Exogenous Application of Alpha-Lipoic Acid Mitigates Salt-Induced Oxidative Damage in Sorghum Plants through Regulation Growth, Leaf Pigments, Ionic Homeostasis, Antioxidant Enzymes, and Expression of Salt Stress Responsive Genes

Abstract

In plants, α-Lipoic acid (ALA) is considered a dithiol short-chain fatty acid with several strong antioxidative properties. To date, no data are conclusive regarding its effects as an exogenous application on salt stressed sorghum plants. In this study, we investigated the effect of 20 µM ALA as a foliar application on salt-stressed sorghum plants (0, 75 and 150 mM as NaCl). Under saline conditions, the applied-ALA significantly (p ≤ 0.05) stimulated plant growth, indicated by improving both fresh and dry shoot weights. A similar trend was observed in the photosynthetic pigments, including Chl a, Chl b and carotenoids. This improvement was associated with an obvious increase in the membrane stability index (MSI). At the same time, an obvious decrease in the salt induced oxidative damages was seen when the concentration of H2O2 and malondialdehyde (MDA) was reduced in the salt stressed leaf tissues. Generally, ALA-treated plants demonstrated higher antioxidant enzyme activity than in the ALA-untreated plants. A moderate level of salinity (75 mM) induced the highest activities of superoxide dismutase (SOD), guaiacol peroxidase (G-POX), and ascorbate peroxidase (APX). Meanwhile, the highest activity of catalase (CAT) was seen with 150 mM NaCl. Interestingly, applied-ALA led to a substantial decrease in the concentration of both Na and the Na/K ratio. In contrast, K and Ca exhibited a considerable increase in this respect. The role of ALA in the regulation of K+/Na+ selectivity under saline condition was confirmed through a molecular study (RT-PCR). It was found that ALA treatment downregulated the relative gene expression of plasma membrane (SOS1) and vacuolar (NHX1) Na+/H+ antiporters. In contrast, the high-affinity potassium transporter protein (HKT1) was upregulated.

Alpha Lipoic Acid as a Protective Mediator for Regulating the Defensive Responses of Wheat Plants against Sodic Alkaline Stress: Physiological, Biochemical and Molecular Aspects

“…As well as Chl a, Chl a + b and Chl a/b ratio were substantially decreased; while Chl b was more stable under sodic alkaline stress ( Figure 1 C–F). Affecting the chlorophyll content and reducing of Chl a/b ratio under various stress conditions are common biochemical signs to induce the oxidative damage and destruction of photosynthetic machinery [ 25 , 28 , 48 , 49 ]. Due to its essential role as a major electron donor in the electron transport chain, reducing Chl a can be considered an important regulatory step under stress conditions.…”

“…Exogenous ALA can enhance plant growth under osmotic stress conditions, i.e., drought and salinity [ 25 , 28 ]. This effect could be attributed to its high efficiency as a strong soluble antioxidant in both hydrophilic and hydrophobic phases [ 53 ].…”

“…Moreover, it is very effective in both its forms of reduced dihydrolipoic acid (DHLA) and oxidized α-lipoic acid (ALA) [ 27 , 53 ]. ALA can repair photosynthesis and stimulate the biosynthesis of photosynthetic pigments under stress conditions [ 25 , 27 , 28 ]. In a previous study on salt-stressed canola seedlings, exogenous ALA was found to promote the root system [ 54 ].…”

“…Its two sulfhydryl moieties, which enable it to scavenge free radicals and bind metals, are responsible for its antioxidant capacity [ 24 ]. Exogenous ALA can improve photosynthetic performance, mitigate lipid peroxidation, stimulate the antioxidant systems and regulate the osmolytes under several abiotic stresses [ 25 , 26 , 27 , 28 ]. Moreover, ALA can decrease the cytotoxic effects of Na and maintain the ionic homeostasis in the salt-stressed sorghum plants; these influences were associated with altering the expression of ionic transport-related genes, i.e., plasma membrane Na + /H + antiporters protein ( SOS1 ), vacuolar Na + /H + antiporters protein ( NHX1 ) and the high-affinity potassium transporter protein ( HKT1 ) [ 28 ].…”

“…Exogenous ALA can improve photosynthetic performance, mitigate lipid peroxidation, stimulate the antioxidant systems and regulate the osmolytes under several abiotic stresses [ 25 , 26 , 27 , 28 ]. Moreover, ALA can decrease the cytotoxic effects of Na and maintain the ionic homeostasis in the salt-stressed sorghum plants; these influences were associated with altering the expression of ionic transport-related genes, i.e., plasma membrane Na + /H + antiporters protein ( SOS1 ), vacuolar Na + /H + antiporters protein ( NHX1 ) and the high-affinity potassium transporter protein ( HKT1 ) [ 28 ]. Under osmotic stress, exogenous ALA increased the activities of enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), glutathione reductase (GR), and monodehydroascorbate reductase (MDHAR) [ 21 , 26 , 29 ].…”