Photosynthetic bradyrhizobia are natural endophytes of the African wild rice Oryza breviligulata

We investigated the presence of endophytic rhizobia within the roots of the wetland wild rice Oryza breviligulata, which is the ancestor of the Africa n cultivated rice Oryza glaberrima, This primitive rice species grows in th e same wetland sites as Aeschynomene sensitiva, an aquatic stem-nodulated l egume associated with photosynthetic strains of Bradyrhizobium. Twenty endo phytic and aquatic isolates were obtained st three different sites in West Africa (Senegal and Guinea) from nodal roots of O. breviligulata and surrou nding water by using A. sensitiva as a trap legume. Most endophytic and aqu atic isolates were photosynthetic and belonged to the same phylogenetic Bra dyrhizobium/Blastobacter subgroup as the typical photosynthetic Bradyrhizob ium strains previously isolated from Aeschynomene stem nodules, Nitrogen-fi xing activity, measured by acetylene reduction, was detected in rice plants inoculated with endophytic isolates. A 20% increase in the shoot growth an d grain yield of O. breviligulata grown in a greenhouse was also observed u pon inoculation with one endophytic strain and one Aeschynomene photosynthe tic strain. The photosynthetic Bradyrhizobium sp, strain ORS278 extensively colonized the root surface, followed by intercellular, and rarely intracel lular, bacterial invasion of the rice roots, which was determined with a la cZ-tagged mutant of ORS278. The discovery that photosynthetic Bradyrhizobiu m strains, which are usually known to induce nitrogen-fixing nodules on ste ms of the legume Aeschynomene, are also natural hue endophytes of the primi tive rice O. breviligulata could significantly enhance cultivated rice prod uction.