Xylanase, a novel elicitor of pathogenesis-related proteins in tobacco, uses a non-ethylene pathway for induction.

Antisera to acidic isoforms of pathogenesis-related proteins were used to measure the induction of these proteins in tobacco (Nicotiana tabacum) leaves. Endo-(1-4)-beta-xylanase purified from culture filtrates of Trichoderma viride was a strong elicitor of pathogenesis-related protein synthesis in tobacco leaves. The synthesis of these proteins was localized to tissue at the area of enzyme application. The inhibitors of ethylene biosynthesis and ethylene action, 1-aminoethoxyvinylglycine and silver thiosulfate, inhibited accumulation of pathogenesis-related proteins induced by tobacco mosaic virus and alpha-aminobutyric acid, but did not inhibit elicitation by xylanase. Likewise, the induction of these proteins by the tobacco pathogen Pseudomonas syringae pv. tabaci was not affected by the inhibitors of ethylene biosynthesis and action. The leaf response to tobacco mosaic virus and alpha-aminobutyric acid was dependent on light in normal and photosynthetically incompetent leaves. In contrast, the response of leaves to xylanase was independent of light. Tobacco mosaic virus and alpha-aminobutyric acid induced concerted accumulation of pathogenesis-related proteins. However, xylanase elicited the accumulation of only a subset of these proteins. Specifically, the plant (1-3)-beta-glucanases, which are normally a part of the concerted response, were underrepresented. These experiments have revealed the presence of a novel ethylene-independent pathway for pathogenesis-related protein induction that is activated by xylanase.