and Growth Development of Tissues (parts of Chapters 1, 15 and 16) Growth change in mass (size) Development change in state (juvenile, vegetative, reproductive, etc) G and D is due to: 1) division followed by 2) expansion and then 3) differentiation. Growth is ordered resulted in specific morphology (shape), not random. Thus morphogenesis is the sum of all these ordered divisions and differential expansions. Any environmental factor that changes cell division, expansion or differentiation can alter the way a plant looks e.g. photomorphogenesis. (note vegetative growth can culminate in production of reproductive structures determinate growth or it may continue afterward this - indeterminate growth patterns. 1. DIVISION: Division: occurs in specific regions called meristems primary (growth in the long axis): apical shoot or root tips, secondary growth in
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circumference): vascular cambium, cork cambium, pericycle, axillary buds and a few other places (grasses?) (will come back to these) Note that you can follow cell lineage in micrographs, See diagram in 16.28 and elsewhere. Anticlinal (against the axis of growth - length) vs periclinal (with the axis of growth radial growth) division. See stem and root l.s. and x.s. e.g. Figs 1.2 and 16.28. Note small areas for cell division and larger areas or regions of elongation
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Plants generally 1) have axial development- have a basal and an apical end, thus these ends move away from each other, 2) have a radial pattern in c.s. distinct characteristics on outside and inside of the circle and 3) generate most of their body from primary meristems Axial growth determined at first cell division in the zygote which is not equal this sets up embryo formation in the axial cell or terminal cell and the suspensor (root) in the basal cell. Fig. 16.3 and 16.4
(note - double fertilization where one sperm (pollen) unites to form the zygote and another unites with two polar nuclei to form the triploid endosperm food for the developing embryo)
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Green shoot apical meristem and most of the cotyledon Yellow- lower tier of the apical cell hypocotyl and part of the cotyledon Beige lower part of basal cell, makes the suspensor or nonembryonic chain of cells that attach the embryo to the embryo sac (nutrients) Blue- first division of the basal cell, roots
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Radial growth: also determined early on at the globular stage (ball or glob of cells) Fig. from old text - combination of anticlinal and periclinal divisions allows radial growth
Studies with mutants have shown that this is under genetic control Chapter 16 work on Arabidopsis. Most rapid growth occurs by Cell Expansion in regions behind the apical meristems.
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2. EXPANSION: Expansion is quite complex and related to orientation of microfibrils rings on a barrel. These help give polarity to cell division (e.g. phragmoplast formation) and expansion by orientation and crosslinks. Fig. 15.20 p. 365.
Water drives the expansion by exerting pressure on the walls but cells dont continue to expand, so
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they stiffen and can continue to maintain Turgor (living cells anyway). The walls must loosen and then expand by continued water uptake in order to relieve the stress on them. This is called stress relaxation (loosening) followed by Yielding (long-term irreversible stretching) and this results in permanent cell expansion. Once this stops (secondary well wall formation) the expansion ceases as in older cells that dont Grow anymore. Loosening and expansion is affected by the action of acidity, auxins, specific proteins called expansins , and probably glucanases as well. Auxins (or H+ release by them) may help loosen the cellulose-hemicellulose crosslinks and changes in other wall chemical elements (e.g. ferulic acid) may alter cell expansion. Glucanases have been shown to stimulate growth as well and may do this by digesting xyloglucans but these may be only involved indirectly by enhancing expansin activity or effectiveness in wall creep. How do we know this?
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Use of extensometers show this Figs 15.26 and 15.27.
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3. DIFFERENTIATION: 3 basic tissues Dermal: from outler the layer of the developing embryo form the epidermis and root or leaf hairs, guard cells, etc. Ground: from the next layer, forms the cortical cells and endodermis in roots Vascular: from the inner cells and this forms the vascular cambium and pericycle (xylem and phloem) Remember - all develop from a single cell progenetor following specialization or differentiation plant cells are totipotent. Now were back to our starting point that a few meristems, regions of non-specialized dividing cells produce the cells for the entire plant, which expand and then differentiate.
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Primary: shoot and root or apical meristems, produce the primary axis of the plant and primary growth Secondary: Axillary: produce axillary leaves, branches or roots, activity may be suppressed by primary meristems through hormones why are some plants more shrubby than others? This is under genetic (tree forms) and environmental control (light and other factors). Lateral: increase girth or secondary growth, monocots vs dicots?? Vascular cambium that produces xylem, phloem and ray cells and cork cambium that produces the periderm or bark and the Pericycle roots. So put all these together and we have what is generally called: GROWTH cell division and expansion -irreversible change in something How do we measure growth?
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generally measured as dx/dt where x is cell number, cell size, biomass, (fresh weight?) etc. In doing this we may get typical sigmoid growth curves.
Several ways to look at this: from the perspective of a particular cell or position (material) that changes over time growth trajectory, velocity or relative growth rate.
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Also may be viewed from the perspective of the general shape of moving particles (like a waterfall), spatial perspective --- or kinematics. View growth as displacement of cells. Old Figs. 16.34 p. 368.
Consider putting your initials on a tree trunk -Take home message specific regions account for growth, mostly by cell division followed by expansion. Can be quantified in a variety of ways by agriculture, plant physiologists, ecologists, etc. Lets put this together for a root and a stem:
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Review of Figs on general plant structures (e.g. parts of the plant, leaves, roots, etc.) this is the end product of what weve discussed.
ts.
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Root: anchors plant, water and nutrient uptake, mycorrhizal associations, etc. root cap, meristematic zone, elongation zone and maturation zone (differentiation complet...