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UNIT PLANT FORM AND FUNCTION Interview: JOANNE CHORY Chapter 35: PLANT STRUCTURE AND GROWTH Chapter 36: TRANSPORT IN PLANTS Chapter 37: PLANT NUTRITION Chapter 38: PLANT REPRODUCTION AND BIOTECHNOLOGY Chapter 39: PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS Chapter 35 PLANT STRUCTURE AND GROWTH THE PLANT BODY - Both genes and environment affect plant structure - Plants have three basic organs: roots, stems, and leaves - Plant organs are composed of three tissue systems: dermal, vascular, and ground - Plant tissues are composed of three basic cell types: parenchyma, collenchyma, and sclerenchyma THE PROCESS OF PLANT GROWTH AND DEVELOPMENT - Meristems generate cells for new organs throughout the lifetime of a plant: an overview of plant growth - Primary growth: Apical meristems extend roots and shoots by giving rise to the primary plant body - Secondary growth: Lateral meristems add girth by producing secondary vascular tissue and periderm MECHANISMS OF PLANT GROWTH AND DEVELOPMENT - Molecular biology is revolutionizing the study of plants - Growth, morphogenesis, and differentiation produce the plant body - Growth involves both cell division and cell expansion - Morphogenesis depends on pattern formation - Cellular differentiation depends on the control of gene expression - Clonal analysis of the shoot apex emphasizes the importance of a cell’s location in its developmental fate - Phase changes mark major shifts in development - Genes controlling transcription play key roles in a meristem’s change from a vegetative to a floral phase THE PLANT BODY * Both genes and environment affect plant structure * Plants have three basic organs: roots, stems, and leaves * Plant organs are composed of three tissue systems: dermal, vascular, and ground * Plant tissues are composed of three basic cell types: parenchyma, collenchyma, and sclerenchyma 1.1 Both genes and environment affect plant structure A plant’s structure reflects interactions with the environment on time scales: Over the long term, entire plant species have, by natural selection, accumulated morphological adaptations that enhance survival and reproductive success in the environments in which they grow Ex: cacti (desert plant) Over the short term, individual plants, far more than individual animals, exhibit structural responses to their specific environment Ex: camboba (aquatic plant) Even faster than a plant’s structural responses to environmental changes are its physiological (functional) adjustments Unlike cacti, most plants are rarely exposed to severe drought conditions and rely mainly on physiological adaptations to cope with drought stress In the most common response, the plant produces a hormone that causes the stomata (the pores in the leaves through which most water is lost, to close) 1.2. Plants have three basic organs: roots, stems, and leaves In particular, the two plant groups called the monocots and the dicots differ in many anatomical details (fig 35.1) The evolutionary solution to this separation of resources was differentiation of the plant body into two main systems: a subterranean root system and an aerial shoot system consisting of stems and leaves (fig 35.2) (Flowers are shoots consisting of leaves and stems highly modified for sexual reproduction) 1.2.1 The Root System Monocots, including grasses, generally have fibrous root systems consisting of a mat of thin roots that spread out below the soil surface Many dicots have a taproot system, consisting of one large, vertical root (the taproot) that produces many smaller lateral, or branch, roots (figs 35.1 and 35.2) Most absorption of water and minerals in both monocots and dicots occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root enormously (fig 35.3) Some plants have roots arising aboveground from stems or even from leaves, called the adventitious roots (from the Latin adventicius , extraneous) of some plants, including corn, function as props that help support tall stems Fig 35-1. A comparison of monocots and dicots These two groups of angiosperms are named for the number of cotyledons, or seed leaves, present on the embryo of the plant Monocots include orchids, bamboos, palms, lilies, and yuccas, as well as the grasses, such as wheat, corn, and rice A few examples of dicots are roses, beans, sunflowers, and oaks (which are all eudicots, the largest class of angiosperms with the dicot-type anatomy) Fig 35-2. Morphology of a flowering plant: an overview The plant body is divided nto a root system and a shoot system, connected by vascular tissue (purple strands in this diagram) that is continuous throughout the plant The plant shown in this diagram is an idealized dicot Fig 35-3. Radish root hairs Growing by the thousands just behind the tip of each root, the hairs increase the surface area for the absorption of water and minerals by the roots (colorized SEM) 1.2.2 The Shoot System: Stems and Leaves Shoots - consist of stems and leaves 1.2.2.a Stems A stem is an alternating system of nodes, the points at which leaves are attached, and internodes, the stem segments between nodes (fig 35.2) In the angle (axil) formed by each leaf and the stem is an axillary bud, a structure that has the potential to form a vegetative branch Most axillary buds of a young shoot are dormant (not growing) Thus, growth of a young shoot is usually concentrated at its apex (tip), where there is a terminal bud with developing leaves and a compact series of nodes and internodes The presence of the terminal bud is partly responsible for inhibiting the growth of axillary buds, a phenomenon called apical dominance Modified shoots with diverse functions have evolved in many plants These modified shoots, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots (fig 35.4) Fig 35-2. Morphology of a flowering plant: an overview The plant body is divided nto a root system and a shoot system, connected by vascular tissue (purple strands in this diagram) that is continuous throughout the plant The plant shown in this diagram is an idealized dicot Fig 35-27. The preprophase band and the plane of cell division The location of the preprophase band predicts the plane of cell division Although the cells dhown on the left and right are similar in shpae They will divide in different planes Eachg cell is represented by two light micrographs, one (top) unstained and the other (bottom) stained with a fluorescent dye that binds specifically to microtubules The stained microtubules form a "halo" (preprophase band) around the nucleus in the outer cytoplasm Fig 35-28. The orientation of plant cell expansion Growing plant cells expand mainly through water uptake In a growing cell, enzymes weaken cross-links in the cell wall, allowing it to expand as water flows in by osmosis The orientation of cell growth is mainly in the plane perpendicular to the orientation of cellulose microfibrils in the wall The microfibrils are embedded in a matrix of other (noncellulose) polysaccharides, some of which form the cross-links visible in the micrograph (TEM) Loosening of the wall occurs 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, which accumulate most of this water, coalesce and form the cell’s central vacuole Fig 35-29. A hypothetical mechanism for how microtubules orient cellulose microfibrils Cellolose microfibrils are synthesized at the cell surface by complexes of enzymes that can move in the plane of the plasma membrane According to one hypothesis, microtubules form "banks" that confine the movement of the enzymes to channels of specified direction Each enzyme complex advances along one of these channels as the microfibril it extends becomes locked in place by cross-linking to other microfibrils 3.3.3 The Importance of Cortical Microtubules in Plant Growth Studies of Arabidopsis mutants have confirmed the importance of cortical microtubules in both cell division and expansion As an example, let’s consider what are called fass mutants of Arabidopsis Despite these abnormalities, fass mutants develop into tiny adult plants with all their parts, including flowers, but these organs are compressed longitudinally (Fig 35.30) Fig 35-30. The fass mutant of Arabidopsis confirms the importance of cortical microtubules to plant growth The squat body of the fass mutant results from cell division and cell elongation being randomly oriented instead of orienting in the direction of the normal plant axis 3.4. Morphogenesis depends on pattern formation Pattern formation depends to a large extent on positional information, signals of some kind that indicate each cell’s location within an embryonic structure, such as a shoot tip One type of positional information is associated with polarity The first division of a plant zygote is normally asymmetrical, initiating polarization of the plant body into shoot and root (fig 35.31) Let’s examine another example of how genes regulate pattern formation and morphogenesis in plants For example, the protein product of the KNOTTED-1 homeotic gene, found in many plant species, is important in the development of leaf morphology, including the production of compound leaves If the KNOTTED-1 gene is overexpressed in tomato plants, the normally compound leaves become "supercompound" (fig 35.32) Fig 35-31. Establishment of axial polarity The normal Arabidopsis seedling (left) has a shoot end and a root end In the genomic mutant (right), the first division of the zygote was not asymmetrical; as a result, the plant is ballshaped and lacks cotyledons (seed leaves) and roots Fig 35-32. Too much "volume" from a homeotic gene KNOTTED-1 is a homeotic gene involved in leaf and leaflet formation Its overexpression in tomato plants results in leaves that are "supercompound" (right) compared to normal leaves (left) 3.5. Cellular differentiation depends on the control of gene expression It is remarkable that cells as diverse as guard cells, sieve-tube members (phloem), and xylem vessel elements all share the same DNA and all descend from a common cell, the zygote This cellular differentiation occurs continuously throughout a plant’s life, as meristems sustain indeterminate growth Differentiation reflects the synthesis of different proteins in different types of cells (see Chapter 19) For example, A homeotic gene called GLABRA-2 is required for appropriate root hair distribution GLABRA-2 is normally expressed only in epidermal cells that will not develop root hairs If GLABRA-2 is rendered dysfunctional by mutation, every root epidermal cell develops a root hair Researchers demonstrated this by coupling the GLABRA-2 gene to a "reporter gene" that causes every cell expressing GLABRA- in the root to turn blue following a certain treatment (fig 35.33) Fig 35-33. Example of cellular differentiation Two distinct cell types are formed in the root epidermis of Arabidopsis : root hair cells and hairless epidermal cells 3.6. Clonal analysis of the shoot apex emphasizes the importance of a cell’s location in its developmental fate 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 relationships among these processes is clonal analysis, in which the cell lineages (clones) derived from each cell in an apical meristem are mapped as organs develop Researchers can this by using radiation or chemicals to induce somatic mutations that alter chromosome number or otherwise tag a cell in some way that distinguishes it from its neighbors in the shoot tip The lineage of cells derived by mitosis from the mutant meristematic cell will also be "marked." For example, a single cell in the shoot apical meristem may undergo a somatic mutation that prevents chlorophyll from being produced This cell and all of its descendants will be "albino," and they will appear as a linear file of colorless cells running down the long axis of the otherwise green shoot One of the important questions that clonal analysis can address is: How early is the developmental fate of a cell determined by its position in an embryonic structure? 3.7. Phase changes mark major shifts in development The apical meristem can change from one developmental phase to another during its history a process called a phase change One of these phase changes is a gradual transition in vegetative (leaf-producing) growth from a juvenile state to a mature state in some species Usually, the most obvious sign of this phase change is a change in the morphology of the leaves produced The leaves of juvenile versus mature shoot regions differ in shape and other features (fig 35.34) Fig 35-34. Phase change in the shoot system of Eucalyptus A silver-dollar eucalyptus (Eucalyptus polyanthemos ) has both (a) juvenile leaves (the round "silver dollars") and (b) mature leaves (the lance-shaped leaves) This dual foliage reflects a phase change in the development of the apical meristem of each shoot In its juvenile vegetative phase, a meristem lays down modules on which round leaves develop As the meristem changes gradually to the mature vegetative phase, the leaves become more and more lance-shaped Once a module forms, its developmental phase juvenile or mature is fixed; that is, round leaves not mature into lanceolate leaves 3.8. Genes controlling transcription play key roles in a meristem’s change from a vegetative to a floral phase Another particularly striking passage in plant development is the transition of a vegetative shoot tip into a floral meristem A combination of environmental cues, such as day length, and internal signals, such as hormones, trigger this transition The transition from vegetative growth to flowering is associated with the switching on of floral meristem identity genes The protein products of these genes are transcription factors that in turn help activate the genes required for development of the floral meristem Once a shoot meristem is induced to flower, positional information commits each primordium that arises on the flanks of the shoot tip to develop into an organ of specific structure and function Mutations in these organ identity genes substitute one type of floral organ where another would normally form (fig 35.35) Positional information determines which organ identity genes are expressed in a particular floral-organ primordium fig 35.36 illustrates a hypothesis for how regulatory products of organ identity genes are responsible for normal flower development (capel) (stamen) (petal) (sepal) Fig 35-35. Organ identity genes and pattern formation in flower development (a) Arabidopsis normally has four whorls of flower parts: sepals (Se), petals (Pe), stamens (St), and carpels (Ca) (b) Researchers have identified several mutations of organ identity genes that cause abnormal flowers to develop This flower has an extra set of petals in place of stamens and an internal flower where normal plants have carpels Fig 35-36. The ABC hypothesis for the functioning of organ identity genes in flower development Based on their studies of mutations that affect floral morphology in Arabidopsis and other plants (a generalized flower is illustrated here), researchers have determined that three classes of organ identity genes are responsible for the spatial pattern of floral organs (Chapter 21 presents the experimental support for this hypothesis.) The three classes of organ identity genes are designated A, B, and C in the upper drawing, a schematic diagram of a floral meristem in transverse view The products of these genes are transcription factors that regulate expression of other genes responsible for the development of the specific floral organs: sepals, petals, stamens, and carpels Sepals develop from the region of the meristem where only the A genes are active Petals develop from the region where both A and B genes are expressed Stamens arise from the meristem where both the B and C genes are active And carpels are derived from the region where only the C genes are expressed .. .Chapter 35 PLANT STRUCTURE AND GROWTH THE PLANT BODY - Both genes and environment affect plant structure - Plants have three basic organs: roots, stems, and leaves - Plant organs... many plants These modified shoots, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots (fig 35. 4) Fig 35- 2. Morphology of a flowering plant: an overview The plant. .. photosynthesis, some plants have leaves that have become adapted by evolution for other functions (fig 35. 6) Fig 35- 2. Morphology of a flowering plant: an overview The plant body is divided