Functional analysis of a tomato salicylic acid methyl transferase and its role in synthesis of the flavor volatile methyl salicylate

Abstract

SUMMARYMethyl 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 mechanism of MeSA synthesis in tomato, we screened a set of O-methyltransferases for activity against multiple 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 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 control of the SA-related metabolite pool. Taken together, the results indicate that SlSAMT is critical for methyl salicylate synthesis and methyl salicylate, in turn, likely has an important role in controlling SA synthesis.

Ectopic Expression ofAtJMTinNicotiana attenuata: Creating a Metabolic Sink Has Tissue-Specific Consequences for the Jasmonate Metabolic Network and Silences Downstream Gene Expression

“…However, these results contrast with studies in Arabidopsis 35S-jmt plants reporting constitutively higher transcript levels for JA biosynthesis genes (Seo et al, 2001;Jung et al, 2003). Similarly, a positive feedback effect on SA biosynthesis mediated either by the increase of methyl SA itself or by a disruption in the control of the SA-related pool or flux has been observed in SAMT overexpressing tomato plants (Tieman et al, 2010). We cannot therefore completely exclude the possibility that the flux of compounds through the JA pathway may have been accelerated in N. attenuata in a transcription-independent manner that could not be captured by quantifying individual metabolites.…”

“…However, we cannot fully rule out that changes had occurred in the activity of enzymes, since in N. attenuata, critical steps within the JA pathway are not transcriptionally regulated (Kallenbach et al, 2010). The strength of the metabolic diversion was sustained in 35S-jmt by the remethylation of deesterified MeJA; this recycling reaction may also explain SA metabolism patterns in salicylic acid O-methyltransferase (SAMT) overexpressing lines (Tieman et al, 2010).…”

“…Conversely, manipulating the scale of this reaction and characterizing its consequences at the metabolic and organismic level has been shown to be a valuable approach to uncover new signaling outputs and to revisit the importance of homeostatic control over metabolite fluxes derived from hormonal signals (Qin et al, 2005;Varbanova et al, 2007;Tieman et al, 2010). By ectopically expressing AtJMT, we engineered a massive redirection of the JA flux toward the formation of MeJA.…”

Veratrole Biosynthesis in White Campion

“…In tomato, guaiacol is derived via the methylation of catechol by catechol-Omethyltransferase (CTOMT1; Mageroy et al, 2012); however, the origin of catechol is still unknown. Although there is little overlap between the aroma compounds found in tomato fruit and white campion flowers, both produce guaiacol and methyl salicylate (MeSA), the latter of which is made from salicylic acid (SA; Ross et al, 1999;Tieman et al, 2010;Mageroy et al, 2012). Interestingly, tomato introgression lines with elevated fruit guaiacol levels, due in part to higher CTOMT1 expression, also exhibit a modest decline in SA, suggesting a shared pathway between these two aroma compounds.…”

The genome of the stress-tolerant wild tomato species Solanum pennellii

“…pennellii has additionally been widely documented to show phenotypes divergent from S. lycopersicum in relation to fruit development, maturation and metabolism [27][28][29][30][31][32][33] (Supplementary Note and Supplementary Data Set 13). In terms of fruit maturation, several differences in the sequence and expression of some key regulatory genes were apparent.…”