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 conditions in two different soils. One soil was from a flooded field in Sierra de Huautla, the native habitat of this legume, while the other soil was from a well-drained field in Cuernavaca, where rhizobia were found to nodulate the introduced S. herbacea plants. Nodulation of the plants was completely eliminated by flooding in the Cuernavaca soil, whereas nodules were obtained in the same soil under nonflooded conditions. In contrast, nodules were formed in Huautla soil under both flooded and non-flooded conditions. Most isolates, except isolate HS2, from Huautla soil and water were identified as R. huautlense by colony morphology, growth rate, PCR-RFLP of 16S rRNA genes, MLEE, cellular plasmid contents, and RFLP of nifH and nodDAB genes. Isolate HS2 was identified as Mesorhizobium sp. Isolates from Cuernavaca soil were different from R. huautlense in many aspects and were classified into five rDNA types within the genera Mesorhizobium, Rhizobium, and Sinorhizobium by PCR-RFLP of 16S rRNA genes. R. huautlense is a water Rhizobium species. Growth by denitrification under oxygen limitation or with ethanol was observed for R. huautlense bacteria but not for the isolates from Cuernavaca. In an interstrain nodulation competitive assay under both flooded and non-hooded conditions, R. huautlense strain SO2 completely inhibited the nodulation of Mesorhizobium sp. Sn2, an isolate from Cuernavaca. From these results, we conclude that R. huautlense has the unique ability to nodulate S. herbacea not only in flooded soils, but in nonflooded soils as well.