Type B strains of Rhizobium tropici induce severe foliar
chlorosis when applied at planting to seeds of symbiotic host and non-host dicotyledonous plants. A Tn5-induced mutant, designated CT4812, or R. tropici strain CIAT899 that was unable to induce
chlorosis was isolated. Cloning and sequencing of the
DNA flanking the transposon in CT4812 revealed that the Tn5 insertion is located in a gene similar to glnD, which encodes
uridylyltransferase/uridylyl-removing enzyme in enteric bacteria. Two marker-exchange mutants with insertions in glnD also failed to induce
chlorosis in bean (Phaseolus vulgaris) plants. The 5'-most insertion in glnD (in mutant strain ME330) abolished the ability of R. tropici to utilize
nitrate as a sole
carbon source, whereas a mutation in glnD further downstream (in mutant strain ME245) did not have an obvious effect on
nitrate utilization. A gene similar to the Salmonella typhimurium virulence gene mviN overlaps the 3' end of the R. tropici glnD homologue. A mutation in mviN had no effect on the ability of CIAT899 to induce
chlorosis in bean plants. Therefore the glnD homologue, but not mviN, appears to be required for induction of
chlorosis in plants by R. tropici strain CIAT899. A high
nitrogen:
carbon ratio in the rhizosphere of bean plants also prevented R. tropici from inducing
chlorosis in bean plants. Mutations in either the glnD homologue or mviN had no significant effect on root nodule formation or
acetylene reduction activity. A mutation in mviN eliminated motility in R. tropici. The sequence data, the inability of the glnD mutant to utilize
nitrate, and the role of the R. tropici glnD gene in
chlorosis induction in plants, a process that is
nitrogen regulated, suggest that glnD plays a role in
nitrogen sensing in R. tropici as its homologues do in other organisms.