phytoalexins, compounds produced by plants in response to bacterial or fungal attack. They are not normally present in significant quantities until a plant is invaded by disease. In grapevines, phytoalexins belong to a class of phenolics called stilbenes (see resveratrol). When a fungus such as downy mildew or botrytis invades the vine, resveratrol is synthesized and accumulates very rapidly near the infection point. At sufficient concentrations, resveratrol can slow or even stop the growth of the disease. Some grape varieties that are known to be resistant to fungi (Castor, for example) also have a high potential for stilbene production. Resveratrol was discovered in vines in 1976, and a related compound viniferin was discovered in 1977. This phytoalexin was studied with renewed interest when it was discovered also to be a constituent of the Asian medicinal herb Polygonum cuspidatum. The gene for resveratrol synthesis, stilbene synthase, has been cloned from vinifera vines and inserted into the rootstock 41 B. The transgenic plants able to produce high concentrations of resveratrol were more resistant to botrytis infection, showing its value as a natural defence agent. However, botrytis and possibly other pathogenic fungi manufacture a laccase which can detoxify stilbenes and allow infection to proceed, but phenolics such as catechin can inhibit this enzyme’s activity, thus cancelling out the fungi’s defence.
