Elucidating the crosstalk between tocopherol biosynthesis and chlorophyll degradation in tomato plants

ALMEIDA J; AZEVEDO M; MIRANDA L; GLAUSER G; GUYER L; ASIS R.; KESSLER F; HÖRTENSTEINER S; PEREIRA PERES LE; CARRARI F; ROSSI M

Conferencia; The 11th Solanaceae Conference SOL 2014; 2014

Tocopherols, isoprenoid derived-compounds with vitamin E activity, are lipid-soluble antioxidants synthesized only by photosynthetic organisms. Understanding the mechanisms underlying synthesis, transport and accumulation of vitamin E in crops is of great interest because of its implications for human health. Tocopherol synthesis requires a prenyl moiety, phytyl diphosphate, derived from 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, which is central in chloroplast metabolism leading to production of chlorophyll, carotenoids, prenylquinones and others. Apart from de novo synthesis, the prenyl precursor can also be originated from chlorophyll degradation. In this case, after the phytol residue is removed from tetrapyrrolic ring by action of dephytylases such as pheophytinase and chlorophyllase (CLH), it is sequentially phosphorylated by two enzymes, including phytol kinase (VTE5). The contribution of this alternative pathway via chlorophyll degradation for tocopherol biosynthesis has not been completely elucidated particularly in fruits. To further the understanding of regulatory points underlying the vitamin E production, our strategy involves dissecting the crosstalk between tocopherol and chlorophyll metabolism in tomato. We evaluated CLH- and VTE5-deficient tomato plants for variation in prenylquinones and chlorophylls profile, gene expression and photosynthetic parameters. The obtained data clearly show that chlorophyll-derived phytol is the major source of prenyl precursor for tomato tocopherol biosynthesis in which approximately 80% of tocopherol content is VTE5-dependent both in leaves and fruits. Recently analyses of our group have suggested that maintenance of the de novo phytyl diphosphate synthesis in ripening and senescence-related tomato mutants might compensate for the lack of chlorophyll-derived phytol used for tocopherol production in later ripening stages. In contrast, SlCLH(1) does not seem to play a role in phytol hydrolysis for vitamin E synthesis. The observed changes in plastoquinone and its derivatives of CLH-silenced transgenic lines however linked this gene to regulation of prenylquinone content in tomato. The combined results bring another level of complexity to the tomato vitamin E regulatory network, unveiling new strategies for metabolic engineering.