Fungal xylanases from Trichoderma spp. are potent elicitors of defense responses in various plants. To determine whether enzymatic activity is necessary for elicitor activity, we used site-directed mutagenesis to reduce the catalytic activity of xylanase II from Trichoderma reesei. For this, the glutamic acid residue at position 210, which is part of the active center in this family of enzymes, was changed to either aspartic acid (E210D) or serine (E210S). Wildtype and mutated forms of xylanase II were expressed in yeast cells and purified to homogeneity. Compared with the wild-type form of xylanase II, E210D had >100-fold and E210S 1,000-fold lower enzymatic activity. In contrast, these mutated forms showed no comparable drop in elicitor activity. They fully stimulated medium alkalinization and ethylene biosynthesis in suspension-cultured tomato (Lycopersicon esculentum) cells, as well as hypersensitive necrosis in leaves of tomato and tobacco (Nicotiana tabacum) plants. These results provide direct evidence that enzyme activity is not necessary for elicitor activity of fungal xylanase.

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Total membranes prepared from roots of soybean (Glycine max 1.) seedlings have previously been shown to contain proteinaceous binding site(s) for a hepta-8-glucoside elicitor of phytoalexin accumulation. l h e hepta-&glucoside elicitor-binding proteins have now been shown to co-migrate with a plasma membrane marker enzyme (vanadate-sensitive H+-AlPase) on linear sucrose density gradients. With the use of detergents, the elicitor-binding proteins have been solubilized in fundional form from soybean root membranes. l h e nonionic detergents n-dodecylsucrose, n-dodecylmaltoside, and Triton X-114, at concentrations of 5 to 10 mg/mL, each solubilizes between 50 and 60% of the elicitor-binding adivity in a single extradion of the membranes. A zwitterionic detergent, Ndodecyl-N,N-dimethyl-3-ammonio-l-propane-sulfonate (ZW 3-12), also solubilizes about 40% of the total binding activity at detergent concentrations between 1 and 2 mg/mL, but the total binding adivity recovered is only approximately 50% of that recovered with the nonionic detergents. The elicitor-binding proteins solubilized with either n-dodecylsucrose or ZW 3-12 retain the high affinity for radiolabeled hepta-í3-glucoside elicitor (apparent dissociation constant [Kd] = 1.8 nM and 1.4 nM, respectively) that was observed with the membrane-localized binding proteins (apparent Kd = 1 nM). Competitive ligand-binding experiments with severa1 strudurally related synthetic oligoglucosides demonstrate that the solubilized binding proteins retain specificity for elicitoractive oligosaccharides, irrespedive of the detergent used for solubilization. Moreover, the binding affinities of the oligoglucosides for the solubilized binding proteins correlate well with their abilities to induce phytoalexin accumulation in soybean cotyledon tissue. Cel-permeation chromatography of n-dodecylsucrose-solubilized elicitor-binding proteins demonstrate that the bulk of the elicitor-binding adivity is associated with large detergent-protein micelles (relative molecular weight > 400,000). Our results suggest that n-dodecylsucrose is a suitable detergent for solubilizing elicitor-binding proteins from soybean root membranes with minimal losses of binding activity. More importantly, we demonstrate that solubilization does not significantly alter the binding properties of the proteins for elicitor-adive oligoglucosides.