RHIZOBIUM SP, NGR234 CONTAINS A FUNCTIONAL COPY OF THE SOYBEAN CULTIVAR SPECIFICITY LOCUS, NOLXWBTUV

Rhizobium sp, NGR234, a broad host range nitrogen-fixing symbiont of many legumes, is closely allied to Rhizobium fredii, but there are conflicting reports of its ability to nodulate soybean, the normal host of R. fredii, We evaluated the symbiotic interaction of NGR234 with 89 soybean cultivars and detected only disorganized structures, which had central vascular connections and lacked bacteroids. NGR234 contains nolXWBTUV, a locus responsible for cultivar-specific nodulation of soybean by R. fredii USDA257, We cloned the NGR234 locus as an 8.0-kb EcoRI fragment and found that its restriction map differs from that of the corresponding fragment from USDA257 at only two sites, RNA dot blot analysis indicated that the NGR234 locus is expressed in response to flavonoid signals, This observation was confirmed by probing Western blots with polyclonal antibodies directed against NolX of USDA257 and by monitoring levels of expression of a fusion between the nolX allele of NGR234 and lacZ, Histochemical observations with the gene fusion documented that nolX is expressed by rhizobia in the rhizosphere of soybean roots and in infection threads and mature nodules of cowpea.