ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Thirty-Six Outline INTRODUCTION Plants Respond to Their Environment Plants Undergo Continuous Development Genetic blueprint controls various events Events greatly influenced by external factors Differentiation of specific tissues controlled by hormones fig 36.1 Hormones mediated by genes and environmental factors DIFFERENTIATION IN PLANTS: EXPERIMENTAL EVIDENCE Totipotency of Single Cells Plant differentiation is fully reversible Gene expression reactivated in cells retaining protoplast and nucleus at maturity Reactivation may lead to alternative differentiation or complete plant Haberlandt proposed that all living plant cells are totipotent Possess full genetic potential of the organism Hypothesis not confirmed until cells could be grown in culture Cell Culture Relatively easy to isolate individual cells Repeated division could not be stimulated Solution utilized filter paper floating on established cell culture Single cell in culture media placed on filter paper Isolated from other cells, but still influenced by them Isolated cell obtained various growth promoting substances Established mass of undifferentiated cells called callus Some plants require the addition of coconut milk to culture medium Tissue Culture Steward supplied differentiated cells with substances from dividing cells Small bits of carrot phloem tissue isolated and placed in flask fig 36.2 Growth media contained sucrose, minerals and vitamins New cell clumps differentiated roots fig 36.3a Developed shoots when placed on agar fig 36.3b Grew into whole plants, confirmed Haberlandt's hypothesis fig 36.3c Stages resembled embryonic development of normal zygotes = "embryoids" Regeneration in Nature Common practice uses cuttings of plants to produce whole new plants Formation of adventitious roots from mature pericyle tissue Adventitious shoots do not readily form Some plant cuttings root if simply placed in water or wet sand fig 36.4 Other plants do not readily produce roots Stems with leaves form roots more readily than those without Buds produce auxins that stimulate root growth Regeneration from other tissues Bits of succulent leaf tissue may produce entire plants fig 35.28b Tiny plantlets differentiate along leaf edges of some plants Propagation from rhizomes, stolons or other horizontal roots Century plants may form plantlets among flowers PLANT HORMONES Expression of Plant Genes Controlled by Plant Hormones Differentiated tissue capable of expressing hidden genetic complement Must provide suitable environmental signal Chemical Nature of Hormone Chemical substances produced in small quantities in one location Transported to another location to effect physiological response Response can be stimulatory or inhibitory Animal hormones produced at definite sites, organs of hormone production Plant hormones not produced in such specialized tissues Five major kinds of plant hormones tbl 36.1 Auxin Cytokinins Gibberellins Ethylene Abscisic acid Auxins Basic effects Control growth of lateral buds on stem Regulates stem elongation in young grass seedlings and herbs Discovery of auxin Experiments by Charles and Francis Darwin Observed phototropism: bending of seedlings toward light Response prevented in seedling tips covered with foil fig 36.5 Response occurred in seedling tips covered with gelatin Conclusion: substance produced in response to light was transmitted downward causing shoot to bend toward the light Experiments by Boysen-Jensen and Paal Identified substance as a chemical Normal response if tip separated from shoot by agar block In darkness or normal illumination chemical passed down shoot evenly on all sides, thus no bending occurred Experiments by Went fig 36.6 Cut tips from illuminated seedlings Placed them on cut seedlings grown in the dark Seedlings bent away from side on which block was placed Conclusion: substance enhanced cell elongation Named substance auxin from Greek "to increase" Dark side of seedling had more auxin, its cells elongated more, which bent the seedling Experiments by Briggs fig 36.7 Vertical mica sheet separated light and dark sides of the tip No bending, same amount of auxin on both sides of barrier Conclusion: auxin migrates laterally from light side to dark side Chemical nature of auxin Only naturally occurring compound is indoleacetic acid (IAA) fig 36.8a Resembles and is synthesized from tryptophan fig 36.8b Produced in shoot apex and diffuses downward suppressing growth of lateral buds Migrates to nonilluminated side of shoot and causes cells to elongate, thus bending the shoot Auxin and plant growth Mechanism of action: increases plasticity of cell wall Hormone degraded by indoleacetic acid oxidase IAA and IAA oxidase balanced to rapidly regulate cell growth Chemical transport sites in plasma membrane at basal end of cell Speed of reaction makes determination of chemical basis difficult Unlikely that reaction results from transcription/translation of genes Must effect already existing system Changes in polysaccharides of plant cell walls Increase in concentration of H+ ions Mediates stimulation of mRNA transcription for long-term growth changes Additional effects Promotes growth of vascular tissue and vascular cambium Increases fruit growth Causes fruit maturation Mechanism for inhibitory effects: suppresses lateral bud growth Auxin influences cells at each node to produce ethylene Ethylene actually inhibits bud growth Removing terminal bud stimulates lateral growth, creates bushy plant Synthetic auxins Primarily used to prevent abscission, separation of organ from plant Commercial applications Prevent fruit drop Promote flowering and fruiting in pineapples Induce formation of roots on cuttings Herbicides to control weeds: 2,4-D and 2,4,5-T fig 36.8c Selectively eliminates broad-leaved dicots Weeds literally grow to death Contaminated with dioxin a toxic by-product from herbicides Cytokinins tbl 36.1 Promote differentiation of organs in masses of cultured plant tissue Induces parenchyma cells to become meristematic Causes differentiation of callus tissue Mechanism of action When combined with auxin, cell division stimulated and differentiation induced Mostly produced in roots and transported throughout plant, also by fruit Chemically derived from adenine fig 36.9 Act opposite of auxin, promote growth of lateral branches, inhibit formation of lateral roots Prevents yellowing of leaves detached from plant Appear to be necessary for mitosis and cell division Gibberellins tbl 36.1 Named for fungus that causes "foolish seedling" disease in rice Causes infected plant to grow abnormally tall Large class of chemicals additionally found in normal plants Mode of action Synthesized in apical portions of stems and roots Promotes internodal elongation, enhanced by auxin Restored normal growth to dwarf plant mutants fig 36.10 Stimulate hydrolytic enzyme production in germinating grain seed fig 36.11 Initiates burst of mRNA and protein synthesis May act directly on DNA or via cytoplasmic chemical intermediates Occurs when radicle has grown through seed coats Induce biennial plants to flower fig 36.12 Speeds seed germination Only gibberellin GA1 is active in shoot elongation Ethylene tbl 36.1 Initial observation of ethylene gas inducing defoliation Acts alone and interacts with other plant hormones Suppresses lateral bud formation when combined with auxin Suppresses stem and root elongation Primary factor in formation of separation layer in abscission (opposite of auxin) Produced in large quantities during climacteric of fruit ripening, hastens ripening (carbon dioxide has opposite effect) Ecological role Ethylene production increased after exposure to adverse conditions Can accelerate abscission of leaves damaged by stresses Damage from exposure to ozone due to ethylene production Abscisic Acid tbl 36.1 Synthesized primarily in mature green leaves, fruit and root caps Actions Stimulates leaf senescence and abscission, but not involved in natural process (opposes gibberellins and auxin) Application on leaves causes yellow spots (opposite effect as cytokinins) May induce formation of winter buds Suppresses growth of dormant lateral buds Controls opening and closing of stomata Physiological effects are extremely rapid Binding site located on proteins on outer surface of plasma membrane Proteins not involved with transport of hormone into cells TROPISM Orientation in Response to External Stimuli Phototropism Bending of plants toward unidirectional sources of light Stems grow toward light and are positively phototropic Roots grow away from light and are negatively phototrophic Response is adaptive for leaves to capture greater amounts of light Response is adaptive for roots to grow toward water and nutrients Most phototrophic responses mediated by auxins Gravitropism fig 36.13 Formerly known as geotropism, response is to gravity not earth Causes stems to grow upward and roots downward Obviously adaptive to both roots and stems Hormonal mechanism of response Differential in auxin concentration develops in horizontal stems More auxin on lower side causes these cells to elongate, stem rises Concentration gradient not well documented in roots Roots in tropical rainforests often grow upward Soil is very nutrient poor Precipitation is more reliable source of nutrients Thigmotropism fig 36.14 Response of plants to touch Causes curling of tendrils, twining of vines Action associated with rapid cellular growth Coiling of tendrils is associated with auxin and ethylene TURGOR MOVEMENTS Movement Via Reversible Turgor Pressure Changes in Specific Cells Types of Movements Changes in position of leaves Prayer plants leaves are horizontal in day, vertical at night fig 36.15 Movement associated with pulvinus, turgor of motor cells Touch sensitive plants like Mimosa fig 36.16 Movements are extremely rapid Controlled by changes in ion concentrations stimulated by electrical currents Carnivorous plants like Venus flytrap Not caused by changes in turgor pressure as leaves do not have pulvini With stimulation of two trigger hairs, certain cells irreversibly enlarge Initiated by drop in pH in cell walls Walls most flexible at ph 3 to 4 Expends ATP Cells on opposite side grow slowly to open leaf Flower movements Flowers have structures similar to pulvini Track the position of the sun to keep flower head warm and attract pollinators Closing of flowers at night controlled by pulvini PHOTOPERIODISM Mechanism to Measure Seasonal Changes in Day and Night Length Flowering Responses Significant stimulus fig 36.17 Length of darkness not length of day Critical day length for both types at 12 to 14 hours Short-day plants Form flowers when days get shorter Bloom in late summer and autumn Long-day plants Form flowers when days get longer Bloom in spring and early summer Day neutral plants Produce flowers whenever environmental conditions are suitable No reference to day length Light artificially controlled to force plants to flower out of season Helps control distribution of plants The Chemical Basis of the Photoperiodic Response Interruption of normal responses Brief period of light within dark period cancels flowering response Effective wavelength is at 660 nanometers, red light Effect canceled if followed by far-red light at 730 nanometers Chemical basis of effect Presence of two forms of phytochrome: Pr and Pfr Pr absorbs red light and is converted to Pfr Pfr absorbs far-red light and is converted to Pr Pr is biologically inactive, Pfr is biologically active In short-day plants Pfr leads to suppression of flowering fig 36.18 In darkness Pfr is converted to Pr When darkness is long enough, suppression is removed, plants flower Single flash of red light converts Pr to Pfr, flowering blocked Conversion of Pr and Pfr not sole factor controlling flowering Chemical nature of phytochrome Composed of small, light sensitive part and large protein part Pigment is blue, similar to phycobilins in algae and cyanobacteria Phytochrome involved in other growth responses Seed germination inhibited by far-red light, stimulated by red light Slender, colorless seedlings exposed to red light regain shoot length fig 36.19 Effects canceled by far-red light The Flowering Hormone: Does It Exist? Removal of leaves affects response to day length and inhibits flowering Substance produced in leaves passes to apices to promote flowering Substance does not pass through agar block Requires living plant parts to translocate Substance not identified after 50 years of searching DORMANCY In Temperate Climates Associate dormancy with winter Low temperatures and unavailability of water prevent growth Tree buds are dormant, perennials reduced to underground parts, other plants exist as only seeds In Seasonally Dry Climates Dormancy occurs during dry season Strategies similar to temperate plants In Areas with Seasonal Drought Predominance of annual plants Seeds are capable of surviving indeterminate dry seasons Rapidly germinate, grow and flower when water becomes available fig 36.20 Seeds may contain chemicals that must leach out with sufficient water Seed Dormancy Remain viable for long periods of time, especially legumes Period of cold may be required to initiate germination