Catalyzes the methylation of the free carboxyl end of the plant hormone indole-3-acetic acid (IAA). Converts IAA to IAA methyl ester (MeIAA). Regulates IAA activities by IAA methylation. Methylation of IAA plays an important role in regulating plant development and auxin homeostasis. Required for correct leaf pattern formation. MeIAA seems to be an inactive form of IAA. (indol-3-yl)acetate + S-adenosyl-L-methionine = methyl (indol-3-yl)acetate + S-adenosyl-L-homocysteine Binds 1 Mg(2+) ion per subunit. Homodimer. Expressed in seedling roots and leaves. Expressed in the stigma, funiculus, and vascular bundles in sepals, petals and stamens. After the eighth true leaves emerge, the expression gradually fades away from the center of the leaves toward the edges, as leaf development proceedes. In fully expanded leaves, expressed only at the edge of the leaf blade. Plant silencing IAMT1 are smaller than the wild-type, show epinastic leaves and smaller siliques, and have low fertility. Belongs to the methyltransferase superfamily. SABATH family. Truncated N-terminus.