Excision of Nodules Induced by Rhizobium meliloti Exopolysaccharide Mutants Releases Autoregulation in Alfalfa

Nodulation in alfalfa (Medicago sativa L.) is controlled by a systemic feedback regulatory mechanism that suppresses nodule initiation in younger portions of the root system. Excision of primary root nodules induced by wild-type Rhizobium meliloti stimulates the formation of new nodules on lateral roots. In similar experiments we found that excision of bacteria-free nodules from primary roots induced by mutants of R. meliloti deficient in exopolysaccharide synthesis allowed nodules to reappear in lateral roots especially around the root tip at the time of nodule removal. Our results suggest that organized nodular structures trigger autoregulatory responses in legumes, even in the absence of bacterial infection.