Enhancing Rhizobium–Legume Symbiosis Using Signaling Factors

Abstract

Rhizobial symbiosis with leguminous plants affects the supply of organic nitrogen. Soil bacteria comprising members of the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium, and Azorhizobium, commonly referred to as rhizobia, are taxonomically diverse members of the α and β subclasses of the Proteobacteria. They possess the ability to induce root nodules on legume plants and provide these plants with fixed nitrogen, enabling them to grow in nitrogen-limited soils. Rhizobia colonize root nodules, fix nitrogen inside, transport usable form of N to plants, and concurrently facilitate the growth and grain yields of legumes. Rhizobium–legume symbiosis is a multi-step process requiring the exchange of numerous molecular signals between bacteria and the plant host. Precise fulfilling of all stages of this molecular dialogue is prerequisite to the effective symbiosis, allowing bacteria to invade the host and, conversely, enabling the host to derive benefits from the presence of bacteria. Individual legumes are often nodulated by multiple bacterial strains with varying symbiosis-establishing capabilities. Thus, selection of highly effective strains that successfully compete with less effective ones is required when developing legume inoculants. Various factors that influence symbiotic rhizobial interactions under competitive soil environment, including the exchange of plant and bacterial signaling molecules, such as flavonoids and nodulation factor (Nod factor), in the early stages of symbiosis is highlighted. Beneficial responses of rhizobial inoculants on to legumes, as well as manipulations of symbiotic signaling factors, is likely to increase their potential as biofertilizers for sustainable agriculture to promote growth and nodulation of legume plants.