Characterization of a Methyl Jasmonate Specific Esterase in Arabidopsis

Methyl jasmonate (MeJA)-specific methyl esterase of Arabidopsis (AtMJE) was identified and characterized. AtMJE has high substrate specificity to MeJA compared to other related substrates, methyl indole-3-acetate (MeIAA) and methyl salicylate (MeSA). Through enzyme kinetics analysis, we found AtMJE has similar level of substrate affinity to JA carboxyl methyltransferase (AtJMT). However, AtMJE has 10 times lower catalytic efficiency than AtJMT at low substrate concentrations. AtMJE gene expression was suppressed for 2 h after MeJA treatment, even though its expression recovered and was induced to maximum level within 8 h after treatment. AtMJE overexpressing plants (AtMJEox) showed enhanced MeJA methyl esterase activity demonstrating esterase activity of AtMJE in vivo. AtMJEox plants responded differentially to JA and MeJA in root growth. MeJA in the media could be a source for more JA production in AtMJEox plants, which resulted in root growth inhibition. In contrast, AtMJEox plants grown on JA containing media showed similar root growth inhibition as wild-type. These results show that AtMJE functions in altering JA/MeJA ratios in Arabidopsis and increased JA, because the conversion of MeJA to JA enhances JA responsive gene expression.