Two Methylketone Biosynthetic Enzymes from Wild Tomatoes.

Abstract

Many plants make varying amounts of aliphatic 2-methylketones, but their mode of synthesis is unclear. High concentrations of 2-undecanone and 2-tridecanone in the trichomes of a wild tomato species Solanum habrochaites have been found to be toxic to pests. Since cultivated tomato (S. lycopersicum) produces only low levels of methylketones, the creation of genetic hybrids that produce high levels of methylketones was previously attempted but was unsuccessful. Thus, elucidating methylketone biosynthesis may allow the generation of cultivated tomatoes capable of producing methylketones by genetic engineering. A recent study from our laboratory suggested that plants produce methylketones through de novo fatty acid biosynthesis and identified ShMKS1 (S. habrochaites methylketone synthase 1) as a gene involved in methylketone biosynthesis. Further genetic analysis of the segregated progenies of the interspecific crosses between S. habrochaites X S. lycopersicum identified another gene, ShMKS2 (S. habrochaites methylketone synthase 2), which was shown to be necessary for methylketone production. I heterologously expressed of ShMKS2 in Escherichia coli and observed the production of methylketones. Enzymatic studies of ShMKS1 and ShMKS2 showed that ShMKS2 has a thioesterase activity, hydrolyzing the thioester bond in 3-ketoacyl-ACPs to produce the corresponding 3-keto acids, and that ShMKS1 has a decarboxylase activity towards 3-keto acids, resulting in the production of 2-methylketones. Both enzymes were found to be localized in plastids, the site of fatty acid biosynthesis in plants. Constitutive co-expression of ShMKS1 and ShMKS2 led to the synthesis of methylketones in Arabidopsis and tobacco but also disrupted growth and development of the transgenic plants. Trichome-specific expression of these two genes in cultivated tomato, by using the promoters of ShMKS1 and ShMKS2, did not result in detectable methylketone production. All transgenic plants expressing ShMKS2 in leaves produced myristic acid, suggesting that ShMKS2 can hydrolyze myristoyl-ACP as well. The specificity of two tomato trichome-specific promoter was found to be maintained in other species. The results demonstrated the mechanism for methylketone biosynthesis in plants and that any plant species could be engineered to produce methylketones by co-expressing ShMKS1 and ShMKS2 in leaves.