Inhibition of postharvest rachis browning of table grapes by sulfur dioxide: Evidence from phenolic metabolism and sulfur assimilation

Rachis browning is a visual marker of table grape freshness and plays a vital role in consumer preference and food waste. In this study, the role of sulfur dioxide (SO2) in maintenance of quality attributes of grape rachis during shelf-life was systematically studied, and results demonstrated that the rachis browning was significantly inhibited, and endogenous phenolic metabolism and sulfur assimilation were affected by exogenous SO2, at both transcriptional and metabolic levels. Furthermore, SO2 treatment promoted the accumulation of major phenolic compounds, such as (+)-catechin, (-)-epicatechin, (-)-epicatechin gallate, and proanthocyanidins, by increasing the transcript levels of VvLAR and Vv3GT genes involved in flavonoid pathway. Besides, gene expression involved in sulfite assimilation was also activated by exogenous SO2, resulting in the accumulation of sulfur-containing metabolites, including cysteine, hydrogen sulfide, and glutathione. The present study provided the first evidence for the anti-browning effect of SO2 through the accumulation of flavanols and sulfur-containing compounds.