Characterization of Sinorhizobium strains isolated from arid and semi-arid areas of Morocco promoting the growth of Vachellia gummifera

Vachellia gummifera is a nitrogen-fixing shrub endemic to Morocco, and represents a suitable candidate for reforestation programs in dry areas. In this study, we identified seven root microsymbionts of V. gummifera, and determined their phenotypic characteristics, Plant Growth-Promoting (PGP) activities and their symbiotic efficiency. The bacteria were isolated from root nodules of the plant grown in Moroccan forest soils located in arid and semi-arid regions. The 16S rRNA gene sequences analysis of seven isolates showed similarity values ranging from 99.85% to 99.92% with Sinorhizobium fredii USDA205^T. The phenotypic analyses displayed that the strains use different carbohydrates, mainly mono and di-saccharides, as a sole carbon source, and 13 out of 15 amino acids tested as the sole nitrogen source. The strains have variable tolerance to heavy metals, but are sensitive to salinity and high temperature, as no one grew in the presence of 2% NaCl (w/v) or at 40 °C, respectively. On the other hand, the strains grew in a very high osmotic stress environment induced by PEG 6000 (20% w/v), and they all grow in a pH range between 6.5 and 9. The strains possess PGP activities, such as phosphate solubilization, siderophore, and indole acetic acid production. Inoculation under axenic conditions showed that the strains increased the aerial biomass of inoculated plants up to 217%, whereas the relative symbiotic efficiency percentages (74.87–140,3%) showed that most strains are very efficient. From all these results we concluded that our strains may be proposed as inoculum for large-scale applications, such as reforestation programs.