Transcript Chapter 45
Plant Responses to the Environment Chapter 45 Plant Hormones • Hormones – chemicals secreted by cells, transported to other cells where they exert effect • Released in response to an environmental stimuli • Promote growth, development, aging Six Plant Hormones • • • • • • Auxins Gibberellins Cytokinins Ethylene Abscisic acid Florigens Auxins • Promote or inhibit elongation in target cells – Shoot – high conc. causes elongation – Root – low levels stimulate elongation, high conc. inhibit • Synthetic auxin (2,4 D) is used to kill dicots • Commercially used to promote root formation in plant cuttings, stimulate fruit development, delay fruit fall Gibberellins • Primarily in plant shoots • Promote stem elongation by increasing cell elongation and division • Stimulate bud sprouting, flowering, fruit production and development, seed germination • Produced in the shoot apical meristem, young leaves, plant embryos Cytokinins • Promote cell division • Synthesized in root apical meristem • Inhibit formation of root branches, cause nutrients to be transported to leaves, stimulating chlorophyll production and delaying aging • Commercially - sprayed on cut flowers to keep them fresh Ethylene • Gas • Produced in plant tissues, released in response to a range of environmental stimuli • Stress hormone – produced in response to wounding, flooding, drought, extreme temp. • Stimulates weak celled abscission layers leaves, petals, fruit drop off at appropriate times • Commercially – used to ripen fruit Abscisic Acid • Synthesized in tissues throughout the plant • Helps plants to withstand unfavorable environmental conditions • Causes stomata to close when water is scarce • Promotes root growth, inhibits stem growth in dry conditions. • Helps maintain dormancy Florigens • Synthesized in leaves • Control flowering in response to environment • Discovered in 2007 Hormones regulate plant life cycles • Hormones are produced in response to an environmental stimulus • These hormones may influence the activity of genes (activate, repress) Maintaining Dormancy of Seeds • In temperate zones, seeds remain dormant until spring • Cold weather reduces abscisic acid, preparing plant for spring germination • In desert plants, some seeds have high levels of abscisic acid in their coats. It may be washed away by rain • Grasslands, chaparral, forest – require fire for germination Gibberellin Stimulates Germination • -Abscisic acid and +Gibberellin = germination • Gibberellin is produced by the embryo • Enzymes break down starch for energy Auxin controls Orientation • Light and gravity help the seedling figure which way is up • Auxin controls phototrophism – growth towards light, in shoots • Gravitropism – growth to/away from gravity, in shoots and roots • Gravirtopism and phototrophism work together to cause shoot to grow upward Auxin mediates Gravitropism • Vertical stem – auxin distributed evenly • Horizontal stem – position detected and auxin distributed to lower side of stem. • Lower cells elongate, bending stem upwards (-gravitropism) • When stem is vertical auxin is evenly distributed Gravitropism shoot Auxin, produced in the shoot tip, is distributed evenly across the shoot and root as it travels downward seed Auxin from the shoot tip travels down and collects in the root tip root (a) The shoot and root are oriented vertically Gravitropism Auxin is transported to the lower side of the shoot, where it stimulates cell elongation and causes the stem to bend upward Auxin is transported to the lower side of the root, where it inhibits cell elongation and causes the root to bend downward (b) The shoot and root are oriented horizontally Negative Gravitropism Positive Gravitropism Auxin mediates Phototropism • Auxin accumulates in the side of the shoot that is away from the light. • Cells elongate and bend towards the light Animation: Hormone Characteristics Animation: Hormone Transport and Activity Auxin mediates Root Elongation • Toward gravity • If root is horizontal, they sense gravity and cause auxin transport to lower side • Lower side cells elongate, causing root to grow towards gravity • How do they sense gravity? Statoliths – starch filled plastids settle into the lower part of the cell Statoliths May Be Gravity Detectors root cell in root cap nucleus statoliths Plant responds to Environment • When shoot or root push against soil, ethylene is given off. • Elongation slows and cells become thicker and stronger • More able to force their way through soil • Dicots – ethylene causes formation of hook in forming shoot Thimotropism • Directional movement or growth in response to touch • Cell elongation on contact side is inhibited, tendrils grow • Etheylene may be produced by cells touching the object Shoot and Root Branching • Controlled by Auxin and Cytokinin • Growth of shoot must be balanced by root growth • Water, mineral, anchorage of plant • Stems – auxin inhibits later bud growth to form branches, cytokinin promotes this growth • Root – cytokinin stimulates root branching, auxin promotes this growth Apical Dominance • Pinching back the tip of a plant causes bushy growth because apical meristems release auxin which suppresses bud development into branches. Lateral bud sprouting • Auxin is transported from the stem to the root, decreasing in concentration. • Cytokinin is transported from the root to the stem, decreasing in concentration. • Lateral buds closes to shoot receive enough auxin to inhibit growth, very little cytokinin = remain dormant • Lower lateral buds receive less auxin and more cytokinin = stimulate to grow into branches Root branch formation • Auxin, transported down from the stem stimulates branch roots to form • Cytokinin produced in the root apical meristem inhibits root branching and is transported from the root towards the shoot • Roots closer to the shoot develop branch roots • Gradient of hormones keeps size of root and shoot in balance Gradient of Auxin and Cytokinin auxin high shoot tip Lateral buds are inhibited by high auxin levels Lateral buds develop into branches (optimal ratio of auxin to cytokinin) Branch roots develop (optimal ratio of cytokinin to auxin) high cytokinin Branch roots are inhibited by high cytokinin levels root tip Response to Light and Dark • Timing of flowering and seed production is crucial • Environmental clues like water and temperature are unpredictable • Day length is very reliable • Shortening vs. lengthening days It’s about the amount of darkness • Day-neutral plants – flower independently of day length • Roses, tomatoes, cucumbers, corn • Long-day plants – flower when uninterrupted dark is shorter than species-specific duration • Iris, lettuce, spinach, hollyhocks • Short-day plants – flower when uninterrupted dark is longer than species-specific duration • Cockleburs, chrysanthemums, asters, potato, goldenrod • It is really more about the amount of darkness than light The Effects of Darkness on Flowering day-neutral plant (rose) short-day plant (chrysanthemum) long-day plant (iris) long night night day short night interrupted night Phytochrome • Plants measure darkness using a biological clock that isn’t well understood • Each time the phytochrome molecule is exposed to light, the clock resets to 0 • If a plant needs 8 hours of dark to flower but is interrupted with a flash of light at 4 hours, the clock will reset • Light changes the shape of the phytochrome molecule The Light-Sensitive Phytochrome Pigment absorbs red light absorbs far-red light conversion in light Pr (inactive) both forms are present in daylight conversion in dark Pfr (active) Pfr stimulates or inhibits a response Author Animation: Phytochrome Author Animation: Morning Glory Animation: Seedling Elongation Florigen Stimulates Flowering • Leaves produce florigen in response to the biological clock • Transported on phloem to the apical meristem where it activates genes that are responsible for flowering Coordination of Seeds and Fruit • Auxin and gibberellin promote growth of ovary • Apply to fruit and they grow larger and looser Ethylene • Unripe fruit is green, bitter • Ripened fruit turns color & attracts animals • Ethylene gas stimulates ripening, as they ripen fruits give off ethylene gas to stimulate ripening of adjacent fruit • Bananas & tomatoes are picked & shipped green Senscence • Genetically programmed series of events that prepare the plant for winter • Ethylene production increases • Auxin and cytokinin production decreases • Starches and chlorophyll are broken down and stored in the stem and roots The Abscission Layer • A layer located where the fruit or leaf join the stem • Ethylene promotes breakdown of this layer • Leaves/fruit drop at correct time • Can also be triggered by stress Plant Communication • Plants summon insect bodyguards • When attacked by caterpillars, corn releases chemicals, stimulated by volicitin (in caterpillar saliva) • Parasitic wasps are attracted to chemical, lay eggs in caterpillar • Lima beans, attacked by spider mites release chemical that attracts carnivorous mite that preys on spider mite A Chemical Cry for Help 2 Volicitin and leaf damage cause the plant to synthesize and release volatile chemicals 1 A caterpillar chews on a corn leaf, leaving traces of saliva that contains volicitin 3 The released chemicals attract female parasitic wasps 4 The wasps lay their eggs on the caterpillar, which will provide food for their larvae Animation: Chemical Messengers Plant Defense • Some plants, when damaged by insects, produce a signaling molecule that moves through the plant. • The plant then makes a distasteful chemical • Radishes and caterpillars Warning the Neighbors • Healthy plants sense chemicals released by neighbors that have been wounded by insects. • Salicylic acid methyl salicylate (volitile) • Neighbors boost their defenses Mimosa • Thigmotropism - sensitive to touch • Stimulated by electric signals conducted through motor cells at the base of each leaf Carnivorous Plants • Sundew - movement of trapped insects triggers thigmotropism in the hairs, secrete sticky goop, smothering the insect • Bladderwort – trapdoor is sprung by insect, opens inward suddenly, sucking insect into the bladder where it is digested. A Sundew and Its Insect Prey The Bladderwort Snares Tiny Aquatic Organisms Author Animation: Venus Fly Trap