Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2010
Publication Date: 11/1/2010
Citation: Gonda, I., Bar, E., Portnoy, V., Schaffer, A., Tadmor, Y., Gepstein, S., Giovannoni, J.J., Katzir, N., Lewinsohn, E. 2010. Branched-chain and aromatic amino acid catabolism into aroma volatiles in Cucumis melo L. fruit. Journal of Experimental Botany. 61:1111-1123. Interpretive Summary: Although different fruits often share many aroma characteristics, each fruit has a distinctive aroma that is determined by the proportions of key volatiles as well as the presence or absence of unique components. The most important aroma compounds include, among others, amino acid-derived compounds, lipid-derived compounds, phenolic derivatives, mono- and sesquiterpenes. Two new melon genes, CmArAT1 and CmBCAT1, were identified and characterized following bacterial expression, and shown to encode for enzymes possessing aromatic amino acid transaminase (ArAT) and branched-chain amino acid transaminase (BCAT) activities, respectively. Both genes are expressed during late stages of ripening in aromatic fruits, while ripe fruits of non-aromatic cultivars display lower expression levels. Observations that help explain the formation of melon aroma volatiles from amino acids via transamination are summarized in this report.
Technical Abstract: The unique aroma of melons (Cucumis melo L., Cucurbitaceae) is composed of many volatile compounds biosynthetically derived from fatty-acids, carotenoids, amino-acids as well as terpenes. Incubation of melon fruit cubes with amino- and a-keto acids led to the enhanced formation of aroma compounds bearing the side chain of the exogenous amino or keto acid supplied. Moreover, L-[13C6]phenylalanine also incorporated into aromatic volatile compounds. Amino acid transaminase activities extracted from the flesh of mature melon fruits converted L-isoleucine, L-leucine, L-valine, L-methionine or L-phenylalanine into their respective keto acids, utilizing a-keto glutarate as the amine acceptor. Two novel genes were isolated and characterized (CmArAT1 and CmBCAT1) encoding 45.6 and 42.7 KDa proteins respectively, that displayed, aromatic and branched-chain amino acid transaminase activities, respectively, when expressed in E. coli. The expression patterns of CmBCAT1 and CmArAT1 was low in vegetative tissues, but increased in flesh and rind tissues during fruit ripening. In addition, ripe fruits of climacteric aromatic cultivars generally showed high expression of CmBCAT1 and CmArAT1 in contrast to non-climacteric non-aromatic fruits. The results presented here indicate that in melon tissues, the catabolism of amino-acids into aroma volatiles can initiate through a transamination mechanism, rather than decarboxylation or direct aldehyde synthesis, as has been demonstrated in other plants.