ABNORMAL BACTEROID DEVELOPMENT IN NODULES INDUCED BY A GLUCOSAMINE SYNTHASE MUTANT OF RHIZOBIUM-LEGUMINOSARUM

Mutation of the chromosomal gene (glmS) encoding glucosamine synthase in Rhizobium leguminosarum biovar viciae results in a mutant that can induce nodules on peas, but with greatly reduced level of symbiotic nitrogen fixation. Electron microscopy of the nodules revealed that infection and release of the glmS mutant from infection threads was normal. However, the subsequent development of bacteroids was abnormal; bacteroids in the mature zone of the nodule were much larger than controls, were abnormally shaped and highly vacuolated, and underwent rapid senescence. it is proposed that expression of nodM (also encoding a glucosamine synthase), present on the symbiotic plasmid, enabled the mutant to grow in the rhizosphere and within infection threads, but when the bacteria were released from infection threads, the nod genes (including nodM) were no longer expressed, resulting in glucosamine limitation of the bacteroids. Similarly, glucosamine limitation in free-living cultures caused a significant reduction in the amount of cell wall lipopolysaccharide and in qualitative changes to the lipopolysaccharide, as revealed by probing with monoclonal antibodies targeted against lipopolysaccharide epitopes.