Hydrogen Sulfide: A Robust Combatant against Abiotic Stresses in Plants

Abstract

Hydrogen sulfide (H₂S) is predominantly considered as a gaseous transmitter or signaling molecule in plants. It has been known as a crucial player during various plant cellular and physiological processes and has been gaining unprecedented attention from researchers since decades. They regulate growth and plethora of plant developmental processes such as germination, senescence, defense, and maturation in plants. Owing to its gaseous state, they are effectively diffused towards different parts of the cell to counterbalance the antioxidant pools as well as providing sulfur to cells. H₂S participates actively during abiotic stresses and enhances plant tolerance towards adverse conditions by regulation of the antioxidative defense system, oxidative stress signaling, metal transport, Na⁺/K⁺ homeostasis, etc. They also maintain H₂S-Cys-cycle during abiotic stressed conditions followed by post-translational modifications of cysteine residues. Besides their role during abiotic stresses, crosstalk of H₂S with other biomolecules such as NO and phytohormones (abscisic acid, salicylic acid, melatonin, ethylene, etc.) have also been explored in plant signaling. These processes also mediate protein post-translational modifications of cysteine residues. We have mainly highlighted all these biological functions along with proposing novel relevant issues that are required to be addressed further in the near future. Moreover, we have also proposed the possible mechanisms of H₂S actions in mediating redox-dependent mechanisms in plant physiology.

1. Introduction

Abiotic stresses mainly comprise of numerous stresses such as heavy metal, drought, light, flooding, freezing, salinity, and many more abrupt environmental conditions. Plants, due to their non-sessile nature, are exposed to these fluctuations at immediate pace [1]. Therefore, plants are abruptly affected by these abiotic factors in terms of growth and metabolism. The instant reaction in plants occurs in the form of generation of reactive oxygen species (ROS), in the form of superoxide radicals, singlet oxygen species, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and various other free radicals that are notably increased during stressed conditions [2]. ROS generation in plants mostly takes place in different sites such as chloroplasts, peroxisomes, mitochondria, and apoplasts, thereby affecting their normal functioning during abiotic stressed conditions [2]. For instance, the generation of singlet oxygen species in chloroplasts alters gene programming of nuclear genes causing chlorosis and cell programmed death to take place [3]. Interestingly, plants possess a range of tolerance mechanisms to cope with abiotic stresses in the form of antioxidant enzymes namely, catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), glutathione reductase (GR), glutathione-S-transferase (GST), etc., which improve the ROS-scavenging and reduce the stress levels in plants [4]. Apart from this, a series of physiological and biochemical mechanisms are possessed by the plants for improving their stress tolerance.

Owing to numerous geothermal events and anoxic atmosphere, H₂S is present in the environment in abundance and has been thought to be involved in origin of life [5]. For example, sulfur-comprising compounds such as amino acids (cysteine and methionine) can be formed in H₂S enriched environment [6]. From the past hundred years, H₂S is known to be a colorless poisonous gas with unpleasant odour, similar to rotten eggs, known to affect different kingdoms of life [7]. It alters cellular metabolism, and mitochondrial activity by negatively inhibiting cytochrome c oxidases [8]. Since the past decades, the novel function of H₂S has been known to act as a signaling molecule in regulating different biological and physiological plant processes. Specifically with enhanced understanding about various gasotransmitters, identification of H₂S as a novel transmitter was revealed and it was accepted as a biologically active molecule [9]. Evidences in regard to the role of H₂S as signaling molecules in plants have been illustrated. H₂S play a vital role in various processes of plants such as growth, adventitious root branching, development, seed germination, senescence, stress responses, etc. [10]. A large body of literature determined the protective role of H₂S to signal plant acclimation and resistance mechanism against abiotic stresses such as heavy metals, drought, salinity, freezing, flooding, heat, and osmotic stress [7,8,10]. To illustrate, H₂S enhanced Cr-stress tolerance in barley by stimulating photosynthetic attributes and lowering its absorption in the soil [11]. It also enhanced chlorophyll and protein content in plants subjected to salt stress along with inhibiting ROS accumulation, contributing towards salt resistance in rice [12]. It is noteworthy that H₂S donors when applied exogenously induce the endogenous H₂S levels. For instance, endogenous levels of H₂S were also triggered in Arabidopsis exposed to drought, most likely due to higher expression levels of L-desulfhydrase and D-desulfhydrase enzymes [13]. H₂S donors such as NaHS stimulate internal H₂S levels in maize [14].

Meanwhile, it is quite surprising that the role of H₂S is often linked with ROS in plants during stresses conditions. The mechanism of action of H₂S is related to oxidative stress and both antagonistic and synergistic studies with H₂S and ROS have been reported in plants for regulating plant stress responses towards adverse environmental conditions [8]. Several mechanisms have been proposed by which H₂S interact with oxidative stresses, yet their inter-relationships are still required to be elucidated [15]. Given that the vital role of H₂S in plant processes, many researchers are focused to understand the role of H₂S across cell membranes. It has been studied that H₂S gets transported in membranes via the diffusion process without any carrier protein or facilitator [16]. By this nature, H₂S act as signaling molecule with its efficacy to participate in different physiological and metabolic processes for plant protection. However, H₂S is an ideal gas in plants but lesser used due to its intricacies in maintenance of concentrations during experimentation. Subsequently, the compounds that generate H₂S in water, light, thiols and related enzymes are mostly applied in functional studies [17]. The release rate of donors is quite complex to be determined therefore, and optimal concentrations are also difficult to maintain. Taking into account diverse roles of H₂S in plants and their applications in plants, we have shed light on their beneficial aspects, biochemical and functional roles in plants during abiotic stresses. We have also summarized the H₂S and its role during oxidative stresses, antioxidant defense mechanisms, metal transport, and ion homeostasis. Moreover, the H₂S-mediated mechanisms in plants in terms of post-translational modification of cysteine residues and protein persulfidation have also been elucidated.

2. Multifunctional Capacity of H₂S

H₂S can be produced from natural sources such as volcanic eruptions and anaerobic bacterial reduction of sulfur as well as anthropogenic sources such as petroleum extractions, coal mines, natural gas, and biogas processing industries [18,19]. It has low threshold, and humans, having highly developed olfaction, can discern as low as 1 µM of Na₂S in solution [20]. H₂S was first described as a poisonous gas in 1713, and, ever since, many papers have reported its toxicity in nearly all kingdoms of life [21]. The maximum permissible concentration of this toxic gas for a daily 8 h exposure is 20 ppm, whereas inhalation of higher concentrations can cause serious health issues and may prove to be lethal [5]. Though cytotoxicity of H₂S has been noticed at higher concentrations, at low concentrations, it acts as a gaseous signaling molecule and is recognized as the third endogenous gasotransmitor in plants after nitric oxide (NO) and carbon monoxide (CO) [22,23]. H₂S has the ability to interact with thiol (-SH) groups that are present in peptides such as reduced glutathione (GSH), and also with proteins that alter their functions. This sort of interaction, converting cysteine thiols (-SH) into persulfide (-SSH) groups is called persulfidation [15]. Protein persulfidation, an oxidative posttranslational modification of cysteine residues, represents a mechanism of signaling by H₂S. It is also entailed in biosynthetic pathways that need sulphur transfer, for instance, iron-sulphur clusters, biotin, thiamine, lipoic acid, molybdopterin, and sulphur-containing bases in RNA [5]. These posttranslational modifications of cysteine residues can act as a protective mechanism under oxidative stress conditions.

H₂S has a complex biochemistry. It is a weak acid and in aqueous solution, can be dissociated into hydrosulfide (HS⁻) and sulfide (S²⁻) anions with dissociation constants (pKa₁ and pKa₂) of 6.9 and >12, respectively (Chen et al., 2020a). In aqueous environment, equilibrium also depends on temperature, so the following reactions occurs at 20 °C [15]:

H₂S + H₂O ⇌ HS⁻ + H₃O⁺ (pKa = 6.88)

HS⁻ + H₂O ⇌ S²⁻ + H₃O⁺ (pKa = 14.15)

Therefore, in biological samples having a physiological pH of around 7 and at 37 °C, HS⁻ and H₂S are the major forms whereas S²⁻ is present in negligible concentration [5]. Biological activity of H₂S in cellular compartments depends upon its ability to concentrate in and permeate through the lipid bilayer. H₂S is hydrophobic and twice as soluble in the lipid bilayer as in water, so it can diffuse through biological membranes and they foist significant resistance, which slows down diffusion of H₂S leading to its accumulation at the site of formation [24]. In contrast to water molecules, aquaporins or other protein facilitators are not required for the transportation of H₂S across the lipid bilayer [25].

H₂S plays a vital role in biological activities occurring in mammalian and plant tissues (Figure 1). So, it is crucial to measure the endogenous levels of this molecule. Several techniques have been established to determine H₂S levels in biological samples. These include methylene blue colorimetric assays, fluorescent probes, polarographic sensors, ion-selective electrodes (ISEs), liquid chromatography-mass spectrometry (LC-MS/MS), gas chromatography, and HPLC coupled with UV, fluorescence, or electrochemical detection [15,20]. To monitor the levels of H₂S in environmental samples, various sensors such as chemical sensors, optical sensors, colorimetric sensors, and more recently paper-based devices are being utilized [19]. In plant cells, H₂S can be generated through enzymatic as well as non-enzymatic routes and the significance of its metabolism depends upon sub-cellular compartment, plant parts, optimal environmental and stressful conditions involved [15]. H₂S signaling regulates stomatal movement, germination, growth and senescence [26]. H₂S, when applied exogenously, tends to attenuate the negative effects of different abiotic stresses. It was found to be a key factor in sequestering cadmium in Populus euphratica cells under cadmium stress [27]. H₂S interaction with abscisic acid (ABA) helped in drought tolerance in Arabidopsis by mediating stomatal closure [28].

3. Biosynthesis and Physiological Functions of H₂S in Plants

Plants have the ability to synthesize and consume H₂S. Wilson and his coworkers, by using a sulfur-specific flame photometric detector, observed that the leaves of plants such ascorn (Zea mays L.), cucumber (Cucumis sativus L.), pumpkin (Cucurbita pepo L.), and soybean (Glycine max L.) emitted H₂S at a rate of approximately 40 pmol/min [29]. This was the first report that detected the presence of H₂S in plants and showed that H₂S could be generated endogenously [30]. Furthermore, plants were found to continuously emit H₂S following exposure to exogenous sulfate, sulfite, bisulfite, and L-cysteine [29,31]. This showed that plants get rid of excess inorganic sulfur by emitting H₂S and that H₂S production assists in homeostasis of sulfur assimilation [30,32].

In plant cells, H₂S is found in different sub-cellular compartments (chloroplast, cytosol, and mitochondria) where enzymes linked to sulfur and cysteine metabolism have the potential to produce H₂S (Figure 2). Various enzymes that are involved in H₂S metabolism include L/D-cysteine desulfhydrase, sulfite reductase, cyanoalanine synthase, cysteine synthase, and O-acetylserine(thol)lyase isoforms [33]. When it comes to endogenous production of H₂S in plants, chloroplast is an important player. During sulfate reduction pathway, sulfite reductase (SiR), which is present in chloroplast, catalyses the reduction of sulfite to sulfide [21]. As described above, H₂S is a weak acid and can dissociate into H⁺ and HS⁻ ions in aqueous solution. H₂S is mainly present in the form of HS⁻ under neutral pH conditions but at higher pH this HS⁻ can further dissociate to H⁺ and S²⁻ ions [34]. These anionic forms are unable to diffuse freely through the chloroplast membranes. In the chloroplast stroma, pH is increased from neutral to basic (pH 8) [35]. Hence, most of H₂S is dissociated into its anionic form of HS⁻ in the chloroplast. This form of sulfide cannot permeate through the chloroplast envelop and is transported by an unknown active transporter [36].

In the cytosol, H₂S is metabolically generated from cysteine (Figure 2). H₂S is a by-product of cysteine biosynthesis that is catalyzed by O-acetylserine(thiol)lyase (OASTL) enzymes [30,36]. Cysteine biosynthesis occurs in two steps: first an intermediary product O-acetyl-Ser (OAS) is formed from acetyl-CoA and serine by serine acetyltransferase (SAT), then cysteine is formed by the incorporation of sulfide into OAS catalyzed by OASTL [30]. H₂S is released from cysteine by the action of L-cysteine desulfhydrase (L-CDES) enzyme (specific for L-cysteine) and D-cysteine desulfhydrase (D-CDES) enzyme (specific for D-cysteine) accompanied by the production of pyruvate and ammonia [21,36]. L-cysteine desulfurases such as Nifs-like proteins, which are also present in chloroplast and mitochondria, produce H₂S inside the plant cells by catalyzing the conversion of cysteine to alanine and elemental sulfur or sulfide [36].

H₂S can also be produced in mitochondria during cyanide detoxification (Figure 2). β-cyanoalanine synthase (CAS), a mitochondrial based enzyme, generates H₂S by catalyzing the transmutation of cyanide to β-cyanoalanine at the expense of cysteine [36]. The H₂S thus formed is further used by mitochondrial isoform of OASTL to synthesize cysteine, which in turn is used by CAS for the detoxification of cyanide, producing a cyclic pathway in mitochondria [37]. Apart from these sites, the presence of H₂S is also reported in Arabidopsis peroxisomes however, whether it is endogenously generated or imported from other compartments is still unknown [38].

Endogenous generation of H₂S has been observed to be induced in response to several abiotic stresses, and involves different molecules related to signaling pathways. Recently, Fang and his coworkers provided evidence that in response to chromium stress, a transcription factor TGA3 enhances H₂S production in Arabidopsis by regulating LCD expression through calcium/calmodulin-2 dependent pathway [39]. Under stress conditions, sulfide levels have been shown to be increased by the activities of H₂S producing desulfhydrases [40,41,42]. Involvement of nitric oxide (NO), ethylene, ABA and salicylic acid has been reported in the regulation of H₂S production in plants [43,44]. Signaling by H₂S also leads to stomatal closure by regulating the activity of core components of the guard cell network [45].

4. Beneficial Aspects of H₂S in Plants under Abiotic Stressed Conditions

Experimental evidences in the present era depicted that exogenously applied H₂S alleviated the negative effects of various abiotic stressors. Various studies indicating the positive impact of H₂S in plants under abiotic stresses have been represented in

Table 1. Role of H₂S-mediated abiotic stress tolerance in various plants.

S.No	Abiotic Stress	Plants	H2S	Mechanism of Action	References
1.	Cold	Arabidopsis thaliana	NaHS	Induced MPK4 kinase activity.	[46]
2.	Osmotic stress	Arabidopsis thaliana	NaHS	Stomatal closure mediated by enhanced activities of phospholipase Dδ and H2S.	[47]
3.	Salt	Kandelia obovata	NaHS	Improved photosynthesis, quantum efficiency of photosystem II, membrane integrity, hormone biosynthesis, and proteins related to antioxidation, heat-shock proteins, chaperonins, nitrogen metabolism, glycolysis and ascorbate–glutathione (AsA–GSH) cycle.	[48]
4.	Salinity	Malus hupehensis	NaHS	Declined oxidative damage and Na+, and increased antioxidant enzyme activities, K+ content to maintain the homeostasis, and modulated expression of SOS1 and SKOR under salt stress.	[49]
5.	Heavy metal (Cd)	Vigna radiata	Hypotaurine	Improved antioxidant enzymes and components of ascorbate-glutathione cycle, photosynthesis, and carbohydrate metabolism.	[50]
6.	Salinity	Cucumis sativus	NaHS	Modulated expression of genes encoding photosynthesis, carbon metabolism, amino acids, and proteins (Cysteine synthase 1, Glutathione S-transferase U25-like, Protein disulfide-isomerase, and Peroxidase 2).	[51]
7.	Low temperature	Cucumis sativus	NaHS	Enhanced antioxidative defense system with improved levels of cucurbitacin C.	[52]
8.	High temperature	Zea mays	NaHS	Stimulated antioxidative defense actions, seed germination rate, and proline accumulation.	[53]
9.	Heavy metal (Al)	Oryza sativa	NaHS	Enhanced root elongation, antioxidant activities with reduced oxidative stress markers, and Al content in root tips.	[54]
10.	Heavy metal (Cr)	Zea mays	NaHS	Higher antioxidant activities (SOD, POD, CAT) with reduced Cr accumulation within plants.	[55]
11.	Drought	Triticum aestivum	NaHS	Stimulated ABA synthesis and antioxidant enzyme activities (CAT, POD, SOD, GST) with reduced oxidative stress markers in roots as well as shoots.	[56]
12.	Salinity	Oryza sativa	NaHS	Decrease the uptake of Na+ and the Na+:K+ ratio.	[57]

. However, the concentration levels, time of exposure and different kinds of H₂S donor to be used and adapted under diverse conditions vividly shows the external symptoms of recovery after the H₂S treatment. Alongside, at a physiological and biochemical level, the nitro-oxidative stress markers assessed were in the form of proteins, lipid peroxidation, nitration, nitrosylation, and oxidation to modulate in the different manner [15]. Subsequently, it led to coordinated action of antioxidative toolbox in the form of enhanced activities of superoxide dismutase (SOD), ascorbate peroxidase (APOX), catalase (CAT), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), guaiacol peroxidase (POD), and non-enzymatic antioxidants (ascorbic acid, glutathione, tocopherol, etc.). These results clearly indicated the positive linkage among H₂S and ROS; for instance, ascorbate peroxidase and catalase are specific targets for persulfidation and are also altered by NO and post-translational modifications as well as S-nitrosylations and nitration, respectively [33]. Interestingly, it forms a substantial relation among H₂S, ROS and NO, specifically during stressful conditions.

In this milieu, due to the collective reports of the beneficial aspects of H₂S on plants during adverse conditions, it could be explored in the biotechnological applications for enrichment of soils and water sources in agri-ecosystems. Nanoparticles with the tendency to synthesize H₂S during optimal conditions also require further explorations in the near future. In brief, the exogenous H₂S forms a promising technique to palliate the adverse effects in plants subjected to abiotic stresses. Furthermore, it could be extrapolated for their other applicability such as in seed germination, root development, shoot development, branching patterns, climacteric as well as non-climacteric ripening of fruits, etc. Albeit, the fundamental research on gaining knowledge about H₂S metabolism in plants (endogenous/exogenously applied) at a cellular and molecular level needs to be investigated. Moreover, H₂S-targets in cell and signaling cascade with other molecules namely, NO, H₂O₂, as well as phytohormones, is strongly recommended to be explored further.

4.1. H₂S and Oxidative Stress Signaling

H₂S interaction among ROS and oxidative modification to regulate oxidative stress is one of the foremost signaling mechanisms to take place. ROS oxidises protein and cysteine group to sulfenic acid by sulfenylation, and so called sulfenylated proteins are regulated via thioredoxin systems [58]. Further, the excessive ROS induces oxidation of sulfenic acid into sulfinic acid/sulfonic acid that may cause inactivation of proteins [59]. The persulfidation, antioxidants enzymatic and non-enzymatic, and peroxiredoxin also comprises cysteine residues that are vulnerable to oxidative modifications via ROS [60]. Apart from activating antioxidative responses, H₂S also reacts with protein cysteine, sulfenic acid, to form persulfides. This is further reduced to thiols for recovering their cell functions [5] (Figure 3). Henceforth, this process operates as a protective pathway by averting hyper-oxidation of antioxidants and protein thiol group into thiol modification in the form of sulfinic and sulfonic acid. Consequently, it reduces ROS-dispensation ability during stressed conditions. A proteomic study reported in persulfidated or sulfenylated Arabidopsis revealed that nearly 645 proteins were susceptible to modifications [61].

Amid, KEGG pathway investigation along with domain enrichment display that overlapping of various proteins are specifically involved in different metabolic pathways namely, Kreb’s cycle, Glycolysis, Calvin cycle, protein, and amino acid metabolism. Intriguingly, proteins act as putative candidature for H₂S-mediated signaling in plants under stress severity. Strikingly, H₂S also regulates photosynthesis in several plants, photosynthetic bacterial species, and nitrogen metabolism in stressed plants [62]. Moreover, the persulfides are highly reactive towards ROS in contrast to thiols, thereby, they could be oxidised by ROS into perthiosulfenic acids, subsequently leading to the generation of perthiosulfinic and perthiosulfonic acid, respectively [5]. Contrastingly, both sulfinic and sulfonic acids are formed by irreversible reaction, yet a complete toolkit of reducing a system such as thioredoxins are restored in the form of oxidized cysteines that can convert them back into the reduced forms [36]. Henceforth, this implies to the mechanism by which H₂S-mediated oxidative stress and protein modification is protected from stresses.

In addition, autophagy is a process that gets activated during wide categories of stresses such as nitrogen, carbon, or any other nutrient deprivation and this process mainly disrupts the cytoplasmic bridges along with various organelles comprising of ROS-regulating and scavenging enzymes [63] (Figure 3). To elucidate, CAT enzyme localized within peroxisome is degraded by authophagy through H₂O₂ intervention [64]. Alongside, autophagy also denatures oxidised proteins in plants during stressed conditions. On the contrary, the basic autophagic process that degrade cytoplasm and its components for raw materials and energy related metabolic activities, the fine tuner process encompassing H₂S-mediated persulfidation forms a much more conventional, rational, and competent approach in terms of growth, development, and defense related processes in plants, specifically during stressed conditions. In the forging arguments, the previous studies conducted revealed the negative aspect of autophagic regulation by H₂S in Arabidopsis [37]. Additionally, they also suspected that H₂S when applied exogenously mitigated nitrogen deficiency and modulated the levels of anthocyanins along with suppressing autophagy and nitrogen deficiency in plants. Besides, H₂S-mediated responses also enhanced ROS levels, thereby showing the trio among ROS, H₂S, and autophagy during protein modifications in plants [65].

Nevertheless, a positive coordination among H₂S and oxidative signaling has been found in plants exposed to stressed conditions (Figure 3). Apart from the antagonistic responses among H₂S and ROS, they also participate in stress responses. If we visualize at thermodynamic state, H₂S is unable to interact with protein cysteines to convert them into persulfides [5]. However, one descriptive mechanism in persulfide synthesis is H₂S reacting with sulfenic acid. Henceforth, the oxidative stress generated at a specific intensity allows the H₂S-induced signaling pathway to get initiated via persulfidation in response to stresses. In line with this notion, the suitable mechanism of persulfide generation is through the reaction of H₂S and sulfenic acid. Indeed, the oxidative stress markers were also observed in endoplasmic reticulum along with persulfidation in mammalian cells [66]. This is in concomitant with the sulfenic acid role during protein persulfidation. However, the plant guard cell differentiation also depicts the most suitable example to interpret the complexation among H₂S and ROS. Guard cells regulate stomatal movements during harsh environmental conditions such as drought, flooding, freezing, etc. More recently, the studies have also affirmed that H₂S and NADPH oxidases, RBOHD and RBOHF, which generate H₂O₂ near apoplast are a pre-requisite in the ABA-mediated stomatal regulation process in guard cells [67]. Enigmatically, various different proteins that play a crucial role in stomatal processes in their opening and closing are specific targets for persulfidation and sulfenylation [61]. These proteins basically involve Ca-dependent protein kinases (CPK3 and CPK6) along with mitogen-activated protein kinases (MPK3, MPK6, and MPK4), respectively [68,69]. These are specific targets in abilities to acquire the mechanisms underlying H₂O₂-mediated H₂S-based stomatal closure. In another case, H₂O₂ in combination with H₂S is also mediated polyamine-induced UV-B radiation stress tolerance in barley [70]. Strikingly, H₂S involve the use of NADPH-oxidases derived H₂O₂ to trigger tomato root architecture as well as lateral root formation [71].

4.2. Antioxidant Defense System of H₂S

The increment in the levels of ROS due to numerous stresses are directly co-linked to oxidative damage of various biomolecules such as cell membrane integrity, nucleic acids, and base pairing of DNA, protein structures, etc. Plants tend to cope with ROS and its adverse effects of oxidative damage specifically through two different pathways. Firstly, it comprises of scavenging mechanism in which ROS generated within plants is scavenged by a series of antioxidant related compounds such as ascorbate and glutathione and various antioxidative enzymes related to ascorbate-glutathione cycle [1]. A study conducted in Brassica rapa subjected to Cd stress showed a stimulated content of H₂O₂, O₂^•− and lipid peroxidation due to excessive malondialdehyde content [72]. Followed by that, the ROS accrual was considerably reduced by exogenously supplied H₂S. This reduction in ROS is mainly attributed to the higher activities of antioxidant enzymes CAT and SOD, respectively. Likewise, the SOD and APOX activity was also stimulated in barley after application of H₂S donor NaHS to alleviate Al-toxicity [73]. In addition, the protein expression of APOX and APX1 gene was upregulated by H₂S along with the modulated expression levels of C/Zn SOD. All these findings speculated that H₂S-mediated antioxidant enzyme activities regulate the expression of protein transcripts followed by reducing the ROS accrual due to Al-toxicity [73].

In addition, the enzymatic activities of CAT and glycolate oxidase was also observed to be reduced under glyphosate-mediated oxidative stresses in Arabidopsis [33]. With the aid of NaHS gradients and biotin method, it has been reported that CAT reduction is most likely due to persulfidation and post-translational modifications in the conversion of thiol groups into persulfide groups of proteins, respectively [38]. Furthermore, ascorbate-glutathione cycle has been observed to directly contribute towards waning off of ROS and oxidative stress generated, through a series of reactions underlying antioxidative enzymes. Majorly, APOX, MDHAR, DHAR, and GR act concomitantly in a coordinated manner for H₂O₂ quenching and maintaining cellular redox homeostasis [4]. Treatment using NaHS enhanced APOX and GR activities along with induced APOX and glutathione levels in Zea mays subjected to temperature stress [53]. Similar to this, NaHS also triggered the APOX, GR, and DHAR activities with mitigating the declined ratios of ascorbate/DHA and GSH/GSSG in Z. mays under salt stress [74]. Although, the positive action of H₂S on ascorbate-glutathione pools was also annulated by the addition of hypotaurine, H₂S scavenger, thereby depicting the role of H₂S in regulating redox homeostasis in plants under Cd stress via ascorbate-glutathione [12]. All the above discussed studies provided with the fact that exogenously applied H₂S induces resistance against various biotic as well as abiotic stresses through the regulation of ascorbate and glutathione metabolism. Additionally, exogenous H₂S also incline the levels of endogenous H₂S in plants under stressed conditions. Further, a study formulated that H₂S mitigated salt stress and the rendered growth of root elongation in alfalfa plants. Meanwhile, this positive impact was disturbed by an inhibitor or H₂S scavenger. Moreover, they also determined that H₂S-modulated the protein transcripts and gene expressions of genes encoding SOD, CAT, glutathione, ascorbate, etc., which nullified the effects caused by lipid peroxidation in plants [40]. Altogether, H₂S regulates ROS homeostasis and maintain membranal integrity in plants by regulating the plant metabolic activities associated with antioxidant enzymes and enzymes associated with ascorbate-glutathione pools. Henceforth, plant tolerance towards various abiotic stresses is achieved through H₂S-regulated antioxidant defense system of plants (Figure 3).

4.3. Role of H₂S in Metal Uptake and Transport

Heavy metals restrict crop quality and productivity due to their toxicity, as it has adverse effect on various physiological processes of plants. Under heavy metal stress, there is alteration in the absorption and transport of metal ions in plants. It has been reported that H₂S acts as a regulator of plant resistance against heavy metal stress. In rice seedlings, the toxicity imposed by mercury (Hg) is reduced by H₂S, by inhibiting Hg transport to shoots and its sequestering in roots [75]. This is due to the increased levels of metallothioneins and non-protein thiol induced by H₂S, which can further chelate with ions of heavy metal. Phytochelatins (PCs) and metallothionein’s (MTs) are important compounds which act as heavy metal chelators and regulate the solubility and toxicity of heavy metals [76]. Zinc (Zn), one of the essential elements required for growth and development of plant impose toxic effects on plants when in excess. Transport of Zn²⁺ in the cytoplasm is through a specific zinc transporter [77]. Sodium hydrosulfide (NaHS), a H₂S donor that not only inhibits the expression of natural-resistance associated macrophage protein 1 (NRAMP), iron-regulated transporter (IRT), zinc-regulated transporter (ZRT), heavy metal ATPase 4(HMA4), and metal tolerance proteins (MTP) genes (homeostasis related genes), but also decreases the accumulation and uptake of zinc in the roots and shoots of Solanum nigrum.

Cadmium, a water soluble non-redox toxic heavy metal, absorbed through the plant roots and accumulated in edible plants parts, impose harmful effects on human health [78]. In the cytoplasm of Populus euphratica, accumulation of cadmium ion can be significantly reduced by the exogenous application of H₂S, through its increased vacuolar Cd sequestration, further decreasing the cadmium influx across the plasma membranes [27]. Acceleration of cadmium influx in P. euphratica cells by H₂O₂ was also reported in this study. In cell, the influx of Cd across the plasma membrane was also reduced by CAT, suggesting that plasma calcium channels could get activated by H₂O_2, which allows the Cd influx. Additionally, activity of anti-oxidant enzymes was enhanced by H₂S, resulting in inhibition of H₂O₂accumulation in P. euphratica cells [27]. Collectively, these results indicate that H₂O₂ mediated Cd influx through calcium channels is regulated by H₂S.

Furthermore, Al³⁺ toxicity is one of the most common environmental factors that limits crop productivity by root growth inhibition, especially in acidic soil. Under Al³⁺ toxicity, an increase in root length and decrease in Al³⁺ content in root tip of rice can be achieved by exogenous application of H₂S [27]. H₂S scavenger (HT) reversed these effects, suggesting that H₂S is involved in the alleviation of Al³⁺ toxicity in rice. Pectin, a polysaccharide component of cell wall was involved in the tolerance of Al³⁺ in plants and Al³⁺ is mainly accumulated in the hemicellulose component of the cell wall in plants [54]. Hemicellulose and pectin level was significantly decreased by H₂S in order to reduce the Al³⁺ content in the cell wall of rice root [79]. Furthermore, the expression of genes which encode certain proteins necessary for the detoxification of Al³⁺ in plants is regulated by H₂S. On the other hand, the protein level can be enhanced by H₂S to prevent Al³⁺ entry in cytoplasm. Al³⁺ deposition is reduced by UDP glucose, which is transported to the cell wall from cytoplasm via an ATP binding cassette transporter STAR1-STAR2 complex [80]. OsSTAR1 and OsSTAR2 expression was significantly enhanced by H₂S, suggesting that Al³⁺ resistance is increased by H₂S. Additionally, in rice Al³⁺ toxicity can be increased by citric acid secretion [81]. A citrate efflux transport involved in Al-induced citrate secretion is encoded by OsFRDL4 [82]. Under Al³⁺ stress in rice exogenous application of H₂S improved OsFRDL expression and significantly increased citrate content in root exudates. These results suggest that in rice, citrate secretion could be regulated by H₂S to enhance resistance to Al³⁺ toxicity. Similarly, expression of OSNRAT1, which encodes aluminum transporter 1 (NRAMP) is decreased by H₂S by decreasing the Al³⁺ amount, which enter the root cells [54]. These results indicate that activity of some proteins is regulated by H₂S by blocking the Al³⁺ entry in the root cells, resulting in enhanced Al³⁺ tolerance in rice. On the other hand, sequestration of Al³⁺ from cytoplasm to vacuole depends on half- sized ATP binding cassette transporter. OsALS1 stimulated by H₂S results in Al³⁺ sequestration in the vacuole, suggesting that Al³⁺ can be alleviated by H₂S through the reduction of the Al³⁺ content in symplast and apoplast of rice root. Collectively, it would be safe to accord the involvement of H₂S in plant metal tolerance essentially by influencing the absorption of metal ions and their transport [61] (Figure 4).

4.4. Role of H₂S in Na⁺/K⁺ Homeostasis

Salt stress has affected the growth, development, and survival of plants. Numerous negative effects such as oxidative stress and ionic stress (accumulation of Na⁺) are induced by excess salinity [83]. Recently, some studies reported that the salt tolerance of plants can be enhanced by H₂Sthat has been reported to maintain homeostasis of Na⁺/K⁺. In case of rice plant growing under salt stress, the K⁺ content in the cell decreased while the Na⁺ content was found to increase. In other words, the Na⁺/K⁺ ratio increased in the roots and leaves of rice. Whereas, exogenous application ofH₂S resulted in an increase in K⁺ and decrease in Na⁺ levels, thus maintaining the homeostasis of Na⁺ and K⁺ ions in rice. However, the addition of HT (H₂S scavenger) inhibited the ameliorative effect of H₂S [57]. In further studies, reduction in NaCl-induced transient K⁺ efflux by H₂S was also observed. It has been documented that under salt stress, H₂S significantly suppresses the expression of SKOR (gene involved in encoding of outward K⁺ rectifying channel), thus ultimately suggesting that H₂S-restored K⁺ efflux might be dependent on K⁺ rectifying channel. In addition, the level of plasma membrane bounded NADPH oxidase mediated H₂O₂was reported to enhance under the influence of H₂S [84]. However, the activity of H₂S-induced H₂O₂ accumulation was reported to suppress by a non-specific suppressor of plasma membrane bound NADPH oxidase in the roots of Arabidopsis This indicates that H₂S increases the salt tolerance in A. thaliana by maintaining H₂O₂ mediated Na⁺/K⁺ homeostasis [85].

Homeostasis of ions in the cytoplasm is maintained by the Na⁺/H⁺ antiporter, Salt-Overly-Sensitive 1 (SOS1) present on plasma membrane by reducing the concentration of Na⁺ in cytoplasm. The kinetic energy required for this transportation is provided by H⁺-ATPase derived H⁺ gradient [86]. The efficacy of H₂S on maintenance of Na⁺/K⁺ homeostasis has been reported to impede by Vanadate (an PM H⁺-ATPase inhibitor) and Amiloride (an SOSI inhibitor), thus suggesting that the ion homeostasis is regulated by H₂S through plasma membrane Na⁺/H⁺ antiporter system [87]. Furthermore, in case of Arabidopsis growing under salt stress, H₂S has been reported to have a positive impact on gene expression and phosphorylation level of H⁺-ATPase and moreover this effect was suppressed by N,N-dimethylthiourea, which works by inhibiting the endogenous production of H₂O₂ [88]. These results suggest that H₂S regulates the homeostasis of ions through H₂O₂⁻ mediated signaling pathway to synchronize the expression of plasma membrane Na⁺/H⁺ antiporter and activity of H⁺-ATPase in Arabidopsis roots [87].

5. H₂S-Mediated Mechanism of Action in Plants

It has been reported that hydrogen sulfide is generated for performing various important physiological functions, usually by post-translational oxidation of cysteine moiety to per sulfide form [89]. The persulfidation mechanism of various proteins have been well documented in case of mammals [90,91]. Hydrogen sulfide mediated persulfidation of proteins in case of plants for the proper functioning of biological processes has been discussed as follows.

5.1. Role of H₂S in Post-Translational Modification of Cysteine Residues and Protein Sulfidation

Since, H₂S is a type of gasotransmitter in plants as well as in animal cells, it is known to be equally important as other signaling molecules such as carbon monoxide (CO), nitric oxide (NO), and hydrogen peroxide (H₂O₂), etc. [92,93,94]. Moreover, H₂S has been reported to have a significant role in plant growth and also in plant protection against various types of stresses such as drought, heat, heavy metal toxicity, etc. Despite of all this, the main function of H₂S is its potential of acting as a signaling molecule [95,96]. Its role as a signaling molecule can be explained through a post-translational modification of protein, which is frequently known as ‘persulfidation’, which is characterized by upgradation of thiol group of cysteine residues (-SH group) of protein into persulfide (-SSH) group. Previously, this modification was termed as ‘S-sulfhydration’, but in actual practice, there is no hydration reaction that occurs to complete the process, so the process was renamed as persulfidation. Moreover, it has also been documented that modified cysteine has greater reactivity when compared to the unmodified thiol form [91].

5.1.1. Protein Persulfidation

As discussed earlier, H₂S perform its function by promoting the persulfidation of active cysteine moiety of protein into persulfide form via covalent conversion of thiol group into persulfide group [97,98]. However, the studies suggests that there is no direct reactivity between the thiol group of protein and H₂S group. The reason behind this non-reactivity is due to the oxidation of both hydrogen and sulfur atoms in the reaction, the electrons thus produced end up as protons that are not able to form hydrogen gas [89]. Despite this, when the thiol group of protein reacts with hydrogen peroxide, the oxidized product formed is Sulfenic acid (R-SOH), which further reacts with H₂S to form persulfidated (R-SSH) product. Furthermore, the resultant component thus formed reacts with reactive oxygen species (ROS) and generate the product, perthiosulfenic acid (R-SSOH), which has low stability. Further, it has been documented by Filipovic [89] that if the excess number of oxidants are present, R-SSOH may get further converted to two products via oxidation namely, perthiosulfinic (R-SSO₂H) and perthiosulfonic acid (R-SSO₃H). It has been reported that within the cell the level of persulfidation is regulated by thioredoxin, i.e., thioredoxin is involved in catalyzing the reverse reaction of persulfidation (Figure 5) [58,66]. This reversion reaction of protein persulfidation evades the chances of irreversible oxidative damage that normally occurs at the thiol group of the protein [5,66,98].

In addition to this, other signaling molecules such as NO are also capable of manipulating proteins by a process called S-nitrosylation (R-SNO). This reaction involves the covalent attachment of thiol group of cysteine moiety in protein to the NO [99]. The products formed as a result of this reaction are known as S-nitrosothiols [100,101]. These S-nitrosothiols are capable of reacting with H₂S, thus ultimately resulting in protein persulfidation (R-SSG).

Furthermore, it has been well documented that the modified or persulfidated proteins have high reactivity in comparison to normal unmodified form. The valid reason for this reactivity is the enhanced nucleophilicity of -SSH group that can undergo easy chemical reaction with the electrophiles [102]. The main electrophilic agents include S-4bromobenzyl methanothiosulfonate (BBMTS), methanethiosufonate (MMTS), and methylsulfonylbenzothiazole (MSBT) (Figure 5).

5.1.2. Protein Persulfidation in Plants

The first report of persulfidation in plants was reported in Arabidopsis with about 106 protein that are modified at cysteine residues by Aroca et al. [96]. Furthermore, 2015 persulfidated proteins were reported from wild type and des1 mutant Arabidopsis plants with the help of an assay in which an electrophile MSBT was used as blocking agent. All the reported proteins were found mainly involved in amino acid metabolism, protein biosynthesis, glycolysis, and in response to various stress conditions [103].

Similarly, as per the reports of Li et al. [104], H₂S has a role in the regulation of actin and thus ultimately has effect on root hair growth. Basically, the genome of Arabidopsis consists of 8 ACTIN genes, which are further categorized into two major groups on the basis of their functioning in reproductive and vegetative organs [105]. However, whenever there is an overaccumulation ofH₂S, persulfidation at cys 287 residue of one of the vegetative gene, i.e., ACTIN2 occurs. This persulfidation leads to depolymerization of actin cytoskeleton and thus ultimately resulting in root hair growth inhibition [104]. These findings were further proved by introduction of actin 2-1 mutant with Cys-287 mutated ACTIN2; the outcome of the study is the partial suppression of root hair inhibition, which is H₂S dependent [104]. Moreover, according to the literature, H₂S suppresses the activity of aminocyclopropane carboxylate oxidase (ACC oxidase) enzyme (rate limiting enzyme in ethylene biosynthesis), thus ultimately inhibiting the elongation of root hairs [44].

Moreover, H₂S has also reported to have role in the persulfidation of various enzymes that are involved in signaling of abscisic acid (ABA), which make H₂S, a contributor in stomatal closure [96,106]. This whole cascade of interactions has been studied in Arabidopsis, a model plant, and it has been reported that when the level of ABA increases in the cell, it binds to the receptor PYR/PYL/RCAR (PYRABACTIN RESISTANCE/PYR-LIKE/REGULATORY COMPONENT OF ABA RECEPTOR) and suppresses the activity of PP2C (clade A protein phosphatases) [106]. Further, the SnRK2.6 (SNF1-RELATED PROTEIN KINASE2.6) or OST1 (OPEN STOMATA 1) is stimulated to initiate multiple downstream signaling pathways. This is the stage from where H₂S regulates the ABA signaling by persulfidation of Cys-131 and Cys-137 residues of SnRK2.6, present in guard cell [107]. The persulfidation of cysteine residues enhances the kinase potential of SnRK2.6 and also promotes its interaction with ABF2 (ABA RESPONSE ELEMENT-BINDING FACTOR 2), thus resulting in the phosphorylated ABF2, which further stimulates the downstream genes that control the closure of stomata [107]. In addition to this, H₂S is also found involved in persulfidation of Cys-44 and Cys-205 of DES1 in the presence of ABA, resulting in the enhanced level of H₂S in the guard cell. This increment further promotes overproduction of ROS through persulfidation at Cys-825 and Cys-890 residues of RBOHD (NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D). The excessive production of ROS resulted in suppressing the activity of ABA signaling. Conclusively, all these potencies of H₂S have a role in the regulation of ABA signaling in plant tissue (Figure 5).

6. H₂S-Signaling during Abiotic Stresses

Under stressed conditions, various signaling molecules such as ABA, Ca²⁺, various phytohormones, H₂O_2, etc., come into action. Likewise, H₂S levels are also triggered in plants in response to various stressors. This H₂S- is triggered in response to many stresses and forms a signaling cascade. Following sections describe the role of H₂S-signaling pathway under diverse stress conditions.

6.1. H₂S-Signaling during Heavy Metal Stresses

It has been observed that there is an accumulation of H₂S in plants subjected to heavy metal stresses due to their extreme toxic nature. H₂S enhances the number of mitochondria, endoplasmic reticulum, and golgi bodies in plants [108]. Moreover, it also stimulates metal ion fixation, co-related to cell wall functioning, transporter regulation, and closed association of chelators with specific signals. The cell wall acts as a barrier to external metals, and H₂S in turn induces pectin and pectin methylesterases for strengthening the cell wall [109]. A study affirmed by Zhu et al. [54], revealed that Al-stressed rice plants showed stability in the cell wall towards metals by H₂S-mediated reduction of negative charges in cell wall along with plummeting pectin methylesterases, pectins and hemicelluloses within roots and shoots, respectively. Plants also possess specific mechanism to mitigate metal toxicity via transporting metals into vacuoles through H⁺-ATPases and citrate transporters localized onto vacuolar membrane. This is further amplified by H₂S with upregulation of H⁺-ATPases expression in tonoplast followed by reducing cytoplasmic metal accumulation [110]. Alongside, induced expression of MATE13, MATE47, and FRPL4 genes in soybean and rice by H₂S not only alleviates Cd and Al toxicity but also enhanced citrate exudation [109]. Another study reported that rice exposed to Al-stress showed upregulation in NRT1 and ALS1 genes after H₂S treatment along with controlling Al level in cytoplasm by transporting it to vacuoles [110].

Nevertheless, one of the most efficacious mechanism possessed by plants to counteract metal toxicity is to momentarily pause the metals through PCs and MTs, having a close connection to sulfur metabolism (H₂S-cysteine-core). Cysteine is crucial for GSH-biosynthesis through different enzymes, therefore, H₂S-induces the expression profile of genes encoding MTs and PCs through transcriptional regulation [111]. A certain co-related factor of H₂S that works during heavy metal stresses is NO, which is considered a principal partner of H₂S [112]. Sodium nitroprusside show similar action to NaHS in mitigating metal toxicity, depicting the closed relation among NO and H₂S, respectively [113]. In addition, H₂S also works along with Ca²⁺ ions for metal stress amelioration. This is most likely due to blocking of Ca-channels by metal ions followed by their detoxification through Ca²⁺-pathway [39]. Strikingly, NaHS modulated CDPK-transcripts in zucchini exposed to Ni-stress [111]. Apart from this, H₂S-mediated metal stress alleviation is also accompanied by phytohormones such as salicylic acid, jasmonic acid, gaseous molecules, and different mineral elements [114]. All these components trigger H₂S-pathway or H₂S-producing enzymes or endogenous H₂S [114]. The regulatory action of transcripts in promoter sites of vital genes encoding H₂S-biosynthesis have been observed. Certain transcripts such as WRKY18, WRKY13, WRKY60, etc., are enhanced, which further induces H₂S-levels under metal stressed conditions [114]. Likewise, ZIP-transcript TGA also increase the production of H₂S during metal toxicity [115].

6.2. H₂S-Signaling during Salinity Stress

Salinity has caused many adversities towards agricultural crops by reducing plant growth and productivities. Climatic disturbances have altered the agricultural practices, specifically at coastal sites. H₂S has been known to play a pivotal role in ongoing cellular responses in plants against salinity, therefore considered a powerful agricultural intervention. It has been observed that exogenously applied H₂S enhanced salinity resistance through regulating Na⁺/K⁺-homeostasis along with endogenous H₂S levels with boosted antioxidant activities in cucumber [116]. Another study reported by Kaya et al. [117], showed that melatonin mediated salinity tolerance in pepper through triggering H₂S and antioxidant levels. In addition, NaHS induced salinity in cabbage via enhancing antioxidants and enzymes involved in ascorbate/glutathione cycle [118]. Further, it has also been observed that NaHS stimulated salt tolerance and osmotic stress in strawberry through antioxidants and ascorbate/glutathione redox states, thereby minimizing oxidative/nitrosative stress [119]. Interestingly, it has been revealed that H₂S play key role in regulating antioxidants and various transcription factors namely, dehydration responsive element binding factor, ascorbate/glutathione biosynthesis along with salt overly sensitive genes [119]. H₂S on combination with NO also mitigate salt toxicity as H₂S acts downstream of NO in the signaling pathway. Henceforth, accrual of H₂S has a direct impact on the stress-mediated signaling pathway under salt conditions with the motive to alleviate the toxicity.

6.3. H₂S-Signaling during Drought/Osmotic Stress

As climatic conditions are altering on global scale and precipitation is therefore altering due to such weather conditions. Few areas experience high rain, while others perceive lower or very minimal rainfall depending on where there is disaster in the form of either drought or flooding. Overall agriculture faces a huge impact and treatments to such conditions are required. Strategies such as NO-based molecules, H₂S compounds, etc., act as impactful adjuncts. Drought stress has seriously impacted horticultural crops and impediment towards achieving productivity targets [120]. Additionally, limited rainfall and higher evaporation due to enhanced temperature also induces the impact of drought. Therefore, plants possess adaptive measures to survive during such unfavorable situations through regulating stomatal activities by reducing the transpiration rate so as to retain the water within for regulating physiological activities. H₂S also acts up/down stream in NO-signaling pathways, based on activities such as stomatal movement, closure, etc., during stressed conditions [15]. The role of H₂S in stomatal activities has been observed and studies are further conducted to understand its exact mechanism. To elucidate, H₂S causes stomatal opening and closing under varied conditions in response to adverse conditions. Another study reported that short H₂S-exposure in plants led to induce stomatal closure whereas long exposure led to stimulate stomatal activities and H₂S was also mediated by 8-mercapto-cGMP, respectively [121]. cGMP also acts as a downstream mediator of NO in plants and therefore both of them work in corroboration with one another. H₂S treatment in plants regulates the relative water content of plants subjected to drought, however, the H₂S acts as a donor during such conditions followed by inducing the metabolic profiles of plants in the form of polyamines, glycine betaine, osmolytes, proline and H₂S-biosynthesis [122]. Additionally, genes encoding soluble sugars, aquaporins, polyamines, choline monooxygenases, and betaine aldehyde dehydrogenases, etc., are also upregulated after H₂S application in drought stressed plants [122]. In addition to this, plants with H₂S treatment also reduced oxidative stress markers such as MDA and H₂O₂ [122]. NaHS treatment in Bermuda grass also stimulated tolerance against salt, osmotic, and chilling stress and this is mainly due to increased activities of antioxidants and osmolytes [114]. Further, proteomic approaches were used in H₂S-mediated drought resistance. They reported the imperative role of proteins namely, S-nitrosated proteins, photosynthetic proteins, etc., induced by H₂S. Henceforth, the plant-water relations, plant movements, stomatal opening/closing, etc., act as suitable target sites for H₂S for modulating different physiological activities in plants. H₂S-formulations act as the most suitable molecules for stress resistance in plants.

6.4. H₂S-Signaling during Temperature Stress

Global warming has been observed to be the most adverse effect of climate change, basically due to the enhanced average temperature of the Earth. However, there are various regions where temperature extremity is observed both in the form of warming as well as freezing, therefore affecting the normal agricultural patterns. Certainly, there are H₂S based compounds that participate in counteracting temperature extremities. Plants being sessile have to tolerate the varying temperatures of environment. H₂S has been observed to cope up in mediating tolerance towards high/low temperature conditions. To illustrate, Tang, et al. [123], reported that exogenous H₂S and hypotaurine, H₂S-scavenger mediated cooling stress tolerance in blueberry plants. This improvement is mainly due to enhanced tolerance after NaHS treatment owing to regulated activities of leaf gaseous exchange parameters, declined photoinhibition of PSI/PSII and higher proline levels. Concomitantly, the oxidative stress markers such as H₂O₂, MDA, etc., were also declined after H₂S treatment. Meanwhile, hypotaurine enhanced the negative effects of cooling stress. Another study reported exogenously applied NaHS boosted chilling tolerance in cucumber and the most probable reason behind this was crosstalk among H₂S and auxins during stressed conditions along with higher flavin monooxygenases (FMO) and FMO-like proteins. This in turn inclined auxins that further reduced chilling stress-generated electrolyte leakage and ROS-generation with higher expression of photosynthetic enzymes. They concluded that auxins act downstream in H₂S-mediated chilling stress tolerance in plants [124]. Further, H₂S-mediated chilling stress tolerance also revealed stimulation in cucurbitacin C, a secondary metabolite that enhanced tolerance as well as bitter taste in cucumber [52]. Contrastingly, H₂S also determined an ameliorating agent in high temperature stress that could prove lethal towards agricultural crops. A study carried out in strawberry raised under induced temperatures showed declined oxidative damage and higher heat show defense upon H₂S application. H₂S-mediated heat tolerance was found by antioxidants, aquaporins and heat shock proteins along with upregulation of genes encoding these components along with ascorbate/glutathione pools [119].

6.5. H₂S-Signaling during Nutritional Stress

Agricultural crops are susceptible to nutritional stresses, nutrient deprivation or excessive of nutrients, therefore, plants have to tolerate such conditions when there are either excessive nutrients or there is shortage. Climatic disturbances often alter CO₂ but may also affect the nutritional availability by interfering micro-biotic associations among plants. Exogenously applied NaHS lowers oxidative stresses in plants raised under nitrate stress conditions [52]. The ROS was observed to decline with aggravated activities of antioxidants via mitogen-activated protein kinases and NO-signaling. Sidewise, the expression levels of CsNMAPK transcripts were also found to upregulate in cucumber after H₂S treatment [52]. Moreover, H₂S application also improved the seed germination rate of tomato during nitrate stress through improvement in the levels of antioxidants [125]. Furthermore, Kaya and Ashraf [126] depicted that Fe-deprivation caused chlorosis, which was ameliorated by NaHS. Subsequently, oxidative stress markers were also reduced with promoted plant growth and metabolism. Hence, the inter-relationship among plants, microbiotic environment, and nutrient availability mediated by H₂S is an interesting art of work that should be further explored.

7. Challenges of Utilizing H₂S in Crop Protection

It is quite challenging to understand that H₂S and sulfane sulfur are two of the most reactive species that coexist, and it has been speculated that sulfane sulfur is the signaling molecule that performs biological actions rather than H₂S [127]. Moreover, sulfane sulfur comprises of many reactive molecules such as polysulfides, polythionates, persulfides, elemental sulfur, etc., along with the products of cysteine metabolism such asthiocysteine, thiotaurine, etc. [128]. These compounds alter cysteine residues through S-sulfhydration to generate protein persulfides that are incorporated during the translation of proteins [129]. Henceforth, these molecules have attained much attention due to their signaling, antioxidant and regulatory actions. These compounds also act as a storage unit for H₂S that further releases a gasotransmitter during biological signals. Therefore, this co-relation is of biological significance and needs to be explored. There are many studies going on to unravel this research.

H₂S has been found to play a predominant role in plants as a signaling molecule but sometimes it may become disruptive for cells. This is mainly because of their generation in excess endogenously within plants or they may arrive from exterior and become accumulated within the cells. Their higher concentrations may either affect plays in positive or negative manner [130]. However, organisms possess many ways to remove H₂S generated for proper functioning of cells. Therefore, due to these reasons it is quite challenging for their usage in plants for stress amelioration. Moreover, it also hinders the enzymatic activities in plants of pertinent enzymes such as cytochrome oxidase [131]. Consequently, plants have O-acetylserine thiol lyase enzyme to work against such kind of H₂S. In some cases, mitochondria also metabolizes H₂S to further use it as electron source for electron transport chain through ubiquinone for utilizing H₂S for ATP generation [132]. In addition to this, H₂S is exceeded to such levels that it also impairs complex IV functioning, making mitochondria non-functional due to the lack of electron flow. Accordingly, mitochondria along with controlling H₂S-generation also blocks its working. Meanwhile, H₂S in mitochondria is also maintaining H₂S at optimum levels so that its concentration does not impinge ROS. Therefore, balancing such actions is very substantial so that these reactive molecules could work in accordance with one another. Yet no such literature explains the mitochondrial and H₂S metabolism, therefore, studies are still going on to effectively understand these mechanisms.

8. Conclusions and Future Perspectives

The role of H₂S in plants has been widely known during various abiotic stresses where their accumulation is gradually increased. Various H₂S donors play predominant role in plants and studies pertaining to the same are still required to be strengthened in terms of their suitability and compatibility with no adverse effects towards plants. H₂S regulates the metal uptake and transport of various mineral nutrients to mitigate different abiotic stresses such as heavy metals by inducing chelation through metallothioneins and phytochelatins. H₂S accumulation also alters protein persulfidome, which is crucial in stress signaling, making this molecule a most imperative part to study. The redox changes in protein cysteine residues during initial stress responses are also co-linked to stress-mediated redox perturbation. The mechanistic role and regulatory framework of H₂S-regulated redox regulation and signaling is also a primitive part of this study. Apart from proteomics, the identification of substantial H₂S sensors and donors is quite prevalent. Besides, the sidewise detailed investigations associated with persulfidation is the topic of interest in near future.

Since decades, the complexed role of H₂S during oxidative stress has been studied. H₂S-associates with ROS-induced oxidative stress at various levels to form a network for regulating ROS-processing system at a translational, transcriptional, as well as post-translational level. In addition, the induced activities of antioxidant enzymes were found to be persulfidated, therefore, comprehensive molecular understanding about underlying redox mechanisms that curbs their activities are required to be explored further. In order to unravel such instances, the elucidation of an inter-relationship among persulfidation, sulfenylation, and nitrosation should be untangled with regard to their functional role in activities of redox enzymes and those which are subjected to several modifications. The persulfidation process in mitigating oxidative stress through changes in the cell sulfenylation process through ROS should also be explored further. There are various evidential studies that we have discussed in regard to ROS and H₂S in autophagy, but the fine-tuned molecular understanding pertaining to same would provide us valuable knowledge and novel functions of H₂S and their signaling mechanisms for regulating oxidative stresses. However, the transcripts of genes encoding antioxidant enzymes also adds to the protective role of H₂S against various stresses and these factors are needed to be studied in detail. H₂S also mediate stress responses in plants during stomatal movement and H₂S-targeted signaling molecules are also required to be identified. Uncovering the complexity of H₂S interactions will enable us to gain knowledge about the intricate redox reactions in plants against stresses. The progress in gaining all the information and uncovering the related aspects of H₂S with ROS and stresses in plants have been observed to be accelerated by working on model plants such as Arabidopsis. Therefore, improving our knowledge about such interactions is not just essential for rudimentary research but also for its implementation in crop improvement and breeding programmes in context to provide resistance against fluctuating environment.