Evolutionary and Functional Characterization of an Auxin Methyltransferase (CsIAMT) in Cucumber Reveals Its Role in...

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.