to increased Na+ and Cl− concentrations and decreased K+ concentration inside chloroplasts resulting in the swelling of thylakoid and unstacking of grana, thereby altering the photosynthetic structure (Santos 2004). The regulation of K+ and Cl− across the thylakoid membrane is important in regulating the pH difference, which regulates the photoprotective nonphotochemical quenching mechanism and optimal photosynthesis (Finazzi et al. 2015). Any sort of disorder in this mechanism due to insufficient K+ during salt stress in the chloroplast may hamper its photoprotective mechanism. Accumulation of anions in the chloroplast affects both the acceptor and donor sides of photosystem II (PSII) by binding to a specific site on PSII (Jajoo et al. 1994, 2005). When the anions permeate the membrane during the salt stress, the influx of protons to balance the resultant charge causes structural reorganization in thylakoid membranes resulting in energy redistribution. All these salt stress components ultimately upset the ionic balance within chloroplasts, resulting in poor photosynthetic performance and reduced growth and yield. A shift in anion balance has shifted the solar energy distribution from PSII to PSI, affecting the function of the water‐splitting complex and electron transport to and from the plastocyanin (Singh‐Rawal et al. 2011). At the same time, soluble cations also cause alteration in the excitation energy transfer in the photosynthetic system at physiological temperatures by affecting several primary processes in thylakoids: initial energy distribution or “spillover” from PSII to PSI and rate constant of nonphotochemical quenching (Wong et al. 1980).
During salt stress, the maximum quantum yield of PSII (Fv/Fm), photochemical quenching coefficient, and electron transport rate decreases, whereas nonphotochemical quenching increases (Yang et al. 2020), possibly due to unstacking of the grana. The reduced photochemistry and carbon fixation lead to an increase in ROS production which are potent in damaging chloroplast and its membrane (Hernández et al. 1995). To cope with this situation, the level of enzymatic (catalase, ascorbate peroxidase, and glutathione reductase) and nonenzymatic (ascorbate, total carotenoids, phenolics, and flavonoids) antioxidants increases (Taïbi et al. 2016). The ROS production, especially H2O2, inhibits the synthesis of D1 protein (Murata et al. 2007). Salt stress modifies the QB plastoquinone binding site of D1 at the acceptor side. It stabilizes oxygen‐evolving complex (OEC) in S2 state at the donor side by salt‐induced migration of the PsbO subunit of PSII to the lumen (Sasi et al. 2018). This change in the PSII and OEC slows the water‐splitting process, which may be part of the plant’s defense strategy. The enzyme plastoquinol terminal oxidase (PTOX) in halophyte E. salsugineum chloroplast minimizes the damage by diverting electrons from plastoquinol to oxygen and producing water molecules (Stepien and Johnson 2009).
Salt has an essential role in maintaining enzyme activity. If the salt concentration is below optimum, the charged amino acid side chains of the enzyme will attract each other, thus denaturing it. At the same time, if the salt concentration is too high, regular interaction of charged groups will be blocked, new interactions will occur, and again the enzyme will denature. Some of the enzymes involved in the Calvin–Benson–Bassham cycle share their functional activity with the chloroplast glycolytic pathway. Therefore, one of the direct effects of ionic stress is the denaturation of enzymes involved in the Calvin–Benson–Bassham cycle, leading to the transduction of signals for the upregulation or downregulation of several genes encoding enzymes involved in photosynthesis. We will discuss the effect of salt stress on their activity in the following glycolytic section.
2.4.1.3 Photophosphorylation in Salt Stress
The PSI is relatively less sensitive to salt stress than PSII and participates in the cyclic ETC in algae during the salt stress. The PSI could play a vital role in salt tolerance by increasing cyclic ETC generating ATP by photophosphorylation while avoiding the build‐up of toxic reducing species (Bose et al. 2017). The excess ATP generated through cyclic ETC around PSI has been suggested to prevent Na+ overaccumulation in the chloroplasts of soybean (He et al. 2015). Two chloroplasts envelope antiporters CHX23 and NHD1 help plants to maintain the Na+ homeostasis between the chloroplast and the cytosol (Song et al. 2004). Light‐induced thylakoid swelling in salt‐stressed plants also facilitates the diffusion of the plastocyanin between cytochrome b6f complex and a PSI reaction center, enhancing the overall electron transfer rate (Kirchhoff et al. 2011) by cyclic ETC and producing ATP by photophosphorylation to avoid oxidative damage of chloroplast. In salt stress, the increased accumulation of subunits of soybean NDH complex was found to be involved in cyclic ETC (He et al. 2015) and a gene encoding the protein essential for the assembly of ATP synthase in sorghum (Sui et al. 2015) suggested the importance of cyclic ETC and photophosphorylation during the salt stress.
2.4.2 Glycolysis, Kreb'sCycle Enzymes, Oxidative Phosphorylation, and Other Mitochondrial Functioning
The sugar molecule fixed by photosynthesis is the carbon source of plants used for biosynthesis of structural components to support growth or in the respiration for supplying energy for maintenance of metabolic activity. The regulatory mechanisms involved in allocating the carbon to either growth or the respiratory pathway are defined as the carbon balance of plants (Lambers et al. 2008). Mitochondria are the energy house of the cells producing chemical energy in the form of ATP by oxidation of sugar molecules and supplying energy for metabolic activities. Under optimum growth conditions, energy production from a sugar molecule involves glycolysis, followed by Kreb’s (tricarboxylicacid, TCA) cycle and the mitochondrial electron transport chain (mtETC) coupled with oxidative phosphorylation in mitochondria (Fernie et al. 2004). In salt‐stress conditions, the demand for ATP production increased suddenly for ion transporters to maintain the ionic homeostasis, detoxification of ROS, and synthesis of osmolytes to maintain the cellular osmotic balance (Che‐Othman et al. 2017). The breakdown of glucose by glycolysis in plants operates at the cytoplasm and in the plastids (Plaxton 1996), where some of the isoforms of the chloroplastic glycolytic pathway participate in the Calvin–Benson–Bassham cycle (Dumont and Rivoal 2019).
2.4.2.1 Glycolytic Pathway in Salt Stress
The enzyme fructose‐1,6‐bisphosphatase (FBPase) catalyzing the hydrolysis of fructose‐1,6‐bisphosphate to fructose‐6‐phosphate and fructose‐1,6‐bisphosphatase aldolase (FBP aldolase) catalyzes the breakdown of fructose‐1,6‐bisphosphate into glyceraldehyde 3‐phosphate and dihydroxyacetone phosphate. The FBPase and FBP aldolase expression increased during salt stress (Kim et al. 2005). However, salt‐stress treatment of rice seedlings exhibited increased levels of enzymes involved in ethanolic fermentation and glycolate metabolism (Abbasi and Komatsu 2004), and salt‐stress‐treated soybean and leaves of grass pea also showed a similar response (Chattopadhyay et al. 2011). These findings suggest that salt stress may also induce anaerobic metabolism in plants to fulfill the increased energy demand.
2.4.2.2 TCA Cycle in Salt Stress
The glycolytic breakdown product pyruvate enters the mitochondria from the cytoplasm and serves as the TCA cycle substrate. Another possible source of pyruvate is the synthesis of pyruvate in the mitochondrial matrix with the help of malic enzyme (Che‐Othman et al. 2017). The TCA cycle provides carbon skeleton essential for the biosynthesis of amino acids, fatty acids, nucleic acids, isoprenoids, and secondary metabolites, and reductants to be used in the mtETC (Plaxton 1996; Sweetlove et al. 2010). The TCA cycle is regulated at various steps of the cycle depending upon the environmental conditions, developmental age, and plant species. Only half of the TCA cycle proteins have shown an increased abundance in salt stress (Che‐Othman et al. 2017). The protein abundance of pyruvate dehydrogenase complex (PDC) subunits and the enzyme succinyl coenzyme A synthase enhanced in salt‐sensitive plants upon exposure to salt stress. However, the increased abundance of PDC subunits was not consistent in all the plant species (Che‐Othman et al. 2017). In contrast, the abundance of isocitrate dehydrogenase decreases in salt‐sensitive plants under salt
