Recent Advances in Rhizobium–Legume Interactions: A Proteomic Approach

Abstract

Nitrogen-fixing symbioses between legumes and rhizobia over the years have played a major role in sustainable agricultural ecosystems. Owing to specific interactions with rhizobia, the leguminous plants form specialized nitrogen-fixing organ called as nodule, wherein rhizobia dwell and bring out the conversion of atmospheric nitrogen (N) to its usable form. This symbiosis in turn may abate the demand for external application of nitrogenous fertilizers while growing legumes under natural soil environment. Contemporary genomic research has provided a better understanding of the Rhizobium–legume interaction at molecular level. Several genomic approaches have been employed to define and demonstrate the involvement of rhizobial genomes in the symbiotic events. The genomes of two rhizobial species namely Mesorhizobium loti, the symbiont of several Lotus species, and Sinorhizobium meliloti, the symbiont of alfalfa, have now been completely sequenced, which have revealed interesting information about the genome evolution and structure, plant–microbes communication, and physiological diversity among the microsymbionts of legumes. While for legumes, numerous expressed sequence tags representing tens of thousands of different genes involved in root nodule formation and nitrogen fixation from three major legume species, Glycine max, Medicago truncatula, and Lotus japonicus have been deposited in the public domain. Currently, biological research is directed to understand gene expression and function involved in rhizobia–legume interaction. In this context, proteomics with continually evolving set of novel techniques to study all facets of protein structure and function is being considered as a promising and effective tool in the postgenomic era to explore further the intricacies of symbiotic process. It is likely that the proteomics approach may reveal the newer possibilities for better understanding the complex interactions of rhizobia and legumes, and also the mechanisms as to how rhizobia survive under stressed environment. The major breakthroughs from the contemporary proteome-level investigations into legume–rhizobia interactions are discussed.