ROLES FOR AZORHIZOBIAL NOD FACTORS AND SURFACE POLYSACCHARIDES IN INTERCELLULAR INVASION AND NODULE PENETRATION, RESPECTIVELY

In the symbiotic interaction between Azorhizobium caulinodans and Sesb ania rostrata root and stem-borne nodules are formed. The bacteria ent er the host via intercellular spaces at lateral or adventitious root b ases and form infection pockets in outer cortical layers. Infection th reads guide the bacteria to nodule primordia where plant cells are inv aded. To identify bacterial functions that are required for this infec tion process, two mutants defective in nodulation were studied; one pr oduced no Nod factors (nodA mutant), the other had altered surface pol ysaccharides (SPS) and induced the formation of pseudo-nodules, Bacter ia were visualized with the help of a nodA-uidA reporter fusion that w as functional during nodule development and in bacteroids. In contrast to the SPS mutant, nodA mutants were unable to colonize outer cortica l regions. In mixed inoculations with both mutants, functional nodules were formed, the central tissue of which was occupied by the nodA mut ant. These observations suggest that SPS play a role in deeper invasio n and that Nod factors are necessary for entry. Simultaneous applicati on of purified Nod factors and nodA mutant bacteria restored the forma tion of outer cortical infection pockets leading to the conclusion tha t intercellular infection is an active process that is dependent on ba cterial Nod factor signaling.