Family alliances feeding the carotenoid pathway in tomato

Carotenoids are a family of lipophilic metabolites with essential roles in photosynthesis and photoprotection, and secondary roles as pigments providing color to non-green tissues. In addition, their cleavage generates compounds collectively known as apocarotenoids that can function as flavors, pigments, and bioactive molecules. In plants, carotenoids are synthesized and stored in plastids. Isopentenyl diphosphate and dimethylallyl diphosphate supplied by the methylerythritol 4-phosphate (MEP) pathway are used to produce geranylgeranyl diphosphate (GGPP), a common precursor for carotenoids and other plastidial isoprenoids such as chlorophylls, tocopherols, plastoquinone, phylloquinone, diterpenes, and gibberellins. The first committed step of the carotenoid pathway is the condensation of two GGPP molecules to form phytoene, which is later converted into the rest of the carotenoid products. In stark contrast to other plants such as Arabidopsis thaliana, tomato (Solanum lycopersicum) has gene families encoding the plastidial enzymes that catalyze the main rate-determining step of the MEP pathway [deoxyxylulose 5-phosphate synthase (DXS1 and DXS2)], the production of GGPP [GGPP synthase (GGPPS1-GGPPS3)], and its diversion to the carotenoid pathway [phytoene synthase (PSY1-PSY3)]. Here we review the latest advances in our understanding of how the different tomato paralogs of these three enzymes contribute to carotenoid biosynthesis in different plant tissues, developmental stages, and environmental conditions.