‘Phytoalexins’ are compounds produced by plants in response to bacterial or fungal diseases. ‘Phytoalexins’ are generally not present in large amounts unless a plant is infected with a disease. Phytoalexins are phenolics found in grapevines that belong to the ‘Stilbenes’ class.

How ‘Phytoalexins’ work

When a fungus, such as downy mildew or botrytis, infects the vine, resveratrol is rapidly generated by the vine and accumulates near the infection site. Resveratrol, in sufficient quantities, can then stop the progression of the disease. Studies have shown that some fungi-resistant grape types have a high potential for stilbene synthesis as well.

Resveratrol is probably the most prominent phytoalexin presently. It has been revealed to be a constituent of the famous Asian medicinal herb called ‘Polygonum Cuspidatum’.

‘Phytoalexins’ inhibitors

The resveratrol synthesis gene (the ‘Stilbene Synthase’) has been cloned from Vinifera vines and introduced into tobacco. The resulting transgenic plants were more resistant to botrytis infection, demonstrating the importance of resveratrol as a natural antifungal defense. However, botrytis and possibly other pathogenic fungi can generate an enzyme called Laccase (click here to read more about Laccase). This Laccase has the ability to detoxify and inhibit stilbenes. This action allows the infection to proceed. But, phenolics such as ‘Catechin’ can inhibit the Laccase enzyme activity and therefore cancelling out the Laccase defense.

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