Week of April 3 PLS 360

Week of April 3 PLS 360 - Photoreceptors Light responses...

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Photoreceptors Light responses Light can be a signal, not only an energy source Not all light is equal- wavelength is important Plants respond to red/far red, blue/green, and UV-B light Figure 17.1 Red light response Red light responses are mediated by Phytochrome Phytochrome is a holoprotein- an apoprotein fused with a chromophore The chromophore is phytochromobilin Phytochrome structure Apoproteins are made by PHY genes, mostly PHYA and PHYB (also C, D, E) Phyotochromes made with PHYA and PHYB apoproteins have different light responses 2 holoproteins make a dimer, the functional unit Absorbing light changes the chromophore and alters the holoprotein structure Figure 17.7 Photoreversibility Some red light responses can be reversed by far-red light called Photoreversibility Figure 17.2 Photoreversibility occurs because Phytochrome shifts between two forms, P R and P FR P R binds red light and becomes P FR P FR binds far red light and becomes P R Both can mediate responses to light, but usually P FR is the “active” form Absorption of Light cis/trans isomerization of C and D rings *CQ: If you treat a seedling with red light, what form is most of the Phytochrome in? P FR P FR and P R have overlapping absorption spectrum, causing an equilibrium between the two under any light condition Red light = 88% P FR (730 nm) Far red light= 98% P R (668 nm) Highly sensitive responses are not photoreversible because some P FR remains Phytochrome measuring light Phytochrome responses are based on irradiance and fluence Irradiance: the amount of light absorbed per unit time Fluence: the total amount of light absorbed (irradiance x time) Responses are Very Low Fluence (VLFR), Low Fluence (LFR), or High Irradiance (HIR) VLFRs are so sensitive they are not photoreversible LFRs are triggered by short, bright illumination or by long, dim illumination = law of reciprocity HIRs require continuous illumination (continuous flashes of illumination)
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Timing Light responses “escape” from Photoreversibility when the signal has been passed to the next signaling step Escape time varies based on the signaling mechanism Slow gene expression changes Phosphorylation events Ion fluxes Fast Phytochrome responses The red/far red ratio, irradiance, and duration gives the plant important information about its surroundings Figure 17.3, Table 17.3 Plants have Phytochrome responses throughout their life: o Generation o Photomorphogenesis o Shade avoidance (stem elongation) o Earlier flowering in shade o Setting the circadian block Germination Figure 17.2 Photomorphogenesis Loss of apical hock, suppression of hypocotyl elongation, production of chlorophyll, expression of cotyledons Vs. skotomorphogenesis, aka etiolation Figure 17.1 Shade avoidance
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