Nodule organogenesis in Lotus japonicus

Interactions between legumes and rhizobia are one of the best-characterized symbioses. Legumes develop nodules in response to rhizobia infection. Nod factors are essential for the recognition by legumes of rhizobia. Nodule pr imordia arise from root cortex, where vacuolated cells are redifferentiated . Rhizobia infect legumes via root hairs, penetrating via infection threads provided by the plant. After their release from infection threads into pla nt cells, rhizobia fix aerial nitrogen by differentiating to bacteroids. Th e mechanism of nodule organogenesis is poorly understood. This is partly be cause there has been little molecular analysis on nodulation mutants. Recen t advances in the use of model legumes, however, have enabled one to charac terize nodulation mutants by molecular genetics. In this review, we will fo cus on the early steps of nodule organogenesis in relation to the analysis of a model legume, Lotus japonicus.