Chapter 38 Plant Growth and Regulation

1. Chapter 38 Plant Growth and Regulation Biology 102 Tri-County Technical College Pendleton, SC

2. Seed Dormancy • Seed is dormant if all developmental activity within has been suspended • Cells inside do NOT divide, expand, or differentiate • Insures survival through unfavorable conditions and results in germination when conditions are favorable • Adaptations include: long cold periods, internal clock, need for fire/heat, light and/or dark, annual seeds can skip a year, moisture (cypress)

3. Dormancy, cont. • For embryo to begin developing, dormancy must be broken by physical mechanisms or leeching of inhibitors by water • Exposure to light, mechanical abrasion, fire • As seed germinates (begins to develop), it first imbibes (takes up) water • Growing embryo must obtain monomers for its development by digesting polysaccharides, fats, and proteins stored in cotyledon(s) or endosperm

4. Dormancy, cont. • Release of GIBBERELLINS signals seeds to break dormancy and germinate • Imbibed water stimulates gibberellin release (as does some environmental cues) • In cereal grains, gibberellins stimulate germination and support growth by stimulating synthesis of α-amylase • Will digest stored starch making it available to embryo and seedling

5. Cereal Grass Visual

6. Mono/Dicot Germination • First step is imbibition (absorption of water) for seed germination in many plants • Hydration causes seed to swell and ruptures seed coat • Triggers metabolic changes in embryo that cause it to resume growth • Storage materials of endosperm/cotyledon(s) digested by enzymes and nutrients transferred to growing regions of embryo • Radicle (embryonic root) emerges from seed

7. Germination, cont. • Shoot tip breaks through soil surface • In many dicots, hook forms in the hypocotyl • Growth pushes hypocotyl above ground • Light stimulates hypocotyl to straighten, raising cotyledons and epicotyl • Epicotyl then spreads first leaves which become green and begin photosynthesis

8. Germination, cont. • Germination may follow different methods depending on plant species • In peas, hook forms in epicotyl and shoot tip lifted by elongation of epicotyl and straightening of hook • Cotyledons of peas stay in the ground • In monocots, coleoptile pushes through soil and shoot tip grows up through tunnel of tubular coleoptile

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11. The Hormones are Flowing… • Hormone is regulatory compound that acts at very low [ ] at sites distant from where it is produce • Each plant hormone typically plays multiple regulatory roles • AUXIN(indoleacetic acid, IAA) promotes elongation of young developing shoots or coleoptiles • Affects secondary growth by inducing vascular cambium cell division & differentiation of secondary xylem

12. Hormones, cont. • Auxin promotes formation of adventitious roots • Promotes fruit growth in many plants • Used in herbicides (2,4-D synthetic auxin for dicots) • Apical meristem is major site of production

13. Hormones III • CYTOKININSare modified forms of adenine that stimulate cytokinesis • Affect cell division and differentiation • Influence apical dominance • Serve as anti-aging hormones • Manufactured in the roots

14. Hormones, IV • GIBBERELLINS are produced primarily in roots and young leaves • Stimulate growth in leaves/stems; have little effect on roots • Stimulate cell division & elongation in stems (perhaps in conjunction with auxin) • Causes BOLTING-rapid growth of floral stems which elevates flowers • Fruit development controlled by gibberellins/auxins (Thompson seedless grapes) • Release causes seeds to break dormancy and germinate

15. Hormones, V • ABSCISIC ACID (ABA) produced in terminal bud; prepares plant for winter by suspending both primary/secondary growth • Directs leaf primordia to develop scales to protect dormant buds • Inhibits cell division in vascular cambium • Helps in seed dormancy • Also stress hormone (closing stomata in times of water stress

16. Hormones, VI • ETHYLENE is gaseous hormone that diffuses through air spaces between plant cells • High auxin [ ]s induce release of ethylene which acts as growth inhibitor • Senescence (aging) at cellular, organ, & whole plant level affected by ethylene • Important in fruit ripening and leaf abscission

17. Hormones, VII • BRASSINOSTEROIDS promote elongation of stems/pollen tubes • Promote vascular tissue differentiation • JASMONATES trigger defenses against pathogens and herbivores • OLIGOSACCHARINS trigger defenses against pathogens • Limit effects of high auxin [ ]s • Regulate cell differentiation

18. Hormones, VIII • SALICYLIC ACID triggers resistance to pathogens • SYSTEMIN causes jasmonate production in response to tissue damage • IO #4 “Explain the probable mechanism by which gibberellins trigger seed germination was covered in detail in latter part of IO #1. Enough said……

19. Tropism • Tropism is growth response that results in curvatures of whole plant organs toward or away from the stimuli • Mechanism is differential rate of cell elongation on opposite sides of the organ • Two stimuli that most profoundly affect plant growth are LIGHT and GRAVITY • Phototropism and Gravitropism • Should mention thigmotropism

20. Role of Auxin • Phototropism: cells on darker side of grass coleoptile elongate faster than cells on bright side due to asymmetric distribution of auxins moving down from shoot tip • May be different in other organs • Shoot tip is site of photoreceptioin that triggers growth response • PR sensitive to blue light is present in shoot tip • Believed to be yellow pigment related to riboflavin • Same receptor may be involved in other plant responses to light

21. Role of Auxin, cont. • Gravitropism: even in dark, auxin moves to lower side of tipped-over shoot • Auxin moves downward in response to gravitational stimuli • Higher auxin [ ] causes more rapid growth on lower side • Tip curves upward

22. Role of Auxin, cont. • Apical Dominance is [ ] of growth at tip of plant shoot where terminal bud partially inhibits axillary bud growth • Cytokinins and auxin contribute to apical dominance through antagonistic mechanism • Auxin from terminal bud restrains axillary bud growth causing the shoot to lengthen • Cytokinins (from roots) stimulate axillary bud growth

23. Apical Dominance, cont. • Auxin CANNOT suppress axillary bud growth once it begins • Lower buds grow before higher ones since they are closer to cytokinin source than auxin source • Auxin stimulates lateral root growth while cytokinins restrain it • This stimulation-inhibition action balances plant growth since > in root system would signal plant to produce more shoots

24. Apical Dominance, III • Auxin and cytokinins indirectly change [ ] of ethylene • Levels of different nutrients in bud may also affect response to auxin/cytokinins • Gibberellins also contribute to apical bud dominance • Brassinosteriods are required for normal growth and development

25. Leaf Abscission • High [ ]s of auxin induce release of ethylene which acts as growth inhibitor • Senescence (aging) in plants occurs at cellular, organ, or whole plant level • Ethylene plays important role at each level • Xylem vessel elements/cork cells that die before becoming fully functional • Leaf fall in autumn • Withering of flowers • Death of annuals after flowering

26. Leaf Abscission, cont. • Mechanics controlled by change in balance of ethylene and auxin • Auxin decrease makes cells in abscission layer more sensitive to ethylene • Cells then produce more ethylene which inhibits auxin production • Ethylene induces synthesis of enzymes that digest polysaccharides in cell walls further weakening the abscission layer • Two most important stimuli for leaf abscission are shortening days and cooler temperatures

27. Leaf Abscission Visual

28. Leaf Abscission Visual II

29. Fruit Ripening • Ethylene triggers senescence during fruit ripening • Aging cells then release more ethylene • Breakdown of cell walls and loss of chlorophyll • Signal to ripen then spreads from fruit to fruit since ethylene is a gas • Use of ethylene is single most important use of a plant hormone in agriculture and commerce

30. Parthenocarpy • Fruit development normally depends on prior fertilization of the egg (ovum) • In many species, treatment of unfertilized ovary with auxin or gibberrellins causes parthenocarpy • Fruit formation without fertilization • Dandelions, seedless grapes, and cultivated bananas

31. More of Ethylene • Role of ethylene in leaf abscission covered (IO 6) • Ethylene can be produced in all parts of plant • Called the senescence hormone • As fruit ripens, loses chlorophyll & cell walls break down • Ethylene promotes both processes and causes more ethylene to be produced (apple & barrel) • Also associated with apical hook of eudicots, inhibition of stem elongation in general, and causing stem to lose sensitivity to gravitropic stimulation

32. Abscisic Acid & Stress • Called stress hormone because it accumulates when plants deprived of water & possible role in maintaining winter dormancy of buds • Is most common inhibitor of seed germination • Causes stomata to close and prevents stomatal opening normally caused by light • Both processes involve ion channels in plasma membrane of guard cells

33. Abscisic Acid, cont. • First response of guard cell to abscisic acid is opening of calcium channels and entry of calcium into cell • This calcium causes cell’s vacuole to release calcium too • Increased [ ] of calcium leads to opening of potassium channels • Release of K+ ions and of water causes guard cells to sag together • Results in closing of the stomata

34. Photoreceptors • Light-its presence or absence, intensity, color, and duration-provides cues to various conditions • Light regulates many aspects of plant development • Seed gemination to shoot elongation to flowering to etiolation • Photoreceptors interpret light, its duration, and its wavelength distribution • Five phytochromes mediate effects of red and dim blue light

35. Photoreceptors, cont. • They are bluish proteins (pigments) found in CYTOSOL of plant cells • Phytochromes help plants measure the length of darkness in a photoperiod • Phytochrome is protein containing a chromophore (light-absorbing component) responsible for a plant’s response to photoperiod • Phytochromes alternate between 2 photoreversible forms • Pr (P red) and Pfr (P far red)

36. Photoreceptors, cont. • Plants synthesize Pr and if kept in dark, it remains • If illuminated, some Pr converted to Pfr • Pfr triggers many plant responses to light (seed germination) • Shift in equilibrium indicates relative amounts of red light and far-red light present in sunlight • Shifts in ratio may causes changes such as increased growth

37. Moving right on along… • Far red converted back to red after sunset • Three or more blue-light receptors mediate effects of higher-intensity light • Cryptochromes are yellow photoreceptor pigments that absorb in blue/ultraviolet • Affect some of same developmental processes as phytochromes (seedling development/flowering) • Cryptochromes located in plant nucleus • Phototropin (yellow protein) appears to be photoreceptor for phototropism