Specific, high affinity binding of chitin fragments to tomato cells and membranes. Competitive inhibition of binding by derivatives of chitooligosaccharides and a Nod factor of Rhizobium.

Suspension-cultured tomato cells have a sensitive perception system for chitin fragments with a degree of polymerization (DP) > or = 4 and react to these compounds with a transient alkalinization of their culture medium (Felix, G., Regenass, M., and Boller, T. (1993) Plant. J. 4, 307-316). A chitin fragment with DP 5 was aminated at the reducing end and coupled to t-butoxycarbonyl-L-[35S]methionine via an amidoglycine spacer. The radiolabeled chitin fragment (approximately 1000 Ci/mmol) exhibited specific, saturable, reversible binding to whole tomato cells as well as to tomato microsomal membranes with dissociation constants of 1.4 and 23 nM, respectively. Binding of the radioligand was competed by chitin fragments of different DP with IC50 values (50% inhibition of binding) that closely paralleled the concentrations inducing the alkalinization response half-maximally. Deacetylated chitooligosaccharides and N-propanoyl chitooligosaccharides were weak elicitors of the alkalinization response as well as weak competitors of radioligand binding. A lipochitooligosaccharide (Nod factor) from Rhizobium leguminosarum stimulated the alkalinization response in tomato cells half-maximally at 3 nM and competed radioligand binding to the cells with an IC50 of 8 nM. These results demonstrate the presence of a high affinity binding site for chitin fragments on the tomato cell membrane that may function as a receptor.