Potential of rhizobia for sustainable production of non-legumes

Abstract

Rhizobia are familiar as the symbiotic associates of legumes, forming N2-fixing nodules. However, it has been proven that these bacteria also have the ability to survive and colonize the roots of non-legumes as efficiently as they colonize the roots of their legume host. Although example of N2-fixation in non-legume parasponia has been observed, the researchers are yet unable to extend this nitrogen-fixation symbiosis to major cereals of the planet. Only nodule-like structures or hypertrophies or outgrowths have been recorded with rhizobia on the roots of non-legumes yet without significant N2-fixation. Generally, three types of rhizobial interactions with non-legumes have been demonstrated i.e. interactions which result in growth and yield promotion of the interacting non-legumes, those which result in poor or detrimental effect on the growth and yield of inoculated non-leguminous plants and those which cannot result in any increase or decrease (missing effect/no effect/neutral effect) in the plant growth. Like other PGPR, rhizobia can affect the non-legumes beneficially by solubilizing sparingly soluble organic and inorganic phosphates, by releasing phytohormnes, enzymes, siderophores, lumichromes, lipo-chito-oligosaccharides, exo-polysaccharides and riboflavins. They can also promote the growth of non-legumes by inhibiting the growth of pathogens by sequestering the iron in the rhizosphere with siderophore production, by releasing the antibiotics and/or by the production of cell wall degrading enzymes. They can also play a significant role in alleviating the deleterious effects of various environmental stresses. Even more, they can improve the growth of non-legumes by changing the host-plant susceptibility by releasing different bio-stimulatory agents. However, plant variety, cultural conditions, native micro-flora, soil and other ecological factors have been reported affecting the degree to which rhizobial association benefit the non-legumes. But it has also been recognized that the potential of PGPR strains of rhizobia (regarding colonization to a variety of plants, adoption under variable soil and environmental conditions and against the pathogens that can attack the host plants) can be improved further through dual/mix inoculation with other beneficial microorganisms. Finally, it is recognized that competent rhizobial strains could be used as biofertilizers, stress regulators, and as biocontrol agents to non-legumes to increase their production. However, much more research efforts are needed to develop rhizobial strains which can effectively improve crop productivity under a variety of environments.