Auxin methylation by IAMT1 , duplicated in the legume lineage, promotes root nodule development in Lotus japonicus

Abstract

Significance IAA carboxyl methyltransferase 1 (IAMT1) converts auxin (IAA) into its methyl ester (MeIAA). IAMT1 is reportedly critical for shoot development of the nonsymbiotic plant Arabidopsis . On the other hand, the function of IAMT1 in roots is unknown. Here, we found that IAMT1 is duplicated in the legume lineage, which evolved root nodule symbiosis. In the model legume Lotus japonicus , one of two paralogs (named IAMT1a ) was mainly expressed in root epidermis, but its function is required in the adjacent cell layer, root cortex, where it promotes nodule development. Application of MeIAA, but not IAA, significantly induced NIN , a master regulator of nodule development, without rhizobia. These findings illuminate our understanding of intertissue communication acquired during evolution of root nodule symbiosis.