Isolation and characterization of an endophytic bacterium, Bacillus megaterium BMN1, associated with root-nodules of Medicago sativa L. growing in Al-Ahsaa region, Saudi Arabia

Medicago sativa L. (Alfalfa) is an important forage crop legume in Saudi Arabia due to its high nutritive value and yield. Soil bacteria exist in root or root-nodules of Medicago sativa in either symbiotic relationships or in associations. In the current study, the endophytic bacterium Bacillus megaterium BMN1 was isolated from surface-sterilized root-nodules of Medicago sativa and characterized phenotypically and genotypically. The results indicated that BMN1 consumed a variety of sugars as sole carbon source, and produced catalase and amylase but not urease. BMN1 exhibited some plant growth-promoting traits, such as production of indole acetic acid and acetoin, and solublization of inorganic phosphate. In addition, comparative sequence analysis of the 16S rRNA gene showed that BMN1 exhibited 99 % homology with Bacillus megaterium. In addition, BMN1 could not nodulate alfalfa when re-inoculated but the strain enhanced root growth parameters compared to uninoculated plants. Co-inoculation of BMN1with Sinorhizobium meliloti increased not only the number of nodules formed on roots of alfalfa but also root length and root dry weight under greenhouse conditions. Furthermore, the effects BMN1 inoculation on the growth of Lens esculentus, Phaseolus vulgaris and Pisum sativum were also assessed. The length of the primary root, number of secondary roots and dry weight of roots of the three crop legumes were significantly increased upon inoculation by Bacillus megaterium compared to uninoculated control plants. In conclusion, BMN1 belongs to the group of plant growth promoting rhizobacteria and could have significant agricultural applications.