The carrot monoterpene synthase gene cluster on chromosome 4 harbours genes encoding flavour-associated sabinene synthases

In plants, low molecular weight terpenes produced by terpene synthases (TPS) contribute to multiple ecologically and economically important traits. The present study investigates a carrot terpene synthase gene cluster on chromosome 4 associated with volatile monoterpene production. Two carrot mutants, yellow and cola, which are contrasting in the content of low molecular weight terpenes, were crossed to develop an F(2) mapping population. The mapping analysis revealed overlapping QTLs on chromosome 4 for sabinene, α-thujene, α-terpinene, γ-terpinene, terpinen-4-ol and 4-carene. The genomic region of this locus includes a cluster of five terpene synthase genes (DcTPS04, DcTPS26, DcTPS27, DcTPS54 and DcTPS55). DcTPS04 and DcTPS54 displayed genotype- and tissue-specific variation in gene expression. Based on the QTL mapping results and the gene expression patterns, DcTPS04 and DcTPS54 were selected for functional characterization. In vitro enzyme assays showed that DcTPS54 is a single-product enzyme catalysing the formation of sabinene, whereas DcTPS04 is a multiple-product terpene synthase producing α-terpineol as a major product and four additional products including sabinene, β-limonene, β-pinene and myrcene. Furthermore, we developed a functional molecular marker that could discriminate carrot genotypes with different sabinene content in a set of 85 accessions.