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 Ca2+ 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).
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 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.
2.4.3 Peroxisome Functioning
Peroxisomes play a crucial role in plant physiology. It is the organelle where β‐oxidation of fatty acids, part of the photorespiratory pathway, glyoxylate cycle, biosynthesis of phytohormones: auxin, JA and SA, and ROS metabolism occurs (Fahy et al. 2017). The peroxisome is the primary site of ROS generation, detoxification, and also involved in energy‐producing metabolism. Hypothetically, it is plausible that in salt stress, the peroxisomes' biogenesis and functional activities might be increased to cope with increased photorespiratory pathway, increased energy demand, and increased ROS level. In several previous studies, the increased number of peroxisomes was reported in plants subjected to salt stress (Mitsuya et al. 2010). A peroxisomal membrane protein family PEX11 is known to regulate the size and number of peroxisome in plants, animals, and yeast (Mitsuya et al. 2010). During salt stress in plants, the expression of proteins from the PEX11 families is upregulated (Fahy et al. 2017; Mitsuya et al. 2010). However, there is no evidence found so far, which supports the hypothesis of increased functional activity of the peroxisome and to understand that a detailed study is still missing.
2.5 Halophytes and Their Physiology
Halophytes are the plant species that can survive and complete their lifecycle in the soil with salinity equivalent or higher than 200 mM of NaCl (Flowers and Colmer 2008). The plasticity of halophytes to tolerate saline soil conditions is attributed to genetic, morphological, anatomical, and physiological adaptations (Flowers et al. 1977). Decades of research accumulated knowledge about the halophytes suggest that they evolved to maintain efficient cellular ion homeostasis, better osmotic adjustments, and secretion of excessive salt by the salt gland or by the epidermal bladder cells (EBC). How halophytes enable them to tolerate the salt stress and differed from the glycophytesis, summarized in Figure 2.1.
Figure 2.1 Schematic illustration shows comparison of ion homeostasis and physiological changes in glycophytes and halophytes during salt stress. The illustration shows only some of the important differences between a glycophyte and a halophyte to provide simplicity of the scheme. The arrows represent the direction of Na+ movement during salt stress, whereas the thickness of the arrows indicates the amount of Na+ movement. The number of transporters on the membranes represents the amount of Na+ being transported to the direction of the arrow. ABA, absiscic acid; CAM, Crassulacean acid metabolism; FV, fast‐activating Na+ channel; HKT1, high‐ affinity K+‐1 transporter; NHX, vacuolar Na+/H+ antiporter protein family; SOS1, salt overly sensitive 1 (plasma membrane Na+/H+ antiporter); SV, slow‐activating Na+ channel. Scheme was created using BioRender.com.
Source: Modified from (Bose et al. 2017; Zhao et al. 2020)
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2.5.1 Ion Homeostasis in a Halophyte
The halophytes maintain better cellular ionic homeostasis by efficient Na+ exclusion and vacuolar sequestration, xylem loading and retrieval of the Na+, minimized recirculation of Na+ through the phloem tissue, and secretion of salts through the salt gland and EBCs (Zhao et al. 2020). The constitutively expressing plasma membrane Na+/H+ antiporter SOS1 in halophytes performs the task of Na+ extrusion out of the cell more efficiently than the glycophytes (Shi et al. 2002). The sequestration of the Na+ and Cl− in the vacuole of root and leaf cells of halophytes are facilitated by constitutively expressing Na+/H+ antiporter (NHX), vacuolar H+‐inorganic pyrophosphatase (V‐PPase), and vacuolar H+‐ATPase (V‐ATPase) (Jha et al. 2011). However, to avoid the leakage of the ions
