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Tropisms Growth responses in reaction to external stimuli. Auxin in Dark or Uniform Light Auxin concentration uniform and the coleoptile grows straight. Auxin with Strong Lateral Light Shady side - more auxin; Light side - less auxin. Shady Side elongates more and the light side elongates less. -> Coleoptile bends toward the light. Cholodny-Went Hypothesis Redistribution/migration hypothesis: light causes redistribution of auxin in coleoptile tip Photodestruction Hypothesis Light causes destruction or inactivation of auxin on illuminated side of coleoptile tip. (This hypothesis is false) Phototropically Active Wavelength Blue wavelengths produce the strongest phototrophic response; others produce weaker responses of varying intensity. What is the light receptor (pigment) involved in phototropism? Two Flavoproteins (PHOT1, PHOT2) are the probable photoreceptors of phototropism. They are membrane proteins, autophosphorylate in response to blue light, and are abundant in cells of coleoptile tips. Gravitropism Mechanism Involves differences in auxin concentrations. Upper side has low auxin and the lower side has high auxin. How do cells sense gravity? Root cap has columella cells in the middle which have larger amounts of starch grains. When the root grows normally the starch grains settle to the bottom. Tensegrity Hypothesis (Tension-Integrity) Looks at the tension inside the cell. Heavier things will exert pressures at the bottom of the cell on the cytoskeleton elements. Gravitropism in Roots: Cytokinin Cytokinin unequally distributed. lower side had high cytokinin (growth inhibited) and the upper side had low cytokinin (growth faster). Plant Statoliths Intracellular, plastid based, no neurons, no brain. Generic Statolith Fluid filled vesicle with hair cells inside that function as mechanoreceptors. Thigmotropism Growth response to touch or wind. Stimulated by mechanical forces, pressure or vibration. Circadian Rhythm "Clock" inside of plants telling them what time it is. Entrainment Resetting or adjusting the clock from environmental cues. Biological Clocks of Plants Interlocked loops of transcription feedback and protein turnover. Feedback mechanisms are small metabolites and signaling molecules. Photoperiodism Plant responses to light Long Day Plant Flower when photoperiod is long. Short Day Plant Flower when photoperiod is short. Controlling Photoperiodism Red and far-red act like on and off switches in controlling the flowers response Phytochrome The pigment (light receptor) that mediates photoperiodism and seed germination and detects red/far-red balance. Etiolation Growth response in shoots to dark or extremely low light. Nastic Movements Movement in response to external stimulus. Direction of movement is independent to direction of stimulus.