Host-parasite interactions: elicitation of defense responses in plants with chitosan.

The plant's defense response against pathogens can be elicited by numerous external signals. Plant pathogens known to be incompatible on a given plant species can elicit strong disease resistance responses, whereas an adapted compatible pathogen generates a weaker response and thus can more readily infect the plant tissue. The plant's response can be manipulated genetically by the transfer of "R" genes (single dominant genes for race-specific disease resistance) or by treatment with elicitors such as chitosan. Both of these manipulations can result in the rapid activation of a subset of genes called PR (pathogenesis-related) genes, generally regarded as the genes that functionally develop disease resistance. There appear to be multiple modes by which chitosan can increase PR gene function, including activating cell surface or membrane receptors and internal effects on the plant's DNA conformation that in turn influence gene transcription. A novel strategy for controlling PR gene expression proposes to transform plants with a chitosan-inducible gene promoter linked in line with a single signal gene capable of rapid, intense induction of an entire set of PR genes, thereby enabling the control of disease resistance by external chitosan applications.