Xylem colonization of the legume Sesbania rostrata by Azorhizobium caulinodans

A novel pathway of invasion of the legume Sesbania rostrata by Azorhizobium caulinodans is described that involves colonization of the root xylem, possibly following entry into the natural fissures created during emergence of lateral roots. Azorhizobia were detected microscopically, and their presence confirmed by the expression of a lacZ reporter gene. We have shown that rhizobial Nod factors are not required for either xylem colonization or for crack-entry of lateral roots. We discuss the extent to which this discovery of xylem colonization by azorhizobia is likely to improve our understanding of both symbiosis and of pathogenicity in plant-bacterial interactions.