Rhizobium gallicum sp. nov. and Rhizobium giardinii sp. nov., from Phaseolus vulgaris nodules.

Thirty-one strains of two new genomic species (genomic species 1 and 2) of rhizobia isolated from root nodules of Phaseolus vulgaris and originating from various locations in France were compared with reference strains of rhizobia by performing a numerical analysis of 64 phenotypic features. Each genomic species formed a distinct phenon and was separated from the other rhizobial species. A comparison of the complete 16S rRNA gene sequences of a representative of genomic species 1 (strain R602spT) and a representative of genomic species 2 (strain H152T) with the sequences of other rhizobia and related bacteria revealed that each genomic species formed a lineage independent of the lineages formed by the previously recognized species of rhizobia. Genomic species 1 clustered with the species that include the bean-nodulating rhizobia, Rhizobium leguminosarum, Rhizobium etli, and Rhizobium tropici, and branched with unclassified rhizobial strain OK50, which was isolated from root nodules of Pterocarpus klemmei in Japan. Genomic species 2 was distantly related to all other Rhizobium species and related taxa, and the most closely related organisms were Rhizobium galegae and several Agrobacterium species. On the basis of the results of phenotypic and phylogenetic analyses and genotypic data previously published and reviewed in this paper, two new species of the genus Rhizobium, Rhizobium gallicum and Rhizobium giardinii, are proposed for genomic species 1 and 2, respectively. Each species could be divided in two subgroups on the basis of symbiotic characteristics, as shown by phenotypic (host range and nitrogen fixation effectiveness) and genotypic data. For each species, one subgroup had the same symbiotic characteristics as R. leguminosarum biovar phaseoli and R. etli biovar phaseoli. The other subgroup had a species-specific symbiotic phenotype and genotype. Therefore, we propose that each species should be subdivided into two biovars, as follows: R. gallicum biovar gallicum and R. gallicum biovar phaseoli; and R. giardinii biovar giardinii and R. giardinii biovar phaseoli.

Classification of Austrian rhizobia and the Mexican isolate FL27 obtained from Phaseolus vulgaris L. as Rhizobium gallicum.

The phylogenetic positions of four rhizobial strains obtained from nodules of common bean plants (Phaseolus vulgaris L.) grown in an Austrian soil and of the Mexican bean isolate FL27 are described. Analysis of the 16S rRNA genes revealed sequences almost identical to that of the Rhizobium gallicum type strain, R602sp, with a maximum of two nucleotide substitutions. Comparison of the 16S rRNA gene sequences with those from other bacteria indicated highest similarity to Rhizobium sp. strain OK-50, Rhizobium leguminosarum IAM 12609, and Rhizobium etli. DNA homology determined by DNA-DNA hybridization was high among the Austrian isolates and R602spT (45 to 90%) and ranged from 21 to 65% with FL27, but hybridization analysis revealed very low homology to the recognized common bean-nodulating species, R. leguminosarum bv. phaseoli, R. etli, and Rhizobium tropici. Ribosomal gene organization was studied by Southern hybridization with the 16S rRNA gene and temperature gradient gel electrophoresis, indicating identical organizations and the presence of three identical 16S rRNA copies in the genome of this species. The six strains investigated showed different plasmid profiles based on their geographical origins. We propose that the Austrian isolates and the Mexican strain FL27 are members of the species R. gallicum.

Endocytobiosis: general principles.

The leafhopper Euscelis incisus K (Homoptera) hosts two types of obligate intracellular symbionts (endocytobionts). The DNA molecular weight (approximately 10(8)) of endocytobionts corresponds to that of the mitochondria and plastids. They are transmitted to the next host generation by incorporation between the egg coat and egg cell in the form of an infection mass. Excision of the infection mass results in cephalothorax embryos which lack the abdomen. Endocytobionts synthesize metabolites such as vitamins, amino acids for the hists using their waste products such as urea and uric acid. The endocytobionts regulate pH, osmotic pressure and certain endogenous rhythms of their hosts. This implies that the leafhopper endocytobionts represent for the host cell not only nutrition but also genomic supplement. According to this hypothesis, the structure, function and information of endocytobionts and eukaryotic DNA-containing cell organelles are analogous; these analogies indicate that endocytobionts may provide a model for molecular analysis of eukaryotic cell system. "Endocytobiology" consequently represents a modern field of research between symbiosis and cell biology.