DIVERSE SIGNAL SENSITIVITY OF NODD PROTEIN HOMOLOGS FROM NARROW AND BROAD HOST RANGE RHIZOBIA

The narrow host range plant endosymbiont Rhizobium meliloti exhibited diverse symbiotic properties (nodulation, nitrogen fixation) in concert with 10 host plant species from the genera Medicago, Melilotus, and Trigonella. These plant species released divergent but overlapping sets of compounds from their seeds. Several constituents of the plant exudates exerted inductive or inhibitory effects on nodulation gene expression in R. meliloti, depending on the plasmid-borne NodD protein homolog (NodD1, NodD2, and NodD3 of R. meliloti) introduced into the test cell. These three types of NodD proteins, as well as NodD of the broad host range strain Rhizobium sp. MPIK3030 diverged in terms of specific recognition of flavonoid standards. The NodD(MPIK3030) protein required only the hydroxylation of the C7 atom on the flavonoid ring for a substantial nodulation gene induction ability. Consequently, the NodD(MPIK3030) protein was sensitive to a far broader range of flavonoids than either of three NodD homologs of R. meliloti which, in turn, required the appropriate constellation of additional substitutions on the flavonoid skeleton for effectivity. The aurone sulphuretin was an extremely potent inducer of nodulation genes even without plasmid-borne NodD in R. meliloti. Examination of the effects of individual substitutions and different ring structures suggests that 7,3',4'-hydroxyflavone can be an ideal inducer of nodulation genes in R. meliloti. We conclude that the diverse signal sensitivity of NodD proteins correlates with the narrow or broad host range properties of the rhizobial strains.