Aside from being UV-protectants, in a number of species certain stilbenes act as phytoalexins. Resveratrol (6.24; trans-3,5,4'-trihydroxy-stilbene), its cis-isomer, as well as their glucosides and dehydrodimer trans-s-viniferin (6.25) are present in grape leaves and berries and play a role in the defense against gray rot caused by the fungal pathogen Botrytis cinerea.

Viniferin is synthesized by a grape peroxidase (Morales et al., 1997). The fungus in return is able to inactivate resveratrol through the action of a laccase-like stilbene oxidase (Breuil et al., 1998). This results in the formation of resveratrol trans-dehydrodimer (6.26), as well as the corresponding cis-dehydrodimer (6.27), both of which structurally resemble viniferin.

(6.24) (6.25)

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