# Evolutionary and Functional Characterization of an Auxin Methyltransferase (CsIAMT) in Cucumber Reveals Its Role in Stress Adaptation and Development

## TL;DR

This study identifies and characterizes a cucumber gene, CsIAMT, which helps regulate auxin levels and plays a role in plant stress responses and development.

## Contribution

The study reports the functional characterization of a novel IAA methyltransferase in cucumber and its role in biotic stress adaptation.

## Key findings

- CsIAMT is a conserved IAA methyltransferase that reduces IAA content while increasing MeIAA in transgenic tobacco.
- CsIAMT is strongly upregulated under biotic stresses like powdery mildew and nematode infection.
- Evolutionary analysis shows strong purifying selection on CsIAMT, indicating functional conservation across angiosperms.

## Abstract

The expansion and functional diversification of gene families are key drivers of phenotypic innovation in plants. The SABATH family of methyltransferases, involved in growth regulation and stress responses, provides an ideal system to study such evolutionary processes. In this study, we identified and functionally characterized a novel member of this family in cucumber, designated CsIAMT. Phylogenetic and synteny analyses confirmed that CsIAMT shares a common ancestry with conserved IAA methyltransferase (IAMT) orthologs across angiosperms. Evolutionary analysis revealed that CsIAMT has undergone strong purifying selection (dN/dS < 1), indicating high functional conservation despite deep evolutionary divergence. Functional validation in transgenic tobacco plants revealed a decrease in IAA content accompanied by a significant increase in MeIAA. Expression profiling under various stress conditions showed that CsIAMT is notably up‐regulated under biotic stresses—such as powdery mildew and nematode infection—while exhibiting variable responses to abiotic treatments. These findings collectively establish CsIAMT as a conserved IAA methyltransferase in cucumber and suggest its potential role in integrating auxin metabolism with biotic stress responses, highlighting the evolutionary conservation of core enzymatic functions within the SABATH family amid lineage‐specific regulatory diversification.

The plant SABATH methyltransferase family drives phenotypic diversity evolution through gene duplication and new functionalization. Among them, IAMT, as the most ancient member of this family, is widely present in various species. The study identified a potential auxin methyltransferase homologous to IAMT (CsIAMT) in cucumber, which can catalyze IAA methylation to regulate auxin balance. Moreover, RNA‐seq analysis indicated that this gene responded significantly more strongly to biotic stress than abiotic stress.