期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：10
DOI：10.1073/pnas.2116549119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：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.
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
Lotus japonicus mutant “
daphne” 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,
daphne should be useful for exploring unknown signals that coordinate these events across tissues. Here, we conducted time-course RNA sequencing using
daphne after rhizobial infection. We noticed that
IAA carboxyl methyltransferase 1 (
IAMT1), 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
daphne.
IAMT1 serves an important function in shoot development of
Arabidopsis, a nonsymbiotic plant, but the function of
IAMT1 in roots has not been reported. Phylogenetic tree analysis suggests a gene duplication of
IAMT1 in the legume lineage, and we found that one of the two
IAMT1s (named
IAMT1a) was induced in roots by epidermal infection.
IAMT1a 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 IAA, significantly induced expression of the symbiotic gene
NIN in the absence of rhizobial infection. Our results provide evidence for the role of auxin methylation in an early stage of root nodule development.
