Metabolic, Genomic, and Biochemical Analyses of Glandular Trichomes from the Wild Tomato SpeciesLycopersicon hirsutumIdentify a Key Enzyme in the Biosynthesis of Methylketones

Abstract

Medium-length methylketones (C7-C15) are highly effective in protecting plants from numerous pests. We used a biochemical genomics approach to elucidate the pathway leading to synthesis of methylketones in the glandular trichomes of the wild tomato Lycopersicon hirsutum f glabratum (accession PI126449). A comparison of gland EST databases from accession PI126449 and a second L. hirsutum accession, LA1777, whose glands do not contain methylketones, showed that the expression of genes for fatty acid biosynthesis is elevated in PI126449 glands, suggesting de novo biosynthesis of methylketones. A cDNA abundant in the PI126449 gland EST database but rare in the LA1777 database was similar in sequence to plant esterases. This cDNA, designated Methylketone Synthase 1 (MKS1), was expressed in Escherichia coli and the purified protein used to catalyze in vitro reactions in which C12, C14, and C16 β-ketoacyl–acyl-carrier-proteins (intermediates in fatty acid biosynthesis) were hydrolyzed and decarboxylated to give C11, C13, and C15 methylketones, respectively. Although MKS1 does not contain a classical transit peptide, in vitro import assays showed that it was targeted to the stroma of plastids, where fatty acid biosynthesis occurs. Levels of MKS1 transcript, protein, and enzymatic activity were correlated with levels of methylketones and gland density in a variety of tomato accessions and in different plant organs.

Biosynthesis of plant‐derived flavor compounds

“…Recently, the first methylketone synthase was isolated from tomato, which catalyzes the hydrolysis and subsequent decarboxylation of C 12 , C 14 and C 16 b-ketoacyl ACPs to give C 11 , C 13 and C 15 methylketones, respectively (Fridman et al, 2005). In contrast, in fungi, methylketones are derived from b-oxidative degradation of fatty acids through b-ketoacyl CoA intermediates (Schwab and Schreier, 2002).…”

“…Moreover, as these tissues contain many of the enzymes and significantly express many of the genes involved in the production of such metabolites, the isolation of secretory cells has greatly contributed to characterization of many of the enzymes and genes involved in the formation of many plant natural products in peppermint, sweet and lemon basil, as well as tomato and other crops (Fridman et al, 2005;Gang et al, 2002;Iijima et al, 2004a,b;McConkey et al, 2000). The following sections describe the biosynthesis of plant-derived flavor molecules grouped by their biogenetic origin.…”

Heterologous Expression of Methylketone Synthase1 and Methylketone Synthase2 Leads to Production of Methylketones and Myristic Acid in Transgenic Plants

“…Analysis of a type VI-specific EST database from wild tomato, which is a methylketone-producing line, followed by gene expression comparison with a trichome EST database from wild tomato (accession no. LA1777), which does not produce methylketones, identified the gene Solanum habrochaites methylketone synthase1 (ShMKS1) as correlated with methylketone production (Fridman et al, 2005). Additional genetic and genomic analyses identified a second gene, designated ShMKS2, whose expression was also positively associated with methylketone production (BenIsrael et al, 2009).…”

“…Methylketones accumulate in large amounts in type VI trichomes, specifically in the apical gland cells, with concentrations of up to 8 mg/g fresh leaf weight (FLW) observed (Fridman et al, 2005). Analysis of a type VI-specific EST database from wild tomato, which is a methylketone-producing line, followed by gene expression comparison with a trichome EST database from wild tomato (accession no.…”

“…Biosynthesis and storage of methylketones are confined to dedicated structures in wild tomato termed type VI trichomes (Antonious et al, 2003;Fridman et al, 2005). Trichomes are small protrusions of epidermal originated cells on the surface of aerial parts of approximately one third of vascular land plants (Luckwill, 1943;Schilmiller et al, 2008).…”