Hydrogen sulfide-mediated mitigation and its integrated signaling crosstalk during salinity stress

Unraveling the potential of hydrogen sulfide as a signaling molecule for plant development and environmental stress responses: A state-of-the-art review

Over the past decade, a plethora of research has illuminated the multifaceted roles of hydrogen sulfide (H2S) in plant physiology. This gaseous molecule, endowed with signaling properties, plays a pivotal role in mitigating metal-induced oxidative stress and strengthening the plant's ability to withstand harsh environmental conditions. It fulfils several functions in regulating plant development while ameliorating the adverse impacts of environmental stressors. The intricate connections among nitric oxide (NO), hydrogen peroxide (H2O2), and hydrogen sulfide in plant signaling, along with their involvement in direct chemical processes, are contributory in facilitating post-translational modifications (PTMs) of proteins that target cysteine residues. Therefore, the present review offers a comprehensive overview of sulfur metabolic pathways regulated by hydrogen sulfide, alongside the advancements in understanding its biological activities in plant growth and development. Specifically, it centres on the physiological roles of H2S in responding to environmental stressors to explore the crucial significance of different exogenously administered hydrogen sulfide donors in mitigating the toxicity associated with heavy metals (HMs). These donors are of utmost importance in facilitating the plant development, stabilization of physiological and biochemical processes, and augmentation of anti-oxidative metabolic pathways. Furthermore, the review delves into the interaction between different growth regulators and endogenous hydrogen sulfide and their contributions to mitigating metal-induced phytotoxicity.

Interaction of melatonin and H2S mitigates NaCl toxicity summer savory (Satureja hortensis L.) through Modulation of biosynthesis of secondary metabolites and physio-biochemical attributes

As versatile signaling molecules, melatonin (ML) and hydrogen sulfide (H2S) are well-known for their roles in response to abiotic stresses. However, their cross-talk to the regulation of biochemical defence responses and secondary metabolite synthesis during salinity has received less attention. Here, the role of ML-H2S interplay in inducing defensive responses and the biosynthesis of essential oil compounds in summer savoury plants under NaCl treatment was investigated. NaCl treatment, by increasing Na accumulation, disrupting nitrogen metabolism, and inducing oxidative stress, lowered photosynthetic pigments and savoury growth. NaCl treatment also resulted in a decrease in γ-terpinene (10.3%), α-terpinene (21.9%), and p-cymene (15.3%), while an increase in carvacrol (9.1%) was observed over the control. ML and ML + H2S increased the activity of antioxidant enzymes and the level of total phenols and flavonoids, resulting in decreased levels of hydrogen peroxide and superoxide anion and alleviation of oxidative damage under salinity. ML and ML + H2S increased K uptake and restored K/Na homeostasis, thus protecting the photosynthetic apparatus against NaCl-induced toxicity. ML and ML + H2S treatments also improved nitrate/ammonium homeostasis and stimulated nitrogen metabolism, leading to improved summer savoury adaptation to NaCl stress. ML and ML + H2S changed the composition of essential oils, leading to an increase in the monoterpene hydrocarbons and oxygenated monoterpenes in plants stressed with NaCl. However, the addition of an H2S scavenger, hypotaurine, inhibited the protective effects of the ML and ML + H2S treatments under NaCl stress, which could confirm the function of H2S as a signaling molecule in the downstream defence pathway induced by ML.

Gaseous signaling molecule H2S as a multitasking signal molecule in ROS metabolism of Oryza sativa under thiocyanate (SCN-) pollution

The induction of disruption in the electronic transport chain by thiocyanate (SCN-) leads to an excessive generation of reactive oxygen species (ROS) within rice (Oryza sativa). Hydrogen sulfide (H2S) assumes a crucial role as a gaseous signaling molecule, holding significant potential in alleviating SCN--related stress. Nevertheless, there remains a dearth of understanding regarding the intricate interplay between H2S and ROS in Oryza sativa amidst SCN- pollution. In this investigation, a hydroponics-based experiment was meticulously devised to explore how H2S-mediated modifications influence the genetic feedback network governing ROS metabolism within the subcellular organelles of Oryza sativa when exposed to varying effective concentrations (EC20: 24 mg SCN/L; EC50: 96 mg SCN/L; EC75: 300 mg SCN/L) of SCN-. The findings unveiled the enhanced capacity of Oryza sativa to uptake SCN- under H2S + SCN- treatments in comparison to SCN- treatments alone. Notably, the relative growth rate (RGR) of seedlings subjected to H2S + SCN- exhibited a superior performance when contrasted with seedlings exposed solely to SCN-. Furthermore, the application of exogenous H2S yielded a significant reduction in ROS levels within Oryza sativa tissues during SCN- exposure. To elucidate the intricacies of gene regulation governing ROS metabolism at the mRNA level, the 52 targeted genes were categorized into four distinct types, namely: initial regulatory ROS generation genes (ROS-I), direct ROS scavenging genes (ROS-II), indirect ROS scavenging genes (ROS-III), and lipid oxidation genes (ROS-IV). On the whole, exogenous H2S exhibited the capacity to activate the majority of ROS-I ∼ ROS-IV genes within both Oryza sativa tissues at the EC20 concentration of SCN-. However, genetic positive/negative feedback networks emphasized the pivotal role of ROS-II genes in governing ROS metabolism within Oryza sativa. Notably, these genes were predominantly activated within the cytoplasm, chloroplasts, mitochondria, peroxisomes, and the cell wall.

Hydrogen sulfide priming enhanced salinity tolerance in sunflower by modulating ion hemostasis, cellular redox balance, and gene expression

BACKGROUND

The salinity threat represents an environmental challenge that drastically affects plant growth and yield. Besides salinity stress, the escalating world population will greatly influence the world's food security in the future. Therefore, searching for effective strategies to improve crop salinity resilience and sustain agricultural productivity under high salinity is a must. Seed priming is a reliable, simple, low-risk, and low-cost technique. Therefore, this work aimed to evaluate the impact of seed priming with 0.5 mM NaHS, as a donor of H2S, in mitigating salinity effects on sunflower seedlings. Primed and nonprime seeds were established in nonsaline soil irrigated with tape water for 14 d, and then exposed to 150 mM NaCl for 7 d.

RESULTS

Salinity stress significantly reduced the seedling growth, biomass accumulation, K+, Ca2+, and salinity tolerance index while elevating Na+ uptake and translocation. Salinity-induced adverse effects were significantly alleviated by H2S priming. Upregulation in gene expression (HaSOS2, HaGST) under NaCl stress was further enhanced by H2S priming. Also, H2S reduced lipid peroxidation, electrolyte leakage, and H2O2 content, but elevated the antioxidant defense system. NaCl-induced levels of ascorbate, glutathione, and α tocopherol, as well as the activities of AsA-GSH cycle enzymes: ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and glutathione S-transferase, were further enhanced by H2S priming. Increased level of H2S and total thiol by NaCl was also further stimulated by H2S priming.

CONCLUSION

H2S priming has proved to be an efficient strategy to improve sunflower seedlings' salinity tolerance by retaining ion homeostasis, detoxifying oxidative damage, modulating gene expression involved in ion homeostasis and ROS scavenging, and boosting endogenous H2S. These findings suggested that H2S acts as a regulatory molecule activating the functional processes responsible for sunflower adaptive mechanisms and could be adopted as a crucial crop management strategy to combat saline conditions. However, it would be of great interest to conduct further studies in the natural saline field to broaden our understanding of crop adaptive mechanisms and to support our claims.

Plant salinity stress, sensing, and its mitigation through WRKY

Salinity or salt stress has deleterious effects on plant growth and development. It imposes osmotic, ionic, and secondary stresses, including oxidative stress on the plants and is responsible for the reduction of overall crop productivity and therefore challenges global food security. Plants respond to salinity, by triggering homoeostatic mechanisms that counter salt-triggered disturbances in the physiology and biochemistry of plants. This involves the activation of many signaling components such as SOS pathway, ABA pathway, and ROS and osmotic stress signaling. These biochemical responses are accompanied by transcriptional modulation of stress-responsive genes, which is mostly mediated by salt-induced transcription factor (TF) activity. Among the TFs, the multifaceted significance of WRKY proteins has been realized in many diverse avenues of plants' life including regulation of plant stress response. Therefore, in this review, we aimed to highlight the significance of salinity in a global perspective, the mechanism of salt sensing in plants, and the contribution of WRKYs in the modulation of plants' response to salinity stress. This review will be a substantial tool to investigate this problem in different perspectives, targeting WRKY and offering directions to better manage salinity stress in the field to ensure food security.

Regulation of Mitochondrial Respiration by Hydrogen Sulfide

Hydrogen sulfide (H2S), the third gasotransmitter, has positive roles in animals and plants. Mitochondria are the source and the target of H2S and the regulatory hub in metabolism, stress, and disease. Mitochondrial bioenergetics is a vital process that produces ATP and provides energy to support the physiological and biochemical processes. H2S regulates mitochondrial bioenergetic functions and mitochondrial oxidative phosphorylation. The article summarizes the recent knowledge of the chemical and biological characteristics, the mitochondrial biosynthesis of H2S, and the regulatory effects of H2S on the tricarboxylic acid cycle and the mitochondrial respiratory chain complexes. The roles of H2S on the tricarboxylic acid cycle and mitochondrial respiratory complexes in mammals have been widely studied. The biological function of H2S is now a hot topic in plants. Mitochondria are also vital organelles regulating plant processes. The regulation of H2S in plant mitochondrial functions is gaining more and more attention. This paper mainly summarizes the current knowledge on the regulatory effects of H2S on the tricarboxylic acid cycle (TCA) and the mitochondrial respiratory chain. A study of the roles of H2S in mitochondrial respiration in plants to elucidate the botanical function of H2S in plants would be highly desirable.

Phytomelatonin: A key regulator of redox and phytohormones signaling against biotic/abiotic stresses

Plants being sessile in nature, are exposed to unwarranted threats as a result of constantly changing environmental conditions. These adverse factors can have negative impacts on their growth, development, and yield. Hormones are key signaling molecules enabling cells to respond rapidly to different external and internal stimuli. In plants, melatonin (MT) plays a critical role in the integration of various environmental signals and activation of stress-response networks to develop defense mechanisms and plant resilience. Additionally, melatonin can tackle the stress-induced alteration of cellular redox equilibrium by regulating the expression of redox hemostasis-related genes and proteins. The purpose of this article is to compile and summarize the scientific research pertaining to MT's effects on plants' resilience to biotic and abiotic stresses. Here, we have summarized that MT exerts a synergistic effect with other phytohormones, for instance, ethylene, jasmonic acid, and salicylic acid, and activates plant defense-related genes against phytopathogens. Furthermore, MT interacts with secondary messengers like Ca²⁺, nitric oxide, and reactive oxygen species to regulate the redox network. This interaction triggers different transcription factors to alleviate stress-related responses in plants. Hence, the critical synergic role of MT with diverse plant hormones and secondary messengers demonstrates phytomelatonin's importance in influencing multiple mechanisms to contribute to plant resilience against harsh environmental factors.

Foliar Spraying of NaHS Alleviates Cucumber Salt Stress by Maintaining N+/K+ Balance and Activating Salt Tolerance Signaling Pathways

Hydrogen sulfide (H₂S) is involved in the regulation of plant salt stress as a potential signaling molecule. This work investigated the effect of H₂S on cucumber growth, photosynthesis, antioxidation, ion balance, and other salt tolerance pathways. The plant height, stem diameter, leaf area and photosynthesis of cucumber seedlings were significantly inhibited by 50 mmol·L^-1 NaCl. Moreover, NaCl treatment induced superoxide anion (O₂^·-) and Na⁺ accumulation and affected the absorption of other mineral ions. On the contrary, exogenous spraying of 200 μmol·L^-1 sodium hydrosulfide (NaHS) maintained the growth of cucumber seedlings, increased photosynthesis, enhanced the ascorbate-glutathione cycle (AsA-GSH), and promoted the absorption of mineral ions under salt stress. Meanwhile, NaHS upregulated SOS1, SOS2, SOS3, NHX1, and AKT1 genes to maintain Na⁺/K⁺ balance and increased the relative expression of MAPK3, MAPK4, MAPK6, and MAPK9 genes to enhance salt tolerance. These positive effects of H₂S could be reversed by 150 mmol·L^-1 propargylglycine (PAG, a specific inhibitor of H₂S biosynthesis). These results indicated that H₂S could mitigate salt damage in cucumber, mainly by improving photosynthesis, enhancing the AsA-GSH cycle, reducing the Na⁺/K⁺ ratio, and inducing the SOS pathway and MAPK pathway.

Mechanisms of plant saline-alkaline tolerance

Saline-alkaline soil affects crop growth and development, thereby suppressing the yields. Human activities and climate changes are putting arable land under the threat of saline-alkalization. To feed a growing global population in limited arable land, it is of great urgence to breed saline-alkaline tolerant crops to cope with food security. Plant salt-tolerance mechanisms have already been explored for decades. However, to date, the molecular mechanisms underlying plants responses to saline-alkaline stress have remained largely elusive. Here, we summarize recent advances in plant response to saline-alkaline stress and propose some points deserving of further exploration.

Sustained Release of Hydrogen Sulfide from Di(t-butanol)dithiophosphate Phenethylamine Salt Encapsulated into Poly(lactic acid) Microparticles to Enhance the Growth of Radish Plants

The slow release of hydrogen sulfide has been shown to be beneficial to plants by protecting them from environmental stressors, increasing germination, and extending the lifetime of harvested fruits. A major challenge in this field is controlling the amount and location of release of hydrogen sulfide so that it is available for use by plants at optimal amounts. This article reports a dual method to release hydrogen sulfide near the roots of plants by controlling its release using the hydrolysis of a dithiophosphate and the degradation of poly(lactic acid) [PLA]. Di(t-butanol)dithiophosphate phenylethylamine (tBDPA) was dissolved in a solution of PLA, and the solvent was allowed to evaporate. The resulting solid was crushed in a blender and separated into microparticles with two different size distributions of 250-500 or 500-2000 μm. The microparticles were characterized by powder X-ray diffraction to measure the presence of microcrystals of tBDPA within PLA, and images obtained using scanning electron microscopy with energy dispersive X-ray analysis confirmed the presence of these crystals. Microparticles of tBDPA loaded within PLA were characterized for their release of phosphorus and hydrogen sulfide, which both showed a burst release within 3 days, followed by a steady release. Radish plants grown with microparticles of PLA loaded with tBDPA had up to a 141% increase in harvest yield compared to plants grown in the presence of free tBDPA not loaded into PLA, PLA microparticles without tBDPA, and control plants grown without PLA or tBDPA. These experiments showed that loading hydrogen sulfide-releasing chemicals into PLA is a promising method to improve the effect of hydrogen sulfide on plants.

Hydrogen sulphide-mediated alleviation and its interplay with other signalling molecules during temperature stress

The sessile habit of plants does not provide choices to escape the environmental constraints, leading to negative impacts on their growth and development. This causes significant losses in the agriculture sector and raises serious issues on global food security. Extreme temperatures (high or low) influence several aspects of plant life and can cause reproduction malfunction. Therefore, a strategy for temperature amelioration is necessary for the management of agricultural productivity. Supplementation with various chemicals (e.g. phytohormones, gasotransmitters, osmolytes) is considered a good choice to manage plant stress. Gasotransmitters are well-recognized for stress mitigation in plants, among which hydrogen sulphide (H₂ S) has proved promising to alleviate stress. Temperature (heat/cold) stress can stimulate the endogenous production of H₂ S in plants, and many studies have reported the significance of H₂ S for temperature stress amelioration. Here, H₂ S led to positive changes in plant physiological, biochemical and molecular responses, which are usually compromised during stress. Further, H₂ S also coordinate with other signalling components that act either upstream or downstream during stress mitigation. This review focuses on the significance of H₂ S for mitigation of temperature stress, with a comprehensive discussion on cross-talk with other signalling components or supplements (e.g. NO, H₂ O₂ , salicylic acid, trehalose, proline). Finally, the review provides a rational assessment and holistic understanding of H₂ S-mediated mitigation of extreme temperature stress and addresses the prospects for development of an effective strategy to manage temperature stress.