2-O-β-d-Glucopyranosyl l-Ascorbic Acid, a Stable Form of Vitamin C, Is Widespread in Crop Plants

The ascorbate biosynthesis pathway in plants is known, but there is a way to go with understanding control and functions

Ascorbate (vitamin C) is one of the most abundant primary metabolites in plants. Its complex chemistry enables it to function as an antioxidant, as a free radical scavenger, and as a reductant for iron and copper. Ascorbate biosynthesis occurs via the mannose/l-galactose pathway in green plants, and the evidence for this pathway being the major route is reviewed. Ascorbate accumulation is leaves is responsive to light, reflecting various roles in photoprotection. GDP-l-galactose phosphorylase (GGP) is the first dedicated step in the pathway and is important in controlling ascorbate synthesis. Its expression is determined by a combination of transcription and translation. Translation is controlled by an upstream open reading frame (uORF) which blocks translation of the main GGP-coding sequence, possibly in an ascorbate-dependent manner. GGP associates with a PAS-LOV protein, inhibiting its activity, and dissociation is induced by blue light. While low ascorbate mutants are susceptible to oxidative stress, they grow nearly normally. In contrast, mutants lacking ascorbate do not grow unless rescued by supplementation. Further research should investigate possible basal functions of ascorbate in severely deficient plants involving prevention of iron overoxidation in 2-oxoglutarate-dependent dioxygenases and iron mobilization during seed development and germination.

Protective effect of vitamin C against tetrachlorobenzoquinone-induced 5-hydroxymethylation-dependent apoptosis in HepG2 cells mainly via the mitochondrial apoptosis pathway

Tetrachlorobenzoquinone (TCBQ) is an active metabolite of pentachlorophenol, and stimulates the accumulation of ROS to trigger apoptosis. The preventive effect of vitamin C (Vc) against TCBQ-induced apoptosis in HepG2 cells is unknown. And there is little known about TCBQ-triggered 5-hydromethylcytosine (5hmC)-dependent apoptosis. Here, we confirmed that Vc alleviated TCBQ-induced apoptosis. Through investigating the underlying mechanism, we found TCBQ downregulated 5hmC levels of genomic DNA in a Tet-dependent manner, with a particularly pronounced decrease in the promoter region, using UHPLC-MS-MS analysis and hydroxymethylated DNA immunoprecipitation sequencing. Notably, TCBQ exposure resulted in alterations of 5hmC abundance to ∼91% of key genes at promoters in the mitochondrial apoptosis pathway, along with changes of mRNA expression in 87% of genes. By contrast, 5hmC abundance of genes only exhibited slight changes in the death receptor/ligand pathway. Interestingly, the pretreatment with Vc, a positive stimulator of 5hmC generation, restored 5hmC in the genomic DNA to near-normal levels. More notably, Vc pretreatment further counter-regulated TCBQ-induced alteration of 5hmC abundance in the promoter with 100% of genes, accompanying the reverse modulation of mRNA expressions in 89% of genes. These data from Vc pretreatment supported the relationship between TCBQ-induced apoptosis and the altered 5hmC abundance. Additionally, Vc also suppressed TCBQ-stimulated generation of ROS, and further increased the stability of mitochondria. Our study illuminates a new mechanism of TCBQ-induced 5hmC-dependent apoptosis, and the dual mechanisms of Vc against TCBQ-stimulated apoptosis via reversely regulating 5hmC levels and scavenging ROS. The work also provided a possible strategy for the detoxification of TCBQ.