# The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae
## 2 Regulation of Ascorbate Biosynthesis by Light
They also found that loss of CSN5B function impaired the effect of light on Asc synthesis in response to continuous light or darkness, showing that CSN5B is a posttranslational regulator in Asc biosynthesis. A F-box type repressor, known as Asc acid mannose pathway regulator 1 (AMR1), also regulates Asc biosynthesis in a light-dependent manner [65]. DNA knockout lines for AMR1 accumulated two-fold greater foliar Asc than the wild type. AMR1 also negatively affected the expression levels of most Asc biosynthesis genes, including GMP, GME, GGP, GPP, L-GalDH, and GLDH. In addition, AMR1 expression was higher in aging leaves, and lower at medium light than at low light intensity. A HD-Zip I family transcription factor in tomato, SIHZ24, positively regulates the accumulation of Asc by binding to the promoter of SIGMP3, and it also regulates the expression of GME and GGP. Accordingly, the Asc content fluctuated following the expression of SIHZ24 in a light-dependent manner [66]. GGP also shows a significant response to different light levels due to the regulation by light-responsive cis-elements in its promoter, namely, a G-box motif [67]. Furthermore, light-responsive cis-elements have also been identified in the promoters of GPP and GLDH in rice [68]. It was also proposed that the VTC3 dual protein kinase/protein phosphatase is involved in signal transduction to adjust Asc levels in response to light and temperature changes [69]. VTC3 is probably located in the chloroplast, and it was suggested that the control exerted by VTC3 is post-transcriptional and does not alter the transcript levels of Asc biosynthesis genes in Arabidopsis [69]. The conversion of L-galactono-1,4-lactone to Asc in the mitochondria is light and photosynthesis dependent because photosynthetic inhibitors prevent it [52]. H\({}_{2}\)O\({}_{2}\) produced upon stress effects may inactivate selectively and reversibly Arabidopsis GLDH by oxidizing a cysteine residue (Cys-340) [70]. GLDH is protected from inactivation both by L-galactono-1,4-lactone and Asc; therefore, their availabilities and the level of H\({}_{2}\)O\({}_{2}\) may affect the rate of Asc biosynthesis in vascular plants. Bryophytes, including _Bradytecium veltuinum_, _Marchantia polymorpha_, and _Phycomitrium_ (formerly _Phycomitrella_) _patens_, possess Asc content slightly less than seed plants, in the range of 0.3 to 2 umol/g [71; 72]. [PERSON] et al. [72] presented evidence on the Smirnoff-Wheeler pathway as a source of Asc, whereas the D-galacturonate pathway did not seem to contribute to Asc biosynthesis in _P. patens_. Interestingly, two _VTC2_ paralogs are functional in _P. patens_, and, in contrast to higher plants, they are both equally responsible for Asc biosynthesis. Furthermore, the light-induced fluctuation of the transcript level of the two _VTC2_ genes was comparable to _AtVTC2_. Additionally, DCMU treatment diminished _VTC2_ expression and Asc content, as observed earlier in Arabidopsis. Thus, _VTC2_ expression is a crucial control point of Asc biosynthesis in _P. patens_. Interestingly, knockout mutants with low Asc content exhibited restricted side branch growth in their protonemata; this may be caused by multiple factors related to the various physiological functions of Asc. Green alga grown under benign conditions contain about 100-fold less Asc than vascular plants, i.e., in the range of 100 to 500 uM [73; 74; 75]. Asc is synthesized via the Smirnoff-Wheeler pathway in green algae; however, its regulation significantly differs compared to plants. It has been shown that in _Chlamydomonas reinhardtii_, the expression of the _VTC2_ gene is induced by H\({}_{2}\)O\({}_{2}\) and \({}^{1}\)O\({}_{2}\), resulting in a strong increase in Asc content. On the other hand, photosynthesis is not directly required for Asc biosynthesis. Additionally, in contrast to plants, there is no circadian regulation of Asc biosynthesis, and _C. reinhardtii_ lacks negative feedback regulation by Asc in the physiological concentration range.