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Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.

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Molecular Biology of the Cell. 4th edition.

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Plant Development

Plants and animals are separated by about 1.5 billion years of evolutionary history. They have evolved their multicellular organization independently but using the same initial tool kit—the set of genes inherited from their common unicellular eucaryotic ancestor. Most of the contrasts in their developmental strategies spring from two basic peculiarities of plants. First, they get their energy from sunlight, not by ingesting other organisms. This dictates a body plan different from that of animals. Second, their cells are encased in semirigid cell walls and cemented together, preventing them from moving as animal cells do. This dictates a different set of mechanisms for shaping the body and different developmental processes to cope with a changeable environment.

Animal development is largely buffered against environmental changes, and the embryo generates the same genetically determined body structure unaffected by external conditions. The development of most plants, by contrast, is dramatically influenced by the environment. Because they cannot match themselves to their environment by moving from place to place, plants adapt instead by altering the course of their development. Their strategy is opportunistic. A given type of organ—a leaf, a flower, or a root, say—can be produced from the fertilized egg by many different paths according to environmental cues. A begonia leaf pegged to the ground may sprout a root; the root may throw up a shoot; the shoot, given sunlight, may grow leaves and flowers.

The mature plant is typically made of many copies of a small set of standardized modules, as described in Figure 21-106. The positions and times at which those modules are generated are strongly influenced by the environment, causing the overall structure of the plant to vary. The choices between alternative modules and their organization into a whole plant depend on external cues and long-range hormonal signals that play a much smaller part in the control of animal development.

Figure 21-106. A simple example of the modular construction of plants.

Figure 21-106

A simple example of the modular construction of plants. Each module (shown in different shades of green) consists of a stem, a leaf, and a bud containing a potential growth center, or meristem. The bud forms at the branch point, or node, where the leaf (more...)

But although the global structure of a plant—its pattern of roots or branches, its numbers of leaves or flowers—can be highly variable, its detailed organization on a small scale is not. A leaf, a flower, or indeed an early plant embryo, is as precisely specified as any organ of an animal, possessing a determinate structure, in contrast with the indeterminate pattern of branching and sprouting of the plant as a whole. The internal organization of a plant module raises essentially the same problems in the genetic control of pattern formation as does animal development, and they are solved in analogous ways. In this section we focus on the cellular mechanisms of development in flowering plants. We examine both the contrasts and the similarities with animals.

Arabidopsis Serves as a Model Organism for Plant Molecular Genetics

Flowering plants, despite their amazing variety, are of relatively recent origin. The earliest known fossil examples are 125 million years old, as against 350 million years or more for vertebrate animals. Underlying the diversity of form, therefore, there is a high degree of similarity in molecular mechanisms. As we shall see, a small genetic change can transform a plant's large-scale structure; and just as plant physiology allows survival in many different environments, so also it allows survival of many differently structured forms. A mutation that gives an animal two heads is generally lethal; one that doubles the number of flowers or branches on a plant is generally not.

To identify the genes that govern plant development and to discover how they function, plant biologists have selected a small weed, the common wall cress Arabidopsis thaliana (Figure 21-107), as their primary model organism. Like Drosophila or Caenorhabditis elegans, it is small, quick to reproduce, and convenient for genetics. It can be grown indoors in Petri dishes or tiny plant pots in large numbers and produces hundreds of seeds per plant after 8 to 10 weeks. It has, in common with C. elegans, a significant advantage over Drosophila or vertebrate animals for genetics: like many flowering plants, it can reproduce as a hermaphrodite because a single flower produces both eggs and the male gametes that can fertilize them. Therefore, when a flower that is heterozygous for a recessive lethal mutation is self-fertilized, one-fourth of its seeds will display the homozygous embryonic phenotype. This makes it easy to perform genetic screens (Figure 21-108) and so to obtain a catalog of the genes required for specific developmental processes.

Figure 21-107. Arabidopsis thaliana.

Figure 21-107

Arabidopsis thaliana. This small plant is a member of the mustard (or crucifer) family (see also Figure 1-47). It is a weed of no economic use but of great value for genetic studies of plant development. (From M.A. Estelle and C.R. Somerville, Trends (more...)

Figure 21-108. Production of mutants in Arabidopsis.

Figure 21-108

Production of mutants in Arabidopsis. A seed, containing a multicellular embryo, is treated with a chemical mutagen and left to grow into a plant. In general, this plant will be a mosaic of clones of cells carrying different induced mutations. An individual (more...)

The Arabidopsis Genome Is Rich in Developmental Control Genes

Arabidopsis has one of the smallest plant genomes—125 million nucleotide pairs, on a par with C. elegans and Drosophila—and the complete DNA sequence is now known. It contains approximately 26,000 genes. This total includes many recently generated duplicates, however, so that the number of functionally distinct types of protein represented may be considerably less. Cell culture and genetic transformation methods have been established, as well as vast libraries of seeds carrying mutations produced by random insertions of mobile genetic elements, so that plants with mutations in any chosen gene can be obtained to order. Powerful tools are thus available to analyze gene functions. Although only a small fraction of the total gene set has been characterized experimentally as yet, functions can be tentatively assigned to many genes—about 18,000—on the basis of their sequence similarities to well-characterized genes in Arabidopsis and other organisms.

Even more than the genomes of multicellular animals, the Arabidopsis genome is rich in genes that code for gene regulatory proteins (Table 21-2). Some major families of animal gene regulatory proteins (such as the Myb family of DNA-binding proteins) are greatly expanded, while others (such as nuclear hormone receptors) seem to be entirely absent, and there are large families of gene regulatory proteins in the plant that have no animal homologs.

Table 21-2. Some Major Families of Gene Regulatory Proteins in Arabidopsis, Drosophila, C. elegans, and the Yeast Saccharomyces cerevisiae.

Table 21-2

Some Major Families of Gene Regulatory Proteins in Arabidopsis, Drosophila, C. elegans, and the Yeast Saccharomyces cerevisiae.

Where homologous gene regulatory proteins (such as homeodomain proteins) can be recognized in both plants and animals, they have little in common with regard to the genes they regulate or the types of developmental decisions that they control, and there is very little conservation of protein sequence outside the DNA-binding domains.

Arabidopsis is like multicellular animals in possessing many genes for cell communication and signal transduction (1,900 genes out of 18,000 classified), but the specific details of these gene sets are very different, as discussed in Chapter 15. The Wnt, Hedgehog, Notch, and TGFβ signaling mechanisms are all absent in Arabidopsis. In compensation, other signaling pathways peculiar to plants are highly developed. Cell-surface receptors of the tyrosine kinase class seem to be entirely absent, although many of the signaling components downstream of these receptors in animals are present. Conversely, receptors of the serine/threonine kinase class are very plentiful, but they do not act through the same system of intracellular messengers as the receptor serine/threonine kinases in animals. Substantial sets of genes are devoted to developmental processes of special importance in plants: more than 700 for synthesis and remodelling of the plant cell wall, for example, and more than 100 for detecting and responding to light.

We must now examine how the genes of the plant are used to control plant development.

Embryonic Development Starts by Establishing a Root-Shoot Axis and Then Halts Inside the Seed

The basic strategy of sexual reproduction in flowering plants is briefly summarized in Panel 21-1. The fertilized egg, or zygote, of a higher plant begins by dividing asymmetrically to establish the polarity of the future embryo. One product of this division is a small cell with dense cytoplasm, which will become the embryo proper. The other is a large vacuolated cell that divides further and forms a structure called the suspensor, which in some ways is comparable to the umbilical cord in mammals. The suspensor attaches the embryo to the adjacent nutritive tissue and provides a pathway for the transport of nutrients.

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Panel 21-1

Features of Early Development in Flowering Plants.

During the next step in development the diploid embryo cell proliferates to form a ball of cells that quickly acquires a polarized structure. This comprises two key groups of proliferating cells—one at the suspensor end of the embryo that will collaborate with the uppermost suspensor cell to generate a root, and one at the opposite end that will generate a shoot (Figure 21-109). The main root-shoot axis established in this way is analogous to the head-to-tail axis of an animal. At the same time it begins to be possible to distinguish the future epidermal cells, forming the outermost layer of the embryo, the future ground tissue cells, occupying most of the interior, and the future vascular tissue cells, forming the central core (Panel 21-2). These three sets of cells can be compared to the three germ layers of an animal embryo. Slightly later in development, the rudiment of the shoot begins to produce the embryonic seed leaves, or cotyledons—one in the case of monocots and two in the case of dicots. Soon after this stage, development usually halts and the embryo becomes packaged in a seed (a case formed by tissues of the mother plant), specialized for dispersal and for survival in harsh conditions. The embryo in a seed is stabilized by dehydration, and it can remain dormant for a very long time—even hundreds of years. When rehydrated, the seeds germinate and embryonic development resumes.

Figure 21-109. Two stages of embryogenesis in Arabidopsis thaliana.

Figure 21-109

Two stages of embryogenesis in Arabidopsis thaliana. (From G. Jürgens et al., Development [Suppl.] 1:27–38, 1991. © The Company of Biologists.)

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Panel 21-2

The Cell Types and Tissues From Which Higher Plants Are Constructed.

Genetic screens can be used in Arabidopsis, just as in Drosophila or C. elegans, to identify the genes that govern the organization of the embryo and to group these into categories according to their homozygous mutant phenotypes. Some are required for formation of the seedling root, some for the seedling stem, and some for the seedling apex with its cotyledons. Another class is required for formation of the three major tissue types—epidermis, ground tissue, and vascular tissue—and yet another class for the organized changes of cell shape that give the embryo and seedling their elongated form (Figure 21-110).

Figure 21-110. Mutant Arabidopsis seedlings.

Figure 21-110

Mutant Arabidopsis seedlings. A normal seedling (A) compared with four types of mutant (B-E) defective in different parts of their apico-basal pattern: (B) has structures missing at its apex, (C) has an apex and a root but lacks a stem between them, (D) (more...)

The Parts of a Plant Are Generated Sequentially by Meristems

Roughly speaking, the embryo of an insect or a vertebrate animal is a rudimentary miniature scale model of the later organism, and the details of body structure are filled in progressively as it enlarges. The plant embryo grows into an adult in a quite different way: the parts of the adult plant are created sequentially by groups of cells that proliferate to lay down additional structures at the plant's periphery. These all-important groups of cells are called apical meristems (see Figure 21-106). Each meristem consists of a self-renewing population of stem cells. As these divide, they leave behind a trail of progeny that become displaced from the meristem region, enlarge, and finally differentiate. Although the shoot and root apical meristems generate all the basic varieties of cells that are needed to build leaves, roots, and stems, many cells outside the apical meristems also keep a capacity for further proliferation and retain meristem potential. In this way trees and other perennial plants, for example, are able to increase the girth of their stems and roots as the years go by and can sprout new shoots from dormant regions if the plant is damaged.

The rudiments of the apical meristems of root and shoot are already determined in the embryo. As soon as the seed coat ruptures during germination, a dramatic enlargement of nonmeristematic cells occurs, driving the emergence first of a root, to establish an immediate foothold in the soil, and then of a shoot (Figure 21-111). This is accompanied by rapid and continual cell divisions in the apical meristems: in the apical meristem of a maize root, for example, cells divide every 12 hours, producing 5 × 105 cells per day. The rapidly growing root and shoot probe the environment—the root increasing the plant's capacity for taking up water and minerals from the soil, the shoot increasing its capacity for photosynthesis (see Panel 21-1).

Figure 21-111. A seedling of Arabidopsis.

Figure 21-111

A seedling of Arabidopsis. The brown objects to the right of the young seedling are the two halves of the discarded seed coat. (Courtesy of Catherine Duckett.)

Development of the Seedling Depends on Environmental Signals

From germination onward, the course of plant development is powerfully influenced by signals from the environment. The shoot has to push its way rapidly up through the soil, and must open its cotyledons and begin photosynthesis only after it has reached the light. The timing of this transition from rapid subterranean sprouting to illuminated growth cannot be genetically programmed, because the depth at which the seed is buried is unpredictable. The developmental switch is controlled instead by light, which, among other effects, acts on the seedling by inhibiting production of a class of plant hormones called brassinosteroids, discussed in Chapter 15. Mutations in genes required for production or reception of the brassinosteroid signal cause the stem of the seedling to go green, slow its elongation, and open its cotyledons prematurely, while it is still in the dark.

The Shaping of Each New Structure Depends on Oriented Cell Division and Expansion

Plant cells, imprisoned within their cell walls, cannot crawl about and cannot be shuffled as the plant grows; but they can divide, and they can swell, stretch, and bend. The morphogenesis of a developing plant therefore depends on orderly cell divisions followed by strictly oriented cell expansions. Most cells produced in the root-tip meristem, for example, go through three distinct phases of development—division, growth (elongation), and differentiation. These three steps, which overlap in both space and time, give rise to the characteristic architecture of a root tip. Although the process of cell differentiation often begins while a cell is still enlarging, it is comparatively easy to distinguish in a root tip a zone of cell division, a zone of oriented cell elongation (which accounts for the growth in length of the root), and a zone of cell differentiation (Figure 21-112).

Figure 21-112. A growing root tip.

Figure 21-112

A growing root tip. (A) The organization of the final 2 mm of a growing root tip. The approximate zones in which cells can be found dividing, elongating, and differentiating are indicated. (B) The apical meristem and root cap of a corn root tip, showing (more...)

In the phase of controlled expansion that generally follows cell division, the daughter cells may often increase in volume by a factor of 50 or more. This expansion is driven by an osmotically based turgor pressure that presses outward on the plant cell wall, and its direction is determined by the orientation of the cellulose fibrils in the cell wall, which constrain expansion along one axis (see Figure 19-73). The orientation of the cellulose in turn is apparently controlled by the orientation of arrays of microtubules just inside the plasma membrane, which are thought to guide cellulose deposition (discussed in Chapter 19). These orientations can be rapidly changed by plant growth regulators, such as ethylene and gibberellic acid (Figure 21-113), but the molecular mechanisms underlying these dramatic cytoskeletal rearrangements are still unknown.

Figure 21-113. The different effects of the plant growth regulators ethylene and gibberellic acid.

Figure 21-113

The different effects of the plant growth regulators ethylene and gibberellic acid. These regulators exert rapid and opposing effects on the orientation of the cortical microtubule array in cells of young pea shoots. A typical cell in an ethylene-treated (more...)

Each Plant Module Grows From a Microscopic Set of Primordia in a Meristem

The apical meristems are self-perpetuating: in a perennial plant, they carry on with their functions indefinitely, as long as the plant survives, and they are responsible for its continuous growth and development. But apical meristems also give rise to a second type of outgrowth, whose development is strictly limited and culminates in the formation of a structure such as a leaf or a flower, with a determinate size and shape and a short lifespan. Thus, as a vegetative (non-flowering) shoot elongates, its apical meristem lays down behind itself an orderly sequence of nodes, where leaves have grown out, and internodes (segments of stem). In this way the continuous activity of the meristem produces an ever increasing number of similar modules, each consisting of a stem, a leaf, and a bud (see Figure 21-106). The modules are connected to one another by supportive and transport tissue, and successive modules are precisely located relative to each other, giving rise to a repetitively patterned structure. This iterative mode of development is characteristic of plants and is seen in many other structures besides the stem-leaf system (Figure 21-114).

Figure 21-114. Repetitive patterning in plants.

Figure 21-114

Repetitive patterning in plants. Accurate placing of successive modules from a single apical meristem produces these elaborate but regular patterns in leaves (A), flowers (B), and fruits (C). (A, from John Sibthorp, Flora Graeca. London: R. Taylor, 1806–1840; (more...)

Although the final module is large, its organization, like that of an animal embryo, is mapped out at first on a microscopic scale. At the apex of the shoot, within a space of a millimeter or less, one finds a small, low central dome surrounded by a set of distinctive swellings in various stages of enlargement (Figure 21-115). The central dome is the apical meristem itself; each of the surrounding swellings is the primordium of a leaf. This small region, therefore, contains the already distinct rudiments of several entire modules. Through a well-defined program of cell proliferation and cell enlargement, each leaf primordium and its adjacent cells will grow to form a leaf, a node, and an internode. Meanwhile, the apical meristem itself will give rise to new leaf primordia, so as to generate more and more modules in a potentially unending succession. The serial organization of the modules of the plant is thus controlled by events at the shoot apex. It is the system of local signals within this tiny region that determines the pattern of primordia—the position of one leaf rudiment relative to the next, the spacing between them, and their location relative to the apical meristem itself.

Figure 21-115. A shoot apex from a young tobacco plant.

Figure 21-115

A shoot apex from a young tobacco plant. (A) A scanning electron micrograph shows the shoot apex with two sequentially emerging leaf primordia, seen here as lateral swellings on either side of the domed apical meristem. (B) A thin section of a similar (more...)

Variations on this basic repetitive theme can give rise to more complex architectures, including structures such as tendrils, thorns, branches, and flowers. Thus, by switching on different sets of genes at the shoot apex, the plant can produce different types of primordia, in different spatial patterns.

Cell Signaling Maintains the Meristem

Central to all these phenomena is the question of how the apical meristem maintains itself. The meristem cells must continue to proliferate for weeks, years, or even centuries as a plant grows, replacing themselves while continually generating progeny cells that differentiate. Through all this, the size of the cluster of cells that constitute the meristem remains practically constant (about 100 cells in Arabidopsis, for example). New meristems may arise as the plant branches, but they too preserve the same size.

Genetic screens have identified genes required for meristem maintenance. For example, mutations that disrupt the WUSCHEL gene, which codes for a homeodomain protein, convert the apical meristem into non-meristematic tissue, so that the seedling fails to sprout. Conversely, mutations in the CLAVATA group of genes, coding for components of a cell-cell signaling pathway (see Figure 15-77), make the meristem abnormally big. These genes are expressed in different layers of cells in the meristem region (Figure 21-116A). The two most superficial cell layers, called the L1 and L2 layers, together with the uppermost part of the L3 layer, contain the cells of the meristem proper, capable of dividing indefinitely to give rise to future parts of the plant. The meristematic cells of the L1 and L2 layers express Clavata3, a small secreted signal protein. Just beneath, in the L3 layer, lies a cluster of cells expressing Clavata1 (the receptor for Clavata3). In the center of this Clavata1 patch are cells that express the Wuschel gene regulatory protein.

Figure 21-116. The feedback loops that are thought to maintain the shoot apical meristem.

Figure 21-116

The feedback loops that are thought to maintain the shoot apical meristem. (A) The arrangement of cell layers constituting a shoot apical meristem. (B) The pattern of cell-cell communication that maintains the meristem. Artificial overexpression of Wuschel (more...)

The pattern of cell divisions implies that the cells expressing Wuschel are not themselves part of the meristem proper; new Wuschel-expressing cells are apparently continually recruited from the meristematic part of the L3 population, just above the Wuschel domain. Nevertheless, the Wuschel-expressing cells are at the heart of the mechanism that maintains the meristem. A signal that they produce maintains meristematic behavior in the cells above, stimulates expression of the CLAVATA genes, and, presumably, causes new cells recruited into the Wuschel domain to switch on Wuschel. Negative feedback from the upper meristematic cells, delivered by the Clavata signaling pathway, acts back on the regions below to limit the size of the Wuschel domain, thereby preventing the meristem from becoming too big (Figure 21-116B).

This account of the plant meristem is still uncertain in many details and other genes besides those we have mentioned are also involved. Nevertheless, mathematical modeling shows that systems of a similar sort, based on a feedback loop involving a short-range activating signal and a long-range inhibitory signal, can stably maintain a signaling center of a well-defined size even when there is continual proliferation and turnover of the cells that form that center. Analogous systems of signals are thought to operate in animal development to maintain localized signaling centers—such as the Organizer of the amphibian gastrula, or the zone of polarizing activity in a limb bud.

It is still not known how the Wuschel-expressing cells signal to their neighbors. One possibility is that the Wuschel protein itself diffuses directly from cell to cell through plasmodesmata—a signaling pathway peculiar to plants. Some other gene regulatory proteins have in fact been shown to travel in this way in meristems, spreading from cells that contain the corresponding mRNA into neighboring cells that do not.

Regulatory Mutations Can Transform Plant Topology by Altering Cell Behavior in the Meristem

If a stem is to branch, new shoot apical meristems must be created, and this too depends on events in the neighborhood of the shoot apex. At each developing node, in the acute angle (the axil) between the leaf primordium and the stem, a bud is formed (Figure 21-117). This contains a nest of cells, derived from the apical meristem, that keep a meristematic character. They have the capacity to become the apical meristem of a new branch or the primordium of a structure such as a flower; but they also have the alternative option of remaining quiescent as axillary buds. The plant's pattern of branching is regulated through this choice of fate, and mutations that affect it can transform the structure of the plant. Maize provides a beautiful example.

Figure 21-117. Axillary buds in the neighborhood of a shoot apex.

Figure 21-117

Axillary buds in the neighborhood of a shoot apex. The photograph shows a longitudinal section of Coleus blumei, a common houseplant. (From P.H. Raven, R.F. Evert, and S.E. Eichhorn, Biology of Plants, 6th edn. New York: Freeman/Worth, 1999, used with (more...)

Maize represents one of mankind's most remarkable feats of genetic engineering. Native Americans created it by selective breeding, over a period of several centuries or perhaps millennia between 5,000 and 10,000 years ago. They started from a wild grass known as teosinte, with highly branched leafy stems and tiny ears bearing inedible hard kernels. Detailed genetic analysis has identified a handful of genetic loci—about five—as the sites of the mutations that account for most of the difference between this unpromising ancestor and modern corn. One of these loci, with a particularly dramatic effect, corresponds to a gene called teosinte branched-1 (tb1). In maize with loss-of-function mutations in tb1, the usual simple unbranched stem, with a few large leaves at intervals along it, is transformed into a dense, branching, leafy mass reminiscent of teosinte (Figure 21-118A). The pattern of branching in the mutant implies that axillary buds, originating in normal positions, have escaped from an inhibition that prevents them, in normal maize, from growing into branches.

Figure 21-118. Transformation of plant architecture by mutation: a comparison of teosinte, normal maize, and tb1-defective maize.

Figure 21-118

Transformation of plant architecture by mutation: a comparison of teosinte, normal maize, and tb1-defective maize. (A) Photographs of three types of plants. (B) The architecture of teosinte, normal maize and the tb1-defective maize compared schematically. (more...)

In normal maize, the single stem is crowned with a tassel—a male flower—while a few of the axillary buds along the stem develop into female flowers and, upon fertilization, form the ears of corn that we eat. In the mutant maize with a defective tb1 gene, these fruitful axillary buds are transformed into branches bearing tassels. The wild teosinte plant is like the tb1-defective maize in its leafy, highly branched appearance, but unlike this mutant it makes ears on many of its side branches, as though tb1 were active. DNA analysis reveals the explanation. Both teosinte and normal maize possess a functional tb1 gene, with an almost identical coding sequence, but in maize the regulatory region has undergone a mutation that boosts the level of gene expression. Thus in normal maize the gene is expressed at a high level in every axillary bud, inhibiting branch formation, while in teosinte the expression in many axillary buds is low, so that branches are permitted to form (Figure 21-118B).

This example shows how simple mutations, by switching the behavior of meri-stem cells, can transform plant structure—a principle of enormous importance in the breeding of plants for food. More generally, the case of tb1 illustrates how new body plans, whether of plant or animal, can evolve through changes in regulatory DNA without change in the characters of the proteins made.

Long-Range Hormonal Signals Coordinate Developmental Events in Separate Parts of the Plant

The fate of an axillary bud is dictated not only by its genes, but also by environmental conditions. Separate parts of a plant experience different environments and react to them individually by changes in their mode of development. The plant, however, must continue to function as a whole. This demands that developmental choices and events in one part of the plant affect developmental choices elsewhere. There must be long-range signals to bring about such coordination.

As gardeners know, for example, by pinching off the tip of a branch one can stimulate side growth: removal of the apical meristem relieves the quiescent axillary meristems of an inhibition and allows them to form new twigs. In this case the long-range signal from the apical meristem, or at least a key component has been identified. It is an auxin, a member of one of six known classes of plant growth regulators (sometimes called plant hormones), all of which have powerful influences on plant development. The five other known classes are the gibberellins, the cytokinins, abscisic acid, the gas ethylene, and the brassinosteroids. As shown in Figure 21-119, all are small molecules that readily penetrate cell walls. They are all synthesized by most plant cells and can either act locally or be transported to influence target cells at a distance. Auxin, for example, is transported from cell to cell at a rate of about 1 cm per hour from the tip of a shoot toward its base. Each growth regulator has multiple effects, and these are modulated by the other growth regulators, as well as by environmental cues and nutritional status. Thus auxin alone can promote root formation, but in conjunction with gibberellin it can promote stem elongation, with cytokinin, auxin it can suppress lateral shoot outgrowth, and with ethylene it can stimulate lateral root growth. The receptors that recognize some of these growth regulators are discussed in Chapter 15.

Figure 21-119. Plant growth regulators.

Figure 21-119

Plant growth regulators. The formula of one naturally occurring representative molecule from each of the six groups of plant growth regulatory molecules is shown.

Homeotic Selector Genes Specify the Parts of a Flower

Meristems face other developmental choices besides that between quiescence and growth, as we have already seen in our discussion of maize, and these also are frequently regulated by the environment. The most important is the decision to form a flower (Figure 21-120).

Figure 21-120. The structure of an Arabidopsis flower.

Figure 21-120

The structure of an Arabidopsis flower. (A) Photograph. (B) Drawings. (C) Schematic cross-sectional view. The basic plan, as shown in (C), is common to most flowering dicotyledonous plants. (A, courtesy of Leslie Sieburth.)

The switch from meristematic growth to flower formation is typically triggered by light. By poorly understood mechanisms based on light absorption by phytochrome and cryptochrome proteins (discussed in Chapter 15), the plant can sense very precisely a change in day length. It responds by turning on expression of a set of floral meristem-identity genes in the apical meristem. By switching on these genes, the apical meristem abandons its chances of continuing vegetative growth and gambles its future on the production of gametes. Its cells embark on a strictly finite program of growth and differentiation: by a modification of the ordinary mechanisms for generating leaves, a series of whorls of specialized appendages are formed in a precise order—typically sepals first, then petals, then stamens carrying anthers containing pollen, and lastly carpels containing eggs (see Panel 21-1). By the end of this process the meristem has disappeared, but among its progeny it has created germ cells.

The series of modified leaves forming a flower can be compared to the series of body segments forming a fly. In plants, as in flies, one can find homeotic mutations that convert one part of the pattern to the character of another. The mutant phenotypes can be grouped into at least four classes, in which different but overlapping sets of organs are altered (Figure 21-121). The first or ‘A’ class, exemplified by the apetala2 mutant of Arabidopsis, has its two outermost whorls transformed: the sepals are converted into carpels and the petals into stamens. The second or ‘B’ class, exemplified by apetala3, has its two middle whorls transformed: the petals are converted into sepals and the stamens into carpels. The third or ‘C’ class, exemplified by agamous, has its two innermost whorls transformed, with a more drastic consequence: the stamens are converted into petals, the carpels are missing, and in their place the central cells of the flower behave as a floral meristem, which begins the developmental performance all over again, generating another abnormal set of sepals and petals nested inside the first and, potentially, another nested inside that, and so on, indefinitely. A fourth class, the sepallata mutants, has its three inner whorls all transformed into sepals.

Figure 21-121. Arabidopsis flowers showing a selection of homeotic mutations.

Figure 21-121

Arabidopsis flowers showing a selection of homeotic mutations. (A) In apetala2, sepals are converted into carpels and petals into stamens; (B) In apetala3, petals are converted into sepals and stamens into carpels; (C)In agamous, stamens are converted (more...)

These phenotypes identify four classes of homeotic selector genes, which, like the homeotic selector genes of Drosophila, all code for gene regulatory proteins. These are expressed in different domains and define the differences of cell state that give the different parts of a normal flower their different characters, as shown in Figure 21-122. The gene products collaborate to form protein complexes that drive expression of the appropriate downstream genes. In a triple mutant where the A, B and C genetic functions are all absent, one obtains in place of a flower an indefinite succession of tightly nested leaves (see Figure 21-121D). Conversely, in a transgenic plant where genes of the A, B and sepallata classes are all expressed together outside their normal domains, leaves are transformed into petals. Leaves therefore represent a “ground state” in which none of these homeotic selector genes are expressed, while the other types of organ result from expressing the genes in different combinations.

Figure 21-122. Homeotic selector gene expression in an Arabidopsis flower.

Figure 21-122

Homeotic selector gene expression in an Arabidopsis flower. (A) Diagram of the normal expression patterns of the three genes whose mutant phenotypes are illustrated in Figure 21-121A-C. All three genes code for gene regulatory proteins. The colored shading (more...)

Similar studies have been carried out in other plant species, and a similar set of phenotypes and genes have been identified: plants, no less than animals, have conserved their homeotic selector gene systems. Gene duplication has played a large part in the evolution of these genes: several of them, required in different organs of the flower, have clearly homologous sequences. These are not of the homeobox class but are members of another family of gene regulatory proteins (the so-called MADS family), also found in yeast and in vertebrates.

Clearly, plants and animals, have independently found very similar solutions to many of the fundamental problems of multicellular development.

Summary

The development of a flowering plant, like that of an animal, begins with division of a fertilized egg to form an embryo with a polarized organization: the apical part of the embryo will form the shoot, the basal part, the root, and the middle part, the stem. At first, cell division occurs throughout the body of the embryo. As the embryo grows, however, addition of new cells becomes restricted to small regions known as meristems. Apical meristems, at shoot tips and root tips, will persist throughout the life of the plant, enabling it to grow by sequentially adding new body parts at its periphery. Typically, the shoot generates a repetitive series of modules, each consisting of a segment of stem, a leaf, and an axillary bud. An axillary bud is a potential new meristem, capable of giving rise to a side branch; the environment—and long-range hormonal signals within the plant can control the development of the plant by regulating bud activation. Mutations that alter the rules for activating axillary buds can have a drastic effect on the shape and structure of the plant; a single such mutation—one of about five key genetic alterations—accounts for a large part of the dramatic difference between modern maize and its wild ancestor, teosinte.

The small weed Arabidopsis thaliana is widely used as a model organism for genetic studies and is the first plant to have had its genome completely sequenced. As in animals, genes governing plant development can be identified through genetic screens and their functions tested by genetic manipulations. Such studies have begun to reveal the molecular mechanisms by which the internal organization of each plant module is sketched out on a microscopic scale through cell-cell interactions in the neighborhood of the apical meristem. The meristem itself appears to be maintained by a local feedback loop, in which cells expressing the gene regulatory protein Wuschel provide a positive stimulus, and a negative feedback dependent on the Clavata cell-cell signaling pathway keeps the meristem from becoming too big.

Environmental cues—especially light that is appropriately timed—can cause the expression of genes that switch the apical meristem from a leaf-forming to a flower-forming mode. The parts of a flower—its sepals, petals, stamens and carpels—are formed by a modification of the mechanism for development of leaves, and the differences between these parts are controlled by homeotic selector genes that are closely analogous (although not homologous) to those of animals.

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Copyright © 2002, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter; Copyright © 1983, 1989, 1994, Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson .
Bookshelf ID: NBK26922