Involvement of ROP6 and clathrin in nodulation factor signaling.

The symbiotic association between the legume Lotus japonicus and the nitrogen-fixing bacterium Mesorhizobium loti results in the formation of root nodules. This process begins with the recognition of the rhizobial nodulation factor (NF) by the NF receptors (NFR) at the cell surface of the host roots. The downstream signaling cascades after NFR recognition have not been fully characterized. We recently identified a clathrin heavy chain 1 (CHC1) from L. japonicus as a potential target of the NF signaling cascades. CHC is a known central component in the clathrin-mediated endocytosis (CME) in eukaryotic cells. The CHC1 gene was highly expressed in Rhizobium-infected root hairs and the CHC1 protein was present in cytoplasmic punctate structures near the infection pockets and along the infection thread membrane. Furthermore, expression of a dominant-negative variant of CHC1 or treatment with a chemical inhibitor of CME resulted in impaired phenotypes in the NF signaling, rhizobial infection and nodulation. These findings open a new avenue for future work aiming at understanding the role of endocytosis in NF signaling pathway and rhizobial infection.