Chapter 22

Discovered 19th 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 until1959 …..

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Triple Response of Pea seedlings1. Decrease in longitudinal growth2. Increase in radial expansion3. Horizontal orientation of epicotyl

Epicotyl

Grow

n in Dark

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NOT a class of molecule! -- H2C=CH2 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

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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

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Silver ions CO2 inhibits fruit ripening

Less efficient than silver HIGH concentrations – unlikely in nature

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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

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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

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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

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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

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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

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Spots on Two week old Bananas - Ripening Experiment

Spots where Ethylene is produced

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Leaf epinasty Auxin causes the production

of ethylene Waterlogged and/or

anaerobic root conditions ACC transported to the leaves

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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 O2

Loss of ethylene by diffusion is retarded by H2O Increases sensitivity to GA because of decrease of ABA

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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 …..

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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

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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

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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

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Leaf Maintenance – high auxins reduce ethylene sensitivity

Shedding Induction – reduction in auxin increases ethylene sensitivity

Shedding – enzymes that hydrolyze middle lamella in abscission zone

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STS inhibits ethylene

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Infection and disease – complex process Ethylene increases response to pathogen

attack Ethylene + Jasmonic acid required for

activation of several defense genes

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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.

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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.

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

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Small molecule -- H2C=CH2 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.

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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.

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