Molecular cloning and biochemical characterization of indole-3-acetic acid methyltransferase from Japanese star anise (Illicium anisatum).

SABATH proteins methylate the carboxyl groups or nitrogen atoms of small plant molecules and play important roles in many developmental processes and plant defense responses. Previous studies have shown that indole-3-acetic acid (IAA) carboxyl methyltransferase (IAMT), a member of the SABATH methyltransferase family, converts IAA into its methyl ester (Me-IAA). We used RNA-seq analysis to identify a putative IAMT gene, IaIAMT, in the ancient angiosperm Illicium anisatum. Functional characterization of the recombinant IaIAMT protein expressed in Escherichia coli showed the highest level of activity with IAA, whereas indole-3-propionic acid and indole-3-butyric acid were not used as substrates. The apparent K_m value of IaIAMT using IAA as a substrate was determined to be 122 µM. Phylogenetic analysis and structural modeling of IaIAMT suggested that IaIAMT evolved independently from IAMTs isolated from other plant species, whereas strict substrate specificity toward IAA was conserved in Illicium species, as observed in other plants.