Physiology of Salt Stress in Plants. Группа авторов

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Physiology of Salt Stress in Plants - Группа авторов


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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).

      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 stress.

      2.4.2.3 Salt Stress and Oxidative Phosphorylation

      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


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