Chapter 22

 Discovered 19 th C.  trees around streetlamps senesced earlier …..  1901 – triple mutants in Russian lab  reduced stem elongation, increased lateral growth, abnormal horizontal growth  Commonality?  ethylene gas  Not taken seriously until 1959 ….. 2

Triple Response of Pea seedlings 1.Decrease in longitudinal growth 2.Increase in radial expansion 3.Horizontal orientation of epicotyl Epicotyl Grown in Dark 3

 NOT a class of molecule! -- H 2 C=CH 2  Produced by almost all parts of higher plants  Rate of production depends on tissue type and stage of development  Increases during leaf abscission, flower senescence, and fruit ripening.  Wounding can induce ethylene formation  Physiological stresses  flooding, disease, temperature, drought, infection  Circadian rhythms  peak at midday  Auxins can promote 4

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 Effects occur at transcriptional level via ACC synthase.  (ACC == 1-amino-cyclopropane-1-carboxylic acid)  In tomato … 10 different ACC synthase genes  Depends on auxin, wounding and/or fruit ripening 6

 Silver ions  CO 2  inhibits fruit ripening  Less efficient than silver  HIGH concentrations – unlikely in nature 7

 Ethylene activity can be controlled by  Oxidation to carbon dioxide  Conversion to ethylene oxide  Conversion to ethylene glycol  Big problem – gaseous  Simulate activity with ethephon  2-chloroethylphosphonic acid → decomposes to ethylene.  Known to affect virtually every aspect of growth and development 8

 Characteristic respiration rise before ripening  Ripening → softening due to breakdown of cell walls, starch hydrolysis, sugar accumulation and disappearance of acids and phenolics including tannins 9

 Climacteric fruits – have the climacteric response  Autocatylitic response to ethylene  Climacteric  Apple, avocado, banana, canteloupe, figs, mango, olive, peach, pear, persimmon, plum, tomato  Non-climacteric  Peppers, cherry, citrus, grapes, pineapple, beans, strawberries, watermelon 10

 In climacteric plants – two systems  System 1  vegetative tissue  ethylene inhibits its own biosynthesis  System 2  ripening fruits & senescing petals  ethylene stimulates production of ethylene 11

 Unripe climacteric fruits treated with ethylene  Climacteric response is hastened  Ethylene production increases  Unripe non-climacteric fruits treated with ethylene  Rise in respiration  No increase in ethylene production 12

Spots on Two week old Bananas - Ripening Experiment Spots where Ethylene is produced 13

 Leaf epinasty  Auxin causes the production of ethylene  Waterlogged and/or anaerobic root conditions  ACC transported to the leaves 14

 While usually inhibitor of elongation – promotes elongation of stem and petioles in submerged aquatics (including rice)  Treatment with ethylene mimics submergence  Ethylene synthesis diminished by lack of O 2  Loss of ethylene by diffusion is retarded by H 2 O  Increases sensitivity to GA because of decrease of ABA 15

 Induces lateral cell expansion by changing the orientation of the cellulose microfibrils in the cell wall.  Maintenance of the hypocotyl hook in dark grown seedlings  Break dormancy in certain seeds  Increase seed germination  Promote bud sprouting in potato  Part of the triple response ….. 16

 Induces adventitious formation of roots and root hairs  Primarily function of auxin  Ethylene-insensitive mutants auxin has no effect!  Negative regulator of root nodule formation  Positive regulator of root hair formation 17

 Regulates flowering  Flowering in pineapple and other bromeliads  Mango  Sex-determination in monoecious species  Change sex of developing flowers  Ethylene → female flowers in cucumbers  Gibberellins → male flowers in cucumbers 18

 Exogenous ethylene accelerates leaf senescence; exogenous cytokinin delays it  Appears to effect the rate of senescence rather than a senescence “switch”  Enhanced ethylene production associated with chlorophyll loss and color fading  Triggers breakdown of middle lamella 19

 Leaf Maintenance – high auxins reduce ethylene sensitivity  Shedding Induction – reduction in auxin increases ethylene sensitivity  Shedding – enzymes that hydrolyze middle lamella in abscission zone 20

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 STS inhibits ethylene 22

 Infection and disease – complex process  Ethylene increases response to pathogen attack  Ethylene + Jasmonic acid required for activation of several defense genes 23

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 Crown galls produce ethylene  Limiting and controlling factor -- reduces the diameter of vessels in the host stem adjacent to the tumor and enlarges the gall surface through which high transpiration occurs, thus giving priority in water supply to the growing tumor over the host shoot. 25

 Often produced in the presence of high concentration of auxin  Inhibitory effects of auxin appear to be auxin- induced ethylene …  Ethylene -- all plant tissues in varying conc. 26

 Senescence and Abscission two separate processes!  Senescence – developmental process  Abscission – shedding of dead/dying tissue 27

 Small molecule -- H 2 C=CH 2  NOT a class of molecules!  Not required for normal vegetative growth  Development of roots and shoots  Synthesized in response to stress  Large quantities in tissues senescing or ripening  Often produced in the presence of high concentration of auxin  Inhibitory effects of auxin appear to be auxin- induced ethylene …  Ethylene -- all plant tissues in varying conc. 28

 Ethylene synthesis is influenced by:  Auxin  Wounding  Water stress  Temperature  Inhibitors of RNA and protein synthesis  Ethylene autocatalysis  Effects occur at transcriptional level via ACC synthase. 29