Ch. 39 Plant Hormones I. Plant Hormones Chemical signals that coordinate the parts of an organism. Only minute amounts are needed to get the desired response. Control plant growth and development by affecting the division, elongation and differentiation of cells. Hormones may act by altering expression of genes, affecting activity of enzymes, or by changing properties of membranes. A. comes from the Greek, "to increase" any substance that coleoptiles (sheath enclosing the shoot of a seedling) IAA = indoleaceacetic acid = natural auxin 1. Cell elongation auxin synthesis is an moves to region of elongation promotes cell growth too much auxin will indirectly inhibit cell growth because it stimulates production of which inhibits plant growth moves through parenchyma tissue from shoot tip to base in one direction only = 2. Secondary growth induced cell division in promotes formation of promotes growth of B. 1. Control of cell division and differentation in activley growing tissues (roots embryo, fruit) move in xylem sap act with auxins; alone = equal amount of both growth continues with no differentiation 2. Control of auxins cause suppresion of auxillary buds by terminal buds cytokinkins signal auxillary buds to grow; auxin cannot suppress this growth as roots expand, an increase in cytokinkins causes more shoot branching (it has enough roots to support it) opposite happens in order to develop more lateral roots
3. As hormones may inhibit protein breakdown, stimulate protein synthesis, efficiently gather nutrients from nearby tissues cytokinin spays keep cut flowers fresh longer C. Gibberellins 1. Stem elongation produced in roots as young leaves stimulate growth in, but not in root causes = growth of floral stalk some plants have a form (low to ground with very short inernodes) surge of gibberellins causes stems to elongate, elevating developing flowers 2. Fruit growth some plants need both gib. And auxins for fruit growth 3. Germination may have and increased conc. in embryosignals seed to break dormancy if enough water is available stimulates synthesis of that codes for enzymes that can break down stored nutrients in seed for the embryo D. produced in terminal bud causes onset of in seeds; dormancy is broken when gib. overcome ABA helps plant cope with stressful conditions E. it's a diffuses =through air spaces between cells may inhibit cell elongation = aging a. fruit ripening: ovary becomes the fruit degradation of decrease in chlorophyll content (loss of greenness) b. leaf abscission: nutrients in leaves are moved to storage tissues in stem stop making
Apoptosis: = small, thinwalled parenchyma cells; enzymes hydrolyze polysaccharides in walls to further weaken them wind and weight of leaf causes separation cork forms a protective layer (scar) before leaf falls aging leaf leads to less auxin which means cells of abscission layer are more sensitive to ethylene ex. shedding of leaves, vessel elements, annual flowers, etc. F. Mutant Plants more hormones 1. short sugar chains released by cell wall trigger defense responses by plants that have been infected with a pathogen II. Plant Movements 2. required for normal growth and development A. Tropisms = 1. movement toward light may be induced p.752 may be caused by growth inhibitors on lighted side of stem shoot tip is the site that triggers growth response photoreceptors are (pigments) most sensitive to blue light 2. plant's response to roots have positive gravity; shoots have negative gravity this ensures proper directional growth = specialized plastids with dense starch stains settling of these to the low point of cell helps plant tell up from down 3. directional growth in response to touch ex: tendrilscontact stimulates coiling which is caused by differential growth of cells on opposite side of tendril wind can stimulate this
B. Turgor Movements 1. Rapid leaf movements ex: Mimosa rapid loss of turgor of cells within (specialized motor organs at leaf joints) cells become flaccid because they lose causing water to leave the cell may be for water conservation, or to expose thorns for protection can have a domino responsechemical messengers and/or electrical impulses impulses (action potentials)similar to messages from nervous system in animals 2. Sleep movements lower leaves in evening and raise them in morning cause by changes in horizontal: cells on one side of pulvinus are turgid; this is reversed at night massive migration of K+ from one side of pulvinus to the other K + leads to reversible uptake and loss of water by motor cells III. Control of Daily and Seasonal Responses = a physiological cycle with a frequency of about 24 hours; persists even when sheltered from environmental cues Photoperiod = Photoperiodism = A. Control of Flowering 1. shortday plant 2. longday plants 3. dayneutral plants B. Critical Night Length night length actually controls flowering and other responses to photoperiodism flower industry uses this knowledge to grow plants our of season C. A Flowering Hormone? buds produce flowers, but photoperiodism is detected by if all leaves are gone except one, the plant will still flower if all leaves gone it's probably a that sends this signal, but it is still unidentified
IV. Phytochromes (pigments) Figure 39,18 page 768explanation of red light and farred light flashes' effect on flowering Protein component convalently bonded to a nonprotein part = (the light absorbing part) Figure 39.12 page 769 and explanation is excellent! ex: if a tree that needs sun is shaded, this sensing mechanism tells the tree to use its resources to grow taller and not branch out If photoperiodism requirement is net, a trigger is sent to stimulate flowering Some plants will not lower if the night is even one minute too short! Some flower on the same day every year V. Plant Responses to Environmental Stress A. Water Deficit B. Oxygen Deprivation too much soil fills up air spaces = some plants may grow may form for submerged roots C. Salt Stress if soil is salty, plant will lose water may be toxic to plants plants may produce compatible solutes = D. Heat Stress produce help other proteins fold into their specific shapes help prevent E. Cold Stress cold changes fluidity of membranesalters solute transport plants will alter of mem. to have more unsat. fatty acids plants adapted to very low temps. can change solute composition of their cells; this allows cytosol to cool below 0 Celsius without ice forming in protoplast (may form in cell walls)
F. Herbivores VI. Defense Against Pathogens is the first line of defense, but pathogens may enter in injuries or through stomata Virulent Most plant pathogens inhabit plant without killing itplant is "resistant" to that pathogen Resistance is based on geneforgene recognitionrequires a matchup between an allele in plant and an allele in the pathogen Figure 39.23 page 772 If a virulent pathogen attacks, plant responds to signals sent out from damaged cells activates genes that make PR proteins (pathogenesis related) some are antimicrobial, some spread news of infection (see mutant hormones section) HR to a virulent pathogen everything that happens but faster and more effectively after cells at site of infection are sealed off, cells destroy themselves SAR (systemic acquired resistance)