Restoration of attachment, virulence and nodulation of Agrobacterium tumefaciens chvB mutants by rhicadhesin

In contrast to wild-type Agrobacterium tumefaciens strains, β-1,2-glucan-deficient chvB mutants were found to be unable to attach to pea root hair tips. The mutants appeared to produce rhicadhesin, the protein that mediates the first step in attachment of Rhizobiaceae cells to plant root hairs, but the protein was inactive. Both attachment to root hairs and virulence of the ChvB mutants could be restored by treatment of the plants with active rhicadhesin, whereas treatment of plants with β-1,2-glucan had no effect on attachment or virulence. Moreover, nodulation ability of a chvB mutant carrying a Sym plasmid could be restored by pretreatment of the host plant with rhicadhesin. Apparently the attachment-minus and avirulence phenotype of chvB mutants is caused by lack of active rhicadhesin, rather than directly being caused by a deficiency in β-1,2-glucan synthesis. The results strongly suggest that rhicadhesin is essential for attachment and virulence of A. tumefaciens cells. They also indicate that the mechanisms of binding of Agrobacterium and Rhizobium bacteria to plant target cells are similar, despite differences between these target cells.