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1 Key Laboratory of Agro-Microbial Resources and Application, Ministry of Agriculture of China, College of Biological Sciences, China Agricultural University, Beijing 100094, China
2 Department of Applied Chemistry and Microbiology, Biocenter 1, PO Box 56, FIN-00014, University of Helsinki, Finland
3 Department of Biology, University of York, PO Box 373, York Y01 5YW, UK
4 Departamento de Microbiología, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, México D.F. 11340, Mexico
5 Department of Molecular Genetics, College of Agronomy, South China Agricultural University, Guangzhou 510642, China
Correspondence
Wen Xin Chen
wenxin_chen{at}263.net
Ninety-five rhizobial strains isolated from Astragalus adsurgens growing in the northern regions of China were classified into three main groups, candidate species I, II and III, based on a polyphasic approach. Comparative analysis of full-length 16S rRNA gene sequences of representative strains showed that candidate species I and II were Mesorhizobium, while candidate species III, which consisted of non-nodulating strains, was closely related to Agrobacterium tumefaciens. The phylogenetic relationships of the three candidate species and some related strains were also confirmed by the sequencing of glnA genes, which were used as an alternative chromosomal marker. The DNA–DNA relatedness was between 11·3 and 47·1 % among representative strains of candidate species I and II and the type strains of defined Mesorhizobium species. Candidate III had DNA relatedness of between 4·3 and 25·2 % with type strains of Agrobacterium tumefaciens and Agrobacterium rubi. Two novel species are proposed to accommodate candidate species I and II, Mesorhizobium septentrionale sp. nov. (type strain, SDW014T=CCBAU 11014T=HAMBI 2582T) and Mesorhizobium temperatum sp. nov. (type strain, SDW018T=CCBAU 11018T=HAMBI 2583T), respectively. At least two distinct nodA sequences were identified among the strains. The numerically dominant nodA sequence type was most similar to that from the Mesorhizobium tianshanense type strain and was identified in strains belonging to the two novel species as well as other, as yet, undefined genome types. Host range studies indicate that the different nodA sequences correlate with different host ranges. Further comparative studies with the defined Agrobacterium species are needed to clarify the taxonomic identity of candidate species III.
The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene sequences obtained in this work are AF508207, AF508208 and AF508209 for strains SDW014T, SDW018T and SDW052, respectively. Those for the partial glnA gene sequences determined in this study are AJ579875–AJ579885.
A phylogenetic tree is available as supplementary material in IJSEM Online.
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