Lecture14 - MCDB321 Plant Physiology MCDB321 Mar. 08, 2011...

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Unformatted text preview: MCDB321 Plant Physiology MCDB321 Mar. 08, 2011 Lecture 13 Phytochrome & Lecture Light-Regulated Plant Development Light-Regulated 1. 2. 3. Light-mediated plant responses Phytochromes and their photoreversibility Phytochrome-mediated fluence responses VFLR, LFR, &HIR 4. Ecological functions Seed germination shade-aviodance response PhyA-PhyB interaction Light acts as a signal to induce photomorphogenesis Light 1. Inhibition of stem elongation 2. Expansion of cotyledons and development of true leaves 3. Induction of photosynthetic genes 3. Several light-induced responses of the mustard seedling (Sinapis alba) [All these photomorphisms can be traced back to the active form of phytochrome ] 1. Inhibition of the elongation of the hypocotyl 2. Inhibition of translocation from the cotyledons 3. Increase of the surface area of the cotyledons 4. Unfolding of the cotyledons’ lamina 5. Development of hairs at the hypocotyl 6. Opening of the hypocotyl’s hook 7. Development of the primary leaves 8. Development of mature leaf primordia 9. Increase in the negative geotropic reaction of the hypocotyl 10. Development of xylem elements 11. Differentiation of the stomata within the epidermis of the cotyledons 12. Development of super-etioplasts in the cotyledons’ mesophyll 13. Changes in the intensity of the cell respiration 14. Synthesis of anthocyanin in the cotyledons and the hypocotyl 15. Increase in the synthesis of carotenoids 16. Increase in the capacity of the chlorophyll synthesis 17. Increase in the RNA synthesis within cotyledons 18. Increase in the protein synthesis within cotyledons 19. Intensification of the storage fat breakdown 20. Intensification of the storage protein breakdown 21. Increase in the synthesis of ethylene 22. Acceleration of the Shibata-shift within the cotyledons 23. Determination of the cotyledons’ capacity to photophosphorylate 24. Modulation of the cotyledons’ enzyme synthesis Lettuce seed germination is a typical photoreversible response photoreversible controlled by phytochrome Phytochrome can interconvert between Pr and Pfr forms Phytochrome In dark-grown or etiolated plants, phytochrome is present in a red light-absorbing form [Pr] (blue to In light-absorbing human eye) and can be converted by red-light to a far red light-absorbing form called Pfr (blue human far green to human eye) Photostationary state Under Cont. R, Pfr/Pr = 85%:15% Under Under Cont. FR, Pfr/Pr =3%:97% 1. Native phytochrome is a dimer composed of two identical subunits (125 KDa/each) dimer 2. Each subunit consists of two components: a light-absorbing pigment (chromophore) and a polypeptide chain (apoprotein) and 3. Phytochromobilin is synthesized in plastids Phytochromobilin *mutants that can not synthesize the chromophore are defective in processes that require the action of phytochrome that 4. Light absorption induces the conformation changes in both chromophore and the apoprotein with the chromophore as the photon-receptor. Two types of phytochromes have been identified [biochemically] Type I-main PHY in dark-grown seedlings, may be light quantity sensor; Type II-main PHY in light-grown seedlings, may be the light quality sensors Five PHY genes are known in Arabidopsis PHYA, B, C, D, E PHY A is a Type I phytochrome It accumulates to high levels in dark-grown seedlings when it receives RED light and is converted to Pfr which induces de-etiolation, but then it is rapidly degraded [PHYA]Pfr also inhibits its own expression, so levels of PHYA are much lower in seedlings once they receive light Phytochrome A itself is regulated by light Phytochrome @ several different levels 1. Gene expression (transcriptional regulation) Gene 2. mRNA stability 2. 3. Protein degradation In Arabidopsis, PHY B, C, D, and E are Type II phytochromes 1. The Pfr forms of these are stable in the light 2. PhyB is probably the photoreceptor involved in shade detection and avoidance. This response allows many species to greatly increase their stem extension rate when they become shaded by competitors. 3. PhyB also is considered responsible for daylength detection in flowering and for tuberization in the potato. Phytochrome responses vary in how they respond to fluence and irradiance Fluence is the amount of photons absorbed per unit of surface area. Irradiance is the amount of photons per unit surface area per time (fluence rate). Very-low-fluence Responses (VLFR) are Very-low-fluence Nonphotoconversible Nonphotoconversible Can be initiated by fluences of as low as 0.0001 umol/m2 (one-tenth a firefly flash) Saturated at ~0.05 umol/m2 Coleoptile growth of dark-grown oat seedlings Germination of Arabidopsis seeds Non-FR reversible because amount of Pfr formed by FR saturates VLFR. [The mount of light needed to induce VLFR converts <0.02% of the total PHY to Pfr] The VLFR action spectrum matches the absorption spectrum of Pr The action suggesting that Pfr is the active form for eliciting the VLFR. suggesting LFR: initiated at 1.0 umol/m2, saturated at ~1000 umol/m2 LFR: Obey the law of reciprocity Obey the The total fluence is a function of the fluence rate The the (the amount of photons absorbed per unit of surface area per unit of time) and the irradiation time the Both VLFR and LFR can be induced by brief pulses of light, provided that the total amount of light energy adds up to the required fluence. High Irradiance Response High They are proportional to irradiance (brightness) rather than to fluence They irradiance fluence Saturate at a much higher fluences than LFRs (~100 fold higher) Not photoconversible Do not obey the law Require prolonged or continuous exposure to light Require prolonged The same effect can be either an LFR or an HIR, depending on its history of exposure to light depending The HIR action spectrum of ETIOLATED seedlings has peaks in the Far-Red, Blue, and UV-A regions in why far-red peak? why additional receptors? The HIR action spectrum of green plants has a major Red peak The 1. The response to continuous far-red light declines rapidly as the seedlings light begin to green begin 2. The loss of responsiveness to continuous far-red light strongly correlates with the depletion of the light-labile pool the of type I phytochrome, which consists of mostly of PHYA mostly 3. HIR of etiolated seedlings: PHYA HIR of light-grown seedlings: PHYB HIR Phytochromes & Phytochromes light quality The R/FR ratio and seed germination The A low R/FR ratio --> a low Pfr/Ptotal ratio A light requirement is often observed for small seeds of herbaceous plants light and grassland species and Far-red light imparted by a leaf canopy inhibits seed germination Red light stimulates seed germination To ensure that seedlings are photosynthetically self-sustaining after germination Decreasing the F:FR ratio Decreasing Causes elongation in sun plants A low R/FR ratio= a low Pfr/Pr ratio Pfr is the active form that inhibits stem elongation Less Pfr means stem elongation Mutually Antagonistic Roles of PHYA and PHYB Mutually 85% in Pfr Form 3% in Pfr form 3% Leads to complete degradation of PHYA The Effects of PHYA and PHYB on seedling development The In sunlight vs. canopy shade PhyA=typeI (unstable) PhyB=typeII (stable) In open sunlight, de-etiolation is mainly mediated by the PHYB system A seedling emerging under canopy (FR), initiates de-etiolation primarily through PHYA Because PHYA is labile, the response is taken over by PHYB In switching over to PHYB, the stem is released from growth inhibition, allowing for the accelerated rate of stem elongation (shade-avoidance response) Modern maize varieties are planted at high density ...
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This note was uploaded on 11/30/2011 for the course BIOLOGY 321 taught by Professor Min during the Winter '11 term at University of Michigan.

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