Abscisic acid triggers vitamin E accumulation by transient transcript activation of VTE5 and VTE6 in sweet cherry fruits

Tocopherols are lipophilic antioxidants known as vitamin E and synthesized from the condensation of two metabolic pathways leading to the formation of homogentisate and phytyl diphosphate. While homogentisate derives from tyrosine metabolism, phytyl diphosphate may be formed from geranylgeranyl diphosphate or phytol recycling from chlorophyll degradation. Here we hypothesized that abscisic acid (ABA) could induce tocopherol biosynthesis in sweet cherries by modifying the expression of genes involved in vits biosynthesis, including those from the phytol recycling pathway. Hence, expression of key tocopherol biosynthesis genes was determined together with vitamin E and chlorophyll contents during natural development of sweet cherries on-tree. Moreover, the effects of exogenously applied ABA on the expression of key tocopherol biosynthesis genes were also investigated during on-tree fruit development, and tocopherols and chlorophylls contents analyzed. Results showed that expression of tocopherol biosynthesis genes, including VTE5, VTE6, HPPD and HPT showed contrasting patterns of variation, but in all cases, increased by 2- and 3-fold over time during fruit de-greening. This was not the case for GGDR and VTE4, the first showing constitutive expression during development and the second with marked downregulation at ripening onset. Furthermore, exogenous ABA stimulated production of both α- and γ-tocopherols by 60% and 30%, respectively, promoted chlorophyll degradation and significantly enhanced VTE5 and VTE6 expression, and also that of HPPD and VTE4, altogether increasing total tocopherol accumulation. In conclusion, ABA increases promote the transcription of phytol recycling enzymes, which may contribute to vitamin E biosynthesis during fruit development in stone fruits like sweet cherries.