Regulation of Nodulation by Rhizobium meliloti 102F15 on Its Mutant Which Forms an Unusually High Number of Nodules on Alfalfa.

A mutant (WL3A150) of Rhizobium meliloti 102F51 that elicits an unusually high number of nodules on its host, alfalfa (Medicago sativa), supports the idea that the host may rely on early bacteroid development in the nodule or on metabolites produced in the infection thread as one of the signals to control further nodulation. This mutant was initially isolated because of its Fix phenotype. It consistently formed many more nodules than all the other Fix mutants isolated from strain 102F51 (a total of 11 mutants). Nodules formed by this mutant were small and white and were indistinguishable in appearance from nodules formed by the other Fix mutants. An ultrastructural study of the nodules, however, showed that this mutant, although forming numerous infection threads, failed to develop into bacteroids. The ability of the mutant to form an unusually high number of nodules coulde be suppressed in a time-dependent manner by the presence of the wild type.