The Rhizobium meliloti PII Protein Affects Alfalfa Nodule Development and Mobilisation of Fixed Nitrogen

Differentiation of Rhizobium into nitrogen fixing bacteroids implies a shift in nitrogen metabolism from a non nitrogen-fixing, ammonium assimilation state, to a nitrogen-fixing, ammonium exporting state in which fixed nitrogen is routed toward the host plant cell. Because glutamine synthetase (GS) is a key enzyme of ammonium assimilation, this shift should in principle involve regulation of GS expression or activity. We have therefore investigated regulation of GS in the alfalfa symbiont, Rhizobium meliloti, and evaluated its impact on symbiotic nitrogen fixation.