# The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae
## 2 Regulation of Ascorbate Biosynthesis by Light
These mechanisms enable a rapid and manifold increase in Asc content upon various stress treatments, including light stress and sulfur deprivation [74; 76; 77]. Interaction between Photosynthetic and Mitochondrial Electron Transport in Relation to Asc Biosynthesis  
Photosynthetic electron transport rates (ETR) and NADPH levels are only slightly affected in _vtc2_ mutants containing about 20% Asc in comparison with wild-type Arabidopsis plants when grown under moderate or low light conditions [78; 79; 35; 44], whereas the photosynthetic ETR of _vtc2_ mutants is diminished at high light [44]. The _vtc2_ mutants have a slightly lower stomatal conductance, which may be related to a regulatory effect of Asc [79; 80]; this, however, is compensated by a larger stomatal number and increased RuBisCO content. Therefore, the overall CO\({}_{2}\) assimilation rate is affected by Asc deficiency only in high light, but not under normal growth conditions in Arabidopsis [79; 81]. The last step of Asc biosynthesis, the conversion of L-galactono-lactone into Asc, is catalyzed by GLDH in the mitochondria [82; 83]. GLDH is a protein of 58 kDa, located at Complex I in the mitochondrial intermembrane space, tightly tethered to the membrane through protein-protein interactions [84] (Figure 1). Complex I is organized in two arms: the matrix arm transfers electrons from NADH to ubiquinone, and the membrane arm is responsible for proton translocation [85]. In addition to being essential for Asc biosynthesis, GLDH has a non-enzymatic role in the assembly of the membrane arm of Complex I [86]. This function is likely to be independent of the role of GLDH in Asc biosynthesis because the _vtc2-1_ Arabidopsis mutant accumulates wild-type levels of Complex I [86]. During the oxidation of L-galactono-1,4-lactone to Asc by GLDH, electrons are fed into the mitochondrial electron transport chain via cytochrome c [5; 87], a soluble redox-active heme protein that transfers electrons from Complex III to Complex IV [88] (Figure 1). Cytochrome c knockdown mutants of Arabidopsis had a 60% decrease in GLDH activity without affecting the Asc content, showing that low cytochrome c levels are enough under normal growth conditions to sustain Asc biosynthesis [89]. Plant mitochondria also have an alternative oxidase (AOX) pathway, taking electrons directly from the ubiquinone pool without the contribution of the cytochrome c pathway (Figure 1). [PERSON] et al. [90] found that AOX-overexpressing Arabidopsis lines accumulated more Asc than wild-type plants, particularly at high light. Higher throughput in the cytochrome c pathway would require a larger pool of electron acceptors for L-galactono-1,4-lactone oxidation; therefore, an enhanced capacity of the AOX pathway may favor Asc biosynthesis by maintaining the cytochrome c pool in a more oxidized state. This is particularly relevant under high light conditions to prevent over-reduction of the mitochondrial electron transport chain and, at the same time, to enhance Asc biosynthesis to protect the cells against the damaging effect of ROS [90]. These results show that integration of L-galactono-1,4-lactone oxidation and mitochondrial electron transport chain activity via cytochrome c could coordinate Asc biosynthesis and respiration [88]. It has also been shown that the conversion of L-galactono-1,4-lactone depends on the photosynthetic electron transport chain because DCMU and dibromothymoquinone could effectively inhibit the Asc content increase upon L-galactono-1,4-lactone treatment [52]. This result indicates the occurrence of a crosstalk between photosynthetic and respiratory electron transport chains (Figure 1). It has been suggested that Asc may be a signal connecting the metabolisms of chloroplast and mitochondria [37; 91] based on observations on transgenic tomato plants antisensed in mitochondrial malate dehydrogenase (_mdh_).