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Lecture22 - CDB321 Plant Physiology CDB321 pr 7 2011 ecture...

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Unformatted text preview: CDB321 Plant Physiology CDB321 pr. 7, 2011 ecture 22 thylene, a gaseous hormone . History of ethylene discovery . Ethylene biosynthesis in plants Two key discoveries . Physiological responses of ethylene Fruit ripening Leaf abscission Leaf senescence senescence . Ethylene receptors and signaling triple response as a way to screen ethylene signaling mutant ETR1 as an ethylene receptor the ETR1-CTR1 signaling History of Discovery History 1. 1864, leaks of gas from street lights induced defoliation of trees in the vicinity of streetlamps. 2. 1901, Neljubow discovered ethylene caused a “triple-response” in etiolated pea seedlings 3. 1910, Cousins provided the 1st report that suggested that ethylene was a natural plant product 3. 4. 1934, Gane chemically identified ethylene as a natural product of plant metabolism 4. 5. 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening 5. as well as inhibition of vegetative tissues 6. 1959, the hormonal role of ethylene was rediscovered thanks to Gas Chromatography Reduced stem elongation Increased lateral growth (swelling) Abnormal horizontal growth Ethylene Biosynthesis-General: 1. made by most plants including angiosperms, gymnosperms, ferns, mosses. 2. also synthesized by fungi and bacteria 3. made by all parts of the plant 4. meristematic regions (shoot apex) and senescing tissues are rich sources 5. nodes make more ethylene than internodes 6. ethylene production is stimulated by fruit ripening and senescence and lots of physiological stresses including wounding, anaerobic conditions, flooding, chilling, disease and drought. Ethylene is biologically active at 1 nL/L [Ethylene]i in a ripe apple is ~ 2500 nL/L Two key discoveries Two 1. 1964. Methionine is the precursor of ethylene Methionine 14 14C-Met---> 14C-ethylene and C-Met---> 14C-ethylene the ethylene is derived from carbon 3 and 4 of Methionine carbon 2. 1979, ACC is the immediate precursor of ethylene 2. ACC 1-amino-cyclopropane-1-carboxylic acid A. [14C]-Met -->[14C]-ACC under anaerobic conditions (ethylene is not produced) A. C]-ACC anaerobic B. ACC application leads to overproduction of ethylene B. The synthesis of ACC is the limiting step in the ethylene biosynthetic pathway The The mechanism by which ethylene is produced from methionine is a 3 step process 1. ATP is an essential component in the synthesis of ethylene from methionine. 2. ACC-synthase facilitates the production of ACC from SAM. 3. Oxygen is then needed in order to oxidize ACC and produce ethylene. This reaction is catalyzed by ACC oxidase The recycling of the CH3-S- group Changes in ethylene and ACC content and ACC oxidase activity Changes during fruit ripening during fruit Application of ACC to unripe fruits only slightly enhances slightly ethylene production ethylene ACC oxidase is the ratelimiting step in ripening Developmental and Physiological Effects of Ethylene Developmental 1. Promote fruit ripening in some fruits 1. fruit 2. Leaf epinasty results when ACC from the roots is transported to shoots 2. Leaf 3. Ethylene induces triple response 3. triple Lateral cell expansion and hook formation Lateral 4. Ethylene enhances the rate of leaf senescence 4. leaf (antagonistic to cytokinin) (antagonistic 5. Ethylene/auxin interaction in regulating leaf abscission 5. leaf 6. Initiates germination in grains 6. 7. Activates dormant buds (potatoes in storage) 8. Stem elongation in deep-water rice 9. Induces Flowering in Pineapple 10.Promotes Female Expression in Flowers Fruit ripening Fruit 1. Softening due to enzymatic breakdown of the cell wall 2. Starch hydrolysis and sugar accumulation (sweetening) 3. Disappearance of organic acid and phenolic compounds (no sour tasting) Why juicy? Why Climacteric goes on . . . Signal From Seed Ethylene is produced Ethylene stimulates Enzyme Production Ethylene itself stimulates more ethylene production Not every fruit respond to ethylene 1. Climacteric: (apple, banana, tomato, kiwi…) can be picked from the tree at full size or maturity but before it is 'ripe' and allowed to ripen off the tree All fruits that ripen in response to ethylene exhibit All a characteristic respiratory rise before the ripening phase phase 2. Non-climacteric: (cherry, grape, strawberry, pineapple…) tend to maintain what ever quality they had at harvest do not exhibit the ethylene and respiratory rise Fruit, Softens, Sweetens, etc. Evidence for the importance of ethylene in fruit ripening Evidence 1. Inhibitor of ethylene biosynthesis or signaling can delay fruit ripening 2. Transgenic plants using antisense technique to block ethylene synthesis 2. antisense never ripe until application of ethylene never 3. The tomato never-ripe mutant is defective in a ethylene receptor 3. The Abscission Layer Leaf Petiole Bud Vascular System Layer of cells that actively degrades between-cell layers, leading to tissue separation Stem Cytokinin is also involved, why? 1. A distal-proximal auxin gradient acts as a suppressor of the ethylene effect suppressor 2. Leaf initiates its senescence program reduction or reversal in the auxin gradient reduction abscission zone becomes sensitive to ethylene abscission 3. Cells respond to ethylene synthesize and secret cellulase and other synthesize wall-degrading enzyme wall-degrading resulting in leaf shedding resulting Ethylene promotes senescence Ethylene 1. Exogenous application of ethylene or ACC accelerates leaf senescence 2. Enhanced ethylene production is often associated 2. with chlorophyll loss and color fading with 3. Inhibitor of ethylene synthesis (Co2+ ) or action (Ag+) delay leaf senescence delay 4,.Ethylene-insensitive mutants retain chlorophylls for a longer period 4,.Ethylene-insensitive compared with the wild-type plants compared 5. Antisense-ACC oxidase or ACC synthase 5. ----> delay leaf senescence ----> Ethylene stimulates triple response in Ethylene Arabidopsis Inhibition and swelling of hypocotyl Inhibit of root elongation Exaggeration of the apical hook 1. Prevents cell elongation but promotes 1. lateral cell expansion lateral 2. Apical hook A. Ethylene induces asymmetric growth A. more rapid elongation of the outer side of the more stem compared with the inner side stem B. Auxin is involved in apical hook formation B. inhibitor of auxin transport can block apical hook formation apical Two different genetic screen Two for ethylene signaling mutants 1. ethylene-resistant mutant (ETR) 2. constitutive triple response (CTR) 2. Ethylene Receptors are Negative Regulators of the Ethylene Signaling Process the Different mutations have different effect Different on the downstream signaling events of the Ethylene signaling pathway ETR1 activates CTR1 A crucial negative regulator of the ethylene signaling pathway ethylene ...
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