Control of the Biosynthesis of Ethylene in Senescing Tissues

Abstract

Vegetative tissues usually produce very small amounts of ethylene (2). However, stress and wounding effects, as well as application of some growth regulators, especially auxins, can cause a remarkable increase in ethylene production (1, 10). The rate of ethylene production as positively correlated to the internal level of free IAA, has been demonstrated so far in mung bean hypocotyls (9) and in pea epicotyls (7). Indeed, in these two tissues, IAA-induced ethylene production continued as long as free IAA was present in the medium. On the other hand, in tobacco leaves, IAA-induced ethylene production lasted for several days (3), although IAA was removed from the medium after a 4h-pulse (unpublished results). Such a prolonged increase of the IAA-induced ethylene production occurred in tobacco leaves only when sucrose, or some other carbohydrates, were present in the incubation medium (3, 12). These observations, as well as others, led us to the hypothesis that the natural regulation of ethylene biosynthesis in the leaf is primarily dependent on the type of the endogenous IAA conjugates formed in the tissue and their susceptibility to hydrolytic enzymes. This enzyme-catalyzed hydrolysis of the IAA conjugates enables a frequent, renewed supply of free IAA (3–5), which, in turn, controls ethylene production in vegetative tissues (13). In the present paper we summarize our recent results (3,11,12) regarding the stimulatory effect of carbohydrates both on IAA metabolism and on ethylene biosynthesis.