Exogenous Potassium (K+) Positively Regulates Na+/H+ Antiport System, Carbohydrate Metabolism, and Ascorbate-Glutathione Cycle in H2S-Dependent Manner in NaCl-Stressed Tomato Seedling Roots.

Potassium (K+) is one of the vital macronutrients required by plants for proper growth and blossoming harvest. In addition, K+ also plays a decisive role in promoting tolerance to various stresses. Under stressful conditions, plants deploy their defense system through various signaling molecules, including hydrogen sulfide (H2S). The present investigation was carried out to unravel the role of K+ and H2S in plants under NaCl stress. The results of the study show that NaCl stress caused a reduction in K+ and an increase in Na+ content in the tomato seedling roots which coincided with a lower H+-ATPase activity and K+/Na+ ratio. However, application of 5 mM K+, in association with endogenous H2S, positively regulated the Na+/H+ antiport system that accelerated K+ influx and Na+ efflux, resulting in the maintenance of a higher K+/Na+ ratio. The role of K+ and H2S in the regulation of the Na+/H+ antiport system was validated by applying sodium orthovanadate (plasma membrane H+-ATPase inhibitor), tetraethylammonium chloride (K+ channel blocker), amiloride (Na+/H+ antiporter inhibitor), and hypotaurine (HT, H2S scavenger). Application of 5 mM K+ positively regulated the ascorbate-glutathione cycle and activity of antioxidant enzymes that resulted in a reduction in reactive oxygen species generation and associated damage. Under NaCl stress, K+ also activated carbohydrate metabolism and proline accumulation that caused improvement in osmotic tolerance and enhanced the hydration level of the stressed seedlings. However, inclusion of the H2S scavenger HT reversed the effect of K+, suggesting H2S-dependent functioning of K+ under NaCl stress. Therefore, the present findings report that K+, in association with H2S, alleviates NaCl-induced impairments by regulating the Na+/H+ antiport system, carbohydrate metabolism, and antioxidative defense system.

Hydrogen sulfide (H2S) and potassium (K+) synergistically induce drought stress tolerance through regulation of H+-ATPase activity, sugar metabolism, and antioxidative defense in tomato seedlings.

KEY MESSAGE: Exogenous potassium (K+) and endogenous hydrogen sulfide (H2S) synergistically alleviate drought stress through regulating H+-ATPase activity, sugar metabolism and redox homoeostasis in tomato seedlings. Present work evaluates the role of K+ in the regulation of endogenous H2S signaling in modulating the tolerance of tomato (Solanum lycopersicum L. Mill.) seedlings to drought stress. The findings reveal that exposure of seedlings to 15% (w/v) polyethylene glycol 8000 (PEG) led to a substantial decrease in leaf K+ content which was associated with reduced H+-ATPase activity. Treatment with sodium orthovanadate (SOV, PM H+-ATPase inhibitor) and tetraethylammonium chloride (TEA, K+ channel blocker) suggests that exogenous K+ stimulated H+-ATPase activity that further regulated endogenous K+ content in tomato seedlings subjected to drought stress. Moreover, reduction in H+-ATPase activity by hypotaurine (HT; H2S scavenger) substantiates the role of endogenous H2S in the regulation of H+-ATPase activity. Elevation in endogenous K+ content enhanced the biosynthesis of H2S through enhancing the synthesis of cysteine, the H2S precursor. Synergistic action of H2S and K+ effectively neutralized drought stress by regulating sugar metabolism and redox homoeostasis that resulted in osmotic adjustment, as witnessed by reduced water loss, and improved hydration level of the stressed seedlings. The integrative role of endogenous H2S in K+ homeostasis was validated using HT and TEA which weakened the protection against drought stress induced impairments. In conclusion, exogenous K+ and endogenous H2S regulate H+-ATPase activity which plays a decisive role in the maintenance of endogenous K+ homeostasis. Thus, present work reveals that K+ and H2S crosstalk is essential for modulation of drought stress tolerance in tomato seedlings.

Nitric oxide is involved in nano-titanium dioxide-induced activation of antioxidant defense system and accumulation of osmolytes under water-deficit stress in Vicia faba L.

Nano-titanium dioxide (nTiO2) has been reported to improve tolerance of plants against different environmental stresses by modulating various physiological and biochemical processes. Nitric oxide (NO) has been shown to act as an important stress signaling molecule during plant responses to abiotic stresses. The present work was planned to investigate the involvement of endogenous NO in nTiO2-induced activation of defense system of fava bean (Vicia faba L.) plants under water-deficit stress (WDS) conditions. Water-suffered plants showed increased concentration of hydrogen peroxide (H2O2) and superoxide (O2-) content coupled with increased electrolyte leakage and lipid peroxidation which adversely affected nitrate reductase (NR) activity, chlorophyll content and growth of the plants. However, application of 15 mg L-1 nTiO2 to stressed plants significantly induced NR activity and synthesis of NO which elevated enzymatic and non-enzymatic defense system of the stressed plants and suppressed the generation of H2O2 and O2- content, leakage of electrolytes, and lipid peroxidation. Application of nTiO2, in association with NO, also enhanced the accumulation of osmolytes (proline and glycine betaine) that assisted the stressed plants in osmotic adjustment as witnessed by improved hydration level of the plants. Involvement of NO in nTiO2-induced activation of defense system was confirmed with NO scavenger cPTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide] which caused recurrence of WDS.