Sesbania herbacea-Rhizobium huautlense nodulation in flooded soils and comparative characterization of S. herbacea-nodulating rhizobia in different environments

The nodulation of S. herbacea was compared under flooded and non-flooded co nditions in two different soils. One soil was from a flooded field in Sierr a de Huautla, the native habitat of this legume, while the other soil was f rom a well-drained field in Cuernavaca, where rhizobia were found to nodula te the introduced S. herbacea plants. Nodulation of the plants was complete ly eliminated by flooding in the Cuernavaca soil, whereas nodules were obta ined in the same soil under nonflooded conditions. In contrast, nodules wer e formed in Huautla soil under both flooded and non-flooded conditions. Mos t isolates, except isolate HS2, from Huautla soil and water were identified as R. huautlense by colony morphology, growth rate, PCR-RFLP of 16S rRNA g enes, MLEE, cellular plasmid contents, and RFLP of nifH and nodDAB genes. I solate HS2 was identified as Mesorhizobium sp. Isolates from Cuernavaca soi l were different from R. huautlense in many aspects and were classified int o five rDNA types within the genera Mesorhizobium, Rhizobium, and Sinorhizo bium by PCR-RFLP of 16S rRNA genes. R. huautlense is a water Rhizobium spec ies. Growth by denitrification under oxygen limitation or with ethanol was observed for R. huautlense bacteria but not for the isolates from Cuernavac a. In an interstrain nodulation competitive assay under both flooded and no n-hooded conditions, R. huautlense strain SO2 completely inhibited the nodu lation of Mesorhizobium sp. Sn2, an isolate from Cuernavaca. From these res ults, we conclude that R. huautlense has the unique ability to nodulate S. herbacea not only in flooded soils, but in nonflooded soils as well.