The bacterial genera Rhizobium and Bradyrhizobium, nitrogen-fixing symbionts of legumes, secrete specific lipo-chitooligosaccharides that induce the formation of nodules on their host plants. When preparations of such nodulation-inducing factors (Nod factors) were added to suspension-cultured tomato cells, a rapid and transient alkalinization of the culture medium occurred. Lipo-oligosaccharide preparations from Rhizobium or Bradyrhizobium treated with flavonoids, known inducers of Nod factor synthesis, were up to 100 times more potent in inducing alkalinization than the ones from untreated bacteria. The activity was absent from preparations of the mutant strain Rhizobium sp. NGR234 delta nodABC, unable to produce any Nod factors. Preparations of Nod factors from various bacteria as well as individual, highly purified Nod factors from Rhizobium sp. NGR(pA28) induced alkalinization in the tomato cell cultures at nanomolar concentrations. This demonstrates that Nod factors can be perceived by tomato, a nonhost of rhizobia. Using the alkalinization response as a sensitive bioassay, Nod factors were found to be inactivated by plant chitinases. Root chitinases purified from different legumes differed in their potential to inactivate differently substituted Nod factors produced by Rhizobium sp. NGR(pA28). This indicates 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.

All nitrogen-fixing bacteroids within legume root nodule cells are surrounded by a host-derived peribacteroid membrane. Components of this membrane are supplied directly by the ER and Golgi of the host cell. The peribacteroid space lies between the peribacteroid and bacteroid membranes and contains several activities typically found in vacuoles, namely; protease, acid trehalase, alpha-mannosidase isoenzyme II and protein protease inhibitor. Thus bacteroids inhabit an environment which fulfils the definition of a lysosome. Since the endosymbiotic organelles are morphologically different from the lytic compartment normally present in a root cortex cell (the central vacuole), it is proposed that they represent organ-specific modifications of lysosomes, analogous to the protein bodies of seeds. Perisymbiontic membranes are features common to all known plant endosymbioses (involving rhizobia, cyanobacteria, actinomycetes, vesicular-arbuscular mycorrhiza etc.) and the implications of this lead to the hypothesis that in all these cases the endosymbiont is compartmentalized within a specialized host lysosome.