Observations using light microscopy showed that approximately 30% of B
otrytis cinerea conidia treated with semi-lethal concentrations (i.e.,
60 mu g/ml) of the grapevine phytoalexin resveratrol possessed intrac
ellular brown coloration. This coloration was never observed in the ab
sence of resveratrol or in conidia treated with resveratrol together w
ith sulfur dioxide (antioxidant compound) or sodium diethyldithiocarba
mate (inhibitor of laccase action), suggesting that discoloration resu
lted from the laccase-mediated oxidation of resveratrol. Further studi
es using transmission electron microscopy enabled the observation of p
articular intravacuolar spherical vesicles and of granular material de
posits along the tonoplast. These observations are likely to be relate
d to the oxidation of resveratrol by an intracellular laccase-like sti
lbene oxidase of B. cinerea.