The induction of nitrogenase activity in Rhizobium by non-legume plant cells

Nitrogen fixation was induced in a strain of "cowpea" rhizobia, 32Hl, when it was grown in association with cell cultures of the non-legume, tobacco (Nicotiana tabacum). Rhizobia grown alone on the various media examined did not show nitrogenase activity, indicating the involvement of particular plant metabolites in nitrogenase induction. Nitrogenase activity, as measured by C2H2 reduction, was maximized at an O2 concentration of 20% and at an assay temperature of 30°C, the conditions under which the plant cell-rhizobia associations developed. Glutamine, as a nitrogen source, could be replaced by other organic nitrogen sources, but NH4 (+) and NO3 (-) repressed nitrogenase activity. Nitrogenase activity induced in rhizobia when cultured adjacent to, but not in contact with, the plant cells could be stimulated by providing succinate in the medium. At least 12 other strains of rhizobia also reduced C2H2 in association with tobacco cells; the highest levels of activity were found among cowpea strains.