Expression of ethylene biosynthetic genes in Actinidia chinensis fruit

The fruit of Actinidia chinensis, a diploid relative of kiwifruit, showed an increased rate of ripening in response to the application of exogenous ethylene. Moreover, late in ripening the fruit produced a burst of ethylene biosynthesis. Thus ripening is climacteric, and there is a clear temporal separation of ethylene sensitivity and ethylene production. RNase protection assays were used to monitor transcript levels of ethylene biosynthetic genes during fruit development and ethylene-induced ripening. The application of exogenous ethylene correlated with increased transcript levels for three different S-adenosyl-L-methionine (SAM) synthetase genes and for the 1-aminocyclopropane-1-carboxylate (ACC) oxidase gene family. Transcription of an ACC synthase gene was not affected by exogenous ethylene. However, ACC synthase transcript levels increased during subsequent ethylene production by the fruit, consistent with this being the control step for the onset of climacteric ethylene production. ACC oxidase transcripts increased significantly both prior to and during climacteric ethylene production, while only one of the three SAM synthetase transcripts was induced during the late ethylene burst. We propose that the regulation of SAM synthetase transcripts by ethylene may occur as part of the methionine salvage pathway.