Appraisal of the Role of Gaseous Signaling Molecules in Thermo-Tolerance Mechanisms in Plants.

A significant threat to the ongoing rise in temperature caused by global warming. Plants have many stress-resistance mechanisms, which is responsible for maintaining plant homeostasis. Abiotic stresses largely increase gaseous molecules' synthesis in plants. The study of gaseous signaling molecules has gained attention in recent years. The role of gaseous molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and ethylene, in plants under temperature high-temperature stress are discussed in the current review. Recent studies revealed the critical function that gaseous molecules play in controlling plant growth and development and their ability to respond to various abiotic stresses. Here, we provide a thorough overview of current advancements that prevent heat stress-related plant damage via gaseous molecules. We also explored and discussed the interaction of gaseous molecules. In addition, we provided an overview of the role played by gaseous molecules in high-temperature stress responses, along with a discussion of the knowledge gaps and how this may affect the development of high-temperature-resistant plant species.

Nitric Oxide and Salicylic Acid Regulate Glutathione and Ethylene Production to Enhance Heat Stress Acclimation in Wheat Involving Sulfur Assimilation.

Phytohormones have a role in stress adaptation. The major mechanism underlying the role of exogenously-sourced nitric oxide (NO; as sodium nitroprusside, SNP: 50.0 µM) and salicylic acid (SA; 0.5 mM) in the presence of 2.0 mM SO4-2 was assessed in heat stress (HS; 40 °C for 6 h daily for 15 days) tolerance in wheat (Triticum aestivum L. cv. HD-3226). The cultivar HD-3226 possessed high photosynthetic sulfur use efficiency (p-SUE) among the six cultivars screened. Plants grown under HS exhibited an increased content of reactive oxygen species (ROS; including superoxide radical and hydrogen peroxide) and extent of lipid peroxidation with a consequent reduction in photosynthesis and growth. However, both NO and SA were found to be protective against HS via enhanced S assimilation. Their application reduced oxidative stress and increased the activity of antioxidant enzymes. NO or SA supplementation along with S under HS recovered the losses and improved photosynthesis and growth. The use of SA inhibitor (2-aminoindane-2-phosphonic acid; AIP) and NO scavenger (cPTIO) confirmed that the mitigating effects of SA and NO involved induction in S assimilation.

Hydrogen Sulfide, Ethylene, and Nitric Oxide Regulate Redox Homeostasis and Protect Photosynthetic Metabolism under High Temperature Stress in Rice Plants.

Rising temperatures worldwide due to global climate change are a major scientific issue at present. The present study reports the effects of gaseous signaling molecules, ethylene (200 µL L-1; 2-chloroethylphosphonic acid; ethephon, Eth), nitric oxide (NO; 100 µM sodium nitroprusside; SNP), and hydrogen sulfide (H2S; 200 µM sodium hydrosulfide, NaHS) in high temperature stress (HS) tolerance, and whether or not H2S contributes to ethylene or NO-induced thermo-tolerance and photosynthetic protection in rice (Oryza sativa L.) cultivars, i.e., Taipei-309, and Rasi. Plants exposed to an HS of 40 °C for six h per day for 15 days caused a reduction in rice biomass, associated with decreased photosynthesis and leaf water status. High temperature stress increased oxidative stress by increasing the content of hydrogen peroxide (H2O2) and thiobarbituric acid reactive substance (TBARS) in rice leaves. These signaling molecules increased biomass, leaf water status, osmolytes, antioxidants, and photosynthesis of plants under non-stress and high temperature stress. However, the effect was more conspicuous with ethylene than NO and H2S. The application of H2S scavenger hypotaurine (HT) reversed the effect of ethylene or NO on photosynthesis under HS. This supports the findings that the ameliorating effects of Eth or SNP involved H2S. Thus, the presence of H2S with ethylene or NO can enhance thermo-tolerance while also protecting plant photosynthesis.

The Functional Interplay between Ethylene, Hydrogen Sulfide, and Sulfur in Plant Heat Stress Tolerance.

Plants encounter several abiotic stresses, among which heat stress is gaining paramount attention because of the changing climatic conditions. Severe heat stress conspicuously reduces crop productivity through changes in metabolic processes and in growth and development. Ethylene and hydrogen sulfide (H2S) are signaling molecules involved in defense against heat stress through modulation of biomolecule synthesis, the antioxidant system, and post-translational modifications. Other compounds containing the essential mineral nutrient sulfur (S) also play pivotal roles in these defense mechanisms. As biosynthesis of ethylene and H2S is connected to the S-assimilation pathway, it is logical to consider the existence of a functional interplay between ethylene, H2S, and S in relation to heat stress tolerance. The present review focuses on the crosstalk between ethylene, H2S, and S to highlight their joint involvement in heat stress tolerance.

Exogenously-Sourced Ethylene Positively Modulates Photosynthesis, Carbohydrate Metabolism, and Antioxidant Defense to Enhance Heat Tolerance in Rice.

The effect of exogenously-applied ethylene sourced from ethephon (2-chloroethyl phosphonic acid)was studied on photosynthesis, carbohydrate metabolism, and high-temperature stress tolerance in Taipei-309 and Rasi cultivars of rice (Oryza sativa L.). Heat stress increased the content of H2O2 and thiobarbituric acid reactive substances (TBARS)more in Rasi than Taipei-309. Further, a significant decline in sucrose, starch, and carbohydrate metabolism enzyme activity and photosynthesis was also observed in response to heat stress. The application of ethephon reduced H2O2 and TBARS content by enhancing the enzymatic antioxidant defense system and improved carbohydrate metabolism, photosynthesis, and growth more conspicuously in Taipei-309 under heat stress. The ethephon application enhanced photosynthesis by up-regulating the psbA and psbB genes of photosystem II in heat-stressed plants. Interestingly, foliar application of ethephoneffectively down-regulated high-temperature-stress-induced elevated ethylene biosynthesis gene expression. Overall, ethephon application optimized ethylene levels under high-temperature stress to regulate the antioxidant enzymatic system and carbohydrate metabolism, reducing the adverse effects on photosynthesis. These findings suggest that ethylene regulates photosynthesis via carbohydrate metabolism and the antioxidant system, thereby influencing high-temperature stress tolerance in rice.

The Crosstalk of Melatonin and Hydrogen Sulfide Determines Photosynthetic Performance by Regulation of Carbohydrate Metabolism in Wheat under Heat Stress.

Photosynthesis is a pivotal process that determines the synthesis of carbohydrates required for sustaining growth under normal or stress situation. Stress exposure reduces the photosynthetic potential owing to the excess synthesis of reactive oxygen species that disturb the proper functioning of photosynthetic apparatus. This decreased photosynthesis is associated with disturbances in carbohydrate metabolism resulting in reduced growth under stress. We evaluated the importance of melatonin in reducing heat stress-induced severity in wheat (Triticum aestivum L.) plants. The plants were subjected to 25 °C (optimum temperature) or 40 °C (heat stress) for 15 days at 6 h time duration and then developed the plants for 30 days. Heat stress led to oxidative stress with increased production of thiobarbituric acid reactive substances (TBARS) and hydrogen peroxide (H2O2) content and reduced accrual of total soluble sugars, starch and carbohydrate metabolism enzymes which were reflected in reduced photosynthesis. Application of melatonin not only reduced oxidative stress through lowering TBARS and H2O2 content, augmenting the activity of antioxidative enzymes but also increased the photosynthesis in plant and carbohydrate metabolism that was needed to provide energy and carbon skeleton to the developing plant under stress. However, the increase in these parameters with melatonin was mediated via hydrogen sulfide (H2S), as the inhibition of H2S by hypotaurine (HT; H2S scavenger) reversed the ameliorative effect of melatonin. This suggests a crosstalk of melatonin and H2S in protecting heat stress-induced photosynthetic inhibition via regulation of carbohydrate metabolism.

Nitric Oxide Enhances Photosynthetic Nitrogen and Sulfur-Use Efficiency and Activity of Ascorbate-Glutathione Cycle to Reduce High Temperature Stress-Induced Oxidative Stress in Rice (Oryza sativa L.) Plants.

The effects of nitric oxide (NO) as 100 µM sodium nitroprusside (SNP, NO donor) on photosynthetic-nitrogen use efficiency (NUE), photosynthetic-sulfur use efficiency (SUE), photosynthesis, growth and agronomic traits of rice (Oryza sativa L.) cultivars, Taipie-309 (high photosynthetic-N and SUE) and Rasi (low photosynthetic-N and SUE) were investigated under high temperature stress (40 °C for 6 h). Plants exposed to high temperature stress caused significant reduction in photosynthetic activity, use efficiency of N and S, and increment in H2O2 and thiobarbituric acid reactive substance (TBARS) content. The drastic effects of high temperature stress were more pronounced in cultivar Rasi than Taipie-309. However, foliar spray of SNP decreased the high temperature induced H2O2 and TBARS content and increased accumulation of proline and activity of ascorbate-glutathione cycle that collectively improved tolerance to high temperature stress more effectively in Taipie-309. Exogenously applied SNP alleviated the high temperature induced decrease in photosynthesis through maintaining higher photosynthetic-NUE and photosynthetic-SUE, activity of ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco), and synthesis of reduced glutathione (GSH). The use of 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxy-3-oxide (cPTIO, NO scavenger) substantiated the study that in the absence of NO oxidative stress increased, while NO increased photosynthetic-NUE and photosynthetic-SUE, net photosynthesis and plant dry mass. Taken together, the present investigation reveals that NO increased heat stress tolerance and minimized high temperature stress adversaries more effectively in cultivar Taipie-309 than Rasi by enhancing photosynthetic-NUE and SUE and strengthening the antioxidant defense system.

Ethylene and Sulfur Coordinately Modulate the Antioxidant System and ABA Accumulation in Mustard Plants under Salt Stress.

This study explored the interactive effect of ethephon (2-chloroethyl phosphonic acid; an ethylene source) and sulfur (S) in regulating the antioxidant system and ABA content and in maintaining stomatal responses, chloroplast structure, and photosynthetic performance of mustard plants (Brassica juncea L. Czern.) grown under 100 mM NaCl stress. The treatment of ethephon (200 µL L-1) and S (200 mg S kg-1 soil) together markedly improved the activity of enzymatic and non-enzymatic components of the ascorbate-glutathione (AsA-GSH) cycle, resulting in declined oxidative stress through lesser content of sodium (Na+) ion and hydrogen peroxide (H2O2) in salt-stressed plants. These changes promoted the development of chloroplast thylakoids and photosynthetic performance under salt stress. Ethephon + S also reduced abscisic acid (ABA) accumulation in guard cell, leading to maximal stomatal conductance under salt stress. The inhibition of ethylene action by norbornadiene (NBD) in salt- plus non-stressed treated plants increased ABA and H2O2 contents, and reduced stomatal opening, suggesting the involvement of ethephon and S in regulating stomatal conductance. These findings suggest that ethephon and S modulate antioxidant system and ABA accumulation in guard cells, controlling stomatal conductance, and the structure and efficiency of the photosynthetic apparatus in plants under salt stress.

Hydrogen peroxide modulates activity and expression of antioxidant enzymes and protects photosynthetic activity from arsenic damage in rice (Oryza sativa L.).

We studied the role of H2O2 in the protection of photosynthesis from arsenic (As) damage in rice (Oryza sativa L.) by examining the antioxidant system, photosynthesis, and growth attributes. Among the As concentrations (0, 20, 30, 40 and 50 µM) tested, maximum oxidative stress and inhibition in photosynthesis and growth were found with 50 µM As. The application of 50 µM H2O2 resulted in alleviation of the adverse effects of 50 µM As on Pigment System (PS) II activity, photosynthesis, and growth. Hydrogen peroxide supplementation induced the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) and increased reduced glutathione (GSH) content and proline metabolism. The expression of SOD and APX, PSBA and PSBB was induced in the presence of H2O2 to alleviate the As damage to PS II and maintain photosynthetic activity. The role of H2O2 as a signaling molecule is shown in the protection of photosynthetic activity in rice from As toxicity through regulation on the activity and the expression of antioxidant enzymes.