Sulphur dioxide evokes a large scale reprogramming of the grape berry transcriptome associated with oxidative signalling and biotic defence responses

The grape and wine industries are heavily reliant on sulphite preservatives. However, the view that sulphites act directly on bacterial and fungal pathogens may be simplistic. Mechanisms of sulphur-enhanced defences are largely unknown; many sulphur-rich compounds enhance plant defences and sulphite can also have oxidative consequences via production of H2O2 or sulphitolysis. To investigate the effects of sulphur dioxide (SO2) on fresh table grapes (Vitis vinifera L. ‘Crimson Seedless’), transcriptome analysis was carried out on berries treated with SO2 under commercial conditions for 21 d. We found a broad perturbation of metabolic processes, consistent with a large-scale stress response. Transcripts encoding putative sulphur-metabolizing enzymes indicated that sulphite was directed towards chelation and conjugation, and away from oxidation to sulphate. The results indicated that redox poise was altered dramatically by SO2 treatment, evidenced by alterations in plastid and mitochondrial alternative electron transfer pathways, up-regulation of fermentation transcripts and numerous glutathione S-transferases, along with a down-regulation of components involved in redox homeostasis. Features of biotic stress were up-regulated, notably signalling via auxin, ethylene and jasmonates. Taken together, this inventory of transcriptional responses is consistent with a long-term cellular response to oxidative stress, similar to the effects of reactive oxygen species.