Manipulation of Ascorbate Biosynthetic, Recycling, and Regulatory Pathways for Improved Abiotic Stress Tolerance in Plants

Abstract

Abiotic stresses, such as drought, salinity, and extreme temperatures, are major limiting factors in global crop productivity and are predicted to be exacerbated by climate change. The overproduction of reactive oxygen species (ROS) is a common consequence of many abiotic stresses. Ascorbate, also known as vitamin C, is the most abundant water-soluble antioxidant in plant cells and can combat oxidative stress directly as a ROS scavenger, or through the ascorbate–glutathione cycle—a major antioxidant system in plant cells. Engineering crops with enhanced ascorbate concentrations therefore has the potential to promote broad abiotic stress tolerance. Three distinct strategies have been utilized to increase ascorbate concentrations in plants: (i) increased biosynthesis, (ii) enhanced recycling, or (iii) modulating regulatory factors. Here, we review the genetic pathways underlying ascorbate biosynthesis, recycling, and regulation in plants, including a summary of all metabolic engineering strategies utilized to date to increase ascorbate concentrations in model and crop species. We then highlight transgene-free strategies utilizing genome editing tools to increase ascorbate concentrations in crops, such as editing the highly conserved upstream open reading frame that controls translation of the GDP-L-galactose phosphorylase gene.

Screening and Genetic Network Analysis of Genes Involved in Freezing and Thawing Resistance in DaMDHAR—Expressing Saccharomyces cerevisiae Using Gene Expression Profiling

“…It was examined whether DaMDHAR expression in transgenic yeast affects the cellular response to oxidative stress and high-temperature. As shown in Figure 8, the survival of DaMDHAR-expressing DM cells was better than that of WT cells when exposed to 20 and 25 mM hydrogen peroxide (H 2 O 2 ) for 1 h at 28 • C, or challenged for 4 and 6 min at 50 • C. Further, AsA is the most common antioxidant molecule in the cell; thus, it is a good target for investigating the acquired ability of DaMDHAR-expressing cells to detoxify ROS produced by abiotic stress [19,21]. Yeasts do not typically synthesize AsA; instead, they synthesize a five-carbon analog, D-erythroascorbic acid (eAsA), from Darabinose by D-arabinose dehydrogenase (ARA2) and D-arabino-c-lactone oxidase activity.…”

“…To neutralize ROS generated by cold stress, plants, including those in the Antarctic, have developed a wide range of cell rescue systems such as antioxidant systems [15][16][17]. One includes enzymatic components, such as superoxide dismutase, catalase (CAT), glutathione (GSH) peroxidase, and peroxiredoxins, and especially the ascorbate (AsA)-GSH cycle in plants [18,19]. The other component includes non-enzymatic antioxidants, including ascorbic acid (AsA; vitamin C), GSH, flavonoids, and polyphenol-derived compounds [20].…”

“…Under oxidative stress, AsA directly participates in detoxifying excess ROS and leads to oxidized forms [21]. In this process, AsA is oxidized to monodehydroascorbate (MDHA), and MDHA is further oxidized to dehydroascorbate (DHA) via spontaneous reaction [19]. To enhance the antioxidative capacity and redox state of AsA after an increased AsA pool, oxidized AsA is required to reduce AsA via rapid, effective recycling mediated by DHA reductase (DHAR) and MDHA reductase (MDHAR) in the presence of GSH [11,22].…”

Effect of Rice GDP-L-Galactose Phosphorylase Constitutive Overexpression on Ascorbate Concentration, Stress Tolerance, and Iron Bioavailability in Rice

“…The identification of OsGGP derived small interfering RNAs, for example, may indicate the occurrence of gene silencing in the 35S-OsGGP plants and warrants further investigation. Future efforts to increase ascorbate concentrations in plants could avoid the use of transgene overexpression by utilizing genome editing tools such as the CRISPR/Cas9 system to disrupt transcriptional or translational repressors of ascorbate biosynthetic genes, such as the highly conserved cis-acting upstream open reading frame that controls translation of the GGP gene (Laing et al, 2015;Bulley and Laing, 2016;Macknight et al, 2017;Li et al, 2018;Zhang et al, 2018;Broad et al, 2019Broad et al, , 2020Si et al, 2020). We also examined whether the 35S-OsGGP plants displayed altered salt tolerance at the vegetative growth phase using automated imaging.…”

2-oxoglutarate-dependent dioxygenases: A renaissance in attention for ascorbic acid in plants

“…The enzyme GDP-L galactose phosphorylase (GGP, also known as VTC4) has an important role and is responsible for the sixth enzymatic step of this pathway. Numerous studies have documented that the overexpression of the GGP gene consistently leads to a two-six fold increases in ascorbate levels in a wide range of species including A. thaliana and [3,29]. Given the above premises, a link among 2 (OG)-dioxygenase genes and the GGP gene expression is possible, whereby metabolic reprogramming occurs in the mutants studied.…”