Senescence, ethylene action

Ethylene as a stimulator of growth and development. The most observed actions of ethylene on growing plants involves growth inhibition, or acceleration of senescence. These actions are especially evident in the antagonism or opposition of ethylene to auxins, gibberellins and cytokinins (27), as already outlined above. Actually ethylene stimulates growth in many types of cells, especially in water plants (Table II). When ethylene acts to stimulate cell elongation, as in water plants, auxins and CC>2 enhance the ethylene effect (38,39). This interaction is the reverse of that observed on land plants wherein ethylene opposes the effects of auxin, GA3 and cytokinins. 

The influence of Ag+ and CO on ethylene action was observed in senescing tobacco leaf disks in which loss of chlorophyll was taken as an index of senescence (Fig. 7). 

During drought stress, ABA regulates stomatal closure, whereas increased ethylene production has an inhibitory influence on ABA action. An inhibition of ethylene synthesis delays drought-associated chlorophyll loss, supporting the role of ethylene in drought-induced senescence. °°... 

Ethylene not only hastens the ripening of fruit but also tends to promote senescence in all parts of plants. Its signaling mechanisms are the best-known for any plant hormone.368 369 Tire synthesis and action of ethylene are discussed in Chapter 24, Section D,4. Ethylene is metabolized slowly in plants by oxidation to ethylene oxide. The latter is hydrolyzed to form ethylene glycol, which is metabolized further to C02 (Eq. 30-3). 

Carbon dioxide and Ag+ ions clearly suppressed senescence, as determined by chlorophyll loss. Aminoethoxy vinylglycine (AVG), the inhibitor of ethylene biosynthesis, also significantly suppressed senescence, as determined by preservation of chlorophyll in the leaf disks aging in the dark.Combinations of C0-, Ag ions and AVG were especially effective on preserving chlorophyll, presumably by suppressing both ethylene biosynthesis and action at the two receptor sites. After 6 days aging at 25° in the dark, the controls contained only 7% of the chlorophyll present at the start, whereas 84% of the chlorophyll was retained by the leaf disks treated with a combination of CO-, Ag and AVG. 

There also have been clear indications that interactions of ethylene with auxins, cytokinins, gibberellins and ABA are involved in both ethylene production and action. Generally the effects of ethylene tend to antagonize those of auxins, cytokinins and gibberellins, and tend to reinforce those of ABA, depending, however, on tissue systems involved. Reinforcement of ethylene by ABA and vice versa occurs more frequently in senescence. 

Cycloheximide (CHI), when applied directly to separation zones, will inhibit abscission of citrus fruit explants 24) as it does bean petiole explants (29). By application to citrus fruit surfaces, the rind is injured, evidenced as small pitted areas, and ethylene is produced in quantities that accelerate the abscission processes (41). Apparently, CHI applied to the fruit wall enhances the senescence processes, including ethylene production, thereby stimulating citrus fruit abscission via ethylene production and not by action directly on the tissues in the separation zone (41). Cycloheximide has also been used for harvest of olives (80, 103) and apples (104). 

PLD plays important roles in the response to wounding and other stresses through mediating the actions and production of stress hormones [140]. PLDa is specifically involved in the actions of abscisic acid and ethylene in plant senescence and in the control of water loss [140, 141]. Different stresses in plants lead to different expression patterns for PLD isozymes. 

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