ROOT NODULATION OF SESBANIA-ROSTRATA SUPPRESSES STEM NODULATION BY SINORHIZOBIUM-TERANGA BUT NOT AZORHIZOBIUM-CAULINODANS

Azorhizobium caulinodans ORS571 and Sinorhizobium teranga ORS51 and OR S52 are symbionts of the same host plant Sesbania rostrata. In nature, A. caulinodans nodulates more competitively the stem-located infectio n sires of Sesbania rostrata. Sinorhizobium strains, although frequent ly present in root nodules, are seldom found in stem nodules. One prob able explanation for this phenomenon is the more abundant presence of Azorhizobium on the leaf and stem surfaces of the host plant. Work pre sented here hints at other plausible factors that determine the greate r ''stem specificity'' of Azorhizobium. We found that under experiment al conditions in which roots are not inoculated, all strains nodulated stems very well. However, ORS51 and ORS52 were much more sensitive th an ORS571 to suppression of stem nodulation by previous root inoculati on. The introduction of the regulatory nodD gene from A. caulinodans d iminished the sensitivity to this suppression, probably by enhanced no d gene expression and subsequent Nod factor production. Our hypothesis is that the greater infectivity of ORS571 is due to a more efficient production of mitogenic Nod factors at stem-located infection sites, t hereby more readily overcoming systemic suppression caused by previous root inoculations.