Abstract: Auxin is one of the most important plant hormones in that it regulates almost all aspects of plant growth and development. In plants, free indole-3-acetic acid (IAA) is the major active auxin and can be converted to methyl-IAA (MeIAA) with the activity of an IAA carboxyl methyltransferase 1 (IAMT1). However, MeIAA is also an inactive form of IAA and can be converted to IAA catalyzed by several esterases. MeIAA is a non-polar IAA form and can diffuse between cells, so MeIAA is suitable for screening new components in MeIAA metabolism or IAA biology. Here, we screened ethyl-methane-sulphonate (EMS)-mutagensis seedlings on MeIAA medium in the dark, with longer hypocotyls used as a criterion. We isolated 8 putative MeIAA-resistant mutants and carried out further experiments on one mutant, mir1. These new mutants will help shed light on MeIAA metabolism and the action mechanism of IAA.