Tocopherol metabolism engineering: manipulation of phytol recycling in tomato fruits

Tocopherols are liposoluble compounds that are essential for the maintenance of thylakoid membrane integrity due to their high antioxidant capacity. Moreover, these compounds are also important for human health due to their vitamin E activity, particularly α-tocopherol. As only plants can synthesise these metabolites, unravelling the mechanisms that control their accumulation is of interest for biotechnological purposes. In the past few years, it was revealed the importance of the chlorophyll-derived phytol recycling from tocopherol biosynthesis. This process consists in a two-step phosphorylation sequentially catalysed by the PHYTOL KINASE (VTE5) and the PHYTYL-PHOSPATE KINASE (VTE6) rendering phytyl diphosphate. This molecule not only can be used for the synthesis of new chlorophyll molecules, but also for tocopherols, when condensed with homogentic acid. In this context, the tomato (Solanum lycopersicum) fruit is an interesting model for the study of tocopherol manipulation as, along fruit ripening, chlorophyll is massively degraded and tocopherol accumulates. In order to increase the phytol recycling, transgenic tomato plants overexpressing AtVTE5 or AtVTE6 under the control of a fruit-specific promoter were generated. The content of chlorophylls, carotenoids and tocopherols were addressed at the pedicellar (where chloroplasts are fully developed) and at the stylar (which lacks developed chloroplasts) portion of the fruits. While these metabolites in the latter were almost unaffected, when fully developed chloroplast were presented the scenario was different. The pedicellar portion of immature green fruits had more chlorophylls and carotenoids in both overexpressions, however, only in the case of AtVTE6, this led to an increase in tocopherols as fruits ripen. These results hints that increased phytol recycling increases chlorophyll turnover in green fruits, which in turn increased the photosystem-related carotenoids, leading to an increase in the tocopherol content in the edible fruits.