Quantitative time-course proteome analysis of Mesorhizobium loti during nodule maturation

Rhizobia are nitrogen-fixing bacteria that establish a symbiotic relationship with leguminous plants. To understand the mechanism by which rhizobia alter their metabolism to establish successful nitrogen-fixing symbiotic relationship with hosts, Lotus japonicus were inoculated with Mesorhizobium loti. Bacteroids were isolated from nodules harvested at 2weeks (the early stage of nodule development), and at 3 and 4weeks (the intermediate stage of nodule development) post-inoculation. Using a quantitative time-course proteome analysis, we quantified the variations in the production of 537 proteins in M. loti bacteroids during the course of nodule maturation. The results revealed significant changes in the carbon and amino acid metabolisms by M. loti upon differentiating into bacteroids. Furthermore, our findings suggested that M. loti enters a nitrogen-deficient condition during the early stages of nodule development, and then a nitrogen-rich condition during the intermediate stages of nodule development. In addition, our data indicated that M. loti assimilated ammonia during the intermediate stages of nodule development. Our results provide new insights into the course of physiological transitions undergone by M. loti during nodule maturation.