The exoD gene of Rhizobium meliloti encodes a novel function needed for alfalfa nodule invasion

During the symbiotic interaction between alfalfa and the nitrogen-fixing bacterium Rhizobium meliloti, the bacterium induces the formation of nodules on the plant roots and then invades these nodules. Among the bacterial genes required for nodule invasion are the exo genes, involved in production of an extracellular polysaccharide, and the ndv genes, needed for production of a periplasmic cyclic glucan. Mutations in the exoD gene result in altered exopolysaccharide production and in a nodule invasion defect. In this work we show that the stage of symbiotic arrest of exoD mutants is similar to that of other exo and ndv mutants. However, the effects of exoD mutations on exopolysaccharide production and growth on various media are different from the effects of other exo and ndv mutations. Finally, exoD mutations behave differently from other exo mutations in their ability to be suppressed or complemented extracellularly. The results suggest that exoD represents a new class of Rhizobium genes required for nodule invasion, distinct from the other exo genes and the ndv genes. We discuss models for the function of exoD.