the ground meristem,

which lies beneath the protoderm. The procambium, which forms at the core of the embryo will

give rise to the vascular tissue (xylem and phloem), which will function in support and transport.

The differentiation of each tissue system is at least partially independent. For example, in the

keule mutant of Arabidopsis, the dermal system is defective while the inner tissue systems

develop normally (Mayer et al. 1991).

The globular shape of the embryo is lost as cotyledons ("first leaves") begin to form. Dicots have

two cotyledons, which give the embryo a heart-shaped appearance as they form. The axial body

plan is evident by this heart stage of development. Hormones (specifically, auxins) may mediate

the transition from radial to bilateral symmetry (Liu et al. 1993). In monocots, such as maize,

only a single cotyledon emerges.

In many plants, the cotyledons aid in nourishing the plant by becoming photosynthetic after

germination (although those of some species never emerge from the ground). In some cases

peas, for example the food reserve in the endosperm is used up before germination, and the

cotyledons serve as the nutrient source for the germinating seedling. Even in the presence of

a persistent endosperm (as in maize), the cotyledons store food reserves such as starch, lipids, and

proteins. In many monocots, the cotyledon grows into a large organ pressed against the

endosperm and aids in nutrient transfer to the seedling. Upright cotyledons can give the embryo a

torpedo shape. In some plants, the cotyledons grow sufficiently long that they must bend to fit

within the confines of the seed coat. The embryo then looks like a walking stick. By this point,

the suspensor is degenerating.

The shoot apical meristem and root apical meristem are clusters of stem cells that will persist

in the postembryonic plant and give rise to most of the sporophyte body. The root meristem is

partially derived from the hypophysis in some species. All other parts of the sporophyte body are

derived from the embryo proper. Genetic evidence indicates that the formation of the shoot and

root meristems is regulated independently. This independence is demonstrated by the dek23

maize mutant and the shootmeristemless (STM) mutant of Arabidopsis, both of which form a root

meristem but fail to initiate a shoot meristem (Clark and Sheridan 1986; Barton and Poethig

1993). The STM gene, which has been cloned, is expressed in the late globular stage, before

cotyledons form. Genes have also been identified that specifically affect the development of the

root axis during embryogenesis. Mutations of the HOBBIT gene in Arabidopsis (Willemsen et al.

1998), for example, affect the hypophysis derivatives and eliminate root meristem function.

The shoot apical meristem will initiate leaves after germination and, ultimately, the transition to

reproductive development. In Arabidopsis, the cotyledons are produced from general embryonic

tissue, not from the shoot meristem (Barton and Poethig 1993). In many angiosperms, a few

leaves are initiated during embryogenesis. In the case of Arabidopsis, clonal analysis points to the

presence of leaves in the mature embryo, even though they are not morphologically well

developed (Irish and Sussex 1992). Clonal analysis has demonstrated that the cotyledons and the

first two true leaves of cotton are derived from embryonic tissue rather than an organized

meristem (Christianson 1986).

Clonal analysis experiments provide information on cell fates, but do not necessarily indicate

whether or not cells are determined for a particular fate. Cells, tissues, and organs are shown to be

determined when they have the same fate in situ, in isolation, and at a new position in the

organism (see McDaniel et al. 1992 for more information on developmental states in plants).

Clonal analysis has demonstrated that cells that divide in the wrong plane and "move" to a

different tissue layer often differentiate according to their new position. Position, rather than

clonal origin, appears to be the critical factor in embryo pattern formation, suggesting some type

of cell-cell communication (Laux and Jurgens 1994). Microsurgery experiments on somatic carrot

embryos demonstrate that isolated pieces of embryo can often replace the missing complement of

parts (Schiavone and Racusen 1990; Scheres and Heidstra 1999). A cotyledon removed from the

shoot apex will be replaced. Isolated embryonic shoots can regenerate a new root; isolated root

tissue regenerates cotyledons, but is less likely to regenerate the shoot axis. Although most

embryonic cells are pluripotent and can generate organs such as cotyledons and leaves, only

meristems retain this capacity in the postembryonic plant body.

*Asymmetrical cell division is also important in later angiosperm development, including the

formation of guard cells of leaf stomata and of different cell types in the ground and vascular

tissue systems.

Another intriguing characteristic of these mutants is that cell differentiation occurs in the

absence of morphogenesis. Thus, cell differentiation and morphogenesis can be uncoupled in

plant development.

Mendel's famous wrinkled-seed mutant (the rugosus or r allele) has a defect in a starch

branching enzyme that affects starch, lipid, and protein biosynthesis in the seed and leads to

defective cotyledons (Bhattacharyya et al. 1990).

Dormancy

From the earliest stages of embryogenesis, there is a high level of zygotic gene expression. As the

embryo reaches maturity, there is a shift from constructing the basic body plan to creating a food

reserve by accumulating storage carbohydrates, proteins, and lipids. Genes coding for seed

storage proteins were among the first to be characterized by plant molecular biologists because of

the high levels of specific storage protein mRNAs that are present at different times in embryonic

development. The high level of metabolic activity in the developing embryo is fueled by

continuous input from the parent plant into the ovule. Eventually metabolism slows, and the

connection of the seed to the ovary is severed by the degeneration of the adjacent supporting

sporophyte cells. The seed dessicates (loses water), and the integuments harden to form a tough

seed coat. The seed has entered dormancy, officially ending embryogenesis. The embryo can

persist in a dormant state for weeks or years, a fact that affords tremendous survival value. There

have even been examples of seeds found stored in ancient archaeological sites that germinated

after thousands of years of dormancy!

Maturation leading to dormancy is the result of a precisely regulated program. The viviparous

mutation in maize, for example, produces genetic lesions that block dormancy (Steeves and

Sussex 1989). The apical meristems of viviparous mutants behave like those of ferns, with no

pause before producing postembryonic structures. The embryo continues to develop, and

seedlings emerge from the kernels on the ear attached to the parent plant. Recently a group of

plant genes have been identified that belong to the Polycomb group, which regulates early

development in mammals, nematodes, and insects (Preuss 1999). These genes encode chromatin

silencing factors, which may play an important role in seed formation.

Plant hormones are critical in dormancy, and linking them to genetic mechanisms is an active

area of research. The hormone abscisic acid is important in maintaining dormancy in many

species. Gibberellins, another class of hormones, are important in breaking dormancy.

Germination

The postembryonic phase of plant development begins with germination. Some dormant seeds

require a period of after-ripening during which low-level metabolic activities continue to prepare

the embryo for germination. Highly evolved interactions between the seed and its environment

increase the odds that the germinating seedling will survive to produce another generation.

Temperature, water, light, and oxygen are all key in determining the success of germination.

Stratification is the requirement for chilling (5°C) to break dormancy in some seeds. In

temperate climates, this adaptation ensures germination only after the winter months have passed.

In addition, seeds have maximum germination rates at moderate temperatures of 25° 30°C and

often will not germinate at extreme temperatures. Seeds such as lettuce require light (specifically,

the red wavelengths) for germination; thus seeds will not germinate so far below ground that they

use up their food reserves before photosynthesis is possible.

Desiccated seeds may be only 5 20% water. Imbibition is the process by which the seed

rehydrates, soaking up large volumes of water and swelling to many times its original size. The

radicle (primary embryonic root) emerges from the seed first to enhance water uptake; it is

protected by a root cap produced by the root apical meristem. Water is essential for metabolic

activity, but so is oxygen. A seed sitting in a glass of water will not survive. Some species have

such hard protective seed coats that they must be scarified (scratched or etched) before water and

oxygen can cross the barrier. Scarification can occur by the seed being exposed to the weather

and other natural elements over time, or by its exposure to acid as the seed passes through the gut

of a frugivore. The frugivore thus prepares the seed for germination, as well as dispersing it to a

site where germination can take place.

During germination, the plant draws on the nutrient reserves in the endosperm or cotyledons.

Interactions between the embryo and endosperm in monocots use gibberellin as a signal to trigger

the breakdown of starch into sugar. As the shoot reaches the surface, the differentiation of

chloroplasts is triggered by light. Seedlings that germinate in the dark have long, spindly stems

and do not produce chlorophyll. This environmental response allows plants to use their limited

resources to reach the soil surface, where photosynthesis will be productive.

The delicate shoot tip must be protected as the shoot pushes through the soil. Three strategies for

protecting the shoot tip have evolved 

1. Cotyledons protect the shoot tip.

2. The epicotyl (the stem above the cotyledons) bends so that stem tissue, rather than the shoot

tip, pushes through the soil.

3. In monocots, a special leaflike structure, the coleoptile, forms a protective sheath around the

shoot tip.

Vegetative Growth

When the shoot emerges from the soil, most of the sporophyte body plan remain constant.thebasic parts of the mature sporophyte plant, which will emerge

from meristems.

Meristems

As has been mentioned, meristems are clusters of cells that allow the basic body pattern

established during embryogenesis to be reiterated and extended after germination. Meristematic

cells are similar to stem cells in animals.* They divide to give rise to one daughter cell that

continues to be meristematic and another that differentiates. Meristems fall into three categories:

apical, lateral, and intercalary.

Apical meristems occur at the growing shoot and root tips . Root apical meristems

produce the root cap, which consists of lubricated cells that are sloughed off as the meristem is

pushed through the soil by cell division and elongation in more proximal cells. The root apical

meristem also gives rise to daughter cells that produce the three tissue systems of the root. New

root apical meristems are initiated from tissue within the core of the root and emerge through the

ground tissue and dermal tissue. Root meristems can also be derived secondarily from the stem of

the plant; in the case of maize, this is the major source of root mass.

The shoot apical meristem produces stems, leaves, and reproductive structures. In addition to the

shoot apical meristem initiated during embryogenesis, axillary shoot apical meristems (axillary

buds;) derived from the original one form in the axils (the angles between leaf

and stem). Unlike new root meristems, these arise from the surface layers of the meristem.

Angiosperm apical meristems are composed of up to three layers of cells . One way of investigating the contributions of different layers

to plant structure is by constructing chimeras. Plant chimeras are composed of layers having

distinct genotypes with discernible markers..

Chimeras have also been used to demonstrate classical induction in plants, in which, as in animal

development, one layer influences the developmental pathway of an adjacent layer.

The size of the shoot apical meristem is precisely controlled by intercellular signals, most likely

between layers of the meristem (reviewed by Doerner 1999). Mutations in the Arabidopsis

CLAVATA genes, for example, lead to increased meristem size and the production of extra

organs. STM has the opposite effect, and double mutant phenotypes are consistent with the

hypothesis that the two work together to maintain meristem size (Clark et al. 1996). Perhaps they

balance the rate of cell division (which enlarges the meristem) and the rate of cell differentiation

in the periphery of the meristem (which decreases meristem size) (Meyerowitz 1997).

Lateral meristems are cylindrical meristems found in shoots and roots that result in secondary

growth (an increase in stem and root girth by the production of vascular tissues). Monocot stems

do not have lateral meristems, but often have intercalary meristems inserted in the stems

between mature tissues. The popping sound you can hear in a cornfield on a summer night is

actually caused by the rapid increase in stem length due to intercalary meristems.

Root development

Radial and axial patterning in roots begins during embryogenesis and continues throughout

development as the primary root grows and lateral roots emerge from the pericycle cells deep

within the root. Laser ablation experiments eliminating single cells and clonal analyses have

demonstrated that cells are plastic and that position is the primary determinant of fate in early root

development. Analyses of root radial organization mutants have revealed genes with layerspecific

activity (Scheres et al. 1995; Scheres and Heidstra 1999). We will illustrate these

findings by looking at two Arabidopsis genes that regulate ground tissue fate.

In wild-type Arabidopsis, there are two layers of root ground tissue. The outer layer becomes the

cortex, and the inner layer becomes the endodermis, which forms a tube around the vascular

tissue core. The SCARECROW (SCR) and SHORT-ROOT (SHR) genes have mutant phenotypes

with one, instead of two, layers of root ground tissue (Benfey et al. 1993). The SCR gene is

necessary for an asymmetrical cell division in the initial layer of cells, yielding a smaller

endodermal cell and a larger cortex cell (Figure 20.23). The scr mutant expresses markers for

both cortex and endodermal cells, indicating that differentiation progresses in the absence of cell

division (Di Laurenzio 1996). SHR is responsible for endodermal cell specification. Cells in the

shr mutant do not develop endodermal features.

Axial patterning in roots may be morphogen-dependent, paralleling some aspects of animal

development. A variety of experiments have established that the distribution of the plant hormone

auxin organizes the axial pattern. A peak in auxin concentration at the root tip must be perceived

for normal axial patterning (Sabatini et al. 1999).

As discussed earlier, distinct genes specifying root and shoot meristem formation have been

identified; however, root and shoot development may share common groups of genes that

regulate cell fate and patterning (Benfey 1999). This appears to be the case for the SCR and SHR

genes. In the shoot, these genes are necessary for the normal gravitropic response, which is

dependent on normal endodermis formation (a defect in mutants of both genes; see figure

20.23C). It's important to keep in mind that there are a number of steps between establishment of

the basic pattern and elaboration of that pattern into anatomical and morphological structure.

Uncovering the underlying control mechanisms is likely to be the most productive strategy in

understanding how roots and shoots develop.

Shoot development

The unique aboveground architectures of different plant species have their origins in shoot

meristems. Shoot architecture is affected by the amount of axillary bud outgrowth. Branching

patterns are regulated by the shoot tip a phenomenon called apical dominance and plant

hormones appear to be the factors responsible. Auxin is produced by young leaves and

transported toward the base of the leaf. It can suppress the outgrowth of axillary buds. Grazing

and flowering often release buds from apical dominance, at which time branching occurs.

Cytokinins can also release buds from apical dominance. Axillary buds can initiate their own

axillary buds, so branching patterns can get quite complex. Branching patterns can be regulated

by environmental signals so that an expansive canopy in an open area maximizes light capture.

Asymmetrical tree crowns form when two trees grow very close to each other. In addition to its

environmental plasticity, shoot architecture is genetically regulated. In several species, genes

have now been identified that regulate branching patterns.

Leaf primordia (clusters of cells that will form leaves) are initiated at the periphery of the shoot

meristem (see Figure 20.21). The union of a leaf and the stem is called a node, and stem tissue

between nodes is called an internode (see Figure 20.20). In a simplistic sense, the mature

sporophyte is created by stacking node/internode units together. Phyllotaxy, the positioning of

leaves on the stem, involves communication among existing and newly forming leaf primordia.

Leaves may be arranged in various patterns, including a spiral, 180-degree alternation of single

leaves, pairs, and whorls of three or more leaves at a node (Jean and Barabé 1998).

Experimentation has revealed a number of mechanisms for maintaining geometrically regular

spacing of leaves on a plant, including chemical and physical interactions of new leaf primordia

with the shoot apex and with existing primordia (Steeves and Sussex 1989).

It is not clear how a specific pattern of phyllotaxy gets started. Descriptive mathematical models

can replicate the observed patterns, but reveal nothing about the mechanism. Biophysical models

(e.g., of the effects of stress/strain on deposition of cell wall material, which affects cell division

and elongation) attempt to bridge this gap. Developmental genetics approaches are promising, but

few phyllotactic mutants have been identified. One candidate is the terminal ear mutant in maize,

which has irregular phyllotaxy. The wild-type gene is expressed in a horseshoe-shaped region,

with a gap where the leaf will be initiated (Veit et al. 1998). The plane of the horseshoe is

perpendicular to the axis of the stem

.

Leaf development

Leaf development includes commitment to become a leaf, establishment of leaf axes, and

morphogenesis, giving rise to a tremendous diversity of leaf shapes. Culture experiments have

assessed when leaf primordia become determined for leaf development. Research on ferns and

angiosperms indicates that the youngest visible leaf primordia are not determined to make a leaf;

rather, these young primordia can develop as shoots in culture (Steeves 1966; Smith 1984). The

programming for leaf development occurs later. The radial symmetry of the leaf primordium

becomes dorsal-ventral, or flattened, in all leaves. Two other axes, the proximal-distal and lateral,

are also established. The unique shapes of leaves result from regulation of cell division and cell

expansion as the leaf blade develops. There are some cases in which selective cell death

(apoptosis) is involved in the shaping of a leaf, but differential cell growth appears to be a more

common mechanism (Gifford and Foster 1989).

Leaves fall into two categories, simple and compound.There is much variety in simple leaf shape, from smooth-edged leaves to deeply lobed oak leaves.

Compound leaves are composed of individual leaflets (and sometimes tendrils) rather than a

single leaf blade. Whether simple and compound leaves develop by the same mechanism is an

open question. One perspective is that compound leaves are highly lobed simple leaves. An

alternative perspective is that compound leaves are modified shoots. The ancestral state for seed

plants is believed to be compound, but for angiosperms it is simple. Compound leaves have arisen

multiple times in the angiosperms, and it is not clear if these are reversions to the ancestral state.

Developmental genetic approaches are being applied to leaf morphogenesis. The Class I KNOX

genes are homeobox genes that include STM and the KNOTTED 1 (KN1) gene in maize. Gain-offunction mutations of KN1 cause meristem-like bumps to form on maize leaves.

At a more microscopic level, the patterning of stomata (openings for gas and water exchange) and

trichomes (hairs) across the leaf is also being investigated. In monocots, the stomata form in

parallel files, while in dicots the distribution appears more random. In both cases, the patterns

appear to maximize the evenness of stomata distribution . Genetic analysis is providing insight

into the mechanisms regulating this distribution. A common gene group appears to be working in

both shoots and roots, affecting the distribution pattern of both trichomes and root hairs (Benfey

1999).

*The similarities between plant meristem cells and animal stem cells may extend to the molecular

level, indicating that stem cells existed before plants and animals pursued separate phylogenetic

pathways. Homology has been found between genes required for plant meristems to persist and

genes expressed in Drosophila germ line stem cells (Cox et al. 1998).

This phenomenon, called fasciation, is found in many species, including peas and tomatoes.

The Vegetative-to-Reproductive Transition

Unlike some animal systems in which the germ line is set aside during early embryogenesis, the

germ line in plants is established only after the transition from vegetative to reproductive

development that is, flowering. The vegetative and reproductive structures of the shoot are all

derived from the shoot meristem formed during embryogenesis. Clonal analysis indicates that no

cells are set aside in the shoot meristem of the embryo to be used solely in the creation of

reproductive structures (McDaniel and Poethig 1988). In maize, irradiating seeds causes changes

in the pigmentation of some cells. These seeds give rise to plants that have visually

distinguishable sectors descended from the mutant cells. Such sectors may extend from the

vegetative portion of the plant into the reproductive regions (Figure 20.27), indicating that maize

embryos do not have distinct reproductive compartments.

Maximal reproductive success depends on the timing of flowering and on balancing the number

of seeds produced with resources allocated to individual seeds. As in animals, different strategies

work best for different organisms in different environments. There is a great diversity of

flowering patterns among the over 300,000 angiosperm species, yet there appears to be an

underlying evolutionary conservation of flowering genes and common patterns of flowering

regulation.

A simplistic explanation of the flowering process is that a signal from the leaves moves to the

shoot apex and induces flowering. In some species, this flowering signal is a response to

environmental conditions. The developmental pathways leading to flowering are regulated at

numerous control points in different plant organs (roots, cotyledons, leaves, and shoot apices) in

various species, resulting in a diversity of flowering times and reproductive architectures. The

nature of the flowering signal, however, remains unknown.

Some plants, especially woody perennials, go through a juvenile phase, during which the plant

cannot produce reproductive structures even if all the appropriate environmental signals are

present (Lawson and Poethig 1995). The transition from the juvenile to the adult stage may

require the acquisition of competence by the leaves or meristem to respond to an internal or

external signal (McDaniel et al. 1992; Singer et al. 1992; Huala and Sussex 1993).

Grafting and organ culture experiments, mutant analyses, and molecular analyses give us a

framework for describing the reproductive transition in plants (Figure 20.28). Grafting

experiments have identified the sources of signals that promote or inhibit flowering and have

provided information on the developmental acquisition of meristem competence to respond to

these signals (Lang et al. 1977; Singer et al. 1992; McDaniel et al. 1996; Reid et al. 1996).

Analyses of mutants and molecular characterization of genes are yielding information on the

mechanics of these signal-response mechanisms (Hempel et al. 2000; Levy and Dean 1998).

Leaves produce a graft-transmissible substance that induces flowering. In some species, this

signal is produced only under specific photoperiods (day lengths), while other species are dayneutral

and will flower under any photoperiod (Zeevaart 1984). Not all leaves may be competent

to perceive or pass on photoperiodic signals. The phytochrome pigments transduce these signals

from the external environment. The structure of phytochrome is modified by red and far-red light,

and these changes can initiate a cascade of events leading to the production of either floral

promoter or floral inhibitor . 

Senescence

Flowering and senescence (a developmental program leading to death) are closely linked in many

angiosperms. Individual flower petals in some species senesce following pollination. Orchids,

which stay fresh for long periods of time if they are not pollinated, are a good example. Fruit

ripening (and ultimately over-ripening) is an example of organ senescence. Whole-plant

senescence leads to the death of the entire sporophyte generation. Monocarpic plants flower once

and then senesce. Polycarpic plants, such as the bristlecone pine, can live thousands of years

(4900 years is the current record) and flower repeatedly. In polycarpic plants, death is by

accident; in monocarpic plants, it appears to be genetically programmed. Flowers and fruits play a

key role in the process, and their removal can sometimes delay senescence. In some legumes,

senescence can be delayed by removing the developing seed in other words, the embryo may

trigger senescence in the parent plant. During flowering and fruit development, nutrients are

reallocated from other parts of the plant to support the development of the next generation. The

reproductive structures become a nutrient sink, and this can lead to whole-plant senescence.

Snapshot Summary: Plant Development

1. Plants are characterized by alternation of generations; that is, their life cycle includes both

diploid and haploid multicellular generations.

2. A multicellular diploid sporophyte produces haploid spores via meiosis. These spores divide

mitotically to produce a haploid gametophyte. Mitotic divisions within the gametophyte produce

the gametes. The diploid sporophyte results from the fusion of two gametes.

3. The male gamete, pollen, arrives at the style of the female gametophyte and effects fertilization

through the pollen tube. Two sperm cells move through the pollen tube; one joins with the ovum

to form the zygote, and the other is involved in the formation of the endosperm.

4. Plant embryos develop deeply embedded in parental tissue. The parent tissue provides nutrients

but only minimal patterning information.

5. Early embryogenesis is characterized by the establishment of the shoot-root axis and by radial

patterning yielding three tissue systems. Pattern emerges by regulation of planes of cell division

and the directions of cell expansion, since plant cells do not move during development.

6. As the embryo matures, a food reserve is established. Only the rudiments of the basic body

plan are established by the time embryogenesis ceases and the seed enters dormancy.

7. Pattern is elaborated during postembryonic development, when meristems construct the

reiterative structures of the plant.

8. The germ line is not reserved early in development. Coordination of signaling among leaves,

roots, and shoot meristems regulates the transition to the reproductive state. Reproduction may be

followed by genetically programmed senescence of the parent plant.

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