MUTANTS OF RHIZOBIUM-TROPICI STRAIN CIAT899 THAT DO NOT INDUCE CHLOROSIS IN PLANTS

Type B strains of Rhizobium tropici induce severe foliar chlorosis whe n applied at planting to seeds of symbiotic host and non-host dicotyle donous plants. A Tn5-induced mutant, designated CT4812, of R. tropici strain CIAT899 that was unable to induce chlorosis was isolated. Cloni ng and sequencing of the DNA flanking the transposon in CT4812 reveale d that the Tn5 insertion is located in a gene similar to glnD, which e ncodes uridylyltransferase/uridylyl-removing enzyme in enteric bacteri a. Two marker-exchange mutants with insertions in glnD also failed to induce chlorosis in bean (Phaseolus vulgaris) plants. The 5'-most inse rtion in glnD tin mutant strain ME330) abolished the ability of R. tro pici to utilize nitrate as a sole carbon source, whereas a mutation in glnD further downstream tin mutant strain ME245) did not have an obvi ous effect on nitrate utilization. A gene similar to the Salmonella ty phimurium virulence gene mviN overlaps the 3' end of the R. tropici gl nD homologue. A mutation in mviN had no effect on the ability of CIAT8 99 to induce chlorosis in bean plants. Therefore the glnD homologue. b ut not mviN, appears to be required for induction of chlorosis in plan ts by R. tropici strain CIAT899. A high nitrogen:carbon ratio in the r hizosphere of bean plants also prevented R. tropici from inducing chlo rosis in bean plants. Mutations in either the glnD homologue or mviN h ad no significant effect on root nodule formation or acetylene reducti on activity. A mutation in mviN eliminated motility in R. tropici. The sequence data, the inability of the glnD mutant to utilize nitrate, a nd the role of the R. tropici glnD gene in chlorosis induction in plan ts, a process that is nitrogen regulated, suggest that glnD plays a ro le in nitrogen sensing in R. tropici as its homologues do in other org anisms.