Chitin oligosaccharides elicit lignification in wounded wheat leaves

Oligomers of chitin (monomer to hexamer) and their deacetylated derivatives (monomer to tetramer) were purified from acid hydrolysates of chitin and chitosan, respectively, and assayed for lignification-eliciting activity in wounded wheat leaves. At 40 μg per wound, the chitin monomer and dimer were inactive, whist the chitin trimer possessed very slight activity. In contrast, the chitin tetramer, pentamer and hexamer all possessed significant elicitor activity. Dose-response curves indicated that the activities of the elicitor-active chitin oligomers were similar, and that their activity rapidly diminished at lower doses. Digestion of the tetramer with chitinase abolished elicitor activity. None of the deacetylated oligomers tested (monomer to tetramer) possessed any significant elicitor activity, confirming the requirement of acetyl groups for elicitor activity. The activity of the chitin tetramer was entirely dependent on the inclusion of a pre-wounding step in the assay, which explains why a previous study failed to identify an elicitor-active oligomer. Prior wounding also increased the activity of a number of other known lignification elicitors.