Physiology of Salt Stress in Plants

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.

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).

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.

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 stress.

Apart from being a significant step of respiration, the TCA cycle also contributes to the biosynthesis of amino acids. This role of the TCA cycle links the carbon to nitrogen metabolism. Although the TCA cycle is more efficient in energy production, plants require to maintain the balance between the carbon and nitrogen metabolism even under the increased energy demand under stress. Under salinity stress, the TCA network channelizes the carbon source to the malate/pyruvate pathway (Kazachkova et al. 2013), and the γ‐aminobutyric acid (GABA) shunt (Renault et al. 2010; Zhao et al. 2020) which provides the metabolic flexibility to plants during the stress. In parallel to the increased energy demand, the salt stress exerts pressure on the nitrogen metabolism for the synthesis of polyamines or other nitrogen‐containing osmolytes in plants. The extraction of oxaloacetate for the nitrogen metabolic pathways disturbs the cyclic continuation of the TCA cycle. At this condition, plants activate the alternate malate/pyruvate pathway, where the malic enzyme converts the excess malate into pyruvate replenish and restart the TCA cycle (Che‐Othman et al. 2017). The protein abundance and activity of the ME increased in salt stress in rice and wheat, respectively (Lima et al. 2012). GABA is an amino acid that accumulates in plants during the abiotic stresses and involves in carbon metabolism, pH regulation, nitrogen storage, and functions as osmoticum (Kinnersley and Turano 2010). The GABA shunt pathway involves four enzymes, bypassing the activity of 2‐oxoglutarate dehydrogenase and succinyl‐CoA synthase of the TCA cycle (Che‐Othman et al. 2017). The increase in cytosolic Ca 2+concentration and lower pH, the changes occur during salt stress, activates the enzymes of GABA shunt pathway, whereas the accumulation of sufficient NADH and ATP deactivates the GABA shunt pathway (Busch et al. 2000). The activity of the last enzyme of the GABA shunt, succinate semialdehyde dehydrogenase, synthesizes succinate and thus probably helps to minimize the ROS accumulation in stress and proper functioning of the mtETC (Bao et al. 2015).

Oxidative phosphorylation is the process of ATP production on the mitochondrial membrane by mitochondrial ATP synthase using the electrochemical gradient generated by the mtETC involving protein complexes arranged on the inner membrane of mitochondria. The non‐photosynthetic tissues, like root cells, depend on the mitochondrial oxidative phosphorylation for their energy demand. However, the root cells have limited access to an oxygen supply, have minimal ability to increase the ATP production during stress, and increasing salt ions further suppresses the activity of the complex I and complex II of the mtETC (Jacoby et al. 2011). The negative membrane potential of the mitochondrial membrane assisted the intake of Na +to the mitochondria (Che‐Othman et al. 2017), of which excess accumulation inhibits the mtETC by denaturing the proteins of the mtETC complexes or by disassembly of the complexes (Flowers 1972). At the moderate or lower salt stress in glycophytes, the ATP synthase activity increased without affecting different subunits of the ATP synthase. However, the ATP synthase subunits decreased significantly at higher salt stresses (Kosova et al. 2013). Due to presence of alternative mtETC pathways like NAD(P)H‐dehydrogenases, and alternative oxidase (AOX), the ETC is more complex and flexible in mitochondria (Jacoby et al. 2011). Suppression or inhibition of the mtETC negatively impacts the oxidative phosphorylation and ATP production. Moreover, the altered mtETC facilitates the electron's leakage to the free oxygen molecule and produces an excess amount of the ROS. However, the ionic component of the salt stress is more deleterious to the mtETC than the osmotic, affecting the NADH dehydrogenase (complex I) and succinate dehydrogenase (complex II) activity (Hamilton and Heckathorn 2001). Two competing respiratory chains in plant mitochondria help plants maintain the mtETC up to some extent in moderate salt stress. When salt stress inhibits the cytochrome‐mediated respiratory chain activity, the AOX‐mediated chain remains unaffected (Jacoby et al. 2011) and provides metabolic adjustment to the mitochondria at low or moderate salt stress.