Desensitization of the perception system for chitin fragments in tomato cells

Suspension-cultured tomato (Lycopersicon escolentum) cells react to stimulation by chitin fragments with a rapid, transient alkalinization of the growth medium, but behave refractory to a second treatment with the same stimulus (C. Felix, M. Regenass, T. Boiler [1993] Plant J 4: 307-316). We analyzed this phenomenon and found that chitin fragments caused desensitization in a time-and concentration-dependent manner. Partially desensitized cells exhibited a clear shift toward lower sensitivity of the perception system. The ability of chitin oligomers to induce desensitization depended on the degree of polymerization (DP), with DP5 approximate to DP4 much greater than DP3 much greater than DP2 > DP1. This correlates with the ability of these oligomers to induce the alkalinization response and to compete for the high-affinity binding site on tomato cells and microsomal membranes, indicating that the alkalinization response and the desensitization process are mediated by the same receptor. The dose required for half-maximal desensitization was about 20 times lower than the dose required for half-maximal alkalinization; desensitization could therefore be used as a highly sensitive bioassay for chitin fragments and chitin-related stimuli such as lipochitooligosaccharides (nodulation factors) from Rhizobium leguminosarum. Desensitization was not associated with increased inactivation of the stimulus or with a disappearance of high-affinity binding sites from the cell surface, and thus appears to be caused by an intermediate step in signal transduction.