Melatonin-regulated heat shock proteins and mitochondrial ATP synthase induce drought tolerance through sustaining ROS homeostasis in H2S-dependent manner.

Drought is thought to be one of the major global hazards to crop production. Understanding the role of melatonin (Mel) during plant adaptive responses to drought stress (DS) was the aim of the current investigation. Involvement of hydrogen sulfide (H2S) was also explored in Mel-regulated mechanisms of plants' tolerance to DS. A perusal of the data shows that exposure of tomato plants to DS elevated the activity of mitochondrial enzymes viz. pyruvate dehydrogenase, malate dehydrogenase, and citrate synthase. Whereas the activity of ATP synthase and ATPase was downregulated under stress conditions. Under DS, an increase in the expression level of heat shock proteins (HSPs) and activation level of antioxidant defense system was observed as well. On the other hand, an increase in the activity of NADPH oxidase and glycolate oxidase was observed along with the commencement of oxidative stress and accompanying damage. Application of 30 µM Mel to drought-stressed plants enhanced H2S accumulation and further elevated the activity of mitochondrial enzymes, activation level of the defense system, and expression of HSP17.6 and HSP70. Positive effect of Mel on these attributes was reflected by reduced level of ROS and related damage. However, application of H2S biosynthesis inhibitor DL-propargylglycine reversed the effect of Mel on the said attributes and again the damaging effects of drought were observed even in presence of Mel. This observation led us to conclude that Mel-regulated defense mechanisms operate through endogenous H2S under DS conditions.

Melatonin involves hydrogen sulfide in the regulation of H+-ATPase activity, nitrogen metabolism, and ascorbate-glutathione system under chromium toxicity.

Contamination of soils with chromium (Cr) jeopardized agriculture production globally. The current study was planned with the aim to better comprehend how melatonin (Mel) and hydrogen sulfide (H2S) regulate antioxidant defense system, potassium (K) homeostasis, and nitrogen (N) metabolism in tomato seedlings under Cr toxicity. The data reveal that application of 30 µM Mel to the seedlings treated with 25 µM Cr has a positive effect on H2S metabolism that resulted in a considerable increase in H2S. Exogenous Mel improved phytochelatins content and H+-ATPase activity with an associated increase in K content as well. Use of tetraethylammonium chloride (K+-channel blocker) and sodium orthovanadate (H+-ATPase inhibitor) showed that Mel maintained K homeostasis through regulating H+-ATPase activity under Cr toxicity. Supplementation of the stressed seedlings with Mel substantially scavenged excess reactive oxygen species (ROS) that maintained ROS homeostasis. Reduced electrolyte leakage and lipid peroxidation were additional signs of Mel's ROS scavenging effects. In addition, Mel also maintained normal functioning of nitrogen (N) metabolism and ascorbate-glutathione (AsA-GSH) system. Improved level of N fulfilled its requirement for various enzymes that have induced resilience during Cr stress. Additionally, the AsA-GSH cycle's proper operation maintained redox equilibrium, which is necessary for the biological system to function normally. Conversely, 1 mM hypotaurine (H2S scavenger) abolished the Mel-effect and again Cr-induced impairment on the above-mentioned parameters was observed even in presence of Mel. Therefore, based on the observed findings, we concluded that Mel needs endogenous H2S to alleviate Cr-induced impairments in tomato seedlings.

Crosstalk of hydrogen sulfide and nitric oxide requires calcium to mitigate impaired photosynthesis under cadmium stress by activating defense mechanisms in Vigna radiata.

Hydrogen sulfide (H2S) and nitric oxide (NO) have been known to affect vast number of processes in plants under abiotic stresses. Also, calcium (Ca) works as a second messenger in plants, which underpins the abiotic stress-induced damage. However, the sequence of action of these signaling molecules against cadmium (Cd)-induced cellular oxidative damage remains unidentified. Therefore, we studied the synergistic actions and/or relationship of signaling molecules and Ca-dependent activation of tolerance mechanisms in Vigna radiata seedlings under Cd stress. The present study shows that exogenous Ca supplemented to Cd-stressed V. radiata seedlings reduced Cd accumulation and improved the activity of nitrate reductase, and L/D-cysteine desulfhydrase (LCD/DCD) that resulted in improved synthesis of NO and H2S content. Application of Ca also elevated the level of cysteine (Cys) by upregulating the activity of Cys-synthesizing enzymes serine acetyltransferase and O-acetylserine(thiol)lyase in Cd-stressed seedlings. Maintenance of Cys pool under Cd stress contributed to improved H2S content which together with Ca and NO improved antioxidant enzymes and components of ascorbate-glutathione (AsA-GSH) cycle. All these collectively regulated the activity of NADPH oxidase and glycolate oxidase, resulting in the inhibition of Cd-induced generation of reactive oxygen species. The elevated level of Cys also assisted the Cd-stressed seedlings in maintaining GSH pool which retained normal functioning of AsA-GSH cycle and led to enhanced content of phytochelatins coupled with reduced Cd content. The positive effect of these events manifested in an enhanced rate of photosynthesis, carbohydrate accumulation, and growth attributes of the plants. On the contrary, addition of NO scavenger cPTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide], H2S scavenger HT (Hypotaurine) and Ca-chelator EGTA (Ethylene glycol-bis(b-aminoethylether)-N,N,N',N'-tetraacetic acid) again developed a condition similar to stress and positive effect of the signaling molecules was abolished. The findings of the study postulate that Ca in association with NO and H2S mitigates Cd-induced impairment and enhances the tolerance of the V. radiata plants against Cd stress. The results of the study also substantiate that Ca acts both upstream as well as downstream of NO signals whereas, H2S acts downstream of Ca and NO during Cd-stress responses of the plants.

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.