Synthesis and hydrolysis of methylated plant hormones.

Plant hormones are small molecules that regulate various aspects of plant growth and development. Plant hormones are often chemically modified in the cell to affect their storage, degradation, or transport. We had previously discovered that two genes belonging to the Arabidopsis thaliana SABATH methyltransferase (MT) family encode enzymes that catalyze the methylation of the plant hormones salicylic acid (SA) and jasmonic acid. The Arabidopsis genome contains 22 other genes belonging to the SABATH family, whose catalytic functions at the time were unknown. Using a biochemical genomic approach, I, together with co-workers, have identified a SABATH MT that methylates the plant hormone indole-3-acetic acid (IAA), and two other SABATH MTs that methylate another class of plant hormones, the gibberellins (GAs). Biochemical characterizations of these MTs have shown that they are specific for the respective hormone substrate. In addition, mutant plants with ectopic expression of IAA or GA methyltransferases show a disruption in hormonal homeostasis. The discoveries that IAA and GAs could be methylated are novel. However, the in vivo functions of methylated plant hormones such as SA methylester (MeSA) and IAA methylester (MeIAA) had not yet been determined. I have therefore characterized several plant esterases that hydrolyze these methylated plant hormones. The tobacco protein SABP2 had been originally identified as a SA-binding protein involved in plant defense responses, and I demonstrated that it is an esterase that hydrolyzes McSA to SA. Our results suggested that McSA may function as the long-sought mobile signal for the activation of the SA-dependent pathogen-induced defense responses in plants. The genome of Arabidopsis contains 20 genes encoding enzymes homologous to SABP2. I have shown that several of these enzymes can hydrolyze McIAA in vitro, and that at least one such esterase is capable of hydrolyzing McIAA in vivo to the active hormone IAA. My results suggested that McIAA itself does not possess hormonal functions. Overall, my work contributed to the discoveries and the elucidation of the role of methylated plant hormones, and helped to understand the mechanism by which hormone homeostasis is maintained.PhDBiological SciencesMolecular biologyPlant biologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126596/2/3253437.pd