The evolution of volatile compounds was explored in grape berries at fortnightly intervals from fruit-set to late ripening to identify when biosynthetic pathways may be targeted for enhancement of grape and wine aroma. Stepwise linear discriminant analysis (SLDA) fully recognized patterns in berry physiological developmental stages with most of the variance (>99.0%) explained. The preveraison berry developmental stage was identified as a transition stage for volatile compound biosynthesis when most compounds were potentially sequestered to nonvolatile conjugates and berries lost their potential to synthesize esters and terpenes. Terpenes (predominantly eucalyptol, beta-caryophyllene, and alpha-humulene) characterized early berry development, whereas benzene derivatives (2-phenylethanol and 2-phenylethanal) appeared toward late ripening. Furthermore, C(6) volatile compounds changed from acetate esters to aldehydes and finally to alcohols during early, middle, and late berry developmental stages, respectively. The dominance of alcohols in the late stages of berry development, preceded by aldehydes, offers an opportunity for alcohols to aldehydes ratios to be used in the prediction of harvest timing for enhanced grape and wine aroma. The evolution of volatile compounds during berry development suggests a greater dependency on enzyme activity and specificity than extent of fatty acid unsaturation. The dependence of the stage of berry development on the accumulation of the products of alcohol dehydrogenase (ADH), alcohol acetyl transferase (AAT), and enal isomerase enzyme activity from the lipoxygenase pathway raises possibilities for the manipulation of aroma profiles in grapes and wines.
Methoxypyrazines (MPs) are volatile, grape-derived aroma compounds that contribute to the distinct herbaceous characters of some wines. Although the full pathway leading to MP production has not been elucidated, there is strong evidence that the final step involves the methylation of non-volatile hydroxypyrazine (HP) precursors. Two cDNA encoding O-methyltransferases (OMTs) that have homology to an enzyme previously purified and shown to catalyse the methylation of HPs were isolated from Cabernet Sauvignon. Recombinant protein from the cDNAs (VvOMT1 and VvOMT2) was produced in E. coli and activity assays demonstrated that both encode OMTs able to methylate HPs to produce MPs, however both showed greatest activity against the flavonol quercetin. VvOMT1 has higher catalytic activity against isobutyl hydroxypyrazine compared to isopropyl hydroxypyrazine, whereas the converse is true for VvOMT2. The timing of the expression of VvOMT1 in the skin and the flesh of developing Cabernet Sauvignon grape berries was associated with the period of MP accumulation in these tissues, while VvOMT2 expression was greatest in roots, which were found to contain high levels of MPs. The MP composition of these tissues also reflects the relative levels of expression of these genes and their substrate preference. The identification of genes responsible for MP production in grapevine will help in understanding the effect of different viticultural and environmental factors on MP accumulation.
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