Abstract
Two genes (CM-AAT1 and CM-AAT2) with strong sequence homology (87% identity at the protein level) putatively involved in the formation of aroma volatile esters have been isolated from Charentais melon fruit. They belong to a large and highly divergent family of multifunctional plant acyl-transferases and show at most 21% identity to the only other fruit acyl-transferase characterized so far in strawberry. RT-PCR studies indicated that both genes were specifically expressed in fruit at increasing rates in the early and mid phases of ripening. Expression was severely reduced in ethylene-suppressed antisense ACC oxidase (AS) fruit and in wild-type (WT) fruit treated with the ethylene antagonist 1-MCP. Cloning of the two genes in yeast revealed that the CM-AAT1 protein exhibited alcohol acyl-transferase activity while no such activity could be detected for CM-AAT2 despite the strong homology between the two sequences. CM-AAT1 was capable of producing esters from a wide range of combinations of alcohols and acyl-CoAs. The higher the carbon chain of aliphatic alcohols, the higher the activity. Branched alcohols were esterified at differential rates depending on the position of the methyl group and the nature of the acyl donor. Phenyl and benzoyl alcohols were also good substrates, but activity varied with the position and size of the aromatic residue. The cis/trans configuration influenced activity either positively (2-hexenol) or negatively (3-hexenol). Because ripening melons evolve the whole range of esters generated by the recombinant CM-AAT1 protein, we conclude that CM-AAT1 plays a major role in aroma volatiles formation in the melon.
Highlights
Aroma volatiles are secondary metabolites that play a major role in fruit quality
A BLAST search of these sequences gave the highest homologies with two protein families: (a) hypersensitivity-related proteins of Arabidopsis and tobacco; and (b) acyl transferases such as anthranilate N-hydroxy-cinnamoyl/benzoyl-transferase (NHCBT)-like protein of Arabidopsis and Dianthus caryophyllus, acetyl-CoA benzyl alcohol transferase (BEAT) of Clarkia breweri and other alcohol acyl-transferases (AAT) involved in secondary metabolism
CM-AAT1 and CM-AAT2 are fruit specific and ethyleneregulated genes that belong to a large acyl-transferase multifunctional gene family
Summary
Charentais cantaloupe melon (Cucumis melo L., var cantalupensis Naud.) is characterized by abundant sweetness and very good aromatic flavour. Due to their short storage life, breeders have directed their efforts towards the extension of shelf-life and improving yield, uniformity, and pest resistance. It has been shown that suppression of ethylene production [1] results in a strong inhibition of aroma volatiles in Charentais-type melons [2], suggesting that when new cultivars generated by the breeders are affected in ethylene production and/or sensitivity this may impair flavour. The aroma volatiles of Charentais-type cantaloupe melons, as with other cantaloupes, comprise a complex mixture of compounds including esters, saturated and unsaturated aldehydes and alcohols, and sulphur compounds [5,6]. We report here on the expression pattern and characteristics of two putative AAT genes (CM-AAT1 and CM-AAT2) and on the functional and biochemical characterization of the AAT enzyme encoded by the CM-AAT1 gene
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