# The Significance of Calcium in Photosynthesis
## 7 Other Ca\({}^{2+}\)-Related Chloroplast Proteins  
As a common Ca\({}^{2+}\)-associated chloroplast CAS is associated with stomatal closure and electron chains (mentioned above), and it also participates in the chloroplast-mediated regulation of algae CO\({}_{2}\) concentration [99; 100]. Inorganic carbon in the atmosphere exists as CO\({}_{2}\) for the photosynthesis of terrestrial plants. However, the content of CO\({}_{2}\) in water is very small, and inorganic carbon mostly exists in the form of H\({}_{2}\)CO\({}_{3}\). To adapt to the low concentration of CO\({}_{2}\) in water, algae form a unique mechanism to rapidly absorb inorganic carbon from the external environment and convert it into CO\({}_{2}\) in cells for photosynthesis. This mechanism is called CO\({}_{2}\) concentration mechanism (CCM). This mechanism is involved in multiple proteins, such as the high-light activated 3 (HLA3) and low-CO\({}_{2}\) (LC)-inducible protein A (LCIA), which are both involved in H\({}_{2}\)CO\({}_{3}\) transport. These two proteins act together in H\({}_{2}\)CO\({}_{3}\) transportation by transporting extracellular H\({}_{2}\)CO\({}_{3}\) to the inorganic carbon pool of chloroplast stroma [100], and CAS is mainly used to regulate the expression of nuclear-encoded CO\({}_{2}\)-limiting-inducible genes, including _HLA3_ and _LCIA_[100]. Recent research showed that CAS regulates CCM through the Ca\({}^{2+}\)-dependent pathway, which is not directly regulated by Ca\({}^{2+}\) concentration but acts on the upstream of Ca\({}^{2+}\) signal to regulate CCM [99]. Thylakoid-associated kinase (STN8) is an important photosynthetic protein in chloroplasts. STN8 dysfunction affects the phosphorylation of thylakoid membrane proteins and the expression of photosynthetic proteins encoded by nucleosomes and plastids [101]. STN8 is involved in the phosphorylation of PS2 proteins, including threonine phosphorylation at the N-terminal of D1, D2, and CP43 protein, and Thr-4 of PsbH [39]. Thr-4 is phosphorylated by STN8 only when Thr-2 is phosphorylated by other kinases [39]. The degradation rate of D1 protein in _Arabidopsis_ mutants with STN8 kinase deletion is slower than that of wild type at high light [102], so the phosphorylation of D1 protein mediated by STN8 is involved in regulating PS2 repair mechanism in the case of photoinhibition [103]. CAS is also the phosphorylated substrate of STN8 [7]. [PERSON] et al. [15] found that the phosphorylated level of CAS significantly increases under high-light stress. Phosphorylated CAS is highly likely to participate in signal transduction to respond to environmental stress, so CAS and STN8 may participate in part of the Ca\({}^{2+}\)-dependent signaling pathway. Tic is a nuclear coding input chloroplast protein [40]. It cooperates with chloroplast outer envelope protein complexes (Toc) to pass cytoplasmic material through the chloroplast's bilayer membrane. Tic is composed of several subunits, such as Tic110, Tic40, and Tic32 [104]. Tic110 can interact with Tic32, and they are both regulated by Ca\({}^{2+}\)[41]. The C terminal of Tic32 has a CaM-binding domain, and the N-terminal has an NADP (H)-binding site, which might affect the photosynthetic pathway by regulating the amount of NADP (H) [104]. Guanosine 5'-triphosphate (or 5'-diphosphate) 3'-diphosphate [(p)ppGpp] is an important regulator in chloroplast function [42]. Research shows that (p)ppGpp levels affect photosynthetic capacity and chloroplast development in _A. thaliana_[105]. Genome analysis of _A. thaliana_ found that four kind of ReIA/SpoT homologue (RSH) enzymes from three families RSH1,2,3 can maintain the balance of (p)ppGpp in plants. RSH1 is mainly used as (p)ppGpp hydrolase, whereas RSH2 and RSH3 are mainly used as (p)ppGpp synthase [106]. Chloroplast localization proteins encoded by the _RSH_ gene in rice were found to contain Ca\({}^{2+}\) domains similar to EF-hand motif [42]. Thus, (p)ppGpp may be involved in regulating chloroplast function through Ca\({}^{2+}\) signals. In _A.