Isolation and genetic characterization of Aurantimonas and Methylobacterium strains from stems of hypernodulated soybeans.

The aims of this study were to isolate Aurantimonas and Methylobacterium strains that responded to soybean nodulation phenotypes and nitrogen fertilization rates in a previous culture-independent analysis (Ikeda et al. ISME J. 4:315-326, 2010). Two strategies were adopted for isolation from enriched bacterial cells prepared from stems of field-grown, hypernodulated soybeans: PCR-assisted isolation for Aurantimonas and selective cultivation for Methylobacterium. Thirteen of 768 isolates cultivated on Nutrient Agar medium were identified as Aurantimonas by colony PCR specific for Aurantimonas and 16S rRNA gene sequencing. Meanwhile, among 187 isolates on methanol-containing agar media, 126 were identified by 16S rRNA gene sequences as Methylobacterium. A clustering analysis (>99% identity) of the 16S rRNA gene sequences for the combined datasets of the present and previous studies revealed 4 and 8 operational taxonomic units (OTUs) for Aurantimonas and Methylobacterium, respectively, and showed the successful isolation of target bacteria for these two groups. ERIC- and BOX-PCR showed the genomic uniformity of the target isolates. In addition, phylogenetic analyses of Aurantimonas revealed a phyllosphere-specific cluster in the genus. The isolates obtained in the present study will be useful for revealing unknown legume-microbe interactions in relation to the autoregulation of nodulation.