# The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae
## 1 Introduction  
Ascorbate (Asc) is the most abundant water-soluble metabolite in plants and is essential for plant growth and development. Asc is also required in the human diet; therefore, serious efforts are underway to increase the Asc contents of fruits and vegetables (reviewed, e.g., by [1, 2]). In vascular plants and green algae, Asc is synthesized via the Smirnoff-Wheeler pathway, in which GDP-D-mannose is converted to L-galactono-1,4-lactone in the cytoplasm by GDP-D-mannose 3\({}^{\prime}\),5\({}^{\prime}\) epimerase (GME), GDP-L-galactose phosphorylase (GGP), L-galactose-1-P phosphatase (GPP), and L-galactose dehydrogenase (GDH) [3, 4]. The final step, the conversion of L-galactono-1,4-lactone to Asc, occurs in the mitochondria and is catalyzed by L-galactono-1,4-lactone dehydrogenase (GLDH) at Complex [4, 5] (Figure 1). \(VTC2\), encoding GGP, plays a vital role in the regulation of Asc biosynthesis, both in vascular plants and green algae [6, 7]. In vascular plants, not only its expression but also a feedback mechanism on GGP translation by Asc and a small ORF largely determines the rate of Asc biosynthesis, thereby its cellular concentration [8, 9, 10, 11]. Alternative Asc biosynthesis routes, namely, the galacturonate, the L-glucose pathway, and the myoinositol pathway, have been suggested to contribute to Asc biosynthesis (reviewed by [12]). However, various lines of evidence show that they do not contribute significantly to Asc biosynthesis in plant leaves [13]. The best-known role of Asc is to scavenge reactive oxygen species (ROS), in a highly regulated manner in each cellular compartment (reviewed by [14]). It is linked to its capacity to act as a weak reductant and to the non-toxicity of the generated monoidehydroacor-bate (MDA) and the bicyclic dehydroacorbate (DHA) [15, 16]. Asc effectively eliminates superoxide andtocopherol radicals, though it reacts slowly with H\({}_{2}\)O\({}_{2}\). Plants use Ascperoxidases (APX) to eliminate H\({}_{2}\)O\({}_{2}\) more effectively (reviewed by [14; 15]). The reaction oxidizes Asc into MDA radicals, which can spontaneously disproportionate into DHA and Asc. The MDA radicals are then recycled back to Asc through MDA reductases (MDARs) or reduced ferredoxin produced at the acceptor side of photosystem I (PSI) [14; 17], while DHA is reduced by DHA reductase (DHAR) in glutathione-dependent non-enzymatic or enzymatic reactions [18; 19] (Figure 1). Figure 1: Ascorbate (Asc) biosynthesis and the roles of chloroplastic Asc in vascular plants. Asc is synthesized via the Smirnoff-Wheeler pathway with the majority of the steps taking place in the cytosol and the last step in the mitochondria at Complex I. Asc biosynthesis is regulated by light, circadian clock, photosynthetic electron transport, and a feedback inhibition by Asc. Asc is found in all cell compartments; PHT4\(\downarrow\)4 transports it into the chloroplast. Chloroplastic Asc plays multiple roles: (i) it is an alternative electron donor of photosystem II (PSII) when the oxygen-evolving complex (OEC) is inactive, (ii) it may inactivate the OEC under specific circumstances, (iii) it is a co-substrate of violaxanthin de-epoxidase (VDE) thereby plays a role in non-photochemical quenching, (iv) Asc  
participates in reactive oxygen species management.