Abstract
The infection thread is a tubular structure formed in the root hair infected by rhizobia, which reach the cortex through the IT and differentiate into nitrogen-fixing root nodule bacteroids. alb1 mutants develop short and aberrant ITs. We have reported that ALB1 encodes an atypical receptor kinase with leucine-rich repeats in its extracellular domain at the 2009 annual meeting. ALB1 kinase domain (KD) contains substitution of amino acid residues at positions important for a protein kinase activity. We demonstrated that ALB1 KD, purified form E. coli cells, did not have the kinase activity in vitro. A kinase-dead version of ALB1, in which an amino acid residue of the ATP binding site was mutated, suppressed the alb1 phenotype as well as the wild type ALB1 did also. However, a KD-deleted version of ALB1 failed to suppress the phenotype, suggesting that the KD is important for the ALB1 function, although the kinase activity is not required for. Furthermore, we found that ALB1-GFP fusion protein localized to the IT plasma membrane in L. japonicus. These results suggest that ALB1 positively regulates IT development at the IT membrane without the kinase activity.
