A Chemical Probe for Increasing Leaf Tocopherol Levels by Coordinated Modulation of Biosynthesis, Competition and Storage

Abstract

Plant biofortification with phytonutrients typically relies on metabolic engineering strategies known as ‘push’ (enhancing biosynthetic flux), ‘block’ (inhibiting competing pathways) and ‘pull’ (promoting metabolite storage). Here, we describe a novel synthetic compound, X57, that simultaneously targets biosynthesis, competition and storage to enhance leaf tocopherol content. Tocopherols protect plants against oxidative stress, have a dietary value as vitamin E and are highly appreciated antioxidants in food and cosmetic formulations. X57 exerts a primary ‘push’ effect by inducing tocopherol biosynthesis, in part by reactivating a direct pathway that reduces geranylgeranyl diphosphate (GGPP) to phytyl diphosphate, bypassing the need for chlorophyll-derived phytol. Accordingly, X57 promotes tocopherol accumulation in etiolated seedlings and restores tocopherol synthesis in mutants deficient in phytol phosphorylation. The ‘block’ effect is mediated by down-regulation of GGPP consumption for carotenoid synthesis. X57 also induces a ‘pull’ effect via proliferation of plastoglobules (PG), plastidial lipoprotein bodies that synthesise and store tocopherols. X57-induced PG proliferation is driven by increased tocopherol levels and up-regulation of genes for PG structural proteins such as fibrillins. The unveiled genetic networks simultaneously coordinating plastidial isoprenoid metabolism and plastid differentiation might only be present in higher plants, because X57 does not promote but reduces tocopherol accumulation in Marchantia polymorpha.