# The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae
## 2 Regulation of Ascorbate Biosynthesis by Light  
Under regular growth and moderate light conditions (at approx. 100 umol m\({}^{-2}\)s\({}^{-1}\)), the Asc level of Arabidopsis leaves is in the range of 2 to 5 umol/g fresh weight (FW). The Asc content varies by a factor of two depending on the time of day, with a minimum at night and a maximum in the afternoon [41; 42; 43]. Under stress conditions, including high light [9; 42; 44; 45], ozone [46; 47], salt [48; 49], drought stress [50], and nitrogen starvation [51] a two- to three-fold increase in Asc level occurs on a timescale of days, with an apparent upper limit of about 8-10 umol Asc/g FW. Using stable and inducible expression systems in Arabidopsis, the maximum Asc concentration that can be reached is in the same range of maximum 10 umol Asc/g FW (e.g., [7]; reviewed by [11; 38]). Asc content can be somewhat more effectively increased by L-galactono-1,4-lactone treatment to the range of 15 umol Asc/g FW [52; 53]. These results demonstrate that Asc biosynthesis is highly regulated, to keep Asc in the optimum concentration range during the normal life cycle of plants and under stress conditions. Therefore, understanding the signaling pathways and the control mechanism of Asc biosynthesis are of primary importance to achieve a substantial increase in the Asc content of crops. One major factor controlling Asc biosynthesis is light. Several Smirnoff-Wheeler pathway genes are induced by illumination. These include GDP-D-mannose pyrophosphorylase (GMP), GGP, GPP, and GLDH [54; 55; 56]. Transcript levels of these biosynthesis genes also follow a circadian rhythm [42]. It has been shown that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II (PSII), prevents the increase in Asc pool size in Arabidopsis upon a shift from light-dark cycle to continuous light conditions, along with a decrease in the transcript levels of GMP, GGP, GPP, and GLDH [55]. When Arabidopsis plants were transferred to a medium containing sucrose, the leaf Asc levels decreased along with a decrease in the rate of CO\({}_{2}\) fixation. Asc content diminishment upon sucrose feeding did not occur in a sugar-insensitive Arabidopsis _abi4/sun6_ mutant [55]. Exogenous application of glucose in pea seedlings did not affect Asc content [57], whereas sucrose feeding in detached broccoli inflorescences delayed Asc depletion [58], and in tomato fruit, it increased the Asc content [59]. The Asc content decreases markedly in darkness (at a rate of approx. 2% per hour), while the levels of its degradation products increase [21], and dark-grown plants produce no Asc at all [60]. Interestingly, the downregulation of _VTC_2 expression by DCMU was slightly reversed in Arabidopsis mutants lacking GENOMES UNCOUPLED 1 (GUN1); [61], the master regulator of chloroplast-to-nucleus retrograde signaling. Thus, the results suggest that photosynthesis-derived signal(s) participate in the light activation of Asc biosynthesis, and the role of photosynthesis is not solely to provide a carbon source for Asc biosynthesis. On the other hand, soluble carbohydrates may affect the expression of Asc biosynthesis genes and the regeneration of Asc. The molecular mechanisms underlying the regulation of Asc biosynthesis by photosynthesis remain to be explored. It is conceivable that the redox state of the plastoquinone pool plays a regulatory role in Asc biosynthesis. In principle, H\({}_{2}\)O\({}_{2}\) and other ROS produced during photosynthesis could also influence the expression of Asc biosynthesis genes; however, experimental evidence is still lacking on such roles. Light may also directly affect Asc biosynthesis, degradation, and regeneration. Indeed, light can regulate GMP activity, namely, via CSN5B, a photomorphogenic factor that is part of a CSN complex, negatively regulating photomorphogenesis in Arabidopsis via proteasomal degradation [62; 63; 64]. [PERSON] et al. [64] demonstrated that GMP is polyubiquitinated and degraded in darkness via interaction with CSN5B.