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Plant Hormones AP Biology – LAHS What are Hormones? Chemical signals that coordinate the various parts of an organism Chemicals are made in one region and are target for some other region of the organism The Discovery of Plant Hormones Plant hormones were discovered as scientists were studying how it is that plants grow towards light Phototropism – growth of a shoot towards light Darwin’s Experiments with Phototropism Coleoptile – term for the sheath that encloses a grass seedling Studied growth of the coleoptile in different conditions Darkness – grew straight Illuminated uniformly from all sides – grew straight Illuminated from one side only – grew towards the lighted side Question: What Causes the Coleoptile to Bend towards Light? Hypothesis: cells on the darker side of the coleoptile elongate faster than those on the lighted side. This causes the coleoptile to bend toward light. How does this happen? Darwin’s Ideas The part of the coleoptile responsible for sensing light is the TIP. Growth response that caused curvature of the coleoptile was BELOW the tip. Hypothesis: some signal was transmitted downward from the tip Diagrams of Experiments Testing Darwin’s Hypothesis Peter Boysen-Jensen Tip was separated from the coleoptile Control treatment: A gelatin block separated the tip form the lower parts of the plant The gelatin block allowed the plant to be cut as it would be in the experimental treatment, but still allowed the chemicals from the tip to pass down Resulted in curvature as normal Testing Darwin’s Hypothesis Experimental treatment: An impermeable barrier was placed between the coleoptile tips and the lower parts of the plants Prevented the chemicals made at the tip from moving down the plant Result: Curvature growth did not occur Diagrams of Experiments Went’s Experiments Extracted the chemical messenger from the coleoptile tip Removed the tip and allowed it to diffuse onto a piece of agar Removed and discarded growing tip from other coleoptile seedlings Placed the agar block evenly centered onto the “decapitated” seedlings They grew straight Placed the agar block Uncentered onto other decapitated seedlings Their growth caused them to curve AWAY from the side with the agar block Went’s Experiments Went’s Conclusions The chemical in the tip stimulated growth as it passed down the coleoptile The coleoptile curved toward light because of a HIGHER concentration of the growth-promoting chemical on the DARKER side of the coleoptile Went named the chemical messenger that he studied AUXIN Tropisms Growth responses that result in curvatures of whole plant organs toward or away from some stimulus. Mechanism Elongation of cells on the OPPOSITE side of the organ region that is receiving the stimulus Stimulii Gravity Light Touch How Does Auxin Stimulate Growth? Causes cell walls to become “looser” and more malleable. Then they can be expanded/elongated Plant Hormones - Auxin IAA (indoleacetic acid) Found: Meristems of apical buds Major Functions: stimulation of stem elongation Root growth, differentiation, branching Apical dominance Growth of a stem occurs only at the tip unless the tip is cut off Absence of auxin from tip will allow lateral buds to emerge This is why we prune Actively transorted from cell to cell in a specific direction Plant Hormones - Auxin (IAA) cont. Found Embryos within seeds Major Functions Stimulate growth of fruit from ovary Influences responses to light & gravity Plant Hormones - Cytokinins Found In actively growing tissues Produced in roots, transported elsewhere Major function: Stimulate cytokinesis (cell division) Work with auxins to control plant growth Plant tissue treated with auxin w/o cytokiinin – cells will grow but not divide Control of apical dominance – supports lateral buds (weakens apical dominance) Anti-aging hormones Delays senescence (aging) of leaves Slow deterioration of leaves – used by florists Plant Hormones - Gibberellins Found in: Apical meristems; young leaves/embryos Major function: Simulates growth in leaf and stem Stem bolting – rapid elongation Fruit growth Germination of seeds Grapes are sprayed with gib to cause them to grow larger and further apart After water is imbibed, gibberellins are released in embryo to break from dormancy Inhibition of aging leaves Plant Hormones – Abscisic Acid Found in: Leaves, stems, roots Seeds, green fruit Major function: Slow down growth Dormancy for overwintering Suspends primary and secondary growth Promotes abscision of leaves (falling off) In seeds – inhibits growth until ABA can be overcome or diminished by favorable conditions Heavy rain may wash out ABA Light may degrade Increased gib to ABA ratio may determine germination growth Stress hormone When a plant wilts, ABA accumulates causing stomata to close Plant Hormones - Ethylene Found in Tissues of ripening fruit Nodes of stems Ageing leaves and flowers Major functions Changes of ovary to become fruit Leaf abscission Degradation of cell walls; softening Dropping from plant Loss of leaves to prevent water loss Tissue at base of petiole dies Senescence (aging) Autumn leaves; withering flowers Tropisms Growth responses that result in curvatures of whole plant organs toward or away from some stimulus. Mechanism Elongation of cells on the OPPOSITE side of the organ region that is receiving the stimulus Stimulii Gravity Light Touch Tropisms - Phototropsm Phototropism: response to light Achieved through auxin When all sides equally lit, straight growth When stem unequally lit, differential growth Tropisms - Gravitropism Also geotropism Response to gravity be stems & roots Gibberellins & Auxin involved (relative concentrations) Tropisms - Gravitropism If stem is horizontal: auxin at apical meristem moves down and concentrates on lower side – stem bends upwards If root is horizontal, auxin produced at apical meristem moves up roots and concentrates on lower side – inhibits growth in roots Special starch-storing plastics (staloliths) settle at lower ends of cells to influence auxin movement Tropisms - Thigmotropism Response to touch Seen in climbing vines, venus fly trap, etc. Photoperiodism Response of plants to changes in the photoperiod (relative length of day/night) A plant maintains circadian rhythm: internal clock that measures length of day/night Phytochromes Chemicals that function as photoreceptors in plants and allow plants to “measure” photoperiod Phytochromes The name given to the photoreceptor that is responsible for the reversible effects of red and far-red light is phytochrome Phytochrome = a light absorbing protein 2 forms Pr = absorbs red light Pfr = absorbs far red light The two forms are photoreversible When Pr is exposed to red, it becomes Pfr When Pfr is exposed to far red, it becomes Pr Phytochrome Phytochrome Pr is form of photochrome synthesized in plant cells. Pr synthesized in leaves. Pr and Pfr in equilibrium during daylight. Pr -> Pfr since red light present in sunlight. Pr accumulates at night. No sunlight for Pr -> Pfr. Pfr breaks down faster. Cells continue to make Pr at night. Daybreak, light rapidly converts to accumulated Pr into Pfr. Equilibrium again. Phytochromes The Pr <-> Pfr interconversion acts as a switching mechanism that controls various events in the life of a plant. Phytochromes Red light - 660nm Wavelength of light that is most effective at interrupting the critical night length of a short day (long night) plant. Exposure at night will cause the plant NOT to flower HOWEVER, if this light briefly interrupts the night of a long day (short night) plant, the plant will flower The red flash will shorten the plants perception of night length Phytochromes The shortening of night length can be negated by providing a flash of light at 730nm wavelength. This is called the far-red part of the spectrum Phytochrome Night length is responsible for resetting the circadian rhythm clock If daylight is interrupted with dark there is no effect If dark is interrupted with flashes of red or far-red the clock can be affected Red-light shortens night length Because it converts Pr to Pfr – which would not normally accumulate at night Far-red light restores – as though night was not broken Because far red light flashes convert Pfr back to Pr Phytochrome Plants synthesize phytochrome as Pr If left in the dark, nothing happens to this pigment If the pigment is illuminated with sunlight, Pr changes to Pfr Thus the plant can detect the presence of sunlight Phytochrome Pr = Pfr during daylight If shade of larger trees were to block sunlight from a smaller tree, the radiation most blocked by canopy is red (not far red) Pigments in the plant would be converted to Pr This cue would stimulate the plant to grow taller. Phytochrome If ample sunlight were available, the reverse would happen – Pfr proportions would increase and the plant would “sense” that it was in sun. It would be cued to branch and vertical growth would be inhibited Animated Videos http://bcs.whfreeman.com/thelifewire9e/ default.asp#542578__591513__ http://bcs.whfreeman.com/thelifewire9e/ default.asp#542578__591514__ http://bcs.whfreeman.com/thelifewire9e/ default.asp#542578__591515__
