Diversity of rhizobia associated with Amorpha fruticosa isolated from Chinese soils and description of Mesorhizobium amorphae sp. Nov.

Fifty-five Chinese isolates from nodules of Amorpha fruticosa were characte rized and compared with the type strains of the species and genera of bacte ria which form nitrogen-fixing symbioses with leguminous host plants. A pol yphasic approach, which included RFLP of PCR-amplified 16S rRNA genes, mult ilocus enzyme electrophoresis (MLEE), DNA-DNA hybridization, 16S rRNA gene sequencing, electrophoretic plasmid profiles, cross-nodulation and a phenot ypic study, was used in the comparative analysis. The isolates originated f rom several different sites in China and they varied in their phenotypic an d genetic characteristics. The majority of the isolates had moderate to slo w growth rates, produced acid on YMA and harboured a 930 kb symbiotic plasm id (pSym). Five different RFLP patterns were identified among the 165 rRNA genes of all the isolates. Isolates grouped by PCR-RFLP of the 165 rRNA gen es were also separated into groups by variation in MLEE profiles and by DNA -DNA hybridization. A representative isolate from each of these DNA homolog y groups had a separate position in a phylogenetic tree as determined from sequencing analysis of the 165 rRNA genes. A new species, Mesorhizobium amo rphae, is proposed for the majority of the isolates, which belonged to a mo derately slow- to slow-growing, acid-producing group based upon their disti nct phylogenetic position, their unique electrophoretic type, their low DNA homology with reference strains representing the species within the genus Mesorhirobium and their distinct phenotypic features. Strain ACCC 19665 was chosen as the type strain for M. amorphae sp, nov.