Health-Enhancing Compounds in Carrots: Genetics, Genomics, and Molecular Breeding

Carrot is an economically important vegetable crop worldwide. Its storage root, the consumed organ, varies broadly within the carrot germplasm, exhibiting different colors due to the accumulation of anthocyanin and carotenoid pigments, as well as extensive variation for phytochemicals composition and consumer-quality traits. Anthocyanins and other phenolics, carotenoids, polyacetylenes, and terpenes represent the major carrot nutraceutical classes. In recent years, the use of next-generation sequencing technologies has facilitated the application of “multi-omics” approaches, in combination with transgenics and classical genetic tools, for studying the genetics underlying the accumulation of these phytochemicals in the carrot root. In purple carrot, such approaches allowed the identification and mapping of simply inherited and quantitative trait loci (QTLs) conditioning anthocyanin pigmentation in different tissues and genetic backgrounds, and the discovery of key genes conditioning anthocyanin biosynthesis, glycosylation, and acylation. Glycosylation and acylation influence the chemical stability and bioavailability of anthocyanins, and therefore their potential use as food colorants or nutraceutical agents, respectively. Similarly, important advances were made for two major loci conditioning carotenoids accumulation in white, yellow, and orange roots, namely Y and Y₂. With the sequencing of the carrot genome, a candidate for the Y gene involved in photosystem development and carotenoid storage was described, whereas fine mapping of Y₂ drastically reduced the genomic region of interest to 650-kb, but a clear candidate was not identified. Another gene, Or, which regulates chromoplasts development, was associated with carotenoids presence in the carrot root. Besides these nonstructural genes, progress towards understanding the role of several carotenoid biosynthetic genes has been made. The genetics of carrot polyacetylenes is also becoming increasingly understood. Candidate fatty acid desaturase 2 (FAD2) genes with specific desaturase and/or acetylenase activities have been identified by QTLs analysis and proposed as catalyzers of different steps in the polyacetylene pathway, and their genomic organization was described. Similarly, gene members of the large carrot terpene synthase family were catalogued, partially associated with QTLs for characteristic carrot root monoterpenes like sabinene, and functionally characterized in vitro. This chapter reviews and discusses recent advances in genetics and genomics of the main carrot nutraceuticals.