Auxin methylation by <i>IAMT1</i>, duplicated in the legume lineage, promotes root nodule development in <i>Lotus japonicus</i>.

Abstract

Legumes attract symbiotic bacteria and create de novo root organs called nodules. Nodule development consists of bacterial infection of root epidermis and subsequent primordium formation in root cortex, steps that need to be spatiotemporally coordinated. The <i>Lotus japonicus</i> mutant “<i>daphne</i> ” has uncoupled symbiotic events in epidermis and cortex, in that it promotes excessive bacterial infection in epidermis but does not produce nodule primordia in cortex. Therefore, <i>daphne</i> should be useful for exploring unknown signals that coordinate these events across tissues. Here, we conducted time-course RNA sequencing using <i>daphne</i> after rhizobial infection. We noticed that <i>IAA carboxyl methyltransferase 1 (IAMT1)</i> , which encodes the enzyme that converts auxin (IAA) into its methyl ester (MeIAA), is transiently induced in wild-type roots at early stages of infection but shows different expression dynamics in <i>daphne</i>. <i>IAMT1</i> serves an important function in shoot development of <i>Arabidopsis</i>, a nonsymbiotic plant, but the function of <i>IAMT1</i> in roots has not been reported. Phylogenetic tree analysis suggests a gene duplication of <i>IAMT1</i> in the legume lineage, and we found that one of the two <i>IAMT1s</i> (named <i>IAMT1a</i>) was induced in roots by epidermal infection. <i>IAMT1a</i> knockdown inhibited nodule development in cortex; however, it had no effect on epidermal infection. The amount of root MeIAA increased with rhizobial infection. Application of MeIAA, but not <i>IAA</i> , significantly induced expression of the symbiotic gene <i>NIN</i> in the absence of rhizobial infection. Our results provide evidence for the role of auxin methylation in an early stage of root nodule development.