Root hair deformation of symbiosis-deficient mutants of lotus japonicus by application of nod factor from Mesorhizobium loti

Abstract

In the model leguminous plant Lotus japonicus, the reception of a symbiotic signal called Nod factor (NF), which is secreted by the symbiont bacterium Mesorhizobium loti, induces wavy shaped root hairs. This is called root hair deformation. To dissect the root hair deformation process, we studied symbiosis-deficient mutants of L. japonicus, castor, nup85, ccamk and nsp2. The CASTOR, NUP85, and CCaMK genes are also required for mycorrhizal infection and thus called common symbiotic genes. On the global application of NF, all the mutants except nsp2 exhibited extensive branching of root hairs. The actin cytoskeleton was also observed as a marker for NF-dependent responses in mutant root hairs. At 2 hours of NF treatment, the ccamk mutant showed exaggerated swelling compared with the other mutants, indicating CCaMK to be required to terminate the swelling. In the nsp2 mutant, two hours of NF treatment remarkably induced swelling at root hair tips, although root hair deformation was not apparent at 24 hours of NF treatment. These results showed that common symbiotic components are involved in root hair deformation, which is regulated by a fine tuning mechanism early in the symbiosis between leguminous plants and rhizobia.