AN OSMOREVERTANT OF A RHIZOBIUM-MELILOTI NDVB DELETION MUTANT FORMS INFECTION THREADS BUT IS DEFECTIVE IN BACTEROID DEVELOPMENT

A defined deletion mutant in the ndvB locus of Rhizobium meliloti that was identified by its Inf- phenotype lacked the beta-1,2-glucans and showed osmotic instability. The Inf- phenotype was restored by a spontaneous mutation causing osmotic tolerance. This osmorevertant still lacked the beta-1,2-glucans but induced Fix- nodules containing infection threads. Release of bacteria from the infection thread was observed, but bacterial multiplication in the plant cell and development into bacteroids was blocked. In contrast to the original ndvB mutant, the osmorevertant was able to establish Fix+ nodules following mixed inoculation with a Nod- mutant. Thus, osmotic intolerance seems to be the reason for the Inf- phenotype of ndvB mutants. Reisolation experiments revealed that Fix+ nodules were occupied mainly by the Nod- mutant, indicating that the partially osmotic-tolerant osmorevertant still lacked functions that could not be complemented on the cellular level.