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. 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. Optimum pH is 7.5. Homodimer. Expressed in roots and panicles. Belongs to the methyltransferase superfamily. SABATH family. Truncated N-terminus.