Role of ethylene in biosynthetic pathway of related-aroma volatiles derived from amino acids in oriental sweet melons (Cucumis melo var. makuwa Makino)

Abstract

The catabolizm of amino acid (AA) is regarded as a key pathway of aroma compounds in melon. In this study, we used two cultivars of oriental sweet melon (Cucumis melo var. makuwa Makino cv. Caihong7 and Tianbao) to clarify the critical steps of the biosynthetic pathway of volatile compounds could be regulated by ethylene (ETH). The effects of ETH and 1-methylcyclopropene (1-MCP) on aroma-related metabolites, such as the profile of aroma volatile, content of AAs as aroma precursors, activity of aroma-related enzymes derived from AA pathway and gene expression of key enzymes were investigated. In general, compared with the control, ETH treatment produced different profiles of alcohol, ester and aldehyde; the levels of alcohol and ester increased, whereas the amounts of aldehyde decreased. Moreover, the metabolism of branched-chain AAs, including valine, leucine and isoleucine, as well as the phenylalanine as an aromatic AA and cysteine as a sulfur-containing AA were under the regulation of ETH. The activities of aminotransaminase (AT) included branched-chain AA transaminase (BCAT) and aromatic aminotransferase (ArAT) and pyruvate dehydrogenase (PDH) as well as the expression patterns of CmBCAT1 and CmArAT1 showed a clear pattern concomitant with ETH regulation. Contrarily, pyruvate decarboxylase (PDC) seemed to be independent of ETH modulation. The results presented here indicated that the catabolizm of AAs largely depended on valine, leucine, isoleucine, phenylalanine and cysteine into aroma volatiles, especially the methyl-branched and aromatic esters, through transamination and dehydrogenation activities followed a positive response to ETH and negative response to 1-MCP.