ETHYLENE AND FRUIT RIPENING

The latest advances in our understanding of the relationship between e thylene and fruit ripening are reviewed. Considerable progress has bee n made in the characterisation of genes encoding the key ethylene bios ynthetic enzymes, ACC synthase (ACS) and ACC oxidase (AGO) and in the isolation of genes involved in the ethylene signal transduction pathwa y, particularly those encoding ethylene receptors (ETR). These have al lowed the generation of transgenic fruit with reduced ethylene product ion and the identification of the Ni tomato ripening mutant as an ethy lene receptor mutant. Through these tools, a clearer picture of the ro le of ethylene in fruit ripening is now emerging. In climacteric fruit , the transition to autocatalytic ethylene production appears to resul t from a series of events where developmentally regulated ACO and ACS gene expression initiates a rise in ethylene production, setting in mo tion the activation of autocatalytic ethylene production. Differential expression of ACS and ACO gene family members is probably involved in such a transition. Finally, we discuss evidence suggesting that the N R ethylene perception and transduction pathway is specific to a define d set of genes expressed in ripening climacteric fruit and that a dist inct ETR pathway regulates other ethylene-regulated genes in both imma ture and ripening climacteric fruit as well as in non-climacteric frui t. The emerging picture is one where both ethylene-dependent and - ind ependent pathways coexist in both climacteric and non-climacteric frui ts. Further work is needed in order to dissect the molecular events in volved in individual ripening processes and to understand the regulati on of the expression of both ethylene-dependent and - independent gene s.