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Published on May 8, 2008; 10.1104/pp.108.118224

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Received February 22, 2008
Accepted April 23, 2008

Inactive Methyl Indole-3-Acetic Acid Ester Can Be Hydrolyzed and Activated by Several Esterases Belonging to the AtMES Esterase Family of Arabidopsis thaliana

Yue Yang , Richard Xu , Choong-je Ma , A. Corina Vlot , Daniel F. Klessig , and Eran Pichersky *

Department of Molecular, Cellular and Developmental Biology, University of Michigan, 830 North University Street, Ann Arbor, MI 48109-1048; Boyce Thompson Institute for Plant Research, Ithaca, NY 14853

* Corresponding author; email: lelx{at}umich.edu.

The plant hormone auxin (indole-3-acetic acid, IAA) is found both free and conjugated to a variety of carbohydrates, amino acids and peptides. We have recently shown that IAA could be converted to its methyl ester (MeIAA) by the Arabidopsis thaliana enzyme IAMT1 (IAA carboxyl methyltransferase 1). However, the presence and function of MeIAA in vivo remains unclear. Recently it has been shown that the tobacco protein SABP2 (Salicylic acid binding protein-2) hydrolyzes methyl salicylate to salicylic acid. There are 20 homologs of SABP2 in the genome of Arabidopsis thaliana, which we have named AtMES (for methyl esterases). We tested 15 of the proteins encoded by these genes in biochemical assays with various substrates, and identified several candidate MeIAA esterases that could hydrolyze MeIAA. MeIAA, like IAA, exerts inhibitory activity on the growth of wild type roots when applied exogenously. However, the roots of Arabidopsis plants carrying T-DNA insertions in the putative MeIAA esterase gene AtMES17 (At3g10870) displayed significantly decreased sensitivity to MeIAA compared with wild type roots, while remaining as sensitive to free IAA as wild type roots. Incubating seedlings in the presence of [14C]-MeIAA for 30 min revealed that mes17 mutants hydrolyzed only 40% of the [14C]-MeIAA taken up by plants, whereas wildtype plants hydrolyzed 100% of absorbed [14C]-MeIAA. Roots of Arabidopsis plants overexpressing AtMES17 showed increased sensitivity to MeIAA, but not to IAA. Additionally, mes17 plants have longer hypocotyls, and display increased expression of the auxin-responsive DR5:GUS reporter gene, suggesting a perturbation in IAA homeostasis and/or transport. mes17-1/axr1-3 double mutant plants have the same phenotype as axr1-3, suggesting MES17 acts upstream of AXR1. The protein encoded by AtMES17 had a Km value of 13 µM, and a Kcat value of 0.18 sec-1 for MeIAA. AtMES17 was expressed at the highest levels in shoot apex, stem and root of Arabidopsis. Our results demonstrate that MeIAA is an inactive form of IAA, and the manifestations of MeIAA in vivo activity are due to the action of free IAA that is generated from MeIAA upon hydrolysis by one or more plant esterases.






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