Isolation and characterization of a D-cysteine desulfhydrase protein from Arabidopsis thaliana

Functional interaction of melatonin with gasotransmitters and ROS in plant adaptation to abiotic stresses

Melatonin is considered a multifunctional stress metabolite and a novel plant hormone affecting seed germination, root architecture, circadian rhythms, leaf senescence, and fruit ripening. Melatonin functions related to plant adaptation to stress stimuli of various natures are considered especially important. One of the key components of melatonin's stress-protective action is its ability to neutralise reactive oxygen species (ROS) and reactive nitrogen species directly. However, many of its effects are related to its involvement in the signalling network of plant cells and its influence on the expression of a large number of genes important for adaptation to adverse factors. Insights into the functional relationships of melatonin with gasotransmitters (GT) - gaseous molecules performing signalling functions - are still emerging. This review has analysed and summarised the experimental data that testify to the participation of the main GTs - nitric oxide, hydrogen sulfide, and carbon monoxide - in the implementation of the protective effect of melatonin when plants are exposed to abiotic stimuli of various nature. In addition, modulation by melatonin of one of the most important components in the action of GTs and ROS - post-translational modifications of proteins and the influence of ROS and GTs on melatonin synthesis in plants under stress conditions and the specific physiological effects of exogenous melatonin and GTs have been reviewed. Finally, the prospects of the GTs' practical application to achieve synergistic stress-protective effects on plants have been considered.

Hydrogen sulphide alleviates root growth inhibition induced by phosphate starvation

Phosphorus (P) has crucial roles in plant growth and development. Hydrogen sulphide (H2S) has multiple functions in plants, particularly having the ability to promote tolerance to a variety of adversity stresses. However, it is unclear whether H2S has a function when plants suffer Pi-deficiency stress. DES1, encoding L-cysteine desulfhydrase1, is a crucial source of H2S in Arabidopsis thaliana by catalysing the substrate L-cysteine. Under phosphate starvation, the des1 mutant had a significantly shorter primary root length than the wild-type Col-0, and exogenous application of H2S donor NaHS could compensate for the root growth-sensitive phenotype. In contrast, the transgenic lines DES1ox overexpressing DES1 exhibited less sensitivity to phosphate starvation in terms of longer roots compared to the Col-0. These results demonstrate that H2S is involved in the regulation of Arabidopsis root growth under phosphate starvation. Moreover, using quantitative real-time polymerase chain reaction experiments to analyse the changes in genes induced by phosphate starvation in des1 mutant and Col-0, we screened to find that the expression of the Sulfoquinovosyl diacylglycerol 1 (SQD1) gene was significantly downregulated in the des1 mutant. Consistently, exogenous H2S significantly promoted SQD1 expression levels in roots of Col-0. Taken together, we demonstrate that DES1-mediated H2S participates in alleviating root growth inhibition by promoting the expression of SQD1 under Pi starvation.

Modulation of the Arabidopsis Starch Metabolic Network by the Cytosolic Acetyl-CoA Pathway in the Context of the Diurnal Illumination Cycle

The starch metabolic network was investigated in relation to other metabolic processes by examining a mutant with altered single-gene expression of ATP citrate lyase (ACL), an enzyme responsible for generating cytosolic acetyl-CoA pool from citrate. Previous research has shown that transgenic antisense plants with reduced ACL activity accumulate abnormally enlarged starch granules. In this study, we explored the underlying molecular mechanisms linking cytosolic acetyl-CoA generation and starch metabolism under short-day photoperiods. We performed transcriptome and quantification of starch accumulation in the leaves of wild-type and antisense seedlings with reduced ACL activity. The antisense-ACLA mutant accumulated more starch than the wild type under short-day conditions. Zymogram analyses were conducted to compare the activities of starch-metabolizing enzymes with transcriptomic changes in the seedling. Differential expression between wild-type and antisense-ACLA plants was detected in genes implicated in starch and acetyl-CoA metabolism, and cell wall metabolism. These analyses revealed a strong correlation between the transcript levels of genes responsible for starch synthesis and degradation, reflecting coordinated regulation at the transcriptomic level. Furthermore, our data provide novel insights into the regulatory links between cytosolic acetyl-CoA metabolism and starch metabolic pathways.

Protein persulfidation in plants: mechanisms and functions beyond a simple stress response

Posttranslational modifications (PTMs) can modulate the activity, localization and interactions of proteins and (re)define their biological function. Understanding how changing environments can alter cellular processes thus requires detailed knowledge about the dynamics of PTMs in time and space. A PTM that gained increasing attention in the last decades is protein persulfidation, where a cysteine thiol (-SH) is covalently bound to sulfane sulfur to form a persulfide (-SSH). The precise cellular mechanisms underlying the presumed persulfide signaling in plants are, however, only beginning to emerge. In the mitochondrial matrix, strict regulation of persulfidation and H2S homeostasis is of prime importance for maintaining mitochondrial bioenergetic processes because H2S is a highly potent poison for cytochrome c oxidase. This review summarizes the current knowledge about protein persulfidation and corresponding processes in mitochondria of the model plant Arabidopsis. These processes will be compared to the respective processes in non-plant models to underpin similarities or highlight apparent differences. We provide an overview of mitochondrial pathways that contribute to H2S and protein persulfide generation and mechanisms for H2S fixation and de-persulfidation. Based on current proteomic data, we compile a plant mitochondrial persulfidome and discuss how persulfidation may regulate protein function.

Persulfidation and phosphorylation of transcription factor SlWRKY6 differentially regulate tomato fruit ripening

Cysteine desulfhydrase catalyses the generation of the signaling molecule hydrogen sulfide (H2S) in plants. In this study, we found that H2S can inhibit tomato (Solanum lycopersicum) fruit ripening and SlWRKY6 undergoes differential protein persulfidation in SlLCD1-overexpressing leaves. Then, further study indicated that SlWRKY6 could be persulfidated by H2S at Cys396. By construction of slwrky6 mutants and SlWRKY6-OE lines, we found that SlWRKY6 positively regulates leaf senescence and fruit ripening by activating the transcription of ripening-related genes STAYGREEN 1 (SlSGR1) and Senescence-Associated Gene 12 (SlSAG12). In addition, SlWRKY6 interacted with kinase SlMAPK4 and was phosphorylated at Ser33. Dual-luciferase transient expression assays and electrophoretic mobility shift assays indicated that SlWRKY6 persulfidation attenuated its transcriptional regulation of target genes SlSGR1 and SlSAG12, whereas SlWRKY6 phosphorylation by SlMAPK4 activated the transcription of target genes to promote fruit ripening. Moreover, we provided evidence that SlWRKY6 persulfidation attenuated its SlMAPK4-mediated phosphorylation to inhibit tomato fruit ripening. By transient expression of SlWRKY6, SlWRKY6C396A, SlWRKY6S33A, and SlWRKY6S33D in slwrky6 fruits, we found that SlWRKY6 persulfidation attenuated the expression of SlSGR1 and SlSAG12 thereby delaying tomato fruit ripening, while SlWRKY6 phosphorylation increased the expression of target genes. As tomato fruits ripened, endogenous H2S production decreased, while SlMAPK4 expression increased. Therefore, our findings reveal a model in which SlWRKY6 persulfidation due to higher endogenous H2S levels in un-ripened fruit inhibits its ability to activate SlSGR1 and SlSAG12 expression, while SlWRKY6 phosphorylation by SlMAPK4 activates its transcriptional activity, thereby promoting tomato fruit ripening.

A Bacillus velezensis strain isolated from oats with disease-preventing and growth-promoting properties

Endophytes have been shown to promote plant growth and health. In the present study, a Bacillus velezensis CH1 (CH1) strain was isolated and identified from high-quality oats, which was capable of producing indole-3-acetic acid (IAA) and strong biofilms, and capabilities in the nitrogen-fixing and iron carriers. CH1 has a 3920 kb chromosome with 47.3% GC content and 3776 code genes. Compared genome analysis showed that the largest proportion of the COG database was metabolism-related (44.79%), and 1135 out of 1508 genes were associated with the function "biosynthesis, transport, and catabolism of secondary metabolites." Furthermore, thirteen gene clusters had been identified in CH1, which were responsible for the synthesis of fifteen secondary metabolites that exhibit antifungal and antibacterial properties. Additionally, the strain harbors genes involved in plant growth promotion, such as seven putative genes for IAA production, spermidine and polyamine synthase genes, along with multiple membrane-associated genes. The enrichment of these functions was strong evidence of the antimicrobial properties of strain CH1, which has the potential to be a biofertilizer for promoting oat growth and disease resistance.

Effect of hydrogen sulfide (H2S) on the growth and development of tobacco seedlings in absence of stress

BACKGROUND

Hydrogen sulfide (H2S) is a novel signaling molecule involved in the growth and development of plants and their response to stress. However, the involvement of H2S in promoting the growth and development of tobacco plants is still unclear.

RESULTS

In this study, we explored the effect of pre-soaking or irrigating the roots of tobacco plants with 0.0, 2.0, 4.0, 6.0, and 8.0 mM of sodium hydrosulfide (NaHS) on endogenous H2S production, antioxidant enzymatic and cysteine desulfhydrase activities, seed germination, agronomic traits, photosynthetic pigments contents, and root vigor. The results revealed that exogenous NaHS treatment could significantly promote endogenous H2S production by inducing gene expression of D/L-CD and the activities of D/L-CD enzymes. Additionally, a significant increase in the agronomic traits and the contents of photosynthetic pigments, and no significant difference in carotenoid content among tobacco plants treated with 0.0 to 8.0 mM of NaHS was observed. Additionally, a significant increase in the germination speed, dry weight, and vigor of tobacco seeds, whereas no significant effect on the percentage of seed germination was observed on NaHS treatment. Furthermore, NaHS treatment could significantly increase the activity of superoxide dismutase (SOD) and peroxidase (POD) enzymes, which reduces damage due to oxidative stress by maintaining reactive oxygen species homeostasis.

CONCLUSIONS

These results would aid in enhancing our understanding of the involvement of H2S, a novel signaling molecule to promote the growth and development of tobacco plants.

Hydrogen Sulfide in the Oxidative Stress Response of Plants: Crosstalk with Reactive Oxygen Species

Growing evidence suggests that exposure of plants to unfavorable environments leads to the accumulation of hydrogen sulfide (H2S) and reactive oxygen species (ROS). H2S interacts with the ROS-mediated oxidative stress response network at multiple levels. Therefore, it is essential to elucidate the mechanisms by which H2S and ROS interact. The molecular mechanism of action by H2S relies on the post-translational modification of the cysteine sulfur group (-SH), known as persulfidation. H2S cannot react directly with -SH, but it can react with oxidized cysteine residues, and this oxidation process is induced by H2O2. Evidently, ROS is involved in the signaling pathway of H2S and plays a significant role. In this review, we summarize the role of H2S-mediated post-translational modification mechanisms in oxidative stress responses. Moreover, the mechanism of interaction between H2S and ROS in the regulation of redox reactions is focused upon, and the positive cooperative role of H2S and ROS is elucidated. Subsequently, based on the existing evidence and clues, we propose some potential problems and new clues to be explored, which are crucial for the development of the crosstalk mechanism of H2S and ROS in plants.

Identification, characterization, and application of a d-cysteine desulfhydrase from rice seed (Oryza sativa L.)

Cysteine desulfhydrases decompose cysteine to produce pyruvate, ammonium, and hydrogen sulfide. Using d-cysteine (D-cys) as a substrate, an enzyme with this activity was purified from rice seeds and identified at the native protein level. MALDI-TOF-MS analysis of its tryptic peptides revealed a 426 amino acid protein encoded by the OsDCD1 gene (Os02g0773300). Recombinant OsDCD1 (rOsDCD1) was expressed in Escherichia coli cells and purified as a single protein by column chromatography. Gel filtration column chromatography indicated that the native enzyme was a homodimer. The enzyme exhibited maximum catalytic activity at approximately pH 7.5 and 40 °C and was stable at pH 5.5-7.5 and < 37 °C. Kinetics analysis indicated Km and Vmax values for D-cys of 136 μM and 45.5 μmol/min/mg protein, respectively. In contrast, l-cysteine (L-cys) acted as an inhibitor with mixed non-competitive inhibition. Based on the substrate specificity of rOsDCD1, the amount of D-cys in rice flour was quantified. Even in the presence of up to 1 mM L-cys, the quantification of low concentrations of D-cys was unaffected. We demonstrate for the first time that the amount of D-cys in rice flour varies in the range of 0.76-0.93 μmol/g depending on the variety.

Titanium dioxide nanoparticles require K+ and hydrogen sulfide to regulate nitrogen and carbohydrate metabolism during adaptive response to drought and nickel stress in cucumber

Crop plants face severe yield losses worldwide owing to their exposure to multiple abiotic stresses. The study described here, was conducted to comprehend the response of cucumber seedlings to drought (induced by 15% w/v polyethylene glycol 8000; PEG) and nickel (Ni) stress in presence or absence of titanium dioxide nanoparticle (nTiO2). In addition, it was also investigated how nitrogen (N) and carbohydrate metabolism, as well as the defense system, are affected by endogenous potassium (K+) and hydrogen sulfide (H2S). Cucumber seedlings were subjected to Ni stress and drought, which led to oxidative stress and triggered the defense system. Under the stress, N and carbohydrate metabolism were differentially affected. Supplementation of the stressed seedlings with nTiO2 (15 mg L-1) enhanced the activity of antioxidant enzymes, ascorbate-glutathione (AsA-GSH) system and elevated N and carbohydrates metabolism. Application of nTiO2 also enhanced the accumulation of phytochelatins and activity of the enzymes of glyoxalase system that provided additional protection against the metal and toxic methylglyoxal. Osmotic stress brought on by PEG and Ni, was countered by the increase of proline and carbohydrates levels, which helped the seedlings keep their optimal level of hydration. Application nTiO2 improved the biosynthesis of H2S and K+ retention through regulating Cys biosynthesis and H+-ATPase activity, respectively. Observed outcomes lead to the conclusion that nTiO2 maintains redox homeostasis, and normal functioning of N and carbohydrates metabolism that resulted in the protection of cucumber seedlings against drought and Ni stress. Use of 20 mM tetraethylammonium chloride (K+- channel blocker), 500 μM sodium orthovanadate (PM H+-ATPase inhibitor), and 1 mM hypotaurine (H2S scavenger) demonstrate that endogenous K+ and H2S were crucial for the nTiO2-induced modulation of plants' adaptive responses to the imposed stress.

Ethylene, ACC, and the Plant Growth-Promoting Enzyme ACC Deaminase

Here, a brief summary of the biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) and ethylene in plants, as well as overviews of how ACC and ethylene act as signaling molecules in plants, is presented. Next, how the bacterial enzyme ACC deaminase cleaves plant-produced ACC and thereby decreases or prevents the ethylene or ACC modulation of plant gene expression is considered. A detailed model of ACC deaminase functioning, including the role of indoleacetic acid (IAA), is presented. Given that ACC is a signaling molecule under some circumstances, this suggests that ACC, which appears to have evolved prior to ethylene, may have been a major signaling molecule in primitive plants prior to the evolution of ethylene and ethylene signaling. Due to their involvement in stimulating ethylene production, the role of D-amino acids in plants is then considered. The enzyme D-cysteine desulfhydrase, which is structurally very similar to ACC deaminase, is briefly discussed and the possibility that ACC deaminase arose as a variant of D-cysteine desulfhydrase is suggested.

H2S-Generating Cytosolic L-Cysteine Desulfhydrase and Mitochondrial D-Cysteine Desulfhydrase from Sweet Pepper (Capsicum annuum L.) Are Regulated During Fruit Ripening and by Nitric Oxide

Aims: Pepper fruit is a horticultural product worldwide consumed that has great nutritional and economic relevance. Besides the phenotypical changes that undergo pepper fruit during ripening, there are many associated modifications at transcriptomic, proteomic, biochemical, and metabolic levels. Nitric oxide (NO) and hydrogen sulfide (H2S) are recognized signal molecules that can exert regulatory functions in diverse plant processes. This study aims at analyzing the interrelationship between NO and H2S during fruit ripening. Results: Our data indicate that the H2S-generating cytosolic L-cysteine desulfhydrase (LCD) and the mitochondrial D-cysteine desulfhydrase (DCD) activities are downregulated during ripening but this effect was reverted after NO treatment of fruits. Innovation and Conclusion: Using as a model the non-climacteric pepper fruits at different ripening stages and under an NO-enriched atmosphere, the activity of the H2S-generating LCD and DCD was analyzed. LCD and DCD activities were downregulated during ripening, but this effect was reverted after NO treatment of fruits. The analysis of LCD activity by non-denaturing polyacrylamide gel electrophoresis (PAGE) allowed identifying three isozymes designated CaLCD I to CaLCD III, which were differentially modulated by NO and strictly dependent on pyridoxal 5'-phosphate (PLP). In vitro analyses of green fruit samples in the presence of different compounds including NO donors, peroxynitrite (ONOO-), and reducing agents such as reduced glutathione (GSH) and L-cysteine (L-Cys) triggered an almost 100% inhibition of CaLCD II and CaLCD III. This redox adaptation process of both enzymes could be cataloged as a hormesis phenomenon. The protein tyrosine (Tyr) nitration (an NO-promoted post-translational modification) of the recombinant LCD was corroborated by immunoblot and by mass spectrometry (MS) analyses. Among the 11 Tyr residues present in this enzyme, MS of the recombinant LCD enabled us to identify that Tyr82 and Tyr254 were nitrated by ONOO-, this occurring near the active center on the enzyme, where His237 and Lys260 together with the cofactor PLP are involved. These data support the relationship between NO and H2S during pepper fruit ripening, since LCD and DCD are regulated by NO during this physiological event, and this could also be extrapolated to other plant species.

Biosynthesis of meso-lanthionine in Fusobacterium nucleatum

The opportunistic oral pathogen, Fusobacterium nucleatum contains meso-lanthionine as the diaminodicarboxylic acid in the pentapeptide crosslink of the peptidoglycan layer. The diastereomer, l,l-lanthionine is formed by lanthionine synthase, a PLP-dependent enzyme that catalyzes the β-replacement of l-cysteine with a second equivalent of l-cysteine. In this study, we explored possible enzymatic mechanisms for the formation of meso-lanthionine. Our inhibition studies with lanthionine synthase, described herein, revealed that meso-diaminopimelate, a bioisostere of meso-lanthionine, is a more potent inhibitor of lanthionine synthase compared to the diastereomer, l,l-diaminopimelate. These results suggested that lanthionine synthase could also form meso-lanthionine by the β-replacement of l-cysteine with d-cysteine. Through steady-state and pre-steady state kinetic analysis, we confirm that d-cysteine reacts with the ⍺-aminoacylate intermediate with a k_on that was 2-3-fold faster and K_d value that was 2-3fold lower compared to l-cysteine. However, given that intracellular levels of d-cysteine levels are assumed to be significantly lower than that of l-cysteine, we also determined if the gene product, FN1732, with low sequence identity to diaminopimelate epimerase could convert l,l-lanthionine to meso-lanthionine. Using diaminopimelate dehydrogenase in a coupled spectrophotometric assay, we show that FN1732 can convert l,l-lanthionine to meso-lanthionine with a k_cat of 0.07 ± 0.001 s^-1 and a K_M of 1.9 ± 0.1 mM. In summary, our results provide two possible enzymatic mechanisms for the biosynthesis of meso-lanthionine in F. nucleatum.

Hydrogen sulfide alleviates osmotic stress-induced root growth inhibition by promoting auxin homeostasis

Hydrogen sulfide (H₂ S) promotes plant tolerance against various environmental cues, and d-cysteine desulfhydrase (DCD) is an enzymatic source of H₂ S to enhance abiotic stress resistance. However, the role of DCD-mediated H₂ S production in root growth under abiotic stress remains to be further elucidated. Here, we report that DCD-mediated H₂ S production alleviates osmotic stress-mediated root growth inhibition by promoting auxin homeostasis. Osmotic stress up-regulated DCD gene transcript and DCD protein levels and thus H₂ S production in roots. When subjected to osmotic stress, a dcd mutant showed more severe root growth inhibition, whereas the transgenic lines DCDox overexpressing DCD exhibited less sensitivity to osmotic stress in terms of longer root compared to the wild-type. Moreover, osmotic stress inhibited root growth through repressing auxin signaling, whereas H₂ S treatment significantly alleviated osmotic stress-mediated inhibition of auxin. Under osmotic stress, auxin accumulation was increased in DCDox but decreased in dcd mutant. H₂ S promoted auxin biosynthesis gene expression and auxin efflux carrier PIN-FORMED 1 (PIN1) protein level under osmotic stress. Taken together, our results reveal that mannitol-induced DCD and H₂ S in roots promote auxin homeostasis, contributing to alleviating the inhibition of root growth under osmotic stress.

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

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

Recent advances and mechanistic interactions of hydrogen sulfide with plant growth regulators in relation to abiotic stress tolerance in plants

Adverse environmental constraints such as drought, heat, cold, salinity, and heavy metal toxicity are the primary concerns of the agricultural industry across the globe, as these stresses negatively affect yield and quality of crop production and therefore can be a major threat to world food security. Recently, it has been demonstrated that hydrogen sulfide (H₂S), which is well-known as a gasotransmitter in animals, also plays a potent role in various growth and developmental processes in plants. H₂S, as a potent signaling molecule, is involved in several plant processes such as in the regulation of stomatal pore movements, seed germination, photosynthesis and plant adaptation to environmental stress through gene regulation, post-translation modification of proteins and redox homeostasis. Moreover, a number of experimental studies have revealed that H₂S could improve the adaptation capabilities of plants against diverse environmental constraints by mitigating the toxic and damaging effects triggered by stressful environments. An attempt has been made to uncover recent development in the biosynthetic and metabolic pathways of H₂S and various physiological functions modulated in plants, H₂S donors, their functional mechanism, and application in plants. Specifically, our focus has been on how H₂S is involved in combating the destructive effects of abiotic stresses and its role in persulfidation. Furthermore, we have comprehensively elucidated the crosstalk of H₂S with plant growth regulators.

Whole genome analysis of Enterobacter cloacae Rs-2 and screening of genes related to plant-growth promotion

The genus Enterobacter is widely recognized for its biotechnology potential in improving soil environment and crop growth promotion. To further explore these biotechnological potentials, we sequenced and analyzed the whole genome of Enterobacter cloacae Rs-2. The analysis showed that the total length of the Rs-2 genome was 6,965,070,514 bp, and GC content was 55.80%; the annotation results of GO and COG databases showed that the genome contains a variety of growth-promoting genes, such as iscU, glnA, glnB (nitrogen fixation); iucABCD (siderophore synthesis) and fepA, fcuA, fhuA, and pfeA, etc. (siderophore transport); ipdC (secreted IAA) and gcd, pqqBCDEF (dissolved phosphorus), etc. No pathogenic factors such as virulence genes were found. The application of Rs-2 as a soil inoculant in pot experiments showed great potential for growth promotion. This study proved the plant growth-promoting ability of Rs-2 at the molecular level through genetic screening and analysis, which provided guidance for the further improvement of the strain and laid a foundation for its application in agricultural production.

Bacterial ACC deaminase: Insights into enzymology, biochemistry, genetics, and potential role in amelioration of environmental stress in crop plants

Growth and productivity of crop plants worldwide are often adversely affected by anthropogenic and natural stresses. Both biotic and abiotic stresses may impact future food security and sustainability; global climate change will only exacerbate the threat. Nearly all stresses induce ethylene production in plants, which is detrimental to their growth and survival when present at higher concentrations. Consequently, management of ethylene production in plants is becoming an attractive option for countering the stress hormone and its effect on crop yield and productivity. In plants, ACC (1-aminocyclopropane-1-carboxylate) serves as a precursor for ethylene production. Soil microorganisms and root-associated plant growth promoting rhizobacteria (PGPR) that possess ACC deaminase activity regulate growth and development of plants under harsh environmental conditions by limiting ethylene levels in plants; this enzyme is, therefore, often designated as a "stress modulator." TheACC deaminase enzyme, encoded by the AcdS gene, is tightly controlled and regulated depending upon environmental conditions. Gene regulatory components of AcdS are made up of the LRP protein-coding regulatory gene and other regulatory components that are activated via distinct mechanisms under aerobic and anaerobic conditions. ACC deaminase-positive PGPR strains can intensively promote growth and development of crops being cultivated under abiotic stresses including salt stress, water deficit, waterlogging, temperature extremes, and presence of heavy metals, pesticides and other organic contaminants. Strategies for combating environmental stresses in plants, and improving growth by introducing the acdS gene into crop plants via bacteria, have been investigated. In the recent past, some rapid methods and cutting-edge technologies based on molecular biotechnology and omics approaches involving proteomics, transcriptomics, metagenomics, and next generation sequencing (NGS) have been proposed to reveal the variety and potential of ACC deaminase-producing PGPR that thrive under external stresses. Multiple stress-tolerant ACC deaminase-producing PGPR strains have demonstrated great promise in providing plant resistance/tolerance to various stressors and, therefore, it could be advantageous over other soil/plant microbiome that can flourish under stressed environments.

AtPRMT5-mediated AtLCD methylation improves Cd2+ tolerance via increased H2S production in Arabidopsis

Arabidopsis (Arabidopsis thaliana) PROTEIN ARGININE METHYLTRANSFERASE5 (PRMT5), a highly conserved arginine (Arg) methyltransferase protein, regulates multiple aspects of the growth, development, and environmental stress responses by methylating Arg in histones and some mRNA splicing-related proteins in plants. Hydrogen sulfide (H2S) is a recently characterized gasotransmitter that also regulates various important physiological processes. l-cysteine desulfhydrase (LCD) is a key enzyme of endogenous H2S production. However, our understanding of the upstream regulatory mechanisms of endogenous H2S production is limited in plant cells. Here, we confirmed that AtPRMT5 increases the enzymatic activity of AtLCD through methylation modifications during stress responses. Both atprmt5 and atlcd mutants were sensitive to cadmium (Cd2+), whereas the overexpression (OE) of AtPRMT5 or AtLCD enhanced the Cd2+ tolerance of plants. AtPRMT5 methylated AtLCD at Arg-83, leading to a significant increase in AtLCD enzymatic activity. The Cd2+ sensitivity of atprmt5-2 atlcd double mutants was consistent with that of atlcd plants. When AtPRMT5 was overexpressed in the atlcd mutant, the Cd2+ tolerance of plants was significantly lower than that of AtPRMT5-OE plants in the wild-type background. These results were confirmed in pharmacological experiments. Thus, AtPRMT5 methylation of AtLCD increases its enzymatic activity, thereby strengthening the endogenous H2S signal and ultimately improving plant tolerance to Cd2+ stress. These findings provide further insights into the substrates of AtPRMT5 and increase our understanding of the regulatory mechanism upstream of H2S signals.

Harnessing the power of hydrogen sulphide (H2 S) for improving fruit quality traits

Hydrogen sulphide (H₂ S) is a gaseous molecule and originates endogenously in plants. It is considered a potential signalling agent in various physiological processes of plants. Numerous reports have examined the role of H₂ S in fruit ripening and in enhancing fruit quality traits. H₂ S coordinates the fruit antioxidant system, fruit ripening phytohormones, such as ethylene and abscisic acid, together with other ripening-related signalling molecules, including nitric oxide and hydrogen peroxide. Although many studies have increased understanding of various aspects of this complex network, there is a gap in understanding crosstalk of H₂ S with key players of fruit ripening, postharvest senescence and fruit metabolism. This review focused on deciphering fruit H₂ S metabolism, signalling and its interaction with other ripening-related signalling molecules during fruit ripening and postharvest storage. Moreover, we also discuss how H₂ S can be used as a tool for improving fruit quality and productivity and reducing postharvest loss of perishable fruits.

Crosstalk between H2 S and NO: an emerging signalling pathway during waterlogging stress in legume crops

In legumes, waterlogging is a major detrimental factor leading to huge yield losses. Generally, legumes lack tolerance to submergence, and conventional breeding to develop tolerant varieties are limited due to the lack of tolerant germplasm and potential target genes. Moreover, our understanding of the various signalling cascades, their interactions and key pathways induced during waterlogging is limited. Here, we focus on the role of two important plant signalling molecules, viz. hydrogen sulphide (H₂ S) and nitric oxide (NO), during waterlogging stress in legumes. Plants and soil microbes produce these signalling molecules both endogenously and exogenously under various stresses, including waterlogging. NO and H₂ S are known to regulate key physiological pathways, such as stomatal closure, leaf senescence and regulation of numerous stress signalling pathways, while NO plays a pivotal role in adventitious root formation during waterlogging. The crosstalk between H₂ S and NO is synergistic because of the resemblance of their physiological effects and proteomic functions, which mainly operate through cysteine-dependent post-translational modifications via S-nitrosation and persulfidation. Such knowledge has provided novel platforms for researchers to unravel the complexity associated with H₂ S-NO signalling and interactions with plant stress hormones. This review provides an overall summary on H₂ S and NO, including biosynthesis, biological importance, crosstalk, transporter regulation as well as understanding their role during waterlogging using 'multi-omics' approach. Understanding H₂ S and NO signalling will help in deciphering the metabolic interactions and identifying key regulatory genes that could be used for developing waterlogging tolerance in legumes.

The Interplay between Hydrogen Sulfide and Phytohormone Signaling Pathways under Challenging Environments

Hydrogen sulfide (H₂S) serves as an important gaseous signaling molecule that is involved in intra- and intercellular signal transduction in plant–environment interactions. In plants, H₂S is formed in sulfate/cysteine reduction pathways. The activation of endogenous H₂S and its exogenous application has been found to be highly effective in ameliorating a wide variety of stress conditions in plants. The H₂S interferes with the cellular redox regulatory network and prevents the degradation of proteins from oxidative stress via post-translational modifications (PTMs). H₂S-mediated persulfidation allows the rapid response of proteins in signaling networks to environmental stimuli. In addition, regulatory crosstalk of H₂S with other gaseous signals and plant growth regulators enable the activation of multiple signaling cascades that drive cellular adaptation. In this review, we summarize and discuss the current understanding of the molecular mechanisms of H₂S-induced cellular adjustments and the interactions between H₂S and various signaling pathways in plants, emphasizing the recent progress in our understanding of the effects of H₂S on the PTMs of proteins. We also discuss future directions that would advance our understanding of H₂S interactions to ultimately mitigate the impacts of environmental stresses in the plants.

Abiotic stress-triggered oxidative challenges: Where does H2S act?

Hydrogen sulfide (H₂S) was once principally considered the perpetrator of plant growth cessation and cell death. However, this has become an antiquated view, with cumulative evidence showing that the H₂S serves as a biological signaling molecule notably involved in abiotic stress response and adaptation, such as defense by phytohormone activation, stomatal movement, gene reprogramming, and plant growth modulation. Reactive oxygen species (ROS)-dependent oxidative stress is involved in these responses. Remarkably, an ever-growing body of evidence indicates that H₂S can directly interact with ROS processing systems in a redox-dependent manner, while it has been gradually recognized that H₂S-based posttranslational modifications of key protein cysteine residues determine stress responses. Furthermore, the reciprocal interplay between H₂S and nitric oxide (NO) in regulating oxidative stress has significant importance. The interaction of H₂S with NO and ROS during acclimation to abiotic stress may vary from synergism to antagonism. However, the molecular pathways and factors involved remain to be identified. This review not only aims to provide updated information on H₂S action in regulating ROS-dependent redox homeostasis and signaling, but also discusses the mechanisms of H₂S-dependent regulation in the context of oxidative stress elicited by environmental cues.

Hydrogen sulfide attenuates intracellular oxidative stress via repressing glycolate oxidase activities in Arabidopsis thaliana

BACKGROUND

Hydrogen sulfide (H₂S) has been proposed to exert anti-oxidative effect under many environmental stresses; however, how it influences oxidative stress remains largely unclear.

RESULTS

Here, we assessed the effects of H₂S on oxidative stress responses such as salicylic acid (SA)-dependent cell death, which triggered by increased H₂O₂ availability in Arabidopsis thaliana catalase-deficient mutants cat2 displaying around 20% wild-type catalase activity. H₂S generation and its producing enzyme _L-cysteine desulfhydrase (LCD/DES) were found to transient increase in response to intracellular oxidative stress. Although introducing the mutation of des1, an important LCD, into the cat2 background produced little effect, H₂S fumigation not only rescued the cell death phenotype of cat2 plant, but also attenuated SA accumulation and oxidation of the glutathione pool. Unexpectedly, the activities of major components of ascorbate-glutathione pathway were less affected by the presence of H₂S treatment, but decreased glycolate oxidase (GOX) in combination with accumulation of glycolate implied H₂S treatment impacts the cellular redox homeostasis by repressing the GOX-catalyzed reaction likely via altering the major GOX transcript levels.

CONCLUSIONS

Our findings reveal a link between H₂S and peroxisomal H₂O₂ production that has implications for the understanding of the multifaceted roles of H₂S in the regulation of oxidative stress responses.

Nitric oxide donor, sodium nitroprusside modulates hydrogen sulfide metabolism and cysteine homeostasis to aid the alleviation of chromium toxicity in maize seedlings (Zea mays L.)

The current research aimed to assess the protective role of nitric oxide (NO) against chromium (Cr) toxicity in maize seedlings. Chromium (200 µM) lowered osmotic potential in epicotyls and mostly in radicles (by 38% and 63%, respectively) as compared to the control. Sodium nitroprusside (SNP, NO donor) restored seedling biomass (+90% for both organs) and water potential, whereas application of Nω-nitro-L-arginine methylester (_L-NAME, a NOS inhibitor) increased sensitivity to Cr. SNP suppressed Cr-triggered proline accumulation by inhibiting Δ¹-pyrroline-5-carboxylate synthetase activity and stimulating proline dehydrogenase activity, leading to glutamate over-accumulation (~30% for both organs). Cr stimulated cysteine metabolism and this was further enhanced by SNP which stimulated serine acetyl-transferase and O-acetylserine (thiol) lyase activities. This was followed by an increase in endogenous hydrogen sulfide (H₂S) generation by up-regulating L-cysteine desulfhydrase (+205%), D-cysteine desulfhydrase (+150%) and cyanoalanine synthase (+65%) activities in radicles compared to Cr-treatments plants. These positive effects were reduced in _L-NAME compared to control. Combined Cr+SNP affected the levels of compounds involved in glutathione metabolism (γ-glutamyl-cysteinyl, γ-glutamyl-cysteinyl-clycine, γ-cysteinyl-glycine, and glycine.). All together, our findings indicate that NO and elicited cellular H₂S act synergistically to alleviate Cr stress in maize seedlings by influencing a metabolic interplay between cysteine, proline, and glutathione.

Roles of a Cysteine Desulfhydrase LCD1 in Regulating Leaf Senescence in Tomato

Hydrogen sulfide (H2S), a novel gasotransmitter in both mammals and plants, plays important roles in plant development and stress responses. Leaf senescence represents the final stage of leaf development. The role of H2S-producing enzyme L-cysteine desulfhydrase in regulating tomato leaf senescence is still unknown. In the present study, the effect of an L-cysteine desulfhydrase LCD1 on leaf senescence in tomato was explored by physiological analysis. LCD1 mutation caused earlier leaf senescence, whereas LCD1 overexpression significantly delayed leaf senescence compared with the wild type in 10-week tomato seedlings. Moreover, LCD1 overexpression was found to delay dark-induced senescence in detached tomato leaves, and the lcd1 mutant showed accelerated senescence. An increasing trend of H2S production was observed in leaves during storage in darkness, while LCD1 deletion reduced H2S production and LCD1 overexpression produced more H2S compared with the wild-type control. Further investigations showed that LCD1 overexpression delayed dark-triggered chlorophyll degradation and reactive oxygen species (ROS) accumulation in detached tomato leaves, and the increase in the expression of chlorophyll degradation genes NYC1, PAO, PPH, SGR1, and senescence-associated genes (SAGs) during senescence was attenuated by LCD1 overexpression, whereas lcd1 mutants showed enhanced senescence-related parameters. Moreover, a correlation analysis indicated that chlorophyll content was negatively correlated with H2O2 and malondialdehyde (MDA) content, and also negatively correlated with the expression of chlorophyll degradation-related genes and SAGs. Therefore, these findings increase our understanding of the physiological functions of the H2S-generating enzyme LCD1 in regulating leaf senescence in tomato.

Molybdenum and hydrogen sulfide synergistically mitigate arsenic toxicity by modulating defense system, nitrogen and cysteine assimilation in faba bean (Vicia faba L.) seedlings

Hydrogen sulfide (H₂S) has emerged as a potential gasotransmitter in plants with a beneficial role in stress amelioration. Despite the various known functions of H₂S in plants, not much information is available to explain the associative role of molybdenum (Mo) and hydrogen sulfide (H₂S) signaling in plants under arsenic toxicity. In view to address such lacunae in our understanding of the integrative roles of these biomolecules, the present work attempts to decipher the roles of Mo and H₂S in mitigation of arsenate (AsV) toxicity in faba bean (Vicia faba L.) seedlings. AsV-stressed seedlings supplemented with exogenous Mo and/or NaHS treatments (H₂S donor) showed resilience to AsV toxicity manifested by reduction of apoptosis, reactive oxygen species (ROS) content, down-regulation of NADPH oxidase and GOase activity followed by upregulation of antioxidative enzymes in leaves. Fluorescent localization of ROS in roots reveals changes in its intensity and spatial distribution in response to MO and NaHS supplementation during AsV stress. Under AsV toxicity conditions, seedlings subjected to Mo + NaHS showed an increased rate of nitrogen metabolism evident by elevation in nitrate reductase, nitrite reductase and glutamine synthetase activity. Furthermore, the application of Mo and NaHS in combination positively upregulates cysteine and hydrogen sulfide biosynthesis in the absence and presence of AsV stress. Mo plus NaHS-supplemented seedlings exposed to AsV toxicity showed a substantial reduction in oxidative stress manifested by reduced ELKG, lowered MDA content and higher accumulation of proline in leaves. Taken together, the present findings provide substantial evidence on the synergetic role of Mo and H₂S in mitigating AsV stress in faba bean seedlings. Thus, the application of Mo and NaHS reveals their agronomic importance to encounter heavy metal stress for management of various food crops.

Persulfidation of Nitrate Reductase 2 Is Involved in l-Cysteine Desulfhydrase-Regulated Rice Drought Tolerance

Hydrogen sulfide (H₂S) is an important signaling molecule that regulates diverse cellular signaling pathways through persulfidation. Our previous study revealed that H₂S is involved in the improvement of rice drought tolerance. However, the corresponding enzymatic sources of H₂S and its regulatory mechanism in response to drought stress are not clear. Here, we cloned and characterized a putative l-cysteine desulfhydrase (LCD) gene in rice, which encodes a protein possessing H₂S-producing activity and was named OsLCD1. Overexpression of OsLCD1 results in enhanced H₂S production, persulfidation of total soluble protein, and confers rice drought tolerance. Further, we found that nitrate reductase (NR) activity was decreased under drought stress, and the inhibition of NR activity was controlled by endogenous H₂S production. Persulfidation of NIA2, an NR isoform responsible for the main NR activity, led to a decrease in total NR activity in rice. Furthermore, drought stress-triggered inhibition of NR activity and persulfidation of NIA2 was intensified in the OsLCD1 overexpression line. Phenotypical and molecular analysis revealed that mutation of NIA2 enhanced rice drought tolerance by activating the expression of genes encoding antioxidant enzymes and ABA-responsive genes. Taken together, our results showed the role of OsLCD1 in modulating H₂S production and provided insight into H₂S-regulated persulfidation of NIA2 in the control of rice drought stress.

Synthesis, Stability, and Kinetics of Hydrogen Sulfide Release of Dithiophosphates

The development of chemicals to slowly release hydrogen sulfide would aid the survival of plants under environmental stressors as well as increase harvest yields. We report a series of dialkyldithiophosphates and disulfidedithiophosphates that slowly degrade to release hydrogen sulfide in the presence of water. Kinetics of the degradation of these chemicals were obtained at 85 °C and room temperature, and it was shown that the identity of the alkyl or sulfide group had a large impact on the rate of hydrolysis, and the rate constant varied by more than 10⁴×. For example, using tert-butanol as the nucleophile yielded a dithiophosphate (8) that hydrolyzed 13,750× faster than the dithiophosphate synthesized from n-butanol (1), indicating that the rate of hydrolysis is structure-dependent. The rates of hydrolysis at 85 °C varied from a low value of 6.9 × 10^-4 h^-1 to a high value of 14.1 h^-1. Hydrogen sulfide release in water was also quantified using a hydrogen sulfide-sensitive electrode. Corn was grown on an industrial scale and dosed with dibutyldithiophosphate to show that these dithiophosphates have potential applications in agriculture. At a loading of 2 kg per acre, a 6.4% increase in the harvest yield of corn was observed.

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

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

Hydrogen sulfide: An emerging signaling molecule regulating drought stress response in plants

Hydrogen sulfide (H₂ S) is a small, reactive signaling molecule that is produced within chloroplasts of plant cells as an intermediate in the assimilatory sulfate reduction pathway by the enzyme sulfite reductase. In addition, H₂ S is also produced in cytosol and mitochondria by desulfhydration of l-cysteine catalyzed by l-cysteine desulfhydrase (DES1) in the cytosol and from β-cyanoalanine in mitochondria, in a reaction catalyzed by β-cyano-Ala synthase C1 (CAS-C1). H₂ S exerts its numerous biological functions by post-translational modification involving oxidation of cysteine residues (RSH) to persulfides (RSSH). At lower concentrations (10-1000 μmol L^-1 ), H₂ S shows huge agricultural potential as it increases the germination rate, the size, fresh weight, and ultimately the crop yield. It is also involved in abiotic stress response against drought, salinity, high temperature, and heavy metals. H₂ S donor, for example, sodium hydrosulfide (NaHS), has been exogenously applied on plants by various researchers to provide drought stress tolerance. Exogenous application results in the accumulation of polyamines, sugars, glycine betaine, and enhancement of the antioxidant enzyme activities in response to drought-induced osmotic and oxidative stress, thus, providing stress adaptation to plants. At the biochemical level, administration of H₂ S donors reduces malondialdehyde content and lipoxygenase activity to maintain the cell integrity, causes abscisic acid-mediated stomatal closure to prevent water loss through transpiration, and accelerates the photosystem II repair cycle. Here, we review the crosstalk of H₂ S with secondary messengers and phytohormones towards the regulation of drought stress response and emphasize various approaches that can be addressed to strengthen research in this area.

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

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

Calcium-hydrogen sulfide crosstalk during K+-deficient NaCl stress operates through regulation of Na+/H+ antiport and antioxidative defense system in mung bean roots

Present investigation reports the role of calcium (Ca²⁺) and hydrogen sulfide (H₂S) crosstalk associated with Vigna radiata seedlings subjected to K⁺ deficient conditions under short-term (24 h) and long-term (72 h) NaCl stress. Perusal of the data reveals that under short-term NaCl stress an initial decline in K⁺ level led to the elevation in Ca²⁺ and H₂S levels along with improvement in antioxidant system and reduction in reactive oxygen species (ROS) production. Under long-term NaCl stress a further decline in K⁺ content was deleterious that led to a lower K⁺/Na⁺ ratio. This was followed by reduction in antioxidant system along with excessive accumulation of ROS and methylglyoxal content, and increased membrane damage. However, supplementation of the seedling roots with Ca²⁺ enhanced biosynthesis of H₂S through enhancing cysteine pool. The present findings suggest that synergistic action of Ca²⁺ and H₂S induced the activity of H⁺-ATPase that created H⁺ gradient which in turn induced Na⁺/H⁺ antiport system that accelerated K⁺ influx and Na⁺ efflux. All of these together contributed to a higher K⁺/Na⁺ ratio, activation of antioxidative defense system, and maintenance of redox homeostasis and membrane integrity in Ca²⁺-supplemented stressed seedlings. Role of Ca²⁺ and H₂S in the regulation of Na⁺/H⁺ antiport system was validated by the use of sodium orthovanadate (plasma membrane H⁺-ATPase inhibitor), tetraethylammonium chloride (K⁺ channel blocker), and amiloride (Na⁺/H⁺ antiporter inhibitor). Application of Ca²⁺-chelator EGTA (ethylene glycol-bis(b-aminoethylether)-N,N,N',N'-tetraacetic acid) and H₂S scavenger hypotaurine abolished the effect of Ca²⁺, suggesting the involvement of Ca²⁺ and H₂S in the alleviation of NaCl stress. Moreover, use of EGTA and HT also substantiates the downstream functioning of H₂S during Ca²⁺-mediated regulation of plant adaptive responses to NaCl stress. To sum up, present findings reveal the association of Ca²⁺ and H₂S signaling in the regulation of ion homeostasis and antioxidant defense during K⁺-deficient NaCl stress.

Hydrogen sulfide, a signaling molecule in plant stress responses

Gaseous molecules, such as hydrogen sulfide (H₂ S) and nitric oxide (NO), are crucial players in cellular and (patho)physiological processes in biological systems. The biological functions of these gaseous molecules, which were first discovered and identified as gasotransmitters in animals, have received unprecedented attention from plant scientists in recent decades. Researchers have arrived at the consensus that H₂ S is synthesized endogenously and serves as a signaling molecule throughout the plant life cycle. However, the mechanisms of H₂ S action in redox biology is still largely unexplored. This review highlights what we currently know about the characteristics and biosynthesis of H₂ S in plants. Additionally, we summarize the role of H₂ S in plant resistance to abiotic stress. Moreover, we propose and discuss possible redox-dependent mechanisms by which H₂ S regulates plant physiology.

Hydrogen sulfide (H2S) signaling in plant development and stress responses

ABSTRACT

Hydrogen sulfide (H₂S) was initially recognized as a toxic gas and its biological functions in mammalian cells have been gradually discovered during the past decades. In the latest decade, numerous studies have revealed that H₂S has versatile functions in plants as well. In this review, we summarize H₂S-mediated sulfur metabolic pathways, as well as the progress in the recognition of its biological functions in plant growth and development, particularly its physiological functions in biotic and abiotic stress responses. Besides direct chemical reactions, nitric oxide (NO) and hydrogen peroxide (H₂O₂) have complex relationships with H₂S in plant signaling, both of which mediate protein post-translational modification (PTM) to attack the cysteine residues. We also discuss recent progress in the research on the three types of PTMs and their biological functions in plants. Finally, we propose the relevant issues that need to be addressed in the future research.

GRAPHIC ABSTRACT

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s42994-021-00035-4.

Something smells bad to plant pathogens: Production of hydrogen sulfide in plants and its role in plant defence responses

Background

Sulfur and diverse sulfur-containing compounds constitute important components of plant defences against a wide array of microbial pathogens. Among them, hydrogen sulfide (H₂S) occupies a prominent position as a gaseous signalling molecule that plays multiple roles in regulation of plant growth, development and plant responses to stress conditions. Although the production of H₂S in plant cells has been discovered several decades ago, the underlying pathways of H₂S biosynthesis, metabolism and signalling were only recently uncovered.

Aim of the review

Here we review the current knowledge on the biosynthesis of H₂S in plant cells, with special attention to L-cysteine desulfhydrase (DES) as the key enzyme controlling H₂S levels biosynthesis in the cytosol of plant cells during plant growth, development and diverse abiotic and biotic stress conditions.

Key Scientific Concepts of Review

Recent advances have revealed molecular mechanisms of DES properties, functions and regulation involved in modulations of H₂S production during plant responses to abiotic and biotic stress stimuli. Studies on des mutants of the model plant Arabidopsis thaliana uncovered molecular mechanisms of H₂S action as a signalling and defence molecule in plant-pathogen interactions. Signalling pathways of H₂S include S-persulfidation of protein cysteines, a redox-based post-translational modification leading to activation of downstream components of H₂S signalling. Accumulated evidence shows DES and H2S implementation into salicylic acid signalling and activation of pathogenesis-related proteins and autophagy within plant immunity. Obtained knowledge on molecular mechanisms of H₂S action in plant defence responses opens new prospects in the search for crop varieties with increased resistance to bacterial and fungal pathogens.

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

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

Exogenous nitric oxide requires endogenous hydrogen sulfide to induce the resilience through sulfur assimilation in tomato seedlings under hexavalent chromium toxicity

Nitric oxide (NO) and hydrogen sulfide (H₂S), versatile signaling molecules, play multiple roles in plant growth, physiological and biochemical processes under heavy metal stress. However, the mechanisms through which NO in association with endogenous H₂S mediated hexavalent chromium Cr(VI) toxicity mitigation are still not fully understood. Therefore, we investigated the role of NO and H₂S in sulfur (S)-assimilation and the effect of NO on endogenous H₂S, and cysteine (Cys) biosynthesis and maintenance of cellular glutathione (GSH) pool in tomato seedlings under Cr(VI) stress. Cr(VI) toxicity caused an increase in reactive oxygen species (ROS; O₂^•- and H₂O₂) formation and activity of chlorophyll (Chl) degrading enzyme [Chlorophyllase (Chlase)] and decrease in seedlings growth attributes, Chl a and b content, and activity of Chl synthesizing enzyme [δ-aminolevulinic acid dehydratase (δ-ALAD)], gas exchange parameters, S-assimilation, and Cys and H₂S metabolism. An increase in the content of glycinebetaine (GB), total soluble carbohydrates (TSCs) and total phenols (TPls), and decrease in DNA damage and ROS in NO treated seedlings conferred Cr(VI) toxicity tolerance. Under Cr(VI) toxicity conditions, the inclusion of H₂S scavenger hypotaurine (HT) in growth medium containing NO validated the role of endogenous H₂S in S-assimilation, H₂S and Cys and GSH metabolism by withdrawing activity of enzymes involved in S-assimilation [adenosine 5-phosphosulfatereductase (APS-R), ATP-sulfurylase (ATP-S)], in the biosynthesis of H₂S [L-cysteine desulfhydrase (L-CD) and D-cysteine desulfhydrase (D-CD)], Cys [O-acetylserin (thiol) lyase (OAST-L)], and GSH [glutamylcysteine synthetase (γ-GCS) and glutathione synthetase (GS)], and in antioxidant system. On the other hand, application of cPTIO [2-(4-32 carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide], a NO scavenger and HT diminished the effect of NO on internal H₂S levels, Cys and glutathione homeostasis, and S-assimilation, which resulted in poor immunity against oxidative stress induced by Cr(VI) toxicity. The obtained results postulate that NO-induced internal H₂S conferred tolerance of tomato seedlings to Cr(VI) toxicity and maintained better photosynthesis process and plant growth.

Appraisal of H2S metabolism in Arabidopsis thaliana: In silico analysis at the subcellular level

Hydrogen sulfide (H₂S) has become a new signal molecule in higher plants which seems to be involved in almost all physiological processes from seed germination, root and plant growth until flowering and fruit ripening. Moreover, H₂S also participates in the mechanism of response against adverse environmental stresses. However, its basic biochemistry in plant cells can be considered in a nascent stage. Using the available information of the model plant Arabidopsis thaliana, the goal of the present study is to provide a broad overview of H₂S metabolism and to display an in silico analysis of the 26 enzymatic components involved in the metabolism of H₂S and their subcellular compartmentation (cytosol, chloroplast and mitochondrion) thus providing a wide picture of the cross-talk inside the organelles and amongst them and, consequently, to get a better understanding of the cellular and tissue implications of H₂S. This information will be also relevant for other crop species, especially those whose whole genome is not yet available.

D-amino Acids in Plants: Sources, Metabolism, and Functions

Although plants are permanently exposed to d-amino acids (d-AAs) in the rhizosphere, these compounds were for a long time regarded as generally detrimental, due to their inhibitory effects on plant growth. Recent studies showed that this statement needs a critical revision. There were several reports of active uptake by and transport of d-AAs in plants, leading to the question whether these processes happened just as side reactions or even on purpose. The identification and characterization of various transporter proteins and enzymes in plants with considerable affinities or specificities for d-AAs also pointed in the direction of their targeted uptake and utilization. This attracted more interest, as d-AAs were shown to be involved in different physiological processes in plants. Especially, the recent characterization of d-AA stimulated ethylene production in Arabidopsis thaliana revealed for the first time a physiological function for a specific d-AA and its metabolizing enzyme in plants. This finding opened the question regarding the physiological or developmental contexts in which d-AA stimulated ethylene synthesis are involved in. This question and the ones about the transport characteristics of d-AAs, their metabolism, and their different physiological effects, are the focus of this review.

Hydrogen Sulfide: From a Toxic Molecule to a Key Molecule of Cell Life

Hydrogen sulfide (H2S) has always been considered toxic, but a huge number of articles published more recently showed the beneficial biochemical properties of its endogenous production throughout all regna. In this review, the participation of H2S in many physiological and pathological processes in animals is described, and its importance as a signaling molecule in plant systems is underlined from an evolutionary point of view. H2S quantification methods are summarized and persulfidation is described as the underlying mechanism of action in plants, animals and bacteria. This review aims to highlight the importance of its crosstalk with other signaling molecules and its fine regulation for the proper function of the cell and its survival.

Extracting chemical reactions from text using Snorkel

Background

Enzymatic and chemical reactions are key for understanding biological processes in cells. Curated databases of chemical reactions exist but these databases struggle to keep up with the exponential growth of the biomedical literature. Conventional text mining pipelines provide tools to automatically extract entities and relationships from the scientific literature, and partially replace expert curation, but such machine learning frameworks often require a large amount of labeled training data and thus lack scalability for both larger document corpora and new relationship types.

Results

We developed an application of Snorkel, a weakly supervised learning framework, for extracting chemical reaction relationships from biomedical literature abstracts. For this work, we defined a chemical reaction relationship as the transformation of chemical A to chemical B. We built and evaluated our system on small annotated sets of chemical reaction relationships from two corpora: curated bacteria-related abstracts from the MetaCyc database (MetaCyc_Corpus) and a more general set of abstracts annotated with MeSH (Medical Subject Headings) term Bacteria (Bacteria_Corpus; a superset of MetaCyc_Corpus). For the MetaCyc_Corpus, we obtained 84% precision and 41% recall (55% F1 score). Extending to the more general Bacteria_Corpus decreased precision to 62% with only a four-point drop in recall to 37% (46% F1 score). Overall, the Bacteria_Corpus contained two orders of magnitude more candidate chemical reaction relationships (nine million candidates vs 68,0000 candidates) and had a larger class imbalance (2.5% positives vs 5% positives) as compared to the MetaCyc_Corpus. In total, we extracted 6871 chemical reaction relationships from nine million candidates in the Bacteria_Corpus.

Conclusions

With this work, we built a database of chemical reaction relationships from almost 900,000 scientific abstracts without a large training set of labeled annotations. Further, we showed the generalizability of our initial application built on MetaCyc documents enriched with chemical reactions to a general set of articles related to bacteria.

Stress responsive gene regulation in relation to hydrogen sulfide in plants under abiotic stress

Plants often face a variety of abiotic stresses, which affects them negatively and lead to yield loss. The antioxidant system efficiently removes excessive reactive oxygen species and maintains redox homeostasis in plants. With better understanding of these protective mechanisms, recently the concept of hydrogen sulfide (H₂ S) and its role in cell signaling has become the center of attention. H₂ S has been recognized as a third gasotransmitter and a potent regulator of growth and development processes such as germination, maturation, senescence and defense mechanism in plants. Because of its gaseous nature, H₂ S can diffuse to different part of the cells and balance the antioxidant pools by supplying sulfur to cells. H₂ S showed tolerance against a plethora of adverse environmental conditions like drought, salt, high temperature, cold, heavy metals and flood via changing in level of osmolytes, malonaldialdehyde, Na⁺ /K⁺ uptake, activities of H₂ S biosynthesis and antioxidative enzymes. It also promotes cross adaptation through persulfidation. H₂ S along with calcium, methylglyoxal and nitric oxide, and their cross talk induces the expression of mitogen activated protein kinases as well as other genes in response to stress. Therefore, it is sensible to evaluate and explore the stress responsive genes involved in H₂ S regulated homeostasis and stress tolerance. The current article is aimed to summarize the recent updates on H₂ S-mediated gene regulation in special reference to abiotic stress tolerance mechanism, and cross adaptation in plants. Moreover, new insights into the H₂ S-associated signal transduction pathway have also been explored.

Integrative roles of nitric oxide and hydrogen sulfide in melatonin-induced tolerance of pepper (Capsicum annuum L.) plants to iron deficiency and salt stress alone or in combination

There seems to be no report in the literature on the effect of melatonin (MT) in relieving the detrimental effects of combined application of salt stress (SS) and iron deficiency (ID). Therefore, the effect of MT on the accumulation/synthesis of endogenous nitric oxide (NO) and hydrogen sulphide (H₂ S) and how far these molecules are involved in MT-improved tolerance to the combined application of ID and SS in pepper (Capsicum annuum L) were tested. Hence, two individual trials were set up. The treatments in the first experiment comprised: Control, ID (0.1 mM FeSO₄ ), SS (100 mM NaCl) and ID + SS. The detrimental effects of combined stresses were more prominent than those by either of the single stress, with respect to growth, oxidative stress and antioxidant defense attributes. Single stress or both in combination improved the endogenous H₂ S and NO, and foliar-applied MT (100 µM) led to a further increase in NO and H₂ S levels. In the second experiment, 0.1 mM scavenger of NO, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO) and that of H₂ S, hypotuarine (HT) were applied along with MT to get further evidence whether NO and H₂ S are involved in MT-induced tolerance to ID and SS. MT combined with cPTIO and HT under a single or combined stress showed that NO effect was reversed by the NO scavenger, cPTIO, alone but the H₂ S effect was inhibited by both scavengers. These findings suggested that tolerance to ID and SS induced by MT may be involved in downstream signal crosstalk between NO and H₂ S.

Physiological response and transcription profiling analysis reveal the role of glutathione in H2S-induced chilling stress tolerance of cucumber seedlings

Recent reports have uncovered the multifunctional role of H₂S in the physiological response of plants to biotic and abiotic stresses. Here, we studied whether NaHS (an H₂S donor) pretreatment could provoke the tolerance of cucumber (Cucumis sativus L.) seedlings subsequently exposed to chilling stress and whether glutathione was involved in this process. Results showed that cucumber seedlings sprayed with NaHS exhibited remarkably increased chilling tolerance, as evidenced by the observed plant tolerant phenotype, as well as the lower levels of electrolyte leakage (EL), malondialdehyde (MDA) content, hydrogen peroxide (H₂O₂) content and RBOH mRNA abundance, compared with the control plants. In addition, NaHS treatment increased the endogenous content of the reduced glutathione (GSH) and the ratio of reduced/oxidized glutathione (GSH/GSSG), meanwhile, the higher net photosynthetic rate (Anet), the light-saturated CO₂ assimilation rate (Asat), the photochemical efficiency (Fv/Fm) and the maximum photochemical efficiency of PSII in darkness (ФPSII) as well as the mRNA levels and activities of the key photosynthetic enzymes (Rubisco, TK, SBPase and FBA) were observed in NaHS-treated seedlings under chilling stress, whereas this effect of NaHS was weakened by buthionine sulfoximine (BSO, an inhibitor of glutathione) or 6-Aminonicotinamide (6-AN, a specific pentose inhibitor and thus inhibits the NADPH production), which preliminarily proved the interaction between H₂S and GSH. Moreover, transcription profiling analysis revealed that the GSH-associated genes (GST Tau, MAAI, APX, GR, GS and MDHAR) were significantly up-regulated in NaHS-treated cucumber seedlings, compared to the H₂O-treated seedlings under chilling stress. Thus, novel results highlight the importance of glutathione as a downstream signal of H₂S-induced plant tolerance to chilling stress.

Hydrogen sulfide acts downstream of jasmonic acid to inhibit stomatal development in Arabidopsis

Main conclusion: Jasmonic acid (JA) negatively regulates stomatal development by promoting LCD expression and hydrogen sulfide (H₂S) biosynthesis. H₂S inhibits the initiation of stomata formation and acts upstream of SPEECHLESS. Abstract: Stomatal development is strictly regulated by endogenous signals and environmental cues. We recently revealed that jasmonic acid (JA) negatively regulates stomatal development in Arabidopsis thaliana cotyledons (Han et al., Plant Physiol 176:2871-2885, 2018), but the underlying molecular mechanism remains largely unknown. Here, we uncovered a role for H₂S in regulating stomatal development. The H₂S scavenger hypotaurine reversed the JA-induced repression of stomatal development in the epidermis of wild-type Arabidopsis. The H₂S-deficient mutant lcd displayed increased stomatal density and stomatal index values, which were rescued by treatment with sodium hydrosulfide (NaHS; an H₂S donor) but not JA, suggesting that JA-mediated repression of stomatal development is dependent on H₂S biosynthesis. The high stomatal density of JA-deficient mutants was rescued by exogenous NaHS treatment. Further analysis indicated that JA positively regulates LCD expression, L-cysteine desulfhydrases (L-CDes) activity, and endogenous H₂S content. Furthermore, H₂S represses the expression of stomate-associated genes and functions downstream of stomate-related signaling pathway components TOO MANY MOUTHS (TMM) and STOMATAL DENSITY AND DISTRIBUTION1 (SDD1) and upstream of SPEECHLESS (SPCH). Therefore, H₂S acts downstream of JA signaling to regulate stomatal development in Arabidopsis cotyledons.

Thioredoxin Network in Plant Mitochondria: Cysteine S-Posttranslational Modifications and Stress Conditions

Plants are sessile organisms presenting different adaptation mechanisms that allow their survival under adverse situations. Among them, reactive oxygen and nitrogen species (ROS, RNS) and H₂S are emerging as components not only of cell development and differentiation but of signaling pathways involved in the response to both biotic and abiotic attacks. The study of the posttranslational modifications (PTMs) of proteins produced by those signaling molecules is revealing a modulation on specific targets that are involved in many metabolic pathways in the different cell compartments. These modifications are able to translate the imbalance of the redox state caused by exposure to the stress situation in a cascade of responses that finally allow the plant to cope with the adverse condition. In this review we give a generalized vision of the production of ROS, RNS, and H₂S in plant mitochondria. We focus on how the principal mitochondrial processes mainly the electron transport chain, the tricarboxylic acid cycle and photorespiration are affected by PTMs on cysteine residues that are produced by the previously mentioned signaling molecules in the respiratory organelle. These PTMs include S-oxidation, S-glutathionylation, S-nitrosation, and persulfidation under normal and stress conditions. We pay special attention to the mitochondrial Thioredoxin/Peroxiredoxin system in terms of its oxidation-reduction posttranslational targets and its response to environmental stress.

Characterization of Dialkyldithiophosphates as Slow Hydrogen Sulfide Releasing Chemicals and Their Effect on the Growth of Maize

Hydrogen sulfide is a key gasotransmitter for plants and has been shown to greatly increase their growth and survival in the presence of environmental stressors. Current methods for slowly releasing hydrogen sulfide use chemicals, such as GYY-4137, but these result in the release of chemicals not found in the environment, and chemicals used may lack structures that can be readily tuned to affect the rate of release of hydrogen sulfide. In this article, we describe the synthesis and slow release of hydrogen sulfide from dialkyldithiophosphates, which are a new set of hydrogen sulfide releasing chemicals that can be used in agriculture. The rates of hydrolysis of dibutyldithiophosphate and GYY-4137 were measured in water at 85 °C and compared with each other to investigate their differences. GYY-4137 is widely used as a chemical that slowly releases H₂S, but its rate of release was not previously quantified. The release of hydrogen sulfide in water at room temperature was measured for a series of dialkyldithiophosphates using a hydrogen sulfide electrode. It was shown that the structure of the dialkyldithiophosphate affected the amount of hydrogen sulfide released. The final degradation products of dibutyldithiophosphate were shown to be phosphoric acid and butanol, which are chemicals found in the environment. This result was notable because it demonstrated that dialkyldithiophosphates degrade to safe, natural chemicals that will not pollute the environment. To demonstrate that dialkyldithiophosphates have potential applications in agriculture, maize was grown for 4.5 weeks after exposure to 1-200 mg of dibutyldithiophosphate, and the weight of corn plants increased by up to 39% at low loadings of dibutyldithiophosphate.