10. Sorting of peoxysomal, plastidal and mitochondrial proteins Blue light responses 1. Phototropism 2. Inhibition of hypocotyl elongation 3. Stomatal opening 4. Chlorophyll and carotenoid accumulation 5. Phototaxis 6. Enhancement of respiration Fig. 18-1 Action spectrum for blue light-stimulated phototropism in oat coleoptiles. The photophysiology of Blue-light responses Blue light stimulates asymmetric growth and bending- photopropism Fig.18-2 Relationship btwn direction of growth and unequal incident light. Blue light Fig.18-3 Time-lapse photograph of a corn coleoptile growing toward unilateral blue light given from the right. Dark-grown, etiolated coleoptiles continue to elongate at high rates for several days. The coleoptile stop growing as soon as the stoot has emerges from the soil. Fig. 18.4 Phototropism in wild type (A) and mutant (B) Arabidopsis seedlings. How do plants sense the direction of the light signal? Fig.18-5 Distribution of transmitted, 450 nm blue light in an etiolated corn coleotile. Blue light rapidly inhibits stem elongation -The membrane depolarization of hypocotyl cells is caused by anion channels - Use of anion channel blocker prevent the blue light-dependent membrane depolarization decrease the inhibitory effect of blue light on elongation Fig.18-6 Blue light induced (A) changes in elongation rates of etiolated cucumber seedling and transient membrane depolarization of hypocotyl cells Blue light regulates gene expression - SIG5 in arabidopsis (role in the trascription of chloroplast gene) is specifically activated by blue light. -- The GSA gene is activated by blue light. in photosynthetic unicellular alga Chlamydomonas reinhard Blue light stimulates stomatal opening Several characteristics of blue light-dependent stomatal movements 1. The stomatal response to blue light is rapid and reversible, and it is localized in a single cell type, the guard cells. 2. The stomatal response to blue light regulates stomatal movement throughout the life of plant. This is unlike phototropism or hypocotyl elongation, which are functionally important at early stages of development. 3. The signal transduction cascade that links the perception of blue light with the opening of stomata is understood in considerable detail. Opened Closed Fig. 18.8 Light-stimulated stomatal opening in detached epidermis of Vicia faba Stomatal movements & Solar radiation Blue light response photosynthesis Fig 18-10 The response of stomata to blue light under a red-light background Fig 18.11 The action spectrum for blue light-stimulated stomatal opening (under a red light background) Fig 18-12. Blue light stimulated swelling of guard cell protoplasts. Vanadate: an inhibitor of the H+-ATPase. Blue light activates a proton pump at the guard cell plasma membrane The blue light induced acidification is blocked by inhibitors - CCCP that dissipate pH gradients - vanadate; inhibitor of H+-ATPase Blue light responses have characteristic kinetics and lag times Figure 18.13 Osmoregulatory pathways (A) Potassium and its counterions Figure 18.13 Osmoregulatory pathways; (B) Sucrose accumulation from starch hydrolysis Figure 18.13 Osmoregulatory pathways; (C) Sucrose accumulation from photosynthetic carbon fixation Sucrose is an osmotically active solute in guard cells Four distinct metabolic pathways that can supply osmotically active solutes to guard cells 1. Uptake of K+ and Cl- coupled to the biosynthesis of malate2. The production of sucrose from starch hydrolysis 3. The production of sucrose by photosynthetic carbon fixation in the guard cell chloroplast 4. The uptake of apoplastic sucrose generated by mesophyll photosynthesis Blue light Photoreceptors 3 photoreceptors associated with blue light responses 1. Cryptochrome 2. Phototropins 3. Zeaxanthin Cryptochrome are involved in the inhibition of stem elongation hy4 mutant: lacks the blue light-stimulated inhibition of hypocotyl elongation - The Hy4 protein, later renamed Cryptochrome1 (Cry1), was proposed to be a blue light mediating the inhibition of stem elongation. - Phytolases are pigment proteins that contain a flavin adenine dinucleotide (FAD) and a pterin. - Pterin are light absorbing , pteridine derivatives that often function as pigments in insects, fishes, and birds. Phototropism and stomatal movements appear to be normal in cry1 mutant Overexpression of CRY1 protein in transgenic plants (Fig. 18.17) - stronger light-stimulated inhibition of hypocotyl elongation. - increased accumulation of anthocyanin. CRY2 - CRY1 and CRY2 appear ubiquitous throughout the plant kingdom. - CRY2 is rapidly degraded in the light, whereas CRY1 is stable in the light - OX of CRY2 show a small enhancement of inhibition of HY elongation - OX of CRY2 show a large increase in the cotyledon expansion - CRY1 and CRY2 play a role in the induction of flowering. Phototropins are involved in phototropism and chloroplast movements The N-terminal half of Phototropin binds FMN (flavin mononucleotide) The C-terminal half of Phototropin is a Ser/The protein kinase Blue light activated chloroplast movement The carotenoid zeaxanthin mediates blue-light photoreception in guard cells (A) Zeaxanthine contents - Different in guard & mesophyll cell - Closely follows solar radiation (B) Stomatal aperture Fig.18.19. The absorption spectrum of zeaxanthin in ethanol. Fig.19.20. Stomatal responses to blue light in the wild type and npq1. Compelling evidence that zeaxanthine is a blue-light photoreceptor in guard cell - The absorption spectrum of zeaxanthin closely matches the action spectrum for blue light-stimulated opening. - In daily courses of stomatal opening in intact leaves, incident radiation, zeaxanthin content of guard cells, and stomatal apertures are closely related. - The blue-light sensitivity of guard cells increases as a function of their zeaxanthin concentration. - Blue light-stimulated stomatal opening is completely inhibited by DTT. (DTT inhibits the enzyme that violaxanthin into zeaxanthin.) Role of zeaxanthin in blue light sensing in guard cells The blue light response of the Arabidopsis mutant npq1 In npq1 mutant, no blue light response. Blue light response photosynthesis Fig 18-10 The response of stomata to blue light under a red-light background The phot1 and phot2 mutant lacks blue light-stimulated stomata opening Signal transduction Blue light receptors 1. Phototropism 2. Inhibition of hypocotyl elongation 3. Stomatal opening Cryptochromes accumulate in the nucleus (Cry2 protein, less Cry1 protein) Proteins might be involved in the regulation of gene expression * Cry1 and Cry2 interact with PhyA in vivo, and to be -P by PhyA in vitro. Phototropin (Phot1 and -2) binds FMN Phototropin Protein bound FMN conforamtional change of protein trigger auto-P Start the sensory transduction cascade Sensory transduction cascade of blue light-stimulated inhibition of stem elongation Zeaxanthin isomerization might start a cascade mediating blue light-stimulated stomatal opening Blue light P of C-terminal domain of H+-ATPases 14-3-3 protein binds to P-Ser/ Th r Acivate H+-ATPases Drive ion uptake Increase turgor pressure Stomatal opening The reversal of blue light-stimulated opening by green light It could be related to the sensing of environmental condition such as sun and shade The xanthophyll cycle confers plasticity to the stomatal responses to light 1. Acidification of lumen pH stimulates zeaxanthin formation and luman alkalinization favors violaxanthin formation 2. Lumen pH dependent on levels of incident photosynthetic active radiation Photosynthetic activity, lumen pH, zeaxanthin content, blue light sensitivity, stomatal operture are tightly coupled. The blue light response of the Arabidopsis mutnat npq1 Thanks for your attention.
