Chapter 32

1. Chapter 32 Plant Growth and Development AP Biology Spring 2011

2. Chapter 32.1 Overview of Plant Development

3. Seed Germination • Germination: the resumption of growth after a time of arrested development

4. Environmental Factors Influence Seed Germination • Seasonal Rains: provide water amounts necessary to swell and rupture the seed coat • Water activates enzymes necessary to hydrolyze the stored starch • Starches are converted to sugars • Provides the energy for the meristems to initiate cell division • Oxygen is required, reaches embryo and aerobic respiration provides ATP needed for growth

5. Environmental Factors Influence Seed Germination • Repeated cell divisions produce a seedling with a primary root • When the primary root breaks through the seed coat germination is complete • Seed dormancy and germination is climate specific • Occurs only when conditions are favorable for the seedling to survive

6. Patterns of Early Growth • Growth: an increase in the number, size, and volume of cells • Development: the emergence of specialized, morphologically different body parts • Patterns of germination, growth, and development have a heritable basis dictated by a plant’s genes

7. Patterns of Early Growth • Early cell divisions may result in unequal distribution of cytoplasm • Cytoplasmic differences trigger variable gene expression, which may result in variations in hormone synthesis • Even though all cells have the same genes, it is the selective expression of those genes that results in cell differentiation

8. Patterns of Early Growth • Plant growth and development starts with the selective transcription and translation of genes • Ex. Page 543 Fig. 32.3 and 32.4 • Pattern of growth and development of corn (monocot) and bean plant (dicot)

10. Chapter 32.2 Plant Hormones and Other Signaling Molecules

11. Major Types of Plant Hormones • Plant hormones have central roles in the coordination of plant growth and development

12. Giberellins • Acidic compounds synthesized in seeds and young shoot tissues • Promote stem elongation, germination and starch hydrolysis • Help induce flowering in some plants

14. Auxins • Produced at apical meristems of roots and shoots, coleoptiles in monocots • Influence cell division and elongation either positively or negatively depending on the tissue • Cause leaves to grown in patterns, stems to bend toward light, roots to grow down • Auxins at shoot tips prevent lateral bud growth- apical dominance • Help prevent abscission where leaves, flowers, or fruits drop from plant • Abscission: dropping of leaves, flowers, fruits

15. Cytokinins • Stimulate cell division in root and shoot meristems, where they are most abundant • Can release lateral buds from apical dominance and can stop leaves from aging prematurely • Used commercially to prolong the life of stored vegetables and cut flowers

16. Ethylene (a gas) • Can promote or inhibit cell growth so that tissues expand in the most suitable directions • Induces fruit ripening • Concentrations high when plant is stressed • Ex. Autumn or end of life cycle • Induces abscission of leaves and fruits, and sometimes death of whole plant

17. Abscisic Acid (ABA) • Inhibits cell growth • When growing season ends, ABA overrides gibberellins, auxins, and cytokinins; causes photosynthetic products to be diverted from leaves to seeds • Helps prevent water loss (by promoting stomata closure) • When plant is water stressed, root cells produce more ABA which xylem move to leaves • Promotes seed and bud dormancy

18. Other Signaling Molecules • Brassinosteroids: help promote cell division and elongation • Stems stay short in their absence • Jasmonates: help other hormones control seed germination, root growth, and tissue defense responses to pathogens • FT protein: part of a signaling pathway that induces flower formation

19. Other Signaling Molecules • Salicylic Acid: interacts with nitric oxide in respose to attacks from pathogens • Nitric Oxide: functions in plant defense response • Systemin: peptide that forms when insects attack plant tissues; travels throughout the plant turning on genes for substances that interfere with the insect’s digestion

20. Commercial Uses • Many synthetic and natural plant hormones are used commercially • Ethylene: makes fruits ripen quickly • Gibberellin: promotes larger fruits • Synthetic Auxins: spayed on unpollinated flowers to produce seedles fruits • Synthetic Auxin 2,4-D: used as herbicides • Accelerates the growth of eudicot weeds to a point that the plant cannot sustain it and the weeds die

21. Chapter 32.3 Mechanisms of Plant Hormone Action

22. Signal Transduction • Plants have pathways of cell communication Cell type secretes hormone or signaling molecule Binds with receptor on target cell Signal transduced to a form that may influence a metabolic pathway, gene expression, or membrane properties

23. Hormone Action in Germination • Imbibed water stimulates cells of embryo to release gibberellin • Water moves giberellin to cells of aleurone (protein storing layer) • Water also activates protein digesting enzymes • In aleurone layer, hormone triggers transcription and translation of amylase genes to hydrolyze starch molecules • Digests starch into transportable sugar • Amylase moves into endosperm’s starch rich cells • Sugar monomers released from starch fuel aerobic respiration • ATP from aerobic respiration provides the energy for growth of the primary root and shoot

25. Polar Transport of Auxin • Auxin concentration gradients start forming during early cell divisions of embryo sporophyte • Cells exposed to higher concentrations transcribe different genes than those exposed to lower concentrations • Help form plant parts (leaves) in expected patterns • Helps young cells elongate

26. Polar Transport of Auxin • Auxin concentration highest at source: apical meristem in a shoot (or coleoptile) • Auxin transported down, toward shoot’s base • Polar transport takes place in parenchyma cells

27. Polar Transport of Auxin • Auxin gives up hydrogen in each cell, which alters cytoplasmic pH • Membrane pumps activly transport H+ outside, which lowers pH of moist cell wall • Enzymes in cell wall become active at lower pH

28. Polar Transport of Auxin • Enzymes cleave crosslink's between microfibrils, which support the wall • Water is diffusing into the cell, turgor pressure builds against wall • Microfibrils now free to move apart, wall is free to expand • Ta-dah….cell lengthens! • pH change also activates transcription factors, after auxin exposure, proteins that help cell assume its new shape are synthesized

30. Chapter 32.4 Adjusting the Direction and Rates of Growth

31. Response to Gravity • Gravitroprism: growth response to gravity • Shoots grow up, roots grow down • Auxin, with growth-inhibiting hormone: may play a role in promoting or inhibiting growth in various regions of the plant • Statoliths: are unbound starch grains in plastids, respond to gravity and may trigger redistribution of auxin

32. Response to Light • Phototropism: growth response to light • Bending toward light is caused by elongation of cells (auxin stimulation) on the side of the plant NOT exposed to light • Phototropins: pigments that absorb blue wavelengths of light and signal the redistribution of auxin that initiates the elongation of cells

33. Response to Contact • Thigmotropism: shift in growth triggered by physical contact with surrounding objects • This response to auxin and ethylene is prevalent in climbing vines and in the tendrils that support some plants • Tendrils: new, modified leaves or stems • When cells at shoot tip touch stable object, cells on contact side stop elongating and cells on other side keep growing • Unequal rates of growth make vine or tendril curl around object

35. Response to Mechanical Stress • Responses to the mechanical stress of strong winds explain why plants grown at higher elevations are stubbier than those at lower elevations • Grazing animals, growing outside vs. greenhouse can also inhibit plant growth • Human intervention such as shaking can inhibit plant growth

36. Chapter 32.5 Seasonal Shifts in Growth

37. Seasonal Shifts • Circadian Cycle: completed in 24 hour period • Photoperiodism: refers to biological response to alternations in the length of darkness relative to daylight during a circadian cycle • Ex. The number of hours plant spends in darkness and daylight shifts with seasons

38. Seasonal Shifts • Biological Clocks: internal mechanisms that preset the time for recurring shifts in daily tasks or seasonal patterns of growth, development, and reproduction

39. Seasonal Shifts • Phytochrome: blue-green pigment functions as a receptor for red and far-red light • Red light at sunrise causes phytochrome to shift from its inactive form (Pr) to its active form (Pfr) • Far-red light at sunset shifts to inactive form (Pr) • Longer the nights, longer the interval when phytochrome is inactive • Pfr can induce gene transcription • Can bring about seed germination, shoot elongation, branching, leaf expansion, and flower, fruit and seed formation, then dormancy

41. Chapter 32.6 When to Flower?

42. Response to Hours of Darkness • Flowering process is keyed to changes in day length throughout the year • Cue is length of darkness

43. Response to Hours of Darkness • Short-day plants: flower in early spring or fall • Nights are longer than some critical value • Long-day plants: flower in summer • Nights are shorter than some critical value • Day-neutral plants: flower whenever they are mature enough to do so

44. Response to Hours of Darkness • Phytochrome is trigger for flowering • Detection of photoperiod (alternations in length of darkness relative to daylight) occurs in leaves, where hormones inhibit a shift from leaf growth to flower formation

45. Revisiting the Master Genes • 3 groups of master genes A, B, C control formation of floral structures from whorls of a floral shoot • In response to photoperiods of other environmental cues, leaf cells transcribe a flowering gene • mRNA transcript travels in phloem to as-yet undifferentiated floral buds, where they are translated into FT protein • This signaling molecule with a transcription factor turn on master genes that cause undetermined bud of meristematic tissue to develop into a flower

46. Vernalization • Vernalization: low temperature stimulation of flowering • Unless certain biennials and perennials are exposed to low temperatures, flowers will not form on their stems in spring

47. Chapter 32.7 Entering and Breaking Dormancy

48. Abscission and Senescence • Abscission: the dropping of leaves, flowers, fruits, other parts • Senescence: sum total of the processes leading to the death of plant parts or the whole plant

49. Abscission and Senescence • Recurring cue is decrease in day length that triggers a decrease in auxin production • Cells in abscission zones produce ethylene, which causes cells to deposit suberin in their walls • Simultaneously, enzymes digest cellulose and pectin in the middle lamella to weaken the abscission zone • Lamella: cementing layer between plant cell walls

50. Bud Dormancy • Dormancy occurs in autumn when days shorten, and growth stops in many trees and non-woody perennials • It will not resume until spring