Symbiotic pseudorevertants of Rhizobium meliloti ndv mutants.

Nodule development (ndv) mutants of Rhizobium meliloti cannot invade alfalfa to establish a nitrogen-fixing symbiosis and instead induce the formation of small, white, unoccupied nodules on alfalfa roots. Such mutants also fail to produce the unusual cyclic oligosaccharide beta-(1----2)-glucan and show defects in several aspects of vegetative growth and function. Here we show that ndv mutants are severely reduced, although not totally deficient, in the ability to attach to and initiate infection threads on alfalfa seedlings, and we demonstrate that the symbiotic deficiency can be separated from the rest of the mutant phenotype by isolating second-site pseudorevertants. Pseudorevertants selected for restoration of motility, a vegetative property, regained a substantial amount of attachment capability but only slight infection thread initiation and symbiotic ability. Such strains also regained partial tolerance to growth at low osmolarity, even though they did not recover the ability to synthesize periplasmic beta-(1----2)-glucan. Pseudorevertants selected on alfalfa for restoration of symbiosis were unrestored for beta-(1----2)-glucan production or any other vegetative property and regained little or no attachment or infection thread initiation capability. We take these data to indicate that wild-type R. meliloti normally has considerable excess capability for both attachment and infection thread initiation and that the symbiotic block in ndv mutants lies further along the developmental pathway than either of these processes, probably at the level of infection thread extension. Further, the fact that neither type of pseudorevertant recovered the ability to produce periplasmic beta-(1----2)-glucan raises the possibility that this oligosaccharide is not directly required for nodule development.