Identification at the species and symbiovar levels of strains nodulating Phaseolus vulgaris in saline soils of the Marrakech region (Morocco) and analysis of the otsA gene putatively involved in osmotolerance

Salinity is an increasing problem in Africa affecting rhizobia-legume symbioses. In Morocco, Phaseolus vulgaris is cultivated in saline soils and its symbiosis with rhizobia depends on the presence of osmotolerant strains in these soils. In this study, 32 osmotolerant rhizobial strains nodulating P. vulgaris were identified at the species and symbiovar levels by analysing core and symbiotic genes, respectively. The most abundant strains were closely related to Rhizobium etli and R. phaseoli and belonged to symbiovar phaseoli. A second group of strains was identified as R. gallicum sv gallicum. The remaining strains, identified as R. tropici, belonged to the CIAT 899(T) nodC group, which has not yet been described as a symbiovar. In representative strains, the otsA gene involved in the accumulation of trehalose and putatively in osmotolerance was analysed. The results showed that the phylogeny of this gene was not completely congruent with those of other core genes, since the genus Ensifer was more closely related to some Rhizobium species than others. Although the role of the otsA gene in osmotolerance is not well established, it can be a useful protein-coding gene for phylogenetic studies in the genus Rhizobium, since the phylogenies of otsA and other core genes are coincident at the species level.