# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 4 The Chloroplast is a Site for Ca\({}^{2+}\) Signaling  
Extracellular stimuli induce Ca\({}^{2+}\) spikes in the cytoplasm as well as other organelles such as chloroplasts [275]. PAMP (e.g., flagellin and chitin) treatment was found to induce a rapid Ca\({}^{2+}\) spike in the cytosol, followed by a long-lasting Ca\({}^{2+}\) spike in the chloroplast stroma [276]. The stromal Ca\({}^{2+}\) spike was substantially reduced in _cas-1_, an _Arabidopsis_ mutant lacking a thylakoid-membrane-localized Ca\({}^{2+}\)-sensing protein (CAS) [276]. However, the cytosolic Ca\({}^{2+}\) spike was not impaired in the _cas-1_ mutant [276]. Therefore, it was proposed that CAS is involved in the generation of stromal Ca\({}^{2+}\) spikes by releasing Ca\({}^{2+}\) from thylakoid membranes [276]. As an essential mineral element, Ca\({}^{2+}\) is required in cell wall synthesis, cell division, and membrane functions. It was reported that a low level of Ca\({}^{2+}\) is required for normal photosynthesis in sugar beets [277]. CAS and stromal Ca\({}^{2+}\) spikes participate in the regulation of photosynthesis in response to abiotic stresses. Using _Chlamydomonas reinhardtii_ CAS knockdown lines (_cas-kd_), it was demonstrated that CAS is required for the high light tolerance of photosynthetic light reactions [278]. When transferred under high light, the _cas-kd_ lines could not induce the expression of LHCSR3 (light-harvesting complex stress-related 3), a protein essential for non-photochemical quenching [278]. Under prolonged high light exposure, the _cas-kd_ lines displayed severe light sensitivity and the activity and recovery of PSII were almost abolished [278]. These defects could be fully rescued by a 10-fold increase in the Ca\({}^{2+}\) concentration in the growth medium [278]. Consistent with these observations, foliar Ca\({}^{2+}\) pretreatment was found to alleviate the adverse effects of stress factors and improve photosynthesis in many plant species [279; 280; 281; 282]. Taken together, these results showed that stromal Ca\({}^{2+}\) spikes and CAS are critical for the regulation and photoacclimation of photosynthesis. CAS and stromal Ca\({}^{2+}\) spikes are involved in both PTI and ETI. The _cas-1_ mutant demonstrated severely impaired resistance to virulent and avirulent _Pst_ strains [276]. PTI responses (e.g., stomatal closure, callose deposition, and accumulation of defense-related compounds) were substantially compromised in the _cas-1_ mutant [276]. ETI responses, such as localized cell death, were also delayed and suppressed in the _cas-1_ mutant [276]. Biochemical characterization of _CAS-_silenced _Nicotiana benthamiana_ plants suggested that CAS probably functions downstream of the MAPK signaling cascade and upstream of ROS signaling and SA accumulation [276]. Ca\({}^{2+}\) influences the production and signal transduction of phytohormones (Figure 1). Increases in cytosolic Ca\({}^{2+}\) are a common feature of ABA-mediated stomatal movements and ABA-regulated nuclear gene expression [80]. Ca\({}^{2+}\) is also required in ET-mediated pathogen responses [283]. When Ca\({}^{2+}\) fluxes are blocked by chelators, ET-dependent induction of chitinase accumulation was inhibited, but ET-independent induction was not affected [283]. Interestingly, exogenous Ca\({}^{2+}\) application was found to decrease ABA accumulation but increase ET production in _Fusarium cultorum_ (a fungal pathogen)-treated wheat seedlings [284]. Therefore, it was proposed that Ca\({}^{2+}\) influencesET-ABA balance in plants [284]. Using the Ca\({}^{2+}\) channel blocker heparin, it was shown that changes in the cytosolic Ca\({}^{2+}\) concentration are important in JA signaling [285]. Heparin treatment also promoted the expression of JA-responsive genes, indicating that the release of Ca\({}^{2+}\) from intracellular stores suppresses the expression of JA-responsive genes [285].