Читать книгу Physiology of Salt Stress in Plants - Группа авторов - Страница 35

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 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).