PERCEPTION OF RHIZOBIUM NODULATION FACTORS BY TOMATO CELLS AND INACTIVATION BY ROOT CHITINASES

The bacterial genera Rhizobium and Bradyrhizobium, nitrogen-fixing sym bionts of legumes, secrete specific lipo chitooligosaccharides that in duce the formation of nodules on their host plants. When preparations of such nodulation-inducing factors (Nod factors) were added to suspen sion-cultured tomato cells, a rapid and transient alkalinization of th e culture medium occurred. Lipo oligosaccharide preparations from Rhiz obium or Bradyrhizobium treated with flavonoids, known inducers of Nod factor synthesis, were up to 100 times more potent in inducing alkali nization than the ones from untreated bacteria. The activity was absen t from preparations of the mutant strain Rhizobium sp. NGR234 Delta no dABC, unable to produce any Nod factors. Preparations of Nod factors f rom various bacteria as well as individual, highly purified Nod factor s from Rhizobium sp. NGR(pA28) induced alkalinization in the tomato ce ll cultures at nanomolar concentrations. This demonstrates that Nod fa ctors can be perceived by tomato; a nonhost of rhizobia. Using the alk alinization response as a sensitive bioassay, Nod factors were found t o be inactivated by plant chitinases. Root chitinases purified from di fferent legumes differed in their potential to inactivate differently substituted Nod factors produced by Rhizobium sp. NGR(pA28). This indi cates that the specificity of the bacterium-host plant interaction may be due, at least in part, to differential inactivation of Nod factors by root chitinases.