Ethylene-Dependent and Ethylene-Independent Pathways in a Climacteric Fruit, the Melon

Abstract

Transgenic antisense ACC oxidase melons in which ethylene has been inhibited by more than 99% have been used for discriminating between ethylene-dependent and — independent pathways. In this paper, we have compared wild type and transgenic melons in terms of cell wall-degrading enzymes, ACC synthase activity and gene expression and resistance to chilling injury. The activity of some cell wall-degrading enzymes (pectin methylesterase and exo-polygalacturonase) were identical in wild type and transgenic fruit. These are not regulated by ethylene. On the contrary, the activity of galactanase, α-arabinosidase, β-galactosidase, and endo-polygalacturonase was higher in wild type fruit, indicating a regulatory role for ethylene for at least a portion of activity that could correspond to specific isoforms. The increase in ACC synthase activity at the early stages of ripening occured exactly at the same time in wild type and ethylene-inhibited fruits, indicating that the initiation of ripening associated ethylene biosynthesis could occur as a developmentaly and ethylene-independent phenomenon. An ACS gene (CMe-ACSl) showed strong stimulation during ripening of wild type melons. It was also expressed in transgenic fruits but at a low level that could not directly account for the high ACS activity encountered in these fruits. Ethylene treatment of transgenic fruits stimulated the accumulation of CMe-ACSl transcripts but caused a decrease of ACS activity. These data suggest a complex regulation process of ACS by ethylene at both the transcriptional and post-transcriptional level.