Determining the Methoxypyrazine Biosynthesis Variables Affected by Light Exposure and Crop Level in Cabernet Sauvignon

Abstract

Methoxypyrazines are sensorially potent volatile compounds responsible for herbaceous/vegetal attributes in wines made from certain grape varieties. The biosynthesis of these compounds in grape berries is known to occur via a pathway that involves the methylation of hydroxypyrazine intermediates. Certain viticultural management regimes can be used to alter methoxypyrazine concentrations in fruit of those varieties that have the genetic capability of producing them. This study explored the effect of light exposure and crop level on 3-isobutyl-2-methoxypyrazine (IBMP) concentrations in Cabernet Sauvignon fruit to better understand the effect of these variables on the concentration of the precursor 3-isobutyl-2-hydroxypyrazine (IBHP) and the expression of a methyltransferase gene (VvOMT3) responsible for the final step in methoxypyrazine biosynthesis. Light was found to reduce the expression of VvOMT3 and the concentration of IBHP, suggesting that a combination of these factors reduces IBMP concentration when fruit has greater light exposure. In contrast, reducing the crop level of vines to less than half of that of controls did not have a significant effect on IBHP concentration or VvOMT3 expression, despite the treatment causing a significant increase in IBMP concentration. IBMP appears to be synthesized in the flesh of the berry which suggests differences in berry size may explain the crop level effect on IBMP concentrations.