Chapter 35 - Plant Structure

Chapter 35 Plant Structure, Growth, and Development
Lecture Outline

Overview: No Two Plants Are Alike

• The fanwort, an aquatic weed, demonstrates the great developmental plasticity that is characteristic of plants. The fanwort has feathery underwater leaves and large, flat, floating surface leaves. 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.
• The form of any plant is controlled by environmental and genetic factors. As a result, no two plants are identical.
• In addition to plastic structural responses of individual plants to specific environments, plant species have adaptive features that benefit them in their specific environments.
• For example, cacti have leaves that are reduced as spines and a stem that serves as the primary site of photosynthesis. These adaptations reduce water loss in desert environments.
• Angiosperms comprise 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 are composed of different cell types.
  • A tissue is a group of cells with a common structure and function.
  • 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.
  • Plants obtain water and minerals from the soil.
  • They 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.
  • Lacking chloroplasts and living in the dark, roots would starve without the sugar and other organic nutrients imported from the photosynthetic tissues of the shoot system.
  • Conversely, the shoot system (and its reproductive tissues, flowers) depends on water and minerals absorbed from the soil by the roots.
• A root is an organ that anchors a vascular plant in the soil, absorbs minerals and water, and stores food.
  • Most eudicots and gymnosperms have a taproot system, consisting of one large vertical root (the taproot) that produces many small lateral, or branch, roots.
    • In angiosperms, taproots often store food that supports flowering and fruit production later.
  • Seedless vascular plants and most monocots, including grasses, have fibrous root systems consisting of a mat of thin roots that spread out below the soil surface.
    • A fibrous root system is usually shallower than a taproot system.
    • Grass roots are concentrated in the upper few centimeters of soil. As a result, grasses make excellent ground cover for preventing erosion.
    • Sturdy, horizontal, underground stems called rhizomes anchor large monocots such as palms and bamboo.
  • 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.
  • Root hairs are extensions of individual epidermal cells on the root surface.
    • Absorption of water and minerals is also increased by mutualistic relationships between plant roots and bacteria and fungi.
  • Some plants have modified roots. Some arise from roots while adventitious roots arise aboveground from stems or even from leaves.
    • Some modified roots provide additional support and anchorage. Others store water and nutrients or absorb oxygen or water from the air.
• 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.
• Growth of a young shoot is usually concentrated at its apex, where there is a 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.
  • 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 dominance and give rise to a vegetative branch complete with its own terminal bud, leaves, and axillary buds.
• Modified shoots with diverse functions have evolved in many plants.
  • These shoots, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots.
    • Stolons, such as the “runners” of strawberry plants, are horizontal stems that grow on the surface and enable a plant to colonize large areas asexually as plantlets form at nodes along each runner.
    • Rhizomes, like those of ginger, are horizontal stems that grow underground.
    • Tubers, including potatoes, are the swollen ends of rhizomes specialized for food storage.
    • Bulbs, such as onions, are vertical, underground shoots consisting mostly of the swollen bases of leaves that store food.
• Leaves are the main photosynthetic organs of most plants, although green stems are also photosynthetic.
  • While 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.
  • Grasses and other monocots lack petioles. In these plants, the base of the leaf forms a sheath that envelops the stem.
• Most monocots have parallel major veins that run the length of the blade, while eudicot leaves have a multibranched network of major veins.
• Plant taxonomists use floral morphology, leaf shape, spatial arrangement of leaves, and the pattern of veins to help identify and classify plants.
  • For example, simple leaves have a single, undivided blade, while compound leaves have several leaflets attached to the petiole.
    • The leaflet of a compound leaf has no axillary bud at its base.
  • In a doubly compound leaf, each leaflet is divided into smaller leaflets.
• Most leaves are specialized for photosynthesis.
  • Some plants have leaves that have become adapted for other functions.
  • These include 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.
  • Each system is continuous throughout the plant body.
• The dermal tissue is the outer covering.
• In nonwoody plants, it is a single layer of tightly packed cells, or epidermis, that covers and protects all young parts of the plant.
• 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.
  • 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.
• Vascular tissue, continuous throughout the plant, is involved in the transport of materials between roots and shoots.
  • Xylem conveys water and dissolved minerals upward from roots into the shoots.
  • Phloem transports food made in mature leaves to the roots; to nonphotosynthetic parts of the shoot system; and to sites of growth, such as developing leaves and fruits.
  • The vascular tissue of a root or stem is called the stele.
    • In angiosperms, the vascular tissue of the root forms a solid central vascular cylinder, while stems and leaves have vascular bundles, strands consisting of xylem and phloem.
• Ground tissue is tissue that is neither dermal tissue nor vascular tissue.
  • In eudicot stems, ground tissue is divided into pith, internal to vascular tissue, and cortex, external to the vascular tissue.
  • The functions of ground tissue include photosynthesis, storage, and support.
  • For example, the cortex of a eudicot stem typically consists of both fleshy storage cells and thick-walled support cells.

  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.
  • Cell differentiation may be evident within the protoplast, the cell contents exclusive of the cell wall.
  • Modifications of cell walls also play a role in plant cell differentiation.
• We will consider the major types of differentiated plant cells: 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, and 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.
  • For example, the highly specialized sieve-tube members of the phloem are parenchyma cells.
• Parenchyma cells perform most of the metabolic functions of the plant, synthesizing and storing various organic products.
  • For example, photosynthesis occurs within the chloroplasts of parenchyma cells in the leaf.
  • 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, though the walls are unevenly thickened.
  • 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 and function as supporting elements of the plant.
  • They are much more rigid than collenchyma cells.
  • Unlike parenchyma cells, they 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.
  • 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.
  • Fibers are long, slender, and tapered, and usually occur in groups.
    • Those from hemp fibers are used for making rope, and those from flax are woven into linen.
  • Sclereids are irregular in shape and are 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.
  • 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.
• Tracheids are long, thin cells with tapered ends.
  • Water moves from cell to cell mainly through pits.
  • 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 members.
  • These are alive at functional maturity, although a sieve-tube member 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 member has a nonconducting nucleated companion cell, which is connected to the sieve-tube member by numerous plasmodesmata.
  • The nucleus and ribosomes of the companion cell serve both that cell and the adjacent sieve-tube member.
  • In some plants, companion cells in leaves help load sugar into the sieve-tube members, 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 period.
• Most plants demonstrate indeterminate growth, growing as long as the plant lives.
• In contrast, most animals and certain plant organs, such as flowers and leaves, undergo determinate growth, ceasing to grow after they reach a certain size.
• Indeterminate growth does not mean immortality.
• Annual plants complete their life cycle—from germination through flowering and seed production to death—in a single year or less.
  • Many wildflowers and important food crops, such as cereals and legumes, are annuals.
• The life of a biennial plant spans two years.
  • Often, there is an intervening cold period between the vegetative growth season and the flowering season.
• Plants such as trees, shrubs, and some grasses that live many years are perennials.
  • Perennials do not usually die from old age, but from an infection or some environmental trauma.
• A plant is capable of indeterminate growth because it has perpetually embryonic tissues called meristems in its regions of growth.
  • These cells divide to generate additional cells, some of which remain in the meristematic region, while others become specialized and are incorporated into the tissues and organs of the growing plant.
  • Cells that remain as wellsprings of new cells in the meristem are called initials.
  • Those that are displaced from the meristem, derivatives, continue to divide for some time until the cells they produce differentiate within developing tissues.
• The pattern of plant growth depends on the location of 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 length of roots and shoots.
    • The vascular cambium adds layers of vascular tissue called secondary xylem and phloem.
    • The cork cambium replaces the epidermis with thicker, tougher periderm.
• In woody plants, primary growth produces young extensions of roots and shoots each growing season, while secondary growth thickens and strengthens the older parts of the plant.
• At the tip of a winter twig of a deciduous tree is the dormant terminal bud, enclosed by bud scales that protect its apical meristem.
  • In the spring, the bud will shed its scales and begin a new spurt of primary growth.
  • Along each growth segment, nodes are marked by scars left when leaves fell in 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 terminal bud during the previous winter.
• Each spring and summer, as the primary growth extends the shoot, secondary growth thickens the parts of the shoot that formed in previous years.

Concept 35.3 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.
• An herbaceous plant and the youngest parts of a woody plant 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 thimblelike root cap, which protects the meristem as the root pushes through the abrasive soil during primary growth.
  • 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 maturation—grade together.
• The zone of cell division includes the root apical meristem and its derivatives.
  • 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, ahead.
  • The meristem sustains growth by continuously adding cells to the youngest end of the zone of elongation.
  • In the zone of maturation, cells become differentiated and become functionally mature.
• The primary growth of roots consists of the epidermis, ground tissue, and vascular tissue.
• Water and minerals absorbed from the soil must enter 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 solid core of xylem and phloem. The xylem radiates from the center in two or more spokes, with phloem developing in the wedges between the spokes.
  • In monocot roots, the vascular tissue consists of a central core of parenchyma surrounded by alternating patterns of xylem and phloem.
• 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 food and are active in the uptake of minerals that enter the root with the soil solution.
• 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.
• An established root may sprout lateral roots from the outermost layer of the vascular cylinder, the pericycle.
  • The vascular tissue of the lateral root maintains its connection to the vascular tissue of the primary root.
• The apical meristem of a shoot is a dome-shaped mass of dividing cells at the terminal bud.
  • 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 internodes are very short.
  • Most of the elongation of the shoot occurs by growth in length of slightly older internodes below the shoot apex.
  • This growth is due to cell division and cell elongation within the internode.
  • 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.
    • This explains why grass continues to grow after being mowed.
• Unlike their central position in a root, vascular tissue runs the length of a stem in strands called vascular bundles.
  • 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 gymnosperms and 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 just beneath the epidermis.
  • Sclerenchyma fiber cells within vascular bundles also help support stems.
• The leaf epidermis is composed of cells tightly locked together like pieces of a puzzle.
  • The leaf epidermis is the first line of defense against physical damage and pathogenic organisms, and its waxy cuticle is a barrier to water loss from the plant.
• The epidermal barrier is interrupted only by the stomata, tiny pores flanked by specialized epidermal cells called guard cells.
  • Each stoma is an opening between a pair of guard cells that regulate the opening and closing of the pore.
  • The stomata regulate CO2 exchange between the surrounding air and the photosynthetic cells inside the leaf.
  • They 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.
  • It consists mainly of parenchyma cells with many chloroplasts and specialized for photosynthesis.
  • In many eudicots, a layer or more of columnar palisade mesophyll lies over spongy mesophyll.
    • Carbon dioxide 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 xylem and phloem of the stem.
  • Leaf traces, branches of 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.
  • 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 during this secondary growth in diameter.
  • Primary and secondary growth occur simultaneously but in different regions.
  • While elongation of the stem (primary growth) occurs at the apical meristem, increases in diameter (secondary growth) occur farther down the stem.
• The vascular cambium is a cylinder of meristematic cells that forms secondary vascular tissue.
  • It 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.
  • This meristem forms in a layer between the primary xylem and primary phloem of each vascular bundle and in the ground tissue between the bundles.
• The meristematic bands unite to form a continuous cylinder of dividing cells.
• This ring of vascular cambium consists of regions of ray initials and fusiform initials.
  • The tapered, elongated cells of the fusiform initials form secondary xylem to the inside of the vascular cambium and secondary phloem to the outside.
  • Ray initials produce vascular rays that transfer water and nutrients laterally within the woody stem and also store starch and other reserves.
• As secondary growth continues over the years, layer upon layer of secondary xylem accumulates, 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.
• In temperate regions, secondary growth in perennial plants ceases during the winter.
  • The first tracheid and vessel cells formed in the spring (early wood) have larger diameters and thinner walls than cells produced later in the summer (late wood).
  • The structure of the early wood maximizes delivery of water to new, expanding leaves.
  • The thick-walled cells of later wood provide more physical support.
• This pattern of growth—cambium dormancy, early wood production, and late wood production—produces annual growth rings.
• As a tree or woody shrub ages, the older layers of secondary xylem, known as heartwood, no longer transport water and minerals.
• The outer layers, known as sapwood, continue to transport xylem sap.
• 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.
• The cork cambium acts as a meristem for a tough, thick covering for stems and roots that replaces the epidermis.
• Early in secondary growth, the epidermis produced by primary growth splits, dries, and falls off the stem or root.
  • It 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.
    • 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.
• The cork plus the cork cambium form the periderm, a protective layer that replaces the epidermis.
• In areas called lenticels, spaces develop between the cork cells of the periderm.
  • These areas within the trunk facilitate gas exchange with the outside air.
• Unlike the vascular cambium, cells of the cork cambium do not divide.
• 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.
  • This produces the cracked, peeling bark of many tree trunks.
• Bark refers to all tissues external to the vascular cambium, including secondary phloem, cork cambium, and cork.

Concept 35.5 Growth, morphogenesis, and differentiation produce the plant body

• During plant development, a single cell, the zygote, gives rise to a multicellular plant of particular form with functionally integrated cells, tissues, and organs.
  • An increase in mass, or growth, results from cell division and cell expansion.
  • The development of body form and organization is called morphogenesis.
  • The specialization of cells with the same set of genetic instructions to produce a diversity of cell types is called differentiation.
• Plants have tremendous developmental plasticity.
  • Plant form, including height, branching patterns, and reproductive output, is greatly influenced by environmental factors.
  • A broad range of morphologies can result from the same genotype as three developmental processes—growth, morphogenesis, and differentiation—transform a zygote into an adult plant.

  Molecular biology is revolutionizing the study of plants.

• Modern molecular techniques allow 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.
  • Thousands of these small plants can be cultivated in a few square meters of lab space.
  • With a generation time of about six weeks, it is an excellent model for genetic studies.
• The genome of Arabidopsis is among the tiniest of all known plants.
• Arabidopsis was the first plant to have its genome sequenced, in a six-year multinational project.
• 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 by the year 2010.
  • To aid in this effort, biologists are attempting to create mutants for every gene in the plant’s genome.
  • Study of the function 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 are built.
  • 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 cell number, increasing the potential for growth.
• However, it is cell expansion that accounts for the actual increase in plant mass.
• The plane (direction) and symmetry of cell division are important determinants of plant form.
  • 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.
  • If the planes of cell division of the descendent cells vary at random, an unorganized clump of cells will result.
• While mitosis results in symmetrical redistribution of chromosomes between 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.
  • For example, guard cells form from an unspecialized epidermal cell through an asymmetrical cell division and a change in the plane of 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.
  • While this 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.
  • Animal cells grow by synthesizing a protein-rich cytoplasm, a metabolically expensive process.
  • While growing plant cells add some organic material to their cytoplasm, water uptake by the large central vacuole accounts for 90% of a plant cell’s expansion.
    • This enables plants to grow economically and rapidly.
    • Bamboo shoots can elongate more than 2 m per week.
  • Rapid expansion of shoots and roots increases their exposure to light and soil, an important evolutionary adaptation to the immobile lifestyle of plants.
• The greatest expansion of a plant cell is usually oriented along the plant’s main axis.
  • The orientations 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.
• Studies of Arabidopsis mutants have confirmed the importance of cortical microtubules in both cell division and expansion.
• For example, fass mutants have unusually squat cells, which follow seemingly random planes of cell division.
  • Their roots and stems lack the ordered cell files and layers.
• Fass mutants develop into tiny adult plants with all their organs compressed longitudinally.
• The cortical microtubular organization of fass mutants is abnormal.
  • Although the microtubules involved in chromosome movement and in cell plate deposition are normal, preprophase bands do not form prior to mitosis.
  • In interphase cells, the cortical microtubules are randomly positioned.
    • 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.
  • 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 their position relative to the radial axis of the developing organ.
  • 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.
  • This polarity results in morphological and physiological differences, and it impacts the emergence of adventitious roots and shoots from the appropriate ends of plant cuttings.
  • The first division of the zygote is asymmetrical and may initiate the polarization of the plant body into root and shoot ends.
    • Once the polarity has been induced, it is very difficult to reverse experimentally.
    • The establishment of axial polarity is a critical step in plant morphogenesis.
  • 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 the 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 production of compound leaves.
  • 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 members, 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” to give rise to the diverse cell types of a plant.
  • Cellular differentiation depends, to a large extent, on control of gene expression.
  • Cells with the same genomes follow different developmental pathways because they selectively express certain genes at specific times during differentiation.
• For example, two distinct cell types in Arabidopsis, root hair cells and hairless epidermal cells, develop from immature epidermal cells.
  • 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.
  • 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 a rudimentary 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 induce some change in a cell that tags it in some way such that it (and its descendents) can be distinguished from its neighbors.
  • For example, a somatic mutation in an apical cell that prevents chlorophyll production will produce an “albino” cell.
    • This cell and all its descendants will 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.
  • 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.
• However, it is not possible to pinpoint precisely which cells of the meristem will give rise to specific tissues and organs because random changes in rates and planes of cell division can reorganize the meristem.
  • For example, the outermost cells usually divide in a plane parallel to the surface of the shoot apex.
  • Occasionally, 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.
  • One example of a phase change is the gradual transition from a juvenile phase to an adult phase.
  • In some plants, the result of the phase change is a change in the morphology of the leaves.
  • The leaves of juvenile versus mature shoot regions differ in shape and other features.
  • 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.
  • Though 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.
  • 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.
  • In an animal, this transition occurs at the level of the entire organism, as a larva develops into an adult animal.
  • 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 terminates primary growth of that shoot tip as the apical meristem develops into the flower’s organs.
  • This transition is associated with the switching on of 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.
  • Organ identity genes regulate positional information and function in the development of the floral pattern.
    • 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 model proposes that each class of organ identity genes is switched on in two specific whorls of the floral meristem.
    • 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.
      • Petals arise in those parts of the floral meristems in which A and B genes are active.
      • Stamens arise in those parts of the floral meristems in which B and C genes are active.
      • 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.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 35-1