Phytoalexins and disease resistance mechanisms from a perspective of evolution and adaptation.

Plants respond to cellular injury and infection by accumulating low molecular weight antimicrobial stress metabolites called phytoalexins. The accumulation of phytoalexins, together with lignification, suberization, callose formation and the production of agglutinins and inhibitors of extracellular microbial hydrolases, appears to be part of a multi-component response mechanism associated with disease resistance and wound repair. Compared to the antibody-antigen response in animals, the phytoalexin response in plants has low specificity for induction and activity of the phytoalexins. Plants also contain preformed antimicrobial chemical and physical barriers to infection in their external tissues. The successful pathogen has evolved to cope with preformed inhibitors and barriers and either avoids eliciting the response mechanism, or suppresses the mechanism, or detoxifies its antimicrobial components. Annual plants can be systemically immunized against diseases caused by viruses, bacteria and fungi by limited infection with any one of the respective organisms. As with animals, disease resistance in plants depends on the rate and magnitude of response rather than on the ability to respond. The genetic information for disease resistance is found in all organisms, and disease resistance is the rule in nature. The interactions of plants with microorganisms in their environment are nature's example of diplomacy--compromise, adjustment to change and avoidance of deadly conflict.