A novel auxin methyltransferase of the SABATH family for phenylacetic acid methylation is conserved in potato and tomato

Abstract

The SABATH family of methyltransferases is known for methylating a wide range of substrates, including hormones and secondary metabolites. A notable member of this family is the auxin methyltransferase IAMT which uses indole-3-acetic acid (IAA) as the substrate. This study aims to determine whether methyl phenylacetate (MePAA), the methyl ester of another auxin, phenylacetic acid (PAA), is synthesized by SABATH methyltransferases. Potato (Solanum tuberosum L. cv. Désirée) was chosen as the primary model because it produces MePAA exclusively in flowers. Based on the structural similarity of IAA and PAA, our initial hypothesis was that MePAA is synthesized by an IAMT-like enzyme. The potato genome contains two IAMT-like genes. However, their recombinant enzymes expressed in Escherichia coli were shown to catalyze the methylation of IAA but not PAA, thus rejecting our initial hypothesis. Among the 23 potato SABATH genes, two exhibited flower-specific expression. One was excluded because it had already been identified as an IAMT. In vitro assays of the enzyme encoded by the other gene, StSABATH6, confirmed its catalytic activity against PAA. Consequently, this enzyme was renamed StPAAMT. Notably, StPAAMT has an ortholog in both cultivated and wild tomatoes. The gene from tomato (Solanum lycopersicum), SlPAAMT, was verified to encode PAA methyltransferase. Further genomic and phylogenetic analyses of five Solanum species showed that the PAAMT gene was likely absent in eggplant (Solanum melongena), implying its origin in the common ancestor of potato and tomato. The structural analysis identified key amino acids associated with the substrate specificity of PAAMT. This work provides new insights into the evolution of auxin methyltransferases, particularly PAAMT, as members of the SABATH family.