Ethylene and developmentally-regulated processes in ripening climacteric fruit

Antisense ACC oxidase melons in which ethylene production was greatly suppressed have enabled the discrimination of ethylene-dependent and -independent ripening pathways in climacteric fruit. Yellowing of the rind, softening, climacteric respiration, aroma volatiles production, ABA and putrescine accumulation and peduncle abscision are clearly regulated by ethylene. In contrast, the accumulation of sugars, acids and spermidine, and the synthesis of carotenoids in the flesh are ethylene-independent processes. Although fruit softening was found to be ethylene-dependent overall, certain genes involved in the process are ethylene dependent while others, even different members of the same gene family (e.g. polygalacturonases) are ethylene-independent. Also, the induction of ACC biosynthesis in the early stages of ripening is a developmentally-regulated (ethylene-independent) process. Accumulation of putrescine and ABA was higher in ethylene-inhibited fruit. Therefore, similarly to non-climacteric fruit, many of the processes involved in both the onset and regulation of ripening are controlled by factors other than ethylene. We have demonstrated that chilling injury of fruit held at low temperatures was strongly dependent upon ethylene, while water soaking, another physiological disorder occuring in ripening fruit, was ethylene independent.