Regulation of Ethylene Biosynthesis in Avocado Fruit during Ripening

Abstract

Preclimacteric avocado (Persea americana Mill.) fruits produced very little ethylene and had only a trace amount of l-aminocyclopropane-lcarboxylic acid (ACC) and a very low activity of ACC synthase. In contrast, a significant amount of l-(malonylamino)cyclopropane-l-carboxylic acid (MACC) was detected during the preclimacteric stage. In harvested fruits, both ACC synthase activity and the level of ACC increased markedly during the climacteric rise reaching a peak shortly before the climacteric peak. The level of MACC also increased at the climacteric stage. Cycloheximide and cordycepin inhibited the synthesis of ACC synthase in discs excised from preclimacteric fruits. A low but measurable ethylene forming enzyme (EFE) activity was detected during the preclimacteric stage. During ripening, EFE activity increased only at the beginning of the climacteric rise. ACC synthase and EFE activities and the ACC level declined rapidly after the climacteric peak. Application of ACC to attached or detached fruits resulted in increased ethylene production and ripening of the fruits. Exogenous ethylene stimulated EFE activity in intact fruits prior to the increase in ethylene production. The data suggest that conversion of S-adenosylmethionine to ACC is the major factor limiting ethylene production during the preclimacteric stage. ACC synthase is first synthesized during ripening and this leads to the production of ethylene which in turn induces an additional increase in ACC synthase activity. Only when ethylene reaches a certain level does it induce increased EFE activity.

The Effect of Ethylene and Propylene Pulses on Respiration, Ripening Advancement, Ethylene-Forming Enzyme, and 1-Aminocyclopropane-1-carboxylic Acid Synthase Activity in Avocado Fruit

“…ACC synthase, however, remains at low control levels throughout ethylene treatment, rising only when the fruit manifests autocatalytic ethylene production at the climacteric rise. Thus the induction of ACC synthase is correlated with climacteric ethylene production, confirming earlier observations that the conversion of Sadenosylmethionine to ACC is the rate limiting step in ethylene biosynthesis in ripening avocado fruit (15,22). The failure of avocado fruit to ripen on the tree has been attributed to an inhibitory tree factor (24,27).…”

“…ACC3 synthase and EFE are the key enzymes in the ethylene biosynthetic pathway (27), with ACC synthase activity the apparent rate limiting step (22). Whereas EFE activity has been shown to be induced by an ethylene pulse (22), in normal ripening the activity of both enzymes remains at relatively low levels until the onset of climacteric ethylene production (15). Because both pulsing and wounding (i.e.…”

The fading distinctions between classical patterns of ripening in climacteric and non-climacteric fruit and the ubiquity of ethylene—An overview

“…However, it does not seem to play a regulatory role in ethylene production at the climacteric stage. At this stage, MACC increases only slightly compared to the marked increase in the level of ACC (Sitrit et al 1986). These results suggest that the inability of most avocado cultivars to produce ethylene as long as they remain attached to the tree is mainly due to repression of ACS.…”

“…It has been suggested that some unknown ripening inhibitor moves from the leaves or shoots to the fruit where it inhibits ethylene production and hence ripening (Burg and Burg 1964;Tingwa and Young 1975). Sitrit et al (1986) reported that inability of preclimacteric avocado fruit, to produce ethylene was due to lack of ACC as there was low activity of ACC-synthase (ACS) enzyme [catalyses the conversion o f S -a d e n o s y l m e t h i o n i n e ( S A M ) i n t o 1 -aminocyclopropane-1-carboxylic acid (ACC) during biosynthesis of ethylene]. It is also well accepted that malonylation of ACC may play a role in regulation of endogenous levels of ACC and hence ethylene production (Yang and Hoffman 1984).…”

“…It is also well accepted that malonylation of ACC may play a role in regulation of endogenous levels of ACC and hence ethylene production (Yang and Hoffman 1984). In fact, preclimacteric avocado fruits were reported to contain significant levels of malonylated-ACC (MACC) (Sitrit et al 1986). This indicates that at least a part of the ACC (which is synthesized in the course of fruit growth) is conjugated to form MACC.…”