Screening of stress tolerant bacterial strains possessing interesting multi-plant growth promoting traits isolated from root nodules of Phaseolus vulgaris L.

Rhizobia are soil bacteria known for their ability to nodulate and fix nitrogen with legumes. Some rhizobia also show different plant growth promoting characteristics (PGP); they can therefore be used as effective bio-fertilizers. Phaseolus vulgaris is considered a weak nitrogen fixer; therefore, screening experiments have been designed to select stress-tolerant effective bean rhizobia that have also PGP traits. Surveys of common bean fields in Morocco lead to the isolation of 113 bacteria inhabiting root nodules. Screening tests showed that 45 isolates exhibited positive phosphate solubilization, and auxin and siderophores production as well as high salt and pH tolerance. Correspondence analysis showed that salt tolerance and phosphate solubilization were origin related. Among the screened isolates, 18 were able to renodulate common bean but only 3 were highly effective with their host plant. Interestingly, phylogenetic analysis of 16S rRNA gene showed that the nodulating isolates clustering within the Rhizobium genus were represented by only two isolates related to R. miluonense and R. yanglingense while the remaining strains belonged to the Agrobacterium genus. Our results revealed also the coexistence of several endophytic bacteria with the symbiotic strains inside the nodules. These bacteria are stress tolerant and possess potential PGP traits, which indicate that they could stimulate common bean growth and contribute to N and P plants nutrition.