Progress in multinational collaborative studies on the beneficial association between Rhizobium leguminosarum bv. trifolii and rice

This chapter summarizes our collaborative project to search for natural, intimate associations between rhizobia and rice (Oryza sativa), assess their impact on plant growth, and ultimately exploit those that can enhance grain yield with less dependence on nitrogen fertilizer inputs. Two cycles of field and laboratory studies have indicated that diverse indigenous populations of the clover rootnodule symbiont, Rhizobium leguminosarum bv. trifolii, intimately colonize rice roots in cultivated fields of the Egyptian Nile Delta, where rice has been rotated successfully with berseem clover (Trifolium alexandrinum) since antiquity. Certain strain/variety interactions significantly expand rice root architecture, enhance the uptake of several plant nutrients, and increase plant biomass under laboratory and greenhouse conditions. Preliminary results indicating statistically significant increases in grain yield and agronomic fertilizer N-use efficiency following inoculation have been obtained. We are now examining various basic and applied aspects of this beneficial Rhizobiumrice association, such as its ecology, physiology, biochemistry, and molecular biology, the identification of the underlying mechanisms of plant growth promotion operative in this beneficial association, and further assessment of selected rhizobial strains to perform as beneficial biofertilizer inoculants for rice under field conditions. Certain strains of these rice-adapted rhizobia colonize the surface and within limited regions of the interior of lateral roots of rice seedlings, secrete indoleacetic acid and gibberellin phytohormones in vivo, and extracellularly solubilize precipitated phosphates. Various acetylene reduction assays and 15 N-based studies do not support a role of biological nitrogen fixation in the positive plant growth-promotion response of this Rhizobium-rice association. This natural, intimate Rhizobium-rice association represents a unique experimental system suitable for both basic and applied studies on beneficial rice-bacteria interactions. This association of dissimilar organisms living together may also turn out to offer potential benefits to enhance the sustainable agriculture of rice, the most important cereal crop of the developing world.