Methane enhances aluminum resistance in alfalfa seedlings by reducing aluminum accumulation and reestablishing redox homeostasis

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

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

Regulatory hubs and strategies for improving heavy metal tolerance in plants: Chemical messengers, omics and genetic engineering

Heavy metal (HM) accumulation in the agricultural soil and its toxicity is a major threat for plant growth and development. HMs disrupt functional integrity of the plants, induces altered phenological and physiological responses and slashes down qualitative crop yield. Chemical messengers such as phytohormones, plant growth regulators and gasotransmitters play a crucial role in regulating plant growth and development under metal toxicity in plants. Understanding the intricate network of these chemical messengers as well as interactions of genes/metabolites/proteins associated with HM toxicity in plants is necessary for deciphering insights into the regulatory circuit involved in HM tolerance. The present review describes (a) the role of chemical messengers in HM-induced toxicity mitigation, (b) possible crosstalk between phytohormones and other signaling cascades involved in plants HM tolerance and (c) the recent advancements in biotechnological interventions including genetic engineering, genome editing and omics approaches to provide a step ahead in making of improved plant against HM toxicities.

Methane-induced lateral root formation requires the participation of nitric oxide signaling

Although methane (CH₄)-induced lateral root (LR) formation has been discovered, the identification of downstream signaling compounds has yet to be fully elucidated. Here, we report a unique mechanism for the involvement of nitric oxide (NO) in the above CH₄-mediated pathway in tomato (Solanum lycopersicum L.) and Arabidopsis thaliana. NO was produced rapidly in the root tissues of tomato seedlings when CH₄ was administrated exogenously. The scavenging of NO with its scavengers prevented lateral root primordia formation and thereafter lateral rooting triggered by CH₄. Gene expression analysis revealed that similar to the responses of sodium nitroprusside (SNP; a NO-releasing compound), CH₄-induced SlCYCA2;1, SlCYCA3;1, and SlCDKA1 transcripts, and -downregulated SlKRP2 mRNA, were differentially abolished when endogenous NO was removed by its scavengers. Changes in the lateral root-related miRNA genes (SlmiR160 and SlmiR390a) and their target genes (SlARF16 and SlARF4), exhibited similar tendencies. Similar to those results in tomato, the addition of CH₄ and SNP could obviously induce NO production and LR formation in Arabidopsis seedlings, which were correlated with the transcriptional profiles of representative LR-related genes. Combine with these findings in tomato and Arabidopsis thaliana, our results showed that NO might act, at least partially, as the downstream signaling molecule for CH₄ control of lateral rooting.

The role of methane in plant physiology: a review

Methane (CH₄), one of the most important greenhouse gases, has conventionally been considered as a physiologic inert gas. However, this perspective has been challenged by the observation that CH₄ has diverse biological functions in animals, such as anti-inflammatory, antioxidant, and anti-apoptosis. Meanwhile, it has now been identified as a possible candidate of gaseous signaling molecule in plants, although its biosynthetic and metabolic pathways as well as the mechanism(s) of CH₄ signaling have not fully understood yet. This paper aims to review the available evidence for the biological roles of CH₄ in regulating plant physiology. Although currently available reports do not fully support the notion of CH₄ as a gasotransmitter, they do show that CH₄ might be produced by an aerobic, non-microbial pathway from plants, and plays important roles in enhancing plant tolerance against abiotic stresses, such as salinity, drought, heavy metal exposure, and promoting root development, as well as delaying senescence and browning. Further results showed that CH₄ could interact with reactive oxygen species (ROS), other gaseous signaling molecules [e.g., nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S)], and glutathione (GSH). These reports thus support the idea that plant-produced CH₄ might be a component of a survival strategy of plants. Finally, the possibility of CH₄ application in agriculture is preliminarily discussed.

Research Progress on the Functions of Gasotransmitters in Plant Responses to Abiotic Stresses

Abiotic stress is one of the major threats affecting plant growth and production. The harm of abiotic stresses includes the disruption of cellular redox homeostasis, reactive oxygen species (ROS) production, and oxidative stress in the plant. Plants have different mechanisms to fight stress, and these mechanisms are responsible for maintaining the required homeostasis in plants. Recently, the study of gasotransmitters in plants has attracted much attention, especially for abiotic stress. In the present review, abiotic stressors were mostly found to induce gasotransmitter production in plants. Meanwhile, these gasotransmitters can enhance the activity of several antioxidant enzymes, alleviate the harmfulness of ROS, and enhance plant tolerance under various stress conditions. In addition, we introduced the interaction of gasotransmitters in plants under abiotic stress. With their promising applications in agriculture, gasotransmitters will be adopted in the near future.

Methyl-coenzyme M reductase-dependent endogenous methane enhances plant tolerance against abiotic stress and alters ABA sensitivity in Arabidopsis thaliana

Our study firstly elaborated the underlying mechanism of endogenous CH₄-induced abiotic tolerance, along with an alteration of ABA sensitivity by mimicking the endogenous CH₄ production in MtMCR transgenic Arabidopsis. Endogenous methane (CH₄) production and/or emission have been ubiquitously observed in stressed plants. However, their physiological roles remain unclear. Here, the methyl-coenzyme M reductase gene from Methanobacterium thermoautotrophicum (MtMCR), encoding the enzyme of methanogenesis, was expressed in Arabidopsis thaliana, to mimic the production of endogenous CH₄. In response to salinity and osmotic stress, MtMCR expression was up-regulated in transgenic plants, resulting in significant increase of endogenous CH₄ levels. Similar results were observed in abscisic acid (ABA) treatment. The functions of endogenous CH₄ were characterized by the changes in plant phenotypes related to stress and ABA sensitivity during the germination and post-germination periods. When challenged with osmotic stress, a reduction in water loss and stomatal closure, were observed. Redox homeostasis was reestablished during osmotic and salinity stress, and ion imbalance was also restored in salinity conditions. The expression of several stress/ABA-responsive genes was up-regulated, and ABA sensitivity, in particularly, was significantly altered in the MtMCR transgenic plants. Together, our genetic study for the first time elaborated the possible mechanism of endogenous CH₄-enhanced salinity and osmotic tolerance, along with an alteration of ABA sensitivity. These findings thus provided novel cues for understanding the possible roles of endogenous CH₄ in plants.

Methane Control of Adventitious Rooting Requires γ-Glutamyl Cysteine Synthetase-Mediated Glutathione Homeostasis

Although the key role of methane (CH4) in the induction of cucumber adventitious rooting has been observed previously, the target molecules downstream of the CH4 action are yet to be fully elucidated. Here, we reported that exogenous glutathione (GSH) induced cucumber adventitious root formation; while l-buthionine-sulfoximine (BSO) treatment inhibited it. BSO is a known inhibitor of γ-glutamyl cysteine synthetase (γ-ECS), an enzyme involved in GSH biosynthesis. Further investigations showed that endogenous GSH content was rapidly increased by CH4 application, which was correlated with the increased CsGSH1 transcript and γ-ECS activity. Mimicking the responses of GSH, CH4 could upregulate cell cycle regulatory genes (CsCDC6, CsCDPK1, CsCDPK5 and CsDNAJ-1) and auxin-response genes (CsAux22D-like and CsAux22B-like). Meanwhile, adventitious rooting-related CsmiR160 and CsmiR167 were increased or decreased, respectively, and contrasting tendencies were observed in the changes of their target genes, that included CsARF17 and CsARF8. The responses above were impaired by the removal of endogenous GSH with BSO, indicating that CH4-triggered adventitious rooting was GSH-dependent. Genetic evidence revealed that in the presence of CH4, Arabidopsis mutants cad2 (a γ-ECS-defective mutant) exhibited, not only the decreased GSH content in vivo, but also the defects in adventitious root formation, both of which were rescued by GSH administration other than CH4. Together, it can be concluded that γ-ECS-dependent GSH homeostasis might be required for CH4-induced adventitious root formation.

Hydrogen peroxide is involved in methane-induced tomato lateral root formation

Pharmacological and molecular evidence reveals a novel role of methane (CH₄) gas in root organogenesis, the induction of lateral root (LR) formation, and this response might require hydrogen peroxide (H₂O₂) synthesis. Although plants can produce CH₄ and release this to atmosphere, the beneficial role(s) of CH₄ are not fully elucidated. In this study, the fumigation with CH₄ not only increased NADPH oxidase activity and H₂O₂ production, but also induced tomato lateral root primordial formation and thereafter LR development. However, exogenously applied argon and nitrogen failed to influence LR formation. Above responses triggered by CH₄ were sensitive to the removal of endogenous H₂O₂ with dimethylthiourea (DMTU; a membrane-permeable scavenger of H₂O₂), suggesting the hypothesis that CH₄'s effect on LR formation could be mediated by endogenous H₂O₂. Diphenylene iodonium (DPI) inhibition of the H₂O₂ generating enzyme NADPH oxidase attenuated H₂O₂ synthesis and impaired LR formation in response to CH₄, confirming the requirement of NADPH oxidase-dependent H₂O₂. Meanwhile, the alterations of endogenous H₂O₂ concentrations failed to influence CH₄ production in tomato seedlings. Molecular evidence revealed that CH₄-induced SlCDKA1, SlCYCA2;1, and SlCYCA3;1 transcripts, and -decreased SlKRP2 mRNA were impaired by DMTU or DPI. Contrasting changes in LR formation-related miR390a and miR160 transcripts and their target genes, including SlARF4 and SlARF16, were observed. Together, our pharmacological and molecular evidence suggested the requirement of H₂O₂ synthesis in CH₄-triggered tomato LR formation, partially via the regulation of cell cycle regulatory genes, miRNA-, and tasiRNA-modulated gene expression.

L-Cysteine desulfhydrase-dependent hydrogen sulfide is required for methane-induced lateral root formation

Methane-triggered lateral root formation is not only a universal event, but also dependent on L-cysteine desulfhydrase-dependent hydrogen sulfide signaling. Whether or how methane (CH₄) triggers lateral root (LR) formation has not been elucidated. In this report, CH₄ induction of lateral rooting and the role of hydrogen sulfide (H₂S) were dissected in tomato and Arabidopsis by using physiological, anatomical, molecular, and genetic approaches. First, we discovered that CH₄ induction of lateral rooting is a universal event. Exogenously applied CH₄ not only triggered tomato lateral rooting, but also increased activities of L-cysteine desulfhydrase (DES; a major synthetic enzyme of H₂S) and induced endogenous H₂S production, and contrasting responses were observed in the presence of hypotaurine (HT; a scavenger of H₂S) or DL-propargylglycine (PAG; an inhibitor of DES) alone. CH₄-triggered lateral rooting were sensitive to the inhibition of endogenous H₂S with HT or PAG. The changes in the transcripts of representative cell cycle regulatory genes, miRNA and its target genes were matched with above phenotypes. In the presence of CH₄, Arabidopsis mutant Atdes1 exhibited defects in lateral rooting, compared with the wild-type. Molecular evidence showed that the transcriptional profiles of representative target genes modulated by CH₄ in wild-type plants were impaired in Atdes1 mutant. Overall, our data demonstrate the main branch of the DES-dependent H₂S signaling cascade in CH₄-triggered LR formation.

Hydrogen sulfide acts downstream of methane to induce cucumber adventitious root development

Previous results have shown that hydrogen sulfide (H₂S), mainly catalyzed by l-cysteine desulfhydrase (DES) in plants, triggers adventitious rooting. The objective of this study was to test whether H₂S is involved in methane (CH₄)-induced adventitious root development in cucumber explants. First, we observed that the activities of DES, endogenous H₂S production, and thereafter adventitious root development were induced by CH₄ and NaHS (an H₂S donor). Some responses were sensitive to hypotaurine (HT; a scavenger of H₂S), showing that endogenous H₂S production and adventitious rooting were obviously blocked. The development of adventitious root primordia was also impaired. Further molecular evidence revealed that CH₄-induced gene expression of CsDNAJ-1, CsCDPK1, CsCDPK5, CsCDC6 (a cell-division-related gene), CsAux22D-like, and CsAux22B-like (two auxin-signaling genes), several molecular markers responsible for adventitious rooting, were blocked by the co-treatment with HT. The occurrence of CH₄-elicited S-sulfhydration during the above responses was also sensitive to the removal of endogenous H₂S, suggesting the requirement of H₂S. Taken together, our results reveal a vital role of endogenous H₂S in CH₄-triggered cucumber adventitious root development, and thus provide a comprehensive window into the complex signaling transduction pathway in CH₄-mediated root organogenesis.

Methane alleviates alfalfa cadmium toxicity via decreasing cadmium accumulation and reestablishing glutathione homeostasis

Although methane (CH₄) generation triggered by some environmental stimuli, displays the protective response against oxidative stress in plants, whether and how CH₄ regulates plant tolerance against cadmium stress is largely unknown. Here, we discovered that cadmium (Cd) stimulated the production of CH₄ in alfalfa root tissues. The pretreatment with exogenous CH₄ could alleviate seedling growth inhibition. Less amounts of Cd accumulation was also observed. Consistently, in comparison with Cd stress alone, miR159 transcript was down-regulated by CH₄, and expression levels of its target gene ABC transporter was increased. By contrast, miR167 transcript was up-regulated, showing a relatively negative correlation with its target gene Nramp6. Meanwhile, Cd-triggered redox imbalance was improved by CH₄, evidenced by the reduced lipid peroxidation and hydrogen peroxide accumulation, as well as the induction of representative antioxidant genes. Further results showed that Cd-triggered decrease of the ratio of reduced/oxidized (homo)glutathione was rescued by CH₄. Additionally, CH₄-triggered alleviation of seedling growth was sensitive to a selective inhibitor of glutathione biosynthesis. Overall, above results revealed that CH₄-alleviated Cd accumulation at least partially, required the modulation of heavy metal transporters via miR159 and miR167. Finally, the role of glutathione homeostasis elicited by CH₄ was preliminarily suggested.