Abstract
Over the last couple of decades, rapid progress has been made in understanding the molecular mechanism of legume-microbe endosymbiotic interactions. Forward and reverse genetic approaches, using model legumes Lotus japonicus and Medicago truncatula, allowed the identification of host plant genes, especially responsible for the early stage of infections by micro-symbionts. In the case of root nodule symbiosis, rhizobia release symbiotic signal molecules, Nod factors (NFs). Host-specific recognition of NFs leads to the activation of symbiotic Ca^<2+> spiking. In response to compatible rhizobia, root hair curling and the formation of intracellular tubular structure, infection threads (ITs) occur. In accordance with rhizobial infection through ITs, cortical cell division is triggered and root nodules are developed. In this chapter, we introduce host symbiotic genes classified into three steps of rhizobial infection, i.e., (1) Perception of host-specific NFs by LysM receptor kinases, (2) Induction of Ca^<2+> spiking and transduction of Ca^<2+> signals through common symbiosis pathway, and (3) Rhizobial infection via intracellular root hair ITs.
