Perspectives of rhizobial inoculation for sustainable crop production

Abstract

The need for sustainable agricultural practices is revitalizing the worldwide interest in biological nitrogen fixation (BNF) and rhizobia- legume symbiosis, particularly in organic production system. Rhizobia-legume symbiosis is a complex process involving a set of plant and bacterial genes leading to the formation and development of root nodules on legume roots where rhizobia fix nitrogen from the atmosphere. About 60 % of the Earth’s available nitrogen is fixed via BNF. The success of the symbiotic process depends on the competitiveness, specificity, compatibility, and effectiveness of rhizobia with its host legume under variable soil and environmental conditions. In general, the interaction of rhizobia with nonlegumes has been neglected because of the lack of ability to respond to the rhizobial lipochitin Nod factors, which are the essential signal molecules that trigger legume nodulation. Work on rhizobial interaction with nonlegumes, over the years, has been done progressively; it has been demonstrated that rhizobia can associate with roots of nonlegumes also and can promote their growth by using one or more of the direct or indirect mechanisms of actions. The beneficial association/symbiosis of rhizobia can be negatively affected by many rhizosphere processes and other environmental stresses, resulting in important ecological, economic, and nutritional losses. The application of environment- friendly agricultural techniques, based on the use of plant growth-promoting rhizobacteria (PGPR) and/or plant growth regulators (PGRs), can increase the efficiency of the symbiotic/associative processes. The application of appropriate rhizobia together with PGPR/PGRs is considered an effective and environment-friendly approach which may profoundly increase the crop yield by different mechanisms of actions under variable conditions. Plant growth promotion of legumes/nonlegumes by rhizobia and their practical application as selected commercially available rhizobial inoculants are the major focus of this chapter.