Phytoalexins and disease resistance

Phytoalexins can be considered as anti-microbial components of a range of compounds produced by many plants in response to cellular damage. The chemical structure of phytoalexins differs greatly according to the plant family and sometimes the genus. Demonstrations of the roles of phytoalexins in disease resistance depend upon information on the accumulation of effective concentrations at appropriate times and places in tissues to cause the observed cessation of microbial growth. Phaseollin in Phaseolus accumulates in antifungal amounts as host cells react hypersensitively to incompatible races of Colletotrichum but not when compatible races grow in cells without causing damage. Wyerone acid in Vicia forms as cells are damaged by Botrytis; the highly pathogenic B. fabae causes depletion of wyerone acid levels as it grows through the damaged cells. Two distinct ways in which phytoalexins may be involved in disease resistance are discussed in view of these and other discoveries.