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Mesorhizobium gobiense sp. nov. and Mesorhizobium tarimense sp. nov., isolated from wild legumes growing in desert soils of Xinjiang, China

¹ Key Laboratory of Agro-Microbial Resource and Application, Ministry of Agriculture/College of Biological Sciences, China Agricultural University, Beijing 100193, PR China
² Departamento de Microbiología, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, 11340 México DF, México
³ The Institute of Agricultural Resources and Regional Planning of Chinese Academy of Agricultural Sciences. Beijing 100081, PR China

Correspondence
Wen Xin Chen
wenxin{at}cau.edu.cn

Twenty-four Mesorhizobium strains were isolated from desert soils in the Xinjiang region of China and were characterized by a polyphasic approach. These strains grouped into three clusters in IGS-RFLP, SDS-PAGE analysis of whole-cell proteins and BOX-PCR analysis, corresponding to genomic species V, VI and VII as found in a previous study. The results were supported by sequencing analyses of rrs, IGS, atpD and recA genes. Genospecies VII was most related to Mesorhizobium septentrionale, while genospecies V and VI were both most closely related to Mesorhizobium tianshanense, but were distinct from each other and from M. tianshanense. The DNA–DNA hybridization value between the representative strain CCBAU 83284 (genospecies VII) and the type strain of M. septentrionale was 90.1 %. Genospecies VII was thus defined as M. septentrionale. The DNA–DNA relatedness value for representative strains of genospecies V or VI with the related reference strains of recognized species were always lower than 60 %. Low values of DNA–DNA hybridization (32.79 %) between representative strains of genospecies V (CCBAU 83330^T) and of VI (CCBAU 83306^T) were also observed. Based upon these results, two novel species are proposed: Mesorhizobium gobiense sp. nov. represented by genospecies V (type strain, CCBAU 83330^T=LMG 23949^T=HAMBI 2974^T) and Mesorhizobium tarimense sp. nov. represented by genospecies VI (type strain, CCBAU 83306^T=LMG 24338^T=HAMBI 2973^T). Strain CCBAU 83278 grouped as the most peripheral member with genospecies VI in SDS-PAGE of whole-cell proteins and BOX-PCR analysis and in the phylogenetic tree of 16S-23S rRNA intergenic spacer (IGS) sequences. The results of analyses of rrs, atpD and recA gene sequences, as well as those of DNA–DNA hybridization studies, strongly supported the suggestion that this strain belonged to a species quite different from genospecies V and VI and from any other recognized species of the genus Mesorhizobium. As only one strain has been isolated to date, strain CCBAU 83278 was not proposed as a novel species in this study. Mesorhizobium gobiense sp. nov. and Mesorhizobium tarimense sp. nov. could be differentiated from each other as well as from recognized species of the genus Mesorhizobium on the basis of phenotypic characteristics. The symbiotic loci (nodC and nifH) of the two novel species formed two phylogenetic branches related to Mesorhizobium loti and M. tianshanense. The type strains of the two novel species were able to nodulate Glycyrrhiza uralensis, Lotus corniculatus, Oxytropis glabra and Robinia pseudoacacia but not Astragalus membranaceus, Leucaena leucocephala, Phaseolus vulgaris, Pisum sativum or Medicago sativa.

The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene sequences for M. gobiense sp. nov. strains CCBAU 83330^T, CCBAU 83511 and CCBAU 83346, M. tarimense sp. nov. strains CCBAU 83306^T and CCBAU 83321, strain CCBAU 83278 and M. septentrionale strains CCBAU 83281and 83284 are EF035064, EF549381, EF549388, EF035058, EF549387, EU368831, EF549379 and EF035063, respectively. Those for partial IGS are EF050768, EF549537, EF050769, EF050771, EF050772, EF050770, EF549535 and EF050773, respectively. Accession numbers for the partial atpD gene sequences are EF549409, EF549411, EF549413, EF549410, EF549412, EU371996, EF549407 and EF549415, respectively. For partial recA gene sequences, the accession numbers are EF549481, EF549480, EF549478, EF549482, EF549479, EU371997, EF549484 and EF549485, respectively. The partial nodC gene sequences are EF050784, EF549525, EF549524, EF050786, EF549514, EF549527, EF549511 and EF050783, respectively. The partial nifH gene sequences are EU252606, EU252604, EU252605, EU252607, EU252608, EU252603, EU252601 and EU252602, respectively.

Dendrograms constructed from IGS-RFLP, BOX-PCR and SDS-PAGE analyses and additional phylogenetic trees based on IGS, atpD, recA, nodC and nifH gene sequences are available as supplementary figures with the online version of this paper. A supplementary table giving details of cellular fatty acid contents is also available.