Genetic diversity, symbiotic efficiency, stress tolerance, and plant growth promotion traits of rhizobia nodulating Vachellia tortilis subsp. raddiana growing in dryland soils in southern Morocco

In the present study, we analyzed the genetic diversity, phylogenetic relationships, stress tolerance, phytoben-eficial traits, and symbiotic characteristics of rhizobial strains isolated from root nodules of Vachellia tortilis subsp. raddiana grown in soils collected in the extreme Southwest of the Anti-Atlas Mountains in Morocco. Subsequent to Rep-PCR fingerprinting, 16S rDNA gene sequencing of 15 representative strains showed that all of them belong to the genus Ensifer. Phylogenetic analysis and concatenation of the housekeeping genes gyrB, rpoB, recA, and dnaK revealed that the entire collection (except strain LMR678) shared 99.08 % to 99.92% similarity with Ensifer sp. USDA 257 and 96.92% to 98.79% with Sinorhizobium BJ1. Phylogenetic anal-ysis of nodC and nodA sequences showed that all strains but one (LMR678) formed a phylogenetic group with the type strain E. aridi LMR001T (similarity over 98%). Moreover, it was relevant that most strains belong to the symbiovar vachelliae. In vitro tests revealed that five strains produced IAA, four solubilized inorganic phos-phate, and one produced siderophores. All strains showed tolerance to NaCl concentrations ranging from 2 to 12% and grew at up to 10% of PEG6000. A greenhouse plant inoculation test conducted during five months demonstrated that most rhizobial strains were infective and efficient. Strains LMR688, LMR692, and LMR687 exhibited high relative symbiotic efficiency values (respectively 231.6 %, 171.96 %, and 140.84 %). These strains could be considered as the most suitable candidates for inoculation of V. t. subsp. raddiana, to be used as a pioneer plant for restoring arid soils threatened with desertification.

Automated summary

In this study, we analyzed the genetic diversity, phylogenetic relationships, stress tolerance, phytoben-eficial traits, and symbiotic characteristics of rhizobial strains isolated from root nodules of Vachellia tortilis subsp. raddiana. The majority of the strains belong to the genus Ensifer and showed high similarity with Ensifer sp. USDA 257 and Sinorhizobium BJ1. Most strains belong to symbiovar vachelliae and exhibited plant growth-promoting activities. In vitro tests demonstrated the production of IAA and solubilization of inorganic phosphate, as well as tolerance to high salt and PEG6000 concentrations. Greenhouse inoculation tests showed high infectivity and efficiency of most rhizobial strains, with LMR688, LMR692, and LMR687 exhibiting the highest relative symbiotic efficiency values.