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Download CHAPTER 39-1 NOTES CONTROL SYSTEMS IN PLANTS
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26-2: Plant Responses Tropism = growth responses that result in curvatures of whole plant organs toward or away from stimuli 1) Phototropism = growth toward or away from light Sensitive to blue light o Blue light is absorbed by a pigment called phototropin, which changes shape o Causes the transfer of a phosphate from ATP to a protein portion of the photoreceptor o Phosphorylated photoreceptor triggers a transduction pathway 2) Gravitropism = growth toward or away from gravity’s pull Statoliths = starch-containing plastids that help “sense” gravity by falling to bottom side of cells o In Shoots: Negative (curve upwards away from it) Auxin migrates to lower side of coleoptiles/shoot stimulates growth Ca+2 ions accumulate along upper side inhibits growth When the shoot becomes vertical, differences in concentration disappear and it grows straight up In Roots: Positive (curve down toward it) o Grow down because auxin and Ca+2 have opposite effect on root cells 3) Thigmotropism = growth toward or away from touch Ex. Coiling of tendrils Thigmomorphogenesis = entire plant responds to presence of an environmental stimulus o Ex. Two trees that grow close to each other CONTROL OF DAILY AND SEASONAL RESPONSES Circadian Rhythm = a physiological cycle with a frequency of about 24 hours o Persists even when an organism is sheltered from environmental cues o Is endogenous and set to a 24-hour period by daily signals from the environment o May take days to reset once cues change Biological Clocks = internal clocks by which a circadian rhythm is maintained in the absence of appropriate environmental stimuli Set by the photoperiod = length of daylight compared to length of darkness o Temperature has little to no effect Ex. Spring blooming of trees PHYTOCHROMES Phytochromes = a blue-green leaf pigment responsible for a plant’s response to photoperiod Red light (in daylight) activates phytochrome and Pr Pfr Pfr moves into the nucleus and activates transcription factors, trigger plant functions to increase (“on”) In the dark, Pfr Pr, and pathways shut down (“off”) 1) Control of Flowering o Short-Day Plants = require a light period shorter than a critical length o Usually flower in late summer, fall, and winter o Long-Day Plants = need a light period longer than a certain number of hours to flower o Flower in late spring and summer o Day-Neutral = unaffected by photoperiod o Flower when they reach a certain stage of maturity 2) Critical Night Length Not day length, but actually night length controls flowering! If daytime period is broken by a brief exposure to darkness, there is no effect on flowering. If nighttime period is interrupted by short exposure to light, plants will not flower! o Brief exposure to red light in short-day plants prevents flowering o Brief exposure to red light in long-day plants triggers flowering Therefore, short-day plants flower if night is longer than a critical length, and long-day plants need a night shorter than a critical length. PLANT RESPONSES TO BIOTIC ENVIRONMENT Physical and Chemical Defenses Secondary Metabolites = were originally thought to be waste products, but are actually part of the plant’s defenses against predation. Tannins = coat epidermal cells destroy the outer proteins of bacteria and fungi AND effect herbivores by irritating the mouth and digestive systems Alkaloids (morphine, nicotine, caffeine) in high doses kill insects and fungi by blocking RNA and DNA synthesis Cyanogenic Glycosides break down into cyanide and inhibit cellular respiration Wound Responses After being chewed/injured, plants make Proteinase Inhibitors = chemicals that destroy the digestive enzymes of a predator feeding on them Made throughout the plant, not just at the wound site Caused by systemin (a hormone) in response to predator’s saliva Travels between cells to reach phloem sends it throughout the plant Activates a transduction pathway in cells with systemin receptors these cells then produce proteinase inhibitors Often salicylic acid triggers SAR (Systemic Acquired Resistance) = production of antiherbivore chemicals by defense genes o May excrete these signals for nearby plants as a warning PLANT DEFENSE AGAINST PATHOGENS Depends on Gene-for-Gener Recognition = a precise match-up occurs between an allele in the plant and an allele in the pathogen If resistant, one of the plant’s R genes corresponds to a dominant Avr (avirulent) allele in the pathogen Hypersensitive Response (HR) = when a plant produces a protein that binds to a specific viral, bacterial, or fungal gene product Carries mRNA that is complementary to certain viral mRNA binds it up Allows it to recognize a particular pathogen Triggers the signal transduction pathway that seals off the infected area and initiates wound response Other Responses to Stress 1) Herbivory Physiological defenses include thorns, spines, and distasteful/toxic compounds Some even recruit predatory animals to help defend them. o Ex. Acacia tree 2) Water Stress = when water levels drop too low Stomata close (mesophyll cells release ABA, which helps keep them closed) Growth of new leaves is inhibited Leaves wilt (due to loss of turgor), reducing surface area, slowing transpiration. Roots reduce growth 3) Heat Stress Transpiration provides evaporative cooling (can lower temp. 3-10oC!), but has to prevent too much water loss. Heat-Shock Proteins = help enzymes and other proteins maintain their conformations, preventing them from denaturing PLANT REPRODUCTION BASICS Pollination = transfer of pollen from anther (male part) to stigma (female part) via wind, water, gravity, or animal pollinator Coevolution has occurred between many pollinators and the plants they help Flowering plants use Double Fertilization to create seeds Seed Germination Water and light availability are key in determining when a seed will germinate Often requires a dormancy (cold) period first Wearing away of seed coat and uptake of water break dormancy and trigger germination Gibberellins begin breaking down food (endosperm) for seedling to build new parts Eudicots and monocots form cotyledons differently.