Root nodules of legumes: A suitable ecological niche for isolating non-rhizobial bacteria with biotechnological potential in agriculture

Rhizobial bacteria have been established not to be the only residents of the root nodules of legumes. Non-rhizobial bacteria have also been found in legume root nodules, but their ecological roles are not fully understood. However, the interaction between these bacteria and rhizobial bacteria inside nodules might influence the fitness and behavior of their host legume plants. Although a diverse population of the non-rhizobial bacteria within nodules does not elicit nodulation or nitrogen fixation, they have been found to have multiple-plant growth-promoting (PGP) characteristics. Co-inoculation of legumes with nodule non-rhizobial bacteria has synergistic effects on plant growth and yield, beyond those of single inoculation, in legumes under both stress and non-stress conditions. In this graphical review, the potential importance of beneficial non-rhizobial bacteria within legume root nodules is highlighted, and their benefits to the host plants are also discussed. The use of nodule non-rhizobial strains with multiple PGP potential may be a promising method to boost the effectiveness of rhizobial bacteria biofertilizers in sustainable agriculture, under both stress and non-stress conditions.