The effect of the Agrobacterium tumefaciens attR mutation on attachment and root colonization differs between legumes and other dicots

Infections of wound sites on dicot plants by Agrobacterium tumefaciens resu lt in the formation of crown gall tumors. An early step in tumor formation is bacterial attachment to the plant cells. AttR mutants failed to attach t o wound sites of both legumes and nonlegumes and were avirulent on both gro ups of plants. AttR mutants also failed to attach to the root epidermis and root hairs of nonlegumes and had a markedly reduced ability to colonize th e roots of these plants. However, AttR mutants were able to attach to the r oot epidermis and root hairs of alfalfa, garden bean, and pea. The mutant s howed little reduction in its ability to colonize these roots. Thus, A. tum efaciens appears to possess two systems for binding to plant cells. One sys tem is AttR dependent and is required for virulence on all of the plants te sted and for colonization of the roots of all of the plants tested except l egumes. Attachment to root hairs through this system can be blocked by the acetylated capsular polysaccharide, The second system is AttR independent, is not inhibited by the acetylated capsular polysaccharide, and allows the bacteria to bind to the roots of legumes.