A family of methyl esterases converts methyl salicylate to salicylic acid in ripening tomato fruit

Authors: FRICK, ELIZABETH - University Of Florida; SAPKOTA, MANJO - University Of Georgia; PEREIRA, LARA - University Of Georgia; WANG, YANBING - University Of Georgia; HERMANNS, ANNA - Cornell University; Giovannoni, James; VAN DER KNAAP, ESTHER - University Of Georgia; TIEMAN, DENISE - University Of Florida; KLEE, HARRY - University Of Florida

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2022
Publication Date: 10/31/2023
Citation: Frick, E., Sapkota, M., Pereira, L., Wang, Y., Hermanns, A., Giovannoni, J.J., Van Der Knaap, E., Tieman, D., Klee, H. 2023. A family of methyl esterases converts methyl salicylate to salicylic acid in ripening tomato fruit. Plant Physiology. 191(1):110-124. https://doi.org/10.1093/plphys/kiac509.
DOI: https://doi.org/10.1093/plphys/kiac509

Interpretive Summary: Tomatoes are the second most valuable crop in the USA, valued at over 2 billion USD per year, but are consistently criticized by consumers for a lack of flavor. Consumers’ perception of tomato flavor is influenced by a combination of sugars (fructose and glucose), acids (citric and malic), and approximately 30 volatile aroma compounds. The levels of flavor volatiles can substantially affect fruit liking independent of sugars or acids. Additionally, these flavor volatiles are present at extremely low levels and can vary by more than 1,000-fold between cultivars, representing an attractive target for breeders to enhance flavor without negatively impacting yield. Despite the importance of volatiles to flavor, the biosynthetic pathways for many volatiles are incompletely described. This lack of knowledge complicates breeding efforts to alter targeted volatile levels. One of the few flavor volatiles that has increased in modern tomato is methyl salicylate. Methyl salicylate is a flavor chemical described as tasting medicinal and wintergreen, and it is negatively correlated with liking. Here describes a tomato esterase gene that convert methyl salicylate to salicylic acid in ripe tomato fruit. This esterase gene is an attractive breeding target to decrease the amount of the negative flavor compound methyl salicylate.

Technical Abstract: Methyl salicylate imparts a potent flavor and aroma described as medicinal and wintergreen that is undesirable in tomato (Solanum lycopersicum) fruit. Plants control the quantities of methyl salicylate through a variety of biosynthetic pathways, including the methylation of salicylic acid to form methyl salicylate and subsequent glycosylation to prevent methyl salicylate emission. Here, we identified a subclade of tomato methyl esterases, SALICYLIC ACID METHYL ESTERASE1-4, responsible for demethylation of methyl salicylate to form salicylic acid in fruits. This family was identified by proximity to a highly significant methyl salicylate genome-wide association study locus on chromosome 2. Genetic mapping studies in a biparental population confirmed a major methyl salicylate locus on chromosome 2. Fruits from SlMES1 knockout lines emitted significantly (P'<'0,05, t test) higher amounts of methyl salicylate than wild-type fruits. Double and triple mutants of SlMES2, SlMES3, and SlMES4 emitted even more methyl salicylate than SlMES1 single knockouts—but not at statistically distinguishable levels—compared to the single mutant. Heterologously expressed SlMES1 and SlMES3 acted on methyl salicylate in vitro, with SlMES1 having a higher affinity for methyl salicylate than SlMES3. The SlMES locus has undergone major rearrangement, as demonstrated by genome structure analysis in the parents of the biparental population. Analysis of accessions that produce high or low levels of methyl salicylate showed that SlMES1 and SlMES3 genes expressed the highest in the low methyl salicylate lines. None of the MES genes were appreciably expressed in the high methyl salicylate-producing lines. We concluded that the SlMES gene family encodes tomato methyl esterases that convert methyl salicylate to salicylic acid in ripe tomato fruit. Their ability to decrease methyl salicylate levels by conversion to salicylic acid is an attractive breeding target to lower the level of a negative contributor to flavor.