Submitted to: Plant Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/23/2009
Publication Date: 2/16/2010
Citation: Tieman, D., Zeigler, M., Schmelz, E.A., Taylor, M.G., Rushing, S., Jones, J.B., Klee, H.J. 2010. Functional analysis of a tomato salicylic acid methyl transferase and its role in synthesis of the flavor volatile methyl salicylate. Plant Journal. 62:133-123. Interpretive Summary: Methyl salicylate (MeSA), the major component in oil of wintergreen, is a volatile plant chemical that contributes to the taste and scent of many fruits and flowers. MeSA is synthesized from salicylic acid (SA), a phytohormone that partly regulates plant defense responses to pathogen attack. A family of enzymes, termed O-methyltransferases, converts organic acids (i.e. SA) into corresponding methyl esters (i.e. MeSA). In tomato, MeSA likely influences many aspects of plant physiology and biotic interactions; however, empirical testing of these hypotheses first requires the characterization, cloning and controlled expression of genes encoding key pathway enzymes. Working with the University of Florida (Department of Horticultural Sciences), scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered a specific tomato O-methyltransferase (SlSAMT; Solanum lycopersicum salicylic acid carboxyl methyltransferase) that catalyzes the conversion of SA into MeSA. Genetic mapping and analysis of specific tomato lines supports a role for this gene in regulation MeSA production. Transgenic tomato plants over expressing SlSAMT also produced significant increases in fruit MeSA production. These plants exhibited greatly altered salicylate pools during pathogen infection yet only modest changes in the growth of a disease causing bacteria. These genetic resources will be useful in improving fruit flavor through both traditional and modern approaches and also aid in elucidating MeSA-mediated plant defense responses during pathogen attack.
Technical Abstract: Methyl salicylate (MeSA) is a volatile plant secondary metabolite that is an important contributor to taste and scent of many fruits and flowers. It is synthesized from salicylic acid (SA), a phytohormone that contributes to plant pathogen defense. MeSA is synthesized by members of a family of O-methyltransferases. In order to elaborate the regulation of MeSA synthesis in tomato, we screened a set of O-methyltransferases for activity against a variety of substrates. An enzyme that specifically catalyzes methylation of SA, SlSAMT, as well as enzymes that act upon jasmonic acid and indole-3-acetic acid were identified. Analyses of transgenic over- and under-producing lines validated the function of SlSAMT in vivo. The SlSAMT gene was mapped to a position near the bottom of chromosome 9. Analysis of MeSA emissions from an introgression population derived from a cross with the wild relative Solanum pennellii, revealed a quantitative trait locus (QTL) linked to higher fruit methyl salicylate emissions. The higher MeSA emissions associate with significantly higher SpSAMT expression, consistent with SAMT gene expression being rate-limiting for ripening-associated MeSA emissions. Transgenic plants that constitutively over-produce MeSA exhibited only slightly delayed symptom development following infection with the disease-causing bacterial pathogen, Xanthomonas campestris pv. vesicatoria (Xcv). Unexpectedly, pathogen-challenged leaves accumulated significantly higher levels of SA as well as glycosylated forms of SA and MeSA, indicating a disruption in the normal regulation of the pools of SA-related metabolites. Taken together, the results indicate that SlSAMT is critical for methyl salicylate synthesis and methyl salicylate, in turn, likely has an important role in regulating synthesis of SA.