# The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae
## 1 Introduction
Abbreviations: APX, Asc peroxidase; cp, chloroplast; cs, cytosol; Cytbo\({}_{6}\)f, cytochrome b\({}_{6}\)f; Cytc, cytochrome c; DHA, dehydroacorbate; DHAR, dehydroacorbate reductase; DTX25, vacuolar ascorbate transporter; Fd, ferredoxin; FNR, ferredoxin-NADP oxidoreductase; GDH, L-galactoc dehydrogenase; GGP, GDP-L-galactose phosphorylase (encoded by _VTC_2); GLDH, L-galactono-1,4 lactone dehydrogenase; GME, GDP-D-mannose 3\({}^{\prime}\),5\({}^{\prime}\)-epimerase; GMP, GDP-D-mannose phosphorylase; GPP, L-galactose-1-P phosphatase; GR, glutathione reductase; L-Gal-L, L-galactono-lactone; LHC, Light-harvesting complex; m, mitochondrion; MDA, monodehydrorosorbate; MDAR Monodehydrorosorbate reductase; n, nucleus; PHT4;4, chloroplastic Asc transporter; pm, plasma membrane; PSI, photosystem I; SOD, superoxide dismutase; t, thylakoid; TCA, tricarboxylic acid; UQ, ubiquitin; v, vacuole. DHA is unstable; therefore, if it is not recycled back to Asc, it is rapidly and irreversibly degraded into oxalate, L-threonine, or tartaric acid, depending on the plant species [20; 21]. Nevertheless, the regeneration of DHA and MDA occurs very effectively, as most of the Asc pool is in the reduced state under both non-stress and mild stress conditions. An exception in this respect is the apoplast in which the oxidation of Asc occurs at a relatively high rate due to the activity of Asc oxidase; this peculiarity is most probably related to cell wall loosening and growth [22; 23]. Besides being an effective antioxidant, Asc also acts as a chaperone for 2-oxoglutate-dependent dioxygenases (2-ODDs). Plant 2-ODDs are involved, for instance, in hormone metabolism (ethylene, abscisic acid, gibberell, indole-3-acetic acid) and the synthesis of various secondary metabolites, such as anthocyanins and glucosinolates [24; 25; 26]. Asc is also involved in the epigenetic regulation of gene expression [27] via modifying the activities of ten-eleven translocation (TET) and Jumonji C-domain-containing histone demethylases that are both 2-ODDs [27; 28; 29; 30]. Asc has also been proposed to be involved in iron transport and Ca\({}^{2+}\) signaling [31; 32]. Thus Asc affects plant growth, development, and stress resistance via various mechanisms that are challenging to dissect. Growth defects have been reported for the Ascdeficient _vtc2-1_ Arabidopsis mutant containing about 20% Asc relative to the wild type, but later it turned out to be due at least partially to cryptic mutations [33]. The more recently identified T-DNA mutant _vtc2-4_ did not show any alteration in the phenotype under optimal growth conditions in [33], but a mild phenotype was observed in [34]. Moreover, the approx. 80% reduction in Asc level observed in the _vtc2_ mutants only moderately enhances high light sensitivity [35]; thus, this low level of Asc seems to be sufficient for the housekeeping functions under laboratory conditions. On the other hand, the situation can be different in the field, and it may also be species-dependent because in rice, for instance, Asc content reduction did lead to diminished growth and photosynthesis rate [36]. Several excellent reviews have already been published on the biosynthesis and the various functions of Asc (e.g., [11; 14; 15; 37; 38; 39; 40]). This review focuses on less extensively discussed issues, such as the relationship between photosynthesis and Asc biosynthesis and the effects of Asc on photosynthetic electron transport in vascular plants and algae.