Plasticity and Adaptations in Plant Structure and Growth

Chapter 35 Plant Structure, Growth, and Development Lecture Outline Overview: Plastic Plants? • The fanwort, an aquatic weed, demonstrates the great developmental plasticity that is characteristic of plants. o The fanwort has feathery underwater leaves and large, flat, floating surface leaves. o Both leaf types have genetically identical cells, but the dissimilar environments in which they develop cause different genes involved in leaf formation to be turned on or off. In addition to plastic structural responses of individual plants to specific environments, plant species have adaptations in morphology that benefit them in their specific environments. o For example, cacti have leaves that are reduced to spines and a stem that serves as the primary site of photosynthesis. These adaptations minimize water loss in desert environments. • The form of any plant is controlled by environmental and genetic factors. As a result, no two plants are identical. • Angiosperms make up 90% of plant species and are at the base of the food web of nearly every terrestrial ecosystem. Most land animals, including humans, depend on angiosperms directly or indirectly for sustenance. Concept 35. 1 The plant body has a hierarchy of organs, tissues, and cells. • Plants, like multicellular animals, have organs that are composed of different tissues, and tissues that are composed of different cell types. o A tissue is a group of cells with common structure and function. o An organ consists of several types of tissues that work together to carry out particular functions. Vascular plants have three basic organs: roots, stems, and leaves. The basic morphology of vascular plants reflects their evolutionary history as terrestrial organisms that inhabit and draw resources from two very different environments. o Vascular plants obtain water and minerals from the soil. o Vascular plants obtain CO2 and light above-ground. • To obtain the resources they need, vascular plants have evolved two systems: a subterranean root system and an aerial shoot system of stems and leaves. • Each system depends on the other. o Lacking chloroplasts and living in the dark, roots would starve without photosynthates, the sugar and other carbohydrates imported from the shoot system. Conversely, the shoot system depends on water and minerals that roots absorb from the soil. Roots provide anchorage, absorption, and storage. • A root is an organ that anchors a vascular plant in the soil, absorbs minerals and water, and stores sugars and starches. • Most eudicots and gymnosperms have a taproot system, consisting of one large vertical root (the taproot) that develops from an embryonic root. • The taproot produces many small lateral, or branch, roots. o In angiosperms, taproots often store sugars and starches that later support flowering and fruit production. Taproot systems generally penetrate deeply and are well adapted to deep soils. • In seedless vascular plants and most monocots, including grasses, the embryonic root dies and does not form a main root. • Instead, many small roots grow from the stem. Such roots are adventitious, a terms describing a plant organ that grows in an unusual location. • Each small root forms its own lateral roots, giving rise to a fibrous root system—a mat of thin roots that spread out below the soil surface. o A fibrous root system is usually shallower than a taproot system and is best adapted to shallow soils with light rainfall. Grass roots are concentrated in the upper few centimeters of soil. As a result, grasses make excellent ground cover for preventing erosion. • The root system helps anchor a plant. • In both taproot and fibrous root systems, absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs enormously increase the surface area. o Root hairs are short-lived, tubular extensions of individual root epidermal cells. • Some plants have modified roots. Some arise from roots, while others are adventitious, arising above-ground from stems or even from leaves. Some modified roots provide additional support and anchorage. Others store water and nutrients or absorb oxygen from the air. Stems consist of alternating nodes and internodes. • A stem is an organ consisting of alternating nodes, the points at which leaves are attached, and internodes, the stem segments between nodes. • At the angle formed by each leaf and the stem is an axillary bud with the potential to form a lateral shoot or branch. • The growth of a young shoot is usually concentrated at its apex, where there is an apical bud, or terminal bud, with developing leaves and a compact series of nodes and internodes. The presence of a terminal bud is partly responsible for inhibiting the growth of axillary buds, a phenomenon called apical dominance. o By concentrating resources on growing taller, apical dominance is an evolutionary adaptation that increases the plant’s exposure to light. • In the absence of a terminal bud, the axillary buds break dormancy and give rise to lateral shoots complete with their own apical buds, leaves, and axillary buds. o This is why pruning trees and shrubs makes them bushier. • Modified shoots with diverse functions have evolved in many plants. These shoots, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots. Leaves are the main photosynthetic organs of most plants. • The leaf is the primary site of photosynthetic organs of most plants, although green stems are also photosynthetic. • Although leaves vary extensively in form, they generally consist of a flattened blade and a stalk, the petiole, which joins the leaf to a stem node. o Grasses and other monocots lack petioles. In these plants, the base of the leaf forms a sheath that envelops the stem. Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves. • Most monocots have parallel major veins that run the length of the blade, while eudicot leaves have a branched network of major veins. • Plant taxonomists use floral morphology, leaf morphology, the branching pattern of veins, and the spatial arrangement of leaves to help identify and classify plants. o For example, simple leaves have a single, undivided blade, while compound leaves have several leaflets attached to the petiole. o Many large leaves are compound, which allows them to withstand strong winds without tearing. The structural adaptation of compound leaves also confines pathogens that invade the leaf to one leaflet. • Most leaves are specialized for photosynthesis. • Some plants have leaves that have become adapted for other functions, including tendrils that cling to supports, spines of cacti for defense, leaves modified for water storage, and brightly colored leaves that attract pollinators. Plant organs are composed of three tissue systems: dermal, vascular, and ground. • Each organ of a plant has three tissue systems: dermal, vascular, and ground tissues. Each system is continuous throughout the plant body. • The dermal tissue system is the plant’s outer protective covering. • In nonwoody plants, the dermal tissue system is a single layer of tightly packed cells, or epidermis. • The epidermis of leaves and most stems secretes a waxy coating, the cuticle, which helps the aerial parts of the plant retain water. • In woody plants, protective tissues called periderm replace the epidermis in older regions of stems and roots. • The epidermis has other specialized characteristics consistent with the function of the organ it covers. For example, the root hairs are extensions of epidermal cells near the tips of the roots. o Trichomes, outgrowths of shoot epidermis, reduce water loss and reflect light. They protect against insects with sticky secretions of insecticidal biochemicals. • The vascular tissue system is involved in the transport of materials between roots and shoots. o Xylem conveys water and dissolved minerals upward from roots into the shoots. o Phloem transports sugars, the products of photosynthesis, to the roots and sites of growth, such as developing leaves and fruits. • The vascular tissue of a root or stem is called the stele. In angiosperms, the root stele forms a solid central vascular cylinder, while the stele of stems and leaves consists of vascular bundles, separate strands of xylem and phloem. • Both xylem and phloem are complex tissues with a variety of cell types. • The ground tissue system is tissue that is neither dermal nor vascular. • Ground tissue is divided into pith, internal to vascular tissue, and cortex, external to the vascular tissue. • The functions of specialized cells within ground tissue include photosynthesis, storage, and support. Plant tissues are composed of three basic cell types: parenchyma, collenchyma, and sclerenchyma. Plant cells are differentiated, with each type of plant cell possessing structural adaptations that make specific functions possible. o Cell differentiation may be evident within the protoplast, the cell contents exclusive of the cell wall. o Modifications of cell walls also play a role in plant cell differentiation. • The major types of differentiated plant cells are parenchyma, collenchyma, sclerenchyma, water-conducting cells of the xylem, and sugar-conducting cells of the phloem. • Mature parenchyma cells have primary walls that are relatively thin and flexible; most lack secondary walls. The protoplast of a parenchyma cell usually has a large central vacuole. • Parenchyma cells are often depicted as “typical” plant cells because they generally are the least specialized, but there are exceptions. o For example, the highly specialized sieve-tube elements of the phloem are parenchyma cells. • Parenchyma cells perform most of the metabolic functions of the plant, synthesizing and storing various organic products. o For example, photosynthesis occurs within the chloroplasts of parenchyma cells in the leaf. o Some parenchyma cells in the stems and roots have colorless plastids that store starch. The fleshy tissue of most fruit is composed of parenchyma cells. • Most parenchyma cells retain the ability to divide and differentiate into other cell types under special conditions, such as the repair and replacement of organs after injury to the plant. • In the laboratory, it is possible to regenerate an entire plant from a single parenchyma cell. • Collenchyma cells have thicker primary walls than parenchyma cells, although the walls are unevenly thick. • Grouped into strands or cylinders, collenchyma cells help support young parts of the plant shoot. Young stems and petioles often have strands of collenchyma just below the epidermis, providing support without restraining growth. • Mature collenchyma cells are living and flexible and elongate with the stems and leaves they support. • Sclerenchyma cells have thick secondary walls usually strengthened by lignin; they function as supporting elements of the plant. • Sclerenchyma cells are much more rigid than collenchyma cells. • Unlike parenchyma cells, sclerenchyma cells cannot elongate. • Sclerenchyma cells occur in plant regions that have stopped lengthening. Many sclerenchyma cells are dead at functional maturity, but they produce rigid secondary cells walls before the protoplast dies. o In parts of the plant that are still elongating, secondary walls are deposited in a spiral or ring pattern, enabling the cell wall to stretch like a spring as the cell grows. • Two types of sclerenchyma cells, fibers and sclereids, are specialized entirely for support. o Fibers are long, slender, and tapered, and usually occur in groups. ? Fibers from hemp are used for making rope, and fibers from flax are woven into linen. o Sclereids are irregular in shape and shorter than fibers.

They have very thick, lignified secondary walls. ? Sclereids impart hardness to nutshells and seed coats and the gritty texture to pear fruits. • The water-conducting elements of xylem, the tracheids and vessel elements, are elongated cells that are dead at functional maturity. o The thickened cell walls remain as a nonliving conduit through which water can flow. • Both tracheids and vessels have secondary walls interrupted by pits, thinner regions where only primary walls are present. • Water moves from cell to cell mainly through pits. • Tracheids are long, thin cells with tapered ends. Because their secondary walls are hardened with lignin, tracheids function in support as well as transport. • Vessel elements are generally wider, shorter, thinner-walled, and less tapered than tracheids. • Vessel elements are aligned end to end, forming long micropipes or xylem vessels. • The ends are perforated, enabling water to flow freely. • In the phloem, sucrose, other organic compounds, and some mineral ions move through tubes formed by chains of cells called sieve-tube elements. • Sieve-tube elements are alive at functional maturity, although a sieve-tube element lacks a nucleus, ribosomes, and a distinct vacuole. The end walls, the sieve plates, have pores that facilitate the flow of fluid between cells. • Each sieve-tube element has a nonconducting nucleated companion cell, which is connected to the sieve-tube element by numerous plasmodesmata. • The nucleus and ribosomes of the companion cell serve both that cell and the adjacent sieve-tube element. • In some plants, companion cells in leaves help load sugar into the sieve-tube elements, which transport the sugars to other parts of the plant. Concept 35. 2 Meristems generate cells for new organs. A major difference between plants and most animals is that plant growth is not limited to an embryonic or juvenile period. • Most plants demonstrate indeterminate growth, growing as long as the plant lives. • In contrast, most animals and certain plant organs, such as flowers, leaves, and thorns, undergo determinate growth, ceasing to grow after they reach a certain size. • Indeterminate growth does not mean immortality. • Annuals complete their life cycle—from germination to flowering to seed production to death—in a single year or less. o Many wildflowers and important food crops, such as cereals and legumes, are annuals. The lives of biennials p two years, with flowering and fruiting in the second year. o Radishes and carrots are biennials that are harvested after the first year. • Plants such as trees, shrubs, and some grasses that live many years are perennials. o Some buffalo grass of the North American plains has been growing for 10,000 years from seeds that sprouted at the end of the last ice age. o Perennials do not usually die from old age but from an infection or some environmental trauma, such as fire or drought. • A plant is capable of indeterminate growth because it has perpetually embryonic tissues called meristems. Apical meristems, located at the tips of roots and in the buds of shoots, supply cells for the plant to grow in length. • This elongation, primary growth, enables roots to extend through the soil and shoots to increase their exposure to light and carbon dioxide. • In herbaceous plants, primary growth produces almost all of the plant body. • Woody plants also show secondary growth, progressive thickening of roots and shoots where primary growth has ceased. • Secondary growth is produced by lateral meristems, cylinders of dividing cells that extend along the lengths of roots and shoots. The vascular cambium adds layers of vascular tissue called secondary xylem and phloem. o The cork cambium replaces the epidermis with thicker, tougher periderm. • The cells within meristems divide to generate additional cells, some of which remain in the meristematic region, while others differentiate and are incorporated into the tissues and organs of the growing plant. o Cells that remain as sources of new cells are called initials. o Cells that are displaced from the meristem, called derivatives, continue to divide until the cells they produce become specialized within developing tissues. At the tip of a winter twig of a deciduous tree is the dormant apical bud, enclosed by scales that protect its apical meristem. • In the spring, the bud sheds its scales and begins a new spurt of primary growth. • Along each growth segment, nodes are marked by scars left when leaves fell in the autumn. • Above each leaf scar is either an axillary bud or a branch twig. • Farther down the twig are whorls of scars left by the scales that enclosed the apical bud during the preceding winter. • Each spring and summer, as primary growth extends the shoot, secondary growth thickens the parts of the shoot that formed in earlier years. Concept 35. Primary growth lengthens roots and shoots. • Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems. o Herbaceous plants and the youngest parts of woody plants represent the primary plant body. • Apical meristems lengthen both roots and shoots. However, there are important differences in the primary growth of these two systems. • The root tip is covered by a thimble-like root cap, which protects the meristem as the root pushes through the abrasive soil during primary growth. o The cap also secretes a polysaccharide slime that lubricates the soil around the growing root tip. Growth in length is concentrated just behind the root tip, where three zones of cells at successive stages of primary growth are located. • These zones—the zone of cell division, the zone of elongation, and the zone of differentiation—grade together. • The zone of cell division includes the root apical meristem and its derivatives. o New root cells are produced in this region, including the cells of the root cap. • The zone of cell division blends into the zone of elongation, where cells elongate, sometimes to more than ten times their original length. It is this elongation of cells that is mainly responsible for pushing the root tip, including the meristem, into the soil. o The meristem sustains growth by continuously adding cells to the youngest end of the zone of elongation. • In the zone of differentiation, cells complete differentiation and become distinct cell types. • The primary growth of roots produces the epidermis, ground tissue, and vascular tissue. • Water and minerals absorbed from the soil must enter the plant through the epidermis, a single layer of cells covering the root. Root hairs greatly increase the surface area of epidermal cells. • Most roots have a vascular cylinder of xylem and phloem. o In eudicot roots, xylem radiates from the center like a star, with phloem developing between the arms of the xylem “star. ” o In monocot roots, the vascular tissue consists of a central core of parenchyma surrounded by alternating xylem and phloem. The central region, called pit, is distinct from stem pith. • The ground tissue of roots consists of parenchyma cells that fill the cortex, the region between the vascular cylinder and the epidermis. Cells within the ground tissue store sugars and starches, and their plasma membranes absorb water and minerals from the soil. • The innermost layer of the cortex, the endodermis, is a cylinder one cell thick that forms a selective barrier between the cortex and the vascular cylinder. • Lateral roots may sprout from the outermost layer of the vascular cylinder, the pericycle. o A lateral root pushes through the cortex and epidermis to emerge from the established root. o The vascular tissue of the lateral root is continuous with the vascular cylinder of the primary root. The apical meristem of a shoot is a dome-shaped mass of dividing cells at the shoot tip. • Leaves arise as leaf primordia on the flanks of the apical meristem. • Axillary buds develop from islands of meristematic cells left by apical meristems at the bases of the leaf primordia. • Within a bud, leaf primordia are crowded close together because the internodes are very short. • Most of the elongation of the shoot occurs by growth in length of slightly older internodes below the shoot apex. • In some plants, including grasses, internodes continue to elongate all along the length of the shoot over a prolonged period. These plants have meristematic regions called intercalary meristems at the base of each leaf. o This explains why grass continues to grow after being mowed. • Unlike its central position in a root, vascular tissue runs the length of a stem in strands called vascular bundles. o Because the vascular system of the stem is near the surface, branches can develop with connections to the vascular tissue without having to originate from deep within the main shoot. • In most eudicots, the vascular bundles are arranged in a ring, with pith inside and cortex outside the ring. The vascular bundles have xylem facing the pith and phloem facing the cortex. • In the stems of most monocots, the vascular bundles are scattered throughout the ground tissue rather than arranged in a ring. • In both monocots and eudicots, the stem’s ground tissue is mostly parenchyma. • Many stems are strengthened by collenchyma cells just beneath the epidermis. • Sclerenchyma fiber cells also provide support. • The epidermal barrier of leaves is interrupted only by stomata, tiny pores that regulate gas exchange between the surrounding air and the photosynthetic cells inside a leaf. Each stomatal pore is flanked by two specialized epidermal cells called guard cells. o The term stoma can refer to either the stomatal pore or the entire stomatal complex, the pore and two guard cells. • The stomata are also the major avenues of evaporative water loss from the plant—a process called transpiration. • The ground tissue of the leaf, the mesophyll, is sandwiched between the upper and lower epidermis. • The mesophyll consists mainly of parenchyma cells specialized for photosynthesis. • In many eudicots, a layer or more of columnar palisade mesophyll lies above spongy mesophyll. CO2 and oxygen circulate through the labyrinth of air spaces around the irregularly spaced cells of the spongy mesophyll. • The air spaces are particularly large near stomata, where gas exchange with the outside air occurs. • The vascular tissue of a leaf is continuous with the vascular tissue of the stem. • Leaf traces, connections from vascular bundles in the stem, pass through petioles and into leaves. • Vascular bundles in the leaves are called veins. Each vein is enclosed in a protective bundle sheath consisting of one or more layers of parenchyma. o Bundle-sheath cells are prominent in leaves that undergo C4 photosynthesis. Within a leaf, veins subdivide repeatedly and branch throughout the mesophyll. • The xylem brings water and minerals to the photosynthetic tissues, and the phloem carries sugars and other organic products to other parts of the plant. • The vascular infrastructure also functions to support and reinforce the shape of the leaf. Concept 35. 4 Secondary growth adds girth to stems and roots in woody plants. • The stems and roots of most eudicots increase in girth by secondary growth. • The secondary plant body consists of the tissues produced by the vascular cambium and the cork cambium. The vascular cambium adds secondary xylem (wood) and secondary phloem, increasing vascular flow and support for the shoot system. o The cork cambium produces a tough, thick covering consisting of wax-impregnated cells that protect the stem from water loss and invasion by insects, bacteria, and fungal spores. • Primary and secondary growth occur simultaneously but in different regions. o Elongation of the stem (primary growth) occurs at the apical meristem, but increases in diameter (secondary growth) occur farther down the stem. • All gymnosperms and many eudicots have secondary growth, but it is rare in monocots. The vascular cambium is a cylinder of meristematic cells that may be one cell thick. • The vascular cambium forms successive layers of secondary xylem to its interior and secondary phloem to its exterior. • The accumulation of this tissue over the years accounts for most of the increase in diameter of a woody plant. • The vascular cambium develops from parenchyma cells that retain the capacity to divide. o In a typical woody stem, the vascular cambium forms as a continuous cylinder outside the cortex and primary xylem and inside the pith and primary phloem. In a typical woody root, the vascular cambium forms in segments between the primary phloem, the lobes of primary xylem, and the pericycle. • Viewed in cross section, the vascular cambium appears as a ring of initials. o As these cells divide, they increase the circumference of the vascular cambium, adding secondary xylem to the inside of the cambium and secondary phloem to the outside. • Some initials are elongated, with long axes parallel to the axis of the stem or root. o These initials produce cells such as tracheids, vessel elements, and fibers of the xylem. They also produce sieve-tube elements, companion cells, parenchyma, and fibers of the phloem. • Other initials are shorter, oriented perpendicular to the axis of the stem or root. o These initials produce vascular rays that transfer water and nutrients laterally within the woody stem, store sugars and starches, and aid in wound repair. • As secondary growth continues over the years, layer upon layer of secondary xylem accumulate, producing the tissue we call wood. • Wood consists mainly of tracheids, vessel elements (in angiosperms), and fibers. These cells, dead at functional maturity, have thick, lignified walls that give wood its hardness and strength. • The first tracheid and vessel cells formed in the spring (early wood) have larger diameters and thinner walls than the cells produced later in the summer (late wood). o The structure of the early wood maximizes delivery of water to new, expanding leaves. o The thick-walled cells of later wood provide more physical support. • In temperate regions, secondary growth in perennial plants ceases during the winter. • This pattern of growth—cambium dormancy, early wood production, and late wood production—produces annual growth rings. Dendrochronology is the science of analyzing tree ring growth patterns. o Scientists can use ring patterns to identify climate change. • As a tree or woody shrub ages, the older layers of secondary xylem, known as heartwood, no longer transport water and minerals. o Heartwood contains resins and other compounds that protect the core of the tree from fungi and insects. • The outer layers, known as sapwood, continue to transport xylem sap. • Because each new layer of secondary xylem has a larger circumference, secondary growth enables the xylem to transport more sap each year, supplying more leaves. Only the youngest secondary phloem, closest to the vascular cambium, functions in sugar transport. • The older secondary phloem dies and is sloughed off as part of the bark. • Early in secondary growth, the epidermis produced by primary growth splits, dries, and falls off the stem or root. • The epidermis is replaced by two tissues produced by the first cork cambium, which arises in the outer cortex of stems and in the outer layer of the pericycle of roots. • The first tissue, phelloderm, is a thin layer of parenchyma cells that forms to the interior of the cork cambium. The cork cambium also produces cork cells, which accumulate at the cambium’s exterior. • Waxy material called suberin deposited in the cell walls of cork cells before they die acts as a barrier against water loss, physical damage, and pathogens. • A cork cambium and the tissues it produces make up a layer of periderm, a protective layer that replaces the epidermis. • Because cork cells have suberin and are compacted together, the periderm is impermeable to water and gases. • In most plants, water and minerals are absorbed in the youngest parts of the roots. The older parts of the roots anchor the plant and transport water and solutes between roots and shoots. • In areas called lenticels, spaces develop between the cork cells of the periderm. o These areas within the trunk facilitate gas exchange with the outside air. • The thickening of a stem or root splits the first cork cambium, which loses its meristematic activity and differentiates into cork cells. • A new cork cambium forms to the inside, resulting in a new layer of periderm. • As this process continues, older layers of periderm are sloughed off. o This produces the cracked, peeling bark of many tree trunks. Bark is all tissues external to the vascular cambium, including secondary phloem (produced by the vascular cambium), the most recent periderm, and all the outer layers of periderm. Concept 35. 5 Growth, morphogenesis, and differentiation produce the plant body. • The development of body form and organization is called morphogenesis. • During plant development, a single cell, the zygote, gives rise to a multicellular plant of a particular form with functionally integrated cells, tissues, and organs. • Each cell in the plant body contains the same genomes, but different patterns of gene expression cause cells to differentiate. The three developmental processes of growth, morphogenesis, and cellular differentiation act to transform the fertilized egg into a plant. Molecular biology is revolutionizing the study of plants. • Modern molecular techniques enable plant biologists to investigate how growth, morphogenesis, and cellular differentiation give rise to a plant. • Much of this research has focused on Arabidopsis thaliana, a small weed in the mustard family. o Thousands of these small plants can be cultivated in a few square meters of lab space. o With a generation time of about six weeks, Arabidopsis is an excellent model for genetic studies. Arabidopsis also has one of the smallest genomes of all known plants. • Arabidopsis was the first plant to have its genome sequenced, in a six-year multinational project. o More recently, rice and poplar trees have had their entire genomes sequenced. • Arabidopsis has a total of about 26,000 genes, with fewer than 15,000 different types of genes. • Now that the DNA sequence of Arabidopsis is known, plant biologists are working to identify the functions of every one of the plant’s genes. • To aid in this effort, biologists are attempting to create mutants for every gene in the plant’s genome. Study of the functions of these genes has already expanded our understanding of plant development. • By identifying each gene’s function, researchers aim to establish a blueprint for how plants develop, a major goal of systems biology. • One day it may be possible to create a computer-generated “virtual plant” that will enable researchers to visualize which plant genes are activated in different parts of the plant during the entire course of development. Growth involves both cell division and cell expansion. • Cell division in meristems increases the cell number, thereby increasing the potential for growth. However, it is cell expansion, especially cell elongation, that accounts for the increase in plant mass. • The plane (direction) and symmetry of cell division are important determinants of plant form. o If the planes of division by a single cell and its descendents are parallel to the plane of the first cell division, a single file of cells will be produced. o If the planes of cell division of the descendent cells vary at random, an unorganized clump of cells will result. • Although mitosis results in the equal allocation of chromosomes to daughter cells, cytokinesis may be asymmetrical. Asymmetrical cell division, in which one cell receives more cytoplasm than the other, is common in plant cells and usually signals a key developmental event. o For example, guard cells arise from an unspecialized epidermal cell through an asymmetrical cell division to form a large unspecialized epidermal cell and a small guard cell “mother cell. ” o Guard cells form when the small mother cell divides in a plane perpendicular to the first cell division. • The plane in which a cell will divide is determined during late interphase. • Microtubules in the outer cytoplasm become concentrated into a ring, the preprophase band. Although this ring disappears before metaphase, its “imprint” consists of an ordered array of actin microfilaments that remains after the microtubules disperse and signals the future plane of cell division. • Cell expansion in animal cells is quite different from cell expansion in plant cells. o Animal cells grow by synthesizing a protein-rich cytoplasm, a metabolically expensive process. • Growing plant cells add some protein-rich material to their cytoplasm, but water uptake by the large central vacuole accounts for 90% of a plant cell’s expansion. o This enables plants to grow economically and rapidly. For example, bamboo shoots can elongate more than 2 m per week. • Rapid expansion of shoots and roots increases plants’ exposure to light and soil, an important evolutionary adaptation to the immobile lifestyle of plants. • In a growing plant cell, enzymes weaken cross-links in the cell wall, allowing it to expand as water diffuses into the vacuole by osmosis. • The wall loosens when hydrogen ions secreted by the cell activate cell wall enzymes that break the cross-links between polymers in the wall. • This reduces restraint on the turgid cell, which can take up more water and expand. Small vacuoles coalesce to form the cell’s central vacuole. • The greatest expansion of a plant cell is usually oriented along the plant’s main axis. o The orientation of cellulose microfibrils in the innermost layers of the cell wall cause this differential growth, as the cell expands mainly perpendicular to the “grain” of the microfibrils. o The orientation of microtubules in the cell’s outermost cytoplasm determines the orientation of cellulose microfibrils, the basic structural units of the cell wall. Arabidopsis mutants confirm the role of cytoplasmic microtubules in cell division and growth. Studies of Arabidopsis mutants have confirmed the importance of cytoplasmic microtubules in both cell division and expansion. • For example, fass mutants have unusually squat cells, which follow seemingly random planes of cell division. • The roots and stems of fass mutants lack the ordered cell files and layers. • Fass mutants develop into tiny adult plants with all their organs compressed longitudinally. • The organization of microtubules in fass mutants is abnormal. o In interphase cells, the microtubules are randomly positioned. Preprophase bands do not form prior to mitosis. o Therefore, the cellulose microfibrils deposited in the cell wall cannot be arranged to determine the direction of the cell’s elongation. • Cells with a fass mutation expand in all directions equally and divide in a haphazard arrangement, leading to stout stature and disorganized tissues. Morphogenesis depends on pattern formation. • Morphogenesis organizes dividing and expanding cells into multicellular tissues and organs. • The development of specific structures in specific locations is called pattern formation. Pattern formation depends to a large extent on positional information, signals that continuously indicate each cell’s location within an embryonic structure. • Within a developing organ, each cell responds to positional information by differentiating into a particular cell type. • Developmental biologists are accumulating evidence that gradients of specific molecules, generally proteins or mRNAs, provide positional information. o For example, a substance diffusing from a shoot’s apical meristem may “inform” the cells below of their distance from the shoot tip. A second chemical signal produced by the outermost cells may enable a cell to gauge its position relative to the radial axis of the developing organ. o Developmental biologists are testing the hypothesis that diffusible chemical signals provide plant cells with positional information. • One type of positional information is polarity, the identification of the root end and shoot end along a well-developed axis. • Axial polarity results in morphological and physiological differences. • The unidirectional movement of the hormone auxin causes the emergence of adventitious roots and shoots from the appropriate ends of plant cuttings. The establishment of axial polarity is a critical step in plant morphogenesis. • The first division of a plant zygote is normally asymmetrical and may initiate the polarization of the plant body into root and shoot ends. • Once this polarity has been induced, it is very difficult to reverse experimentally. o In the gnom mutant of Arabidopsis, the first division is symmetrical, and the resulting ball-shaped plant lacks roots and leaves. • Other genes that regulate pattern formation and morphogenesis include master regulatory genes called homeotic genes, which mediate many developmental events, such as organ initiation. For example, the protein product of the KNOTTED-1 homeotic gene is important for the development of leaf morphology, including the production of compound leaves. o Overexpression of this gene causes the compound leaves of a tomato plant to become “supercompound. ” Cellular differentiation depends on the control of gene expression. • The diverse cell types of a plant, including guard cells, sieve-tube elements, and xylem vessel elements, all descend from a common cell, the zygote, and share the same DNA. The cloning of whole plants from single somatic cells demonstrates that the genome of a differentiated cell remains intact and can dedifferentiate in tissue culture and give rise to the diverse cell types of a plant. • Cellular differentiation depends, to a large extent, on the control of gene expression. • Cells with the same genomes follow different developmental pathways because they selectively express certain genes at specific times during differentiation. • The activation or inactivation of specific genes involved in cellular differentiation depends on positional information—where a particular cell is located relative to other cells. For example, two distinct cell types in Arabidopsis, root hair cells and hairless epidermal cells, develop from immature epidermal cells. o Cells in contact with one underlying cortical cell differentiate into mature, hairless cells, while those in contact with two underlying cortical cells differentiate into root hair cells. o The homeotic gene GLABRA-2 is normally expressed only in hairless cells. If it is rendered dysfunctional, every root epidermal cell develops a root hair. Clonal analysis of the shoot apex emphasizes the importance of a cell’s location in its developmental fate. In the process of shaping an organ, patterns of cell division and cell expansion affect the differentiation of cells by placing them in specific locations relative to other cells. • Thus, positional information underlies all the processes of development: growth, morphogenesis, and differentiation. • One approach to studying the relationship among these processes is clonal analysis, mapping the cell lineages (clones) derived from each cell in an apical meristem as organs develop. • Researchers use mutations to distinguish a specific meristematic cell from the neighboring cells in the shoot tip. For example, a somatic mutation in an apical cell that prevents chlorophyll production produces an “albino” cell. o This cell and all its descendants appear as a linear file of colorless cells running down the long axis of the green shoot. • To some extent, the developmental fates of cells in the shoot apex are predictable. o For example, clonal mapping has shown that almost all the cells derived from division of the outermost meristematic cells become part of the dermal tissue of leaves and stems. It is not possible to pinpoint precisely which cells of the meristem will give rise to specific tissues and organs, however, because random changes in rates and planes of cell division can reorganize the meristem. o For example, the outermost cells usually divide in a plane parallel to the surface of the shoot tip. o Occasionally, however, an outer cell divides in a plane perpendicular to this layer, placing one daughter cell beneath the surface, among cells derived from different lineages. • In plants, a cell’s developmental fate is determined not by its membership in a particular lineage but by its final position in an emerging organ.

Phase changes mark major shifts in development. • In plants, developmental changes can occur within the shoot apical meristem, leading to a phase change in the organs produced. o One example of a phase change is the gradual transition from a juvenile phase to an adult phase. o In some plants, the result of the phase change is a change in the size and shape of leaves. o The leaves of juvenile and mature shoot regions differ in shape and other features. o Once the meristem has laid down the juvenile nodes and internodes, they retain that status even as the shoot continues to elongate and the meristem changes to the mature phase. If axillary buds give rise to branches, those shoots reflect the developmental phase of the main shoot region from which they arise. o Although the main shoot apex may have made the transition to the mature phase, the older region of the shoot continues to give rise to branches bearing juvenile leaves if that shoot region was laid down when the main apex was still in the juvenile phase. o A branch with juvenile leaves may actually be older than a branch with mature leaves. • The juvenile-to-mature phase transition points to another difference in the development of plants versus animals. o In an animal, this ransition occurs at the level of the entire organism, as a larva develops into an adult animal. o In plants, phase changes during the history of apical meristems can result in juvenile and mature regions coexisting along the axis of each shoot. Genes controlling transcription play key roles in a meristem’s change from a vegetative to a floral phase. • Another striking phase change in plant development is the transition from a vegetative shoot tip to a floral meristem. • This transition is triggered by a combination of environmental cues, such as day length, and internal signals, such as hormones. Unlike vegetative growth, which is indeterminate, the production of a flower by an apical meristem stops primary growth of that shoot. • This transition is associated with switching “on” floral meristem identity genes. • The protein products of these genes are transcription factors that help activate the genes required for the development of the floral meristem. • Once a shoot meristem is induced to flower, positional information commits each primordium arising from the flanks of the shoot tip to develop into a specific flower organ—a sepal, petal, stamen, or carpel. Viewed from above, the floral organs develop in four concentric circles, or whorls. o Sepals form the fourth (outermost) whorl, petals form the third, stamens form the second, and carpels form the first (innermost) whorl. • Organ identity genes, or plant homeotic genes, regulate positional information and function in the development of the floral pattern. o Mutations in these genes may lead to the substitution of one type of floral organ for the expected one. • Organ identity genes code for transcription factors. • Positional information determines which organ identity genes are expressed in which particular floral-organ primordium. In Arabidopsis, three classes of organ identity genes interact to produce the spatial pattern of floral organs. • The ABC model of flower formation identifies how these genes direct the formation of four types of floral organs. • The ABC model proposes that each class of organ identity genes is switched “on” in two specific whorls of the floral meristem. o A genes are switched on in the two outer whorls (sepals and petals), B genes are switched on in the two middle whorls (petals and stamens), and C genes are switched on in the two inner whorls (stamens and carpels). Sepals arise in those parts of the floral meristems in which only A genes are active. o Petals arise in those parts of the floral meristems in which A and B genes are active. o Stamens arise in those parts of the floral meristems in which B and C genes are active. o Carpels arise in those parts of the floral meristems in which only C genes are active. • The ABC model can account for the phenotypes of mutants lacking A, B, or C gene activity. o When gene A is missing, it inhibits C, and vice versa. o If either A or C is missing, the other takes its place.

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