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  • The metabolism of amino acids into volatile aldehydes involv

    2024-04-11

    The metabolism of amino acids into volatile aldehydes involves both decarboxylation and deamination steps. The order of these reactions and the enzymes performing the catalysis differ among plants [13]. In tomato fruit the conversion of L-Phe to 2-phenylethanal is catalyzed by two enzymatic steps, involving the decarboxylation of L-Phe as the initial reaction [11]. A gene described as a L-phenylalanine decarboxylase was expressed in developing grape berries but its role in volatile formation was unclear [42]. The biosynthetic pathway to benzaldehyde formation from L-Phe has not been reported in grape tissues. Studies performed in other plant species have indicated that benzaldehyde biosynthesis is more complex and requires several additional enzymatic steps involving oxidative and non-oxidative biosynthetic pathways [43]. This biochemical complexity can partially explain the ten-fold lower levels of benzaldehyde generally found as compared to higher levels of 2-phenylethanal detected in L-Phe incubated berries (Fig. 1B). Volatile aldehydes are reportedly converted to the respective alcohols by the action of alcohol dehydrogenase (ADH) enzymes [44]. The ADH family is prominent in the plant kingdom and has been characterized in many fruits and other plant tissues including grapes [[45], [46], [47], [48]]. Most ADH are capable of converting ethanol into acetaldehyde during anaerobic fermentation processes, but other ADH enzymes are known to be involved in aroma volatile formation in melons [49] and other fruits including grapes [45,50]. In melon fruit, two highly divergent ADH genes (15% identity at the amino PNU-120596 level) have been isolated both catalyzing the reduction of aliphatic aldehydes but differ in their specificity to other substrates [49]. In rose flowers, a 2-phenylethanal reductase (PAR) (a member of the ADH gene family) catalyzes the formation of 2-phenylethanol from 2-phenylethanal [51,52]. Functional expression of the V. vinifera VvADH2 indicated that it is involved in the production of multiple volatile alcohols, such as 1-hexanol from the corresponding aldehydes [47]. Although the gene encoding the ADH enzyme(s) involved in the conversions of amino-acid-derived aldehydes into the corresponding alcohols in grape berry tissues are presently unknown, transcriptomic analyses have indicated that several members of the ADH family are expressed in grape berries in all three accessions and could potentially encode for these activities (Supplementary Table 5). Still, more experimental evidence is needed to determine which of the gene(s) are involved in these conversions. Our results indicate that the grape accessions used in this study differ in their ability to accumulate alcohols from exogenous amino acids (Fig. 1A–C). It could be that the lack of sufficient aldehyde prevented the accumulation of the alcohol, as suggested in the case of L-Phe-feeding displaying low 2-phenylethanal levels (Fig. 1B). Alternatively, the lack of alcohol accumulation can be due to low expression of ADH genes responsible for these conversions, and this could biochemically explain the lack of 3-methylbutanol accumulation in ‘Superior Seedless’ berries (Fig. 1A). In any case, the presence of 3-methylbutyl acetate in ‘Superior Seedless’ may indicate that the alcohol 3-methylbutanol was generated but readily converted to the corresponding acetate ester lowering the levels of the alcohol below detection levels. Other possible explanations for the variation observed among accessions could be different possible compartmentalization of the products or the enzymes. Moreover, we noted that esters generally accumulated in L-Leu and L-Phe fed grape berries but no esters were noted upon L-Met incubations. Still, a marked variation in the ability to accumulate acetate esters from exogenous L-Leu and L-Phe was noted among the different grape accessions (Fig. 1A–B). This demonstrates a differentially enhanced concealed potential of grape berries to produce and accumulate volatile acetate esters. The differences observed between the acetyl ester levels and the respective alcohol substrates indicated that there might be differences in the acetyltransferase activities limiting acetyl ester formation in grapes, similarly to reports in melon, apple and other fruit [24,27]. Alternatively, the lack of benzyl acetate or methionyl acetate can also be a result of the low relative amount of the respective alcohols, benzyl alcohol or methionol. Although 2-phenylethyl acetate and 3-methylbutyl acetate are important constituents of many fruits, we could not find reports citing the presence these compounds in fresh berries of any cultivar of V. vinifera grapes including ‘Muscat Hamburg’ or ‘Superior Seedless’. Our studies indicate that there are concealed routes to their formations in grape berries.