Seeds of Phaseolus vulgaris bean carry Rhizobium etli

The presence of soil bacteria on seeds could provide an explanation for bacterial geographical spread. We report that Phaseolus vulgaris bean seeds naturally carry rhizobia on their testa. One hundred eighteen Rhizobium isolates from L-3-1-1-1 seeds were characterized by multilocus enzyme electrophoresis and compared to Rhizobium species which form nitrogen fixing nodules on the roots of bean plants. As a result of this and other analyses, seed isolates were classified as R. etli. Seed borne rhizobia are probably moisture limited and are acquired from contaminating soil during the harvesting process as seeds from non-opened pods do not carry them. Rhizobia on seeds are in a presumably desiccated or dormant state as they require a rehydration process during which they are antibiotic sensitive. With an improved procedure to detect rhizobia on seeds, we found that 5 out of 13 Phaseolus vulgaris bean cultivars tested harbor rhizobia on the seeds.