| To study the regulatory role of ethylene in the metabolites (i.e. sugars, organic acids, aroma compounds and phenolics) that determine overall flavor of apple fruits, ethylene production or action was reduced by using transgenic apple trees suppressed for ACC-synthase or ACC-oxidase enzyme activity, or by the addition of 1-methylcyclopropene (1-MCP), an ethylene action inhibitor. Flavor components were differentially regulated in response to the suppression of both ethylene biosynthesis and action, with organic acids, sugars and especially aroma compounds showing an ethylene-dependent pattern. Due to the significant level of modulation observed on aroma compounds, further studies were performed to elucidate the biochemical and molecular basis of this regulation. A major reduction in ester production was observed under ethylene suppression, with only slight differences in the levels of alcohol and aldehyde volatiles. An important biochemical step in this regulation was identified at the level of alcohol acyltransferase (AAT), in which both enzyme activity and levels of expression of an AAT clone showed an ethylene-dependent pattern.; In general, aroma production, enzyme activity levels and precursor availability were higher in apple peel than flesh tissues, and were differentially affected by ethylene regulation. AAT activity showed a clear pattern concomitant with ethylene regulation in both tissues. Contrarily, alcohol dehydrogenase and lipoxygenase activities seem to be independent of ethylene modulation. Isoleucine, an important precursor of aroma compounds including 2-methylbutanoate esters, showed a major increase in peel tissue during ripening and responded significantly to ethylene regulation. Other important aroma precursors, like linoleic and linolenic acid, showed an accumulation during ripening associated with increases in aldehyde compounds. |
