The Rhizobium meliloti PII protein, which controls bacterial nitrogen metabolism, affects alfalfa nodule development

Symbiotic nitrogen fixation involves the development of specialized organs called nodules within which plant photosynthates are exchanged for combined nitrogen of bacterial origin. To determine the importance of bacterial nitrogen metabolism in symbiosis, we have characterized a key regulator of this metabolism in Rhizobium meliloti, the uridylylatable P(II) protein encoded by glnB. We have constructed both a glnB null mutant and a point mutant making nonuridylylatable P(II). In free-living conditions, P(II) is required for expression of the ntrC-dependent gene glnII and for adenylylation of glutamine synthetase I. P(II) is also required for efficient infection of alfalfa but not for expression of nitrogenase. However alfalfa plants inoculated with either glnB mutant are nitrogen-starved in the absence of added combined nitrogen. We hypothesize that P(II) controls expression or activity of a bacteroid ammonium transporter required for a functional nitrogen-fixing symbiosis. Therefore, the P(II) protein affects both Rhizobium nitrogen metabolism and alfalfa nodule development.