Metabolic and Functional Interactions of H2S and Sucrose in Maize Thermotolerance through Redox Homeodynamics.

Hydrogen sulfide (H2S) is a novel gasotransmitter. Sucrose (SUC) is a source of cellular energy and a signaling molecule. Maize is the third most common food crop worldwide. However, the interaction of H2S and SUC in maize thermotolerance is not widely known. In this study, using maize seedlings as materials, the metabolic and functional interactions of H2S and SUC in maize thermotolerance were investigated. The data show that under heat stress, the survival rate and tissue viability were increased by exogenous SUC, while the malondialdehyde content and electrolyte leakage were reduced by SUC, indicating SUC could increase maize thermotolerance. Also, SUC-promoted thermotolerance was enhanced by H2S, while separately weakened by an inhibitor (propargylglycine) and a scavenger (hypotaurine) of H2S and a SUC-transport inhibitor (N-ethylmaleimide), suggesting the interaction of H2S and SUC in the development of maize thermotolerance. To establish the underlying mechanism of H2S-SUC interaction-promoted thermotolerance, redox parameters in mesocotyls of maize seedlings were measured before and after heat stress. The data indicate that the activity and gene expression of H2S-metabolizing enzymes were up-regulated by SUC, whereas H2S had no significant effect on the activity and gene expression of SUC-metabolizing enzymes. In addition, the activity and gene expression of catalase, glutathione reductase, ascorbate peroxidase, peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, and superoxide dismutase were reinforced by H2S, SUC, and their combination under non-heat and heat conditions to varying degrees. Similarly, the content of ascorbic acid, flavone, carotenoid, and polyphenol was increased by H2S, SUC, and their combination, whereas the production of superoxide radicals and the hydrogen peroxide level were impaired by these treatments to different extents. These results imply that the metabolic and functional interactions of H2S and sucrose signaling exist in the formation of maize thermotolerance through redox homeodynamics. This finding lays the theoretical basis for developing climate-resistant maize crops and improving food security.

Hydrogen sulfide signaling in plant response to temperature stress.

For the past 300 years, hydrogen sulfide (H2S) has been considered a toxic gas. Nowadays, it has been found to be a novel signaling molecule in plants involved in the regulation of cellular metabolism, seed germination, plant growth, development, and response to environmental stresses, including high temperature (HT) and low temperature (LT). As a signaling molecule, H2S can be actively synthesized and degraded in the cytosol, chloroplasts, and mitochondria of plant cells by enzymatic and non-enzymatic pathways to maintain homeostasis. To date, plant receptors for H2S have not been found. It usually exerts physiological functions through the persulfidation of target proteins. In the past 10 years, H2S signaling in plants has gained much attention. Therefore, in this review, based on that same attention, H2S homeostasis, protein persulfidation, and the signaling role of H2S in plant response to HT and LT stress were summarized. Also, the common mechanisms of H2S-induced HT and LT tolerance in plants were updated. These mechanisms involve restoration of biomembrane integrity, synthesis of stress proteins, enhancement of the antioxidant system and methylglyoxal (MG) detoxification system, improvement of the water homeostasis system, and reestablishment of Ca2+ homeostasis and acid-base balance. These updates lay the foundation for further understanding the physiological functions of H2S and acquiring temperature-stress-resistant crops to develop sustainable food and agriculture.

The Essential Role of H2S-ABA Crosstalk in Maize Thermotolerance through the ROS-Scavenging System.

Hydrogen sulfide (H2S) and abscisic acid (ABA), as a signaling molecule and stress hormone, their crosstalk-induced thermotolerance in maize seedlings and its underlying mechanism were elusive. In this paper, H2S and ABA crosstalk as well as the underlying mechanism of crosstalk-induced thermotolerance in maize seedlings were investigated. The data show that endogenous levels of H2S and ABA in maize seedlings could be mutually induced by regulating their metabolic enzyme activity and gene expression under non-heat stress (non-HS) and HS conditions. Furthermore, H2S and ABA alone or in combination significantly increase thermotolerance in maize seedlings by improving the survival rate (SR) and mitigating biomembrane damage. Similarly, the activity of the reactive oxygen species (ROS)-scavenging system, including enzymatic antioxidants catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POD), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and superoxide dismutase (SOD), as well as the non-enzymatic antioxidants reduced ascorbic acid (AsA), carotenoids (CAR), flavone (FLA), and total phenols (TP), was enhanced by H2S and ABA alone or in combination in maize seedlings. Conversely, the ROS level (mainly hydrogen peroxide and superoxide radical) was weakened by H2S and ABA alone or in combination in maize seedlings under non-HS and HS conditions. These data imply that the ROS-scavenging system played an essential role in H2S-ABA crosstalk-induced thermotolerance in maize seedlings.

Gasotransmitter ammonia accelerates seed germination, seedling growth, and thermotolerance acquirement in maize.

Ammonia (NH3), as an intermediate product of nitrogen metabolism, is recognized as a novel gasotransmitter (namely gaseous signaling molecule), its signaling role being revealed in plants. NH3 exists in two different chemical forms, namely the weak base (free molecule: NH3) and the weak acid (ammonium: NH4+), which are generally in equilibrium with each other in plants. However, the effect of NH3 on seed germination, seedling growth, and thermotolerance acquirement in maize remains unclear. Here, maize seeds were imbibed in the different concentrations of NH3·H2O (NH3 donor), and then germinated and calculated seed germination rate at the various time points. Also, the 60-h-old seedlings were irrigated in the different concentrations of NH3·H2O, and then subjected to heat stress and counted survival rate. The data implied that the appropriate concentrations (6, 9, and 12 mM) of NH3·H2O accelerated seed germination as well as increased seedling height and root length compared with the control without NH3 treatment. Also, the suitable concentrations (2 and 4 mM) of NH3·H2O improved tissue vitality, relieved an increase in malondialdehyde content, and enhanced survival rate of maize seedlings under heat stress compared with the control. These results firstly suggest that NH3 could accelerate seed germination, seedling growth, and thermotolerance acquirement in maize.

Key role of reactive oxygen species-scavenging system in nitric oxide and hydrogen sulfide crosstalk-evoked thermotolerance in maize seedlings.

Nitric oxide (NO) and hydrogen sulfide (H2S) are novel signaling molecules, which participate in plant growth, development, and response to stress. In this study root-irrigation with 0.15 mM sodium nitroprusside (SNP, NO donor) up-regulated gene expression of L-CYSTEINE DESULFHYDRASE1 (LCD1), activities of L-cysteine desulfhydrase (LCD) and D-cysteine desulfhydrase (DCD), as well as an endogenous H2S level, compared to control seedlings. The SNP-up-regulated effects were enhanced by 0.5 mM sodium hydrosulfide (NaHS, H2S donor), but weakened by NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) and H2S scavenger hypotaurine (HT) alone. NaHS had no significant effect on gene expression and activity of nitrate reductase (NR, a NO candidate producing enzyme). These data indicate that NO could trigger the LCD/H2S signaling pathway in maize seedlings. To further investigate the effect of NO and H2S crosstalk on thermotolerance in maize seedlings, thermotolerance parameters and reactive oxygen species (ROS)-scavenging system were estimated. The results show that SNP increased survival rate and tissue viability, decreased malondialdehyde (MDA) accumulation, and electrolyte leakage in maize seedlings under heat stress (HS), implying NO could improve thermotolerance in maize seedlings. The NO-improved thermotolerance was impaired by H2S inhibitor DL-propargylglycine (PAG) and scavenger HT alone. Similarly, SNP up-regulated the gene expression of DEHYDROASCORBATE REDUCTASE (DHAR) and GLUTATHIONE REDUCTASE1 (GR1); activities of ascorbate peroxidase, glutathione reductase, and catalase; as well as levels of ascorbic acid, glutathione, flavonoids, carotenoids, and total phenols. SNP also reduced hydrogen peroxide and superoxide radical accumulation in maize seedlings under HS compared to the control. The effects of SNP on ROS and their scavenger system were weakened by PAG and HT alone. These data hint that NO could evoke thermotolerance in maize seedlings by triggering the LCD/H2S signaling pathway, and the ROS-scavenging system played a key role in the NO and H2S crosstalk-evoked thermotolerance.

Sulfur Dioxide: An Emerging Signaling Molecule in Plants.

Sulfur dioxide (SO2) has long been viewed as toxic gas and air pollutant, but now is being verified as a signaling molecule in mammalian cells. SO2 can be endogenously produced and rapidly transformed into sulfur-containing compounds (e.g., hydrogen sulfide, cysteine, methionine, glutathione, glucosinolate, and phytochelatin) to maintain its homeostasis in plant cells. Exogenous application of SO2 in the form of gas or solution can trigger the expression of thousands of genes. The physiological functions of these genes are involved in the antioxidant defense, osmotic adjustment, and synthesis of stress proteins, secondary metabolites, and plant hormones, thus modulating numerous plant physiological processes. The modulated physiological processes by SO2 are implicated in seed germination, stomatal action, postharvest physiology, and plant response to environmental stresses. However, the review on the signaling role of SO2 in plants is little. In this review, the anabolism and catabolism of SO2 in plants were summarized. In addition, the signaling role of SO2 in seed germination, stomatal movement, fruit fresh-keeping, and plant response to environmental stresses (including drought, cold, heavy metal, and pathogen stresses) was discussed. Finally, the research direction of SO2 in plants is also proposed.

Involvement of osmoregulation, glyoxalase, and non-glyoxalase systems in signaling molecule glutamic acid-boosted thermotolerance in maize seedlings.

Glutamic acid (Glu) is not only an important protein building block, but also a signaling molecule in plants. However, the Glu-boosted thermotolerance and its underlying mechanisms in plants still remain unclear. In this study, the maize seedlings were irrigated with Glu solution prior to exposure to heat stress (HS), the seedlings' thermotolerance as well as osmoregulation, glyoxalase, and non-glyoxalase systems were evaluated. The results manifested that the seedling survival and tissue vitality after HS were boosted by Glu, while membrane damage was reduced in comparison with the control seedlings without Glu treatment, indicating Glu boosted the thermotolerance of maize seedlings. Additionally, root-irrigation with Glu increased its endogenous level, reinforced osmoregulation system (i.e., an increase in the levels of proline, glycine betaine, trehalose, and total soluble sugar, as well as the activities of pyrroline-5-carboxylate synthase, betaine dehydrogenase, and trehalose-5-phosphate phosphatase) in maize seedlings under non-HS and HS conditions compared with the control. Also, Glu treatment heightened endogenous methylglyoxal level and the activities of glyoxalase system (glyoxalase I, glyoxalase II, and glyoxalase III) and non-glyoxalase system (methylglyoxal reductase, lactate dehydrogenase, aldo-ketoreductase, and alkenal/alkenone reductase) in maize seedlings under non-HS and HS conditions as compared to the control. These data hint that osmoregulation, glyoxalase, and non-glyoxalase systems are involved in signaling molecule Glu-boosted thermotolerance of maize seedlings.

Transcriptome response of maize (Zea mays L.) seedlings to heat stress.

Among stresses, heat stress (HS) is a prime factor restricting plant growth and productivity. However, the molecular mechanisms of plants' response to HS need to be further uncovered. Here, the transcriptome response of maize seedlings to HS was dissected using transcriptome data analysis. The data exhibited that a total of 43,221 genes in maize seedlings had been found, 37,534 of which were referred, while 5686 were not. Under HS, comparison with the control without HS, there were 13,607 genes that were differentially expressed (DEGs, 6195 upregulated and 7412 downregulated). In addition, Gene Ontology (GO) enrichment analysis indicated that there were 220, 478, and 1300 terms that were enriched in cellular component, molecular function, and biological process, respectively. Significantly enriched GO terms were involved in 23 cellular components, 27 molecular functions, and 124 biological processes. Also, Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis suggested that there were 2613 DEGs that were assigned to 131 pathways, 14 of which (enriched 1068 DEGs in total) were significantly upregulated. These pathways were mainly related to protein renaturation, biomembrane repair, osmotic adjustment, and redox balance. Among them, protein processing in endoplasmic reticulum was the most significantly upregulated. The transcriptome data decoded that protein renaturation, biomembrane repair, osmotic adjustment, and redox balance played a key role in the response of maize seedlings to HS.

Signaling molecule glutamic acid initiates the expression of genes related to methylglyoxal scavenging and osmoregulation systems in maize seedlings.

Glutamic acid (Glu) is not only a protein amino acid, but also a signaling molecule, which takes part in various physiological processes in plants. Our previous study found that root-irrigation with Glu could improve the heat tolerance of maize seedlings by plant Glu receptor-like channels-mediated calcium signaling (Protoplasma, 2019; 256:1165-1169), but its molecular mechanism remains unclear. In this study, based on the our previous work, the maize seedlings were treated with 1 mM Glu prior to be exposed to heat stress (HS), and then the expression of genes related to related to methylglyoxal (MG)-scavenging and osmoregulation systems was quantified. The results showed that Glu treatment up-regulated the gene expression of Zea mays aldo-keto reductase (ZmAKR) under both non-HS and HS conditions. Also, the gene expression of Zea mays alkenal/alkenone reductase (ZmAAR), glyoxalase II (ZmGly II), pyrroline-5-carboxylate synthase (ZmP5CS), betaine dehydrogenase (ZmBADH), and trehalase (ZmTRE) was up-regualted by exogenous Glu treatment under HS conditions. These data imply that signaling molecule Glu initiated the expression of genes related to MG-scavenging and osmoregulation systems in maize seedlings, further supporting the fact that Glu-enhanced heat tolerance in plants.

Regulative role of calcium signaling on methylglyoxal-improved heat tolerance in maize (Zea mays L) seedlings.

Nowadays, calcium (Ca2+) and methylglyoxal (MG) are all deemed to be second messengers in plants, which participate in various physiological processes, such as seed germination, seedling establishment, plant growth and development, as well as response to environmental stress. However, the Ca2+-MG interaction in the development of thermotolerance in maize seedlings remains unclear. Here, using maize seedlings as materials, the crosstalk between Ca2+ and MG signaling in the acquisition of thermotolerance was explored. The results showed that root-irrigation with Ca2+ and MG alone or in combination increased the survival rate of maize seedlings under heat stress, mitigated the decrease in the tissue vitality, and reduced the membrane lipid peroxidation (in term of the content of malondialdehyde), indicating that Ca2+ and MG could improve the thermotolerance in maize seedlings. In addition, MG-improved thermotolerance was impaired by ethylene glycol-bis(b-aminoethylether)-N,N,N΄,N΄-tetraacetic acid (a Ca2+ chelator), La3+ (plasma membrane Ca2+ channel blocker), ruthenium red (a mitochondrial Ca2+ channel blocker), neomycin (vacuole Ca2+ channel blocker), caffeine (an endoplasmic reticulum Ca2+ channel blocker), and calmodulin antagonists (chlorpromazine and trifluoperazine), respectively. Also, MG scavengers (N-acetyl-cysteine, aminoguanidine, and vitamin B6) had no significant effect on Ca2+-triggered thermotolerance (in terms of survival rate, malondialdehyde, and tissue vitality) of maize seedlings. The data illustrated that calcium signaling regulated MG-improved thermotolerance in maize seedlings by mobilizing intracellular and extracellular Ca2+ pools.

Interplay between hydrogen sulfide and methylglyoxal initiates thermotolerance in maize seedlings by modulating reactive oxidative species and osmolyte metabolism.

Hydrogen sulfide (H2S) and methylglyoxal (MG) were supposed to be novel signaling molecules in plants. However, whether interplay between H2S and MG can initiate thermotolerance in maize seedlings and in relation to metabolism of reactive oxygen species (ROS) and osmolytes is little known. In this study, watering with MG and NaHS (H2S donor) alone or in combination elevated survival and tissue vigor of maize seedlings under heat stress and coped with an increase in the biomembrane injury (as indicated in membrane lipid peroxidation and electrolyte leakage). The above-mentioned effects were separately weakened by MG scavengers (N-acetyl cysteine: NAC; aminoguanidine: AG) and H2S inhibitor (DL-propargylglycine, PAG) and scavenger (hypotaurine, HT). These suggested that the interplay between H2S and MG initiated the thermotolerance in maize seedlings. The further data indicated that, under non-heat stress and heat stress conditions, MG and NaHS alone or in combination modulated ROS metabolism by regulating the activities of antioxidant enzymes (catalase, ascorbate peroxidase, guaiacol peroxidase, glutathione reductase, monodehydroascorbate reductase, and dehydroascorbate reductase) and the contents of non-enzymatic antioxidants (ascorbic acid, glutathione, flavonoids, and carotenoids) in maize seedlings. In addition, MG and NaHS alone or in combination also separately modulated the metabolism of osmolytes (proline, trehalose, glycine betaine, and total soluble sugar), H2S (L-cysteine desulfhydrase and O-acetylserine (thione) lyase), and MG (glyoxalase I, glyoxalase II, and MG reductase). These physiological effects also were separately impaired by NAC, AG, PAG, and HT. The current data illustrated that the interplay between H2S and MG initiated the thermotolerance in maize seedlings by modulating ROS, osmolyte, H2S, and MG metabolism.

Electrical stimulation boosts seed germination, seedling growth, and thermotolerance improvement in maize (Zea mays L.).

Electrical signaling, similar to chemical signalings such as calcium (Ca2+), reactive oxygen species (ROS, mainly hydrogen peroxide: H2O2), nitric oxide (NO), and hydrogen sulfide (H2S), regulates many physiological processes. However, the effect of electrical stimulation on seed germination, seedling growth, and thermotolerance improvement in maize was little known. In this study, using maize as materials, the effect of electrical stimulation on seed germination, seedling growth, and thermotolerance improvement in maize was explored. The results suggested that electrical stimulation with optimal intensity boosted germination rate and seedling growth (as indicated in the increase in the length of shoots and roots, as well as fresh weight) under normal germination conditions. In addition, electrical stimulation augmented the survival rate of maize seedlings, mitigated the decrease in the tissue vitality, and reduced the peroxidation of membrane lipids under heat stress. These data suggested that electrical stimulation could boost seed germination, seedling growth, and thermotolerance improvement in maize.

Methylglyoxal triggers the heat tolerance in maize seedlings by driving AsA-GSH cycle and reactive oxygen species-/methylglyoxal-scavenging system.

Traditionally, methylglyoxal (MG) was looked upon as a toxic byproduct of cellular metabolism. Nowadays, MG has been found to be a novel signaling molecule. However, whether MG can trigger the heat tolerance in maize seedlings and the underlying mechanisms is still elusive. In this study, the maize seedlings irrigated with MG increased the survival percentage of seedlings under heat stress (HS), remitted a decrease in tissue vitality and an increase in electrolyte leakage, and reduced membrane lipid peroxidation, implying MG could trigger the heat tolerance of maize seedlings. The further experiments showed that MG drove the ascorbic acid (AsA)-glutathione (GSH) cycle by activating enzymes (glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase, and ascorbate peroxidase) and increasing the contents of antioxidants (AsA and GSH) and the ratio of GSH/(GSH + oxidized glutathione) and AsA/(AsA + dehydroascorbate) under both non-HS and HS. Also, the reactive oxygen species (ROS)-scavenger system (catalase, guaiacol peroxidase, carotenoid, total phenols, and flavonoids) and MG-scavenger system (glyoxalase I and glyoxalas II) also were up-regulated in maize seedlings pretreated with MG under non-HS and HS. This work for the first time reported that MG could trigger the heat tolerance of maize seedlings by driving the AsA-GSH cycle and ROS-/MG-scavenging system.

Glutamate signaling enhances the heat tolerance of maize seedlings by plant glutamate receptor-like channels-mediated calcium signaling.

Glutamate (Glu), a neurotransmitter in animal, is a novel signaling molecule in plants, which takes part in cellular metabolism, seed germination, plant growth, development, and long-distance information transfer. However, whether Glu can enhance the heat tolerance in maize seedlings and its relation to calcium signaling is still elusive. In this study, maize seedlings were pretreated with Glu and then exposed to heat stress. The results showed that Glu pretreatment enhanced the survival percentage of maize seedlings under heat tolerance, indicating that Glu could increase the heat tolerance of maize seedlings. The Glu-induced heat tolerance was weakened by exogenous calcium chloride, plasma membrane Ca2+ channel blocker (LaCl3), Ca2+ chelator (ethylene glycol-bis(b-aminoethylether)-N,N, N΄,N΄-tetraacetic acid), calmodulin antagonists (trifluoperazine and chlopromazine), and plant glutamate receptor-like antagonists (MgCl2 and 6,7-dinitroquinoxaline- 2,3-(1H,4H)- dione). These findings for the first time reported that Glu could increase the heat tolerance of maize seedlings by plant glutamate receptor-like channels-mediated calcium signaling.

Signaling Role of Glutamate in Plants.

It is well known that glutamate (Glu), a neurotransmitter in human body, is a protein amino acid. It plays a very important role in plant growth and development. Nowadays, Glu has been found to emerge as signaling role. Under normal conditions, Glu takes part in seed germination, root architecture, pollen germination, and pollen tube growth. Under stress conditions, Glu participates in wound response, pathogen resistance, response and adaptation to abiotic stress (such as salt, cold, heat, and drought), and local stimulation (abiotic or biotic stress)-triggered long distance signaling transduction. In this review, in the light of the current opinion on Glu signaling in plants, the following knowledge was updated and discussed. 1) Glu metabolism; 2) signaling role of Glu in plant growth, development, and response and adaptation to environmental stress; as well as 3) the underlying research directions in the future. The purpose of this review was to look forward to inspiring the rapid development of Glu signaling research in plant biology, particularly in the field of stress biology of plants.

Melatonin enhances thermotolerance of maize seedlings (Zea mays L.) by modulating antioxidant defense, methylglyoxal detoxification, and osmoregulation systems.

Melatonin (MT), derived from tryptophan, is an amazing signaling molecule with multiple functions in plants. Heat stress (HS) induced by high temperature is a major stress factor that limits metabolism, growth, development, and productivity of plants. However, whether MT could enhance the thermotolerance of maize seedlings and the underlying mechanisms is not completely known. In this study, treatment of maize seedlings with MT enhanced the survival percentage of maize seedlings under HS conditions, mitigated an increase in malondialdehyde (MDA, product of membrane lipid peroxidation) and electrolyte leakage, and improved tissue vitality compared with the control without MT treatment, indicating that MT treatment could enhance the theromotolerance of maize seedlings. To understand the mechanisms underlying MT-enhanced thermotolerance of maize seedlings, the antioxidant defense (guaiacol peroxidease: GPX; glutathione reductase: GR; catalase: CAT; ascorbic acid: AsA; and glutathione: GSH), methylglyoxal (MG) detoxification (glyoxalase I: Gly I; and glyoxalase II: Gly II), and osmoregulation (proline: Pro; trehalose: Tre; and total soluble sugar: TSS) systems were assayed. The results showed that MT treatment stimulated the activities of antioxidant enzymes (GPX, GR, and CAT) and MG detoxification enzymes (Gly I and Gly II), increased the contents of nonenzyme antioxidants (AsA and GSH) and osmolytes (Pro, Tre, and TSS) in maize seedlings under normal culture conditions, and maintained a higher abovementioned enzyme activity and antioxidant and osmolyte contents under HS conditions compared with the control. This work reported that MT could enhance the thermotolerance of maize seedlings by modulating the antioxidant defense, MG detoxification, and osmoregulation systems.

Signaling Molecule Hydrogen Sulfide Improves Seed Germination and Seedling Growth of Maize (Zea mays L.) Under High Temperature by Inducing Antioxidant System and Osmolyte Biosynthesis.

Hydrogen sulfide (H2S) is a novel type signaling molecule in plants. Seed germination is a key stage of life cycle of plants, which is vulnerable to environmental stress including high temperature. However, under high temperature stress, whether pre-soaking of maize seeds with NaHS (a H2S donor) could improve seed germination and seedling growth and the possible mechanisms are not completely clear. In this study, maize seeds pre-soaked with NaHS enhanced germination percentage, sprout length, root length, and fresh weight compared with the control without NaHS treatment, illustrating that H2S could improve maize seed germination and seedling growth under high temperature. In addition, in comparison to the control, NaHS pre-soaking stimulated antioxidant enzymes [ascorbate peroxidase (APX), glutathione reductase (GR), guaiacol peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT)] activities and the contents of water soluble non-enzymatic antioxidants [ascorbic acid (AsA) and glutathione (GSH)], as well as the ratio of reduced antioxidant to oxidized antioxidant. Moreover, pre-soaking with NaHS activated Δ1-pyrroline-5-carboxylate synthetase (P5CS) and ornithine aminotransferase [OAT; both are rate-limiting enzymes in proline (Pro) synthesis], betaine aldehyde dehydrogenase [BADH; a key enzyme in glycine betaine (GB)], and trehalose (Tre)-6-phosphate phosphatase (a key step in Tre synthesis), which in turn accumulated Pro, GB, and Tre in germinating seeds compared with the control. Also, an improved germination by NaHS under high temperature was reinforced by the above osmotic adjustment substances (osmolytes) alone, while deteriorated by the inhibitors of osmolyte biosynthesis [gabaculine (GAB), disulfiram (DSF), and sodium citrate (SC)]. These results imply that H2S could improve maize seed germination and seedling growth under high temperature by inducing antioxidant system and osmolyte biosynthesis.

Methylglyoxal - a signaling molecule in plant abiotic stress responses.

Abiotic stresses are the most common harmful factors, adversely affecting all aspects of plants' life. Plants have to elicit appropriate responses against multifaceted effects of abiotic stresses by reprogramming various cellular processes. Signaling molecules play vital roles in sensing environmental stimuli to modulate gene expression, metabolism and physiological processes in plants to cope with the adverse effects. Methylglyoxal (MG), a dicarbonyl compound, is known to accumulate in cells as a byproduct of various metabolic pathways, including glycolysis. Several works in recent years have demonstrated that MG could play signaling roles via Ca2+, reactive oxygen species (ROS), K+ and abscisic acid. Recently, global gene expression profiling has shown that MG could induce signaling cascades, and an overlap between MG-responsive and stress-responsive signaling events might exist in plants. Once overaccumulated in cells, MG can provoke detrimental effects by generating ROS, forming advanced glycation end products and inactivating antioxidant systems. Plants are also equipped with MG-detoxifying glyoxalase system to save cellular organelles from MG toxicity. Since MG has regulatory functions in plant growth and development, and glyoxalase system is an integral component of abiotic stress adaptation, an in-depth understanding on MG metabolism and glyoxalase system will help decipher mechanisms underlying plant responses to abiotic stresses. Here, we provide a comprehensive update on the current knowledge of MG production and detoxification in plants, and highlight the putative functions of glyoxalase system in mediating plant defense against abiotic stresses. We particularly emphasize on the dual roles of MG and its connection with glutathione-related redox regulation, which is crucial for plant defense and adaptive responses under changing environmental conditions.

Heat or cold priming-induced cross-tolerance to abiotic stresses in plants: key regulators and possible mechanisms.

Plants growing under field conditions are constantly exposed, either simultaneously or sequentially, to more than one abiotic stress factor. Plants have evolved sophisticated sensory systems to perceive a number of stress signals that allow them to activate the most adequate response to grow and survive in a given environment. Recently, cross-stress tolerance (i.e. tolerance to a second, strong stress after a different type of mild primary stress) has gained attention as a potential means of producing stress-resistant crops to aid with global food security. Heat or cold priming-induced cross-tolerance is very common in plants and often results from the synergistic co-activation of multiple stress signalling pathways, which involve reactive nitrogen species (RNS), reactive oxygen species (ROS), reactive carbonyl species (RCS), plant hormones and transcription factors. Recent studies have shown that the signalling functions of ROS, RNS and RCS, most particularly hydrogen peroxide, nitric oxide (NO) and methylglyoxal (MG), provide resistance to abiotic stresses and underpin cross-stress tolerance in plants by modulating the expression of genes as well as the post-translational modification of proteins. The current review highlights the key regulators and mechanisms underlying heat or cold priming-induced cross-stress tolerance in plants, with a focus on ROS, MG and NO signalling, as well as on the role of antioxidant and glyoxalase systems, osmolytes, heat-shock proteins (HSPs) and hormones. Our aim is also to provide a comprehensive idea on the topic for researchers using heat or cold priming-induced cross-tolerance as a mechanism to improve crop yields under multiple abiotic stresses.

Signaling molecule methylglyoxal ameliorates cadmium injury in wheat (Triticum aestivum L) by a coordinated induction of glutathione pool and glyoxalase system.

Methylglyoxal (MG) now is found to be an emerging signaling molecule. It can relieve the toxicity of cadmium (Cd), however its alleviating mechanism still remains unknown. In this study, compared with the Cd-stressed seedlings without MG treatment, MG treatment could stimulate the activities of glutathione reductase (GR) and gamma-glutamylcysteine synthetase (γ-ECS) in Cd-stressed wheat seedlings, which in turn induced an increase of reduced glutathione (GSH). Adversely, the activated enzymes related to GSH biosynthesis and increased GSH were weakened by N-acetyl-L-cysteine (NAC, MG scavenger), 2,4-dihydroxy-benzylamine (DHBA) and 1,3-bischloroethyl-nitrosourea (BCNU, both are specific inhibitors of GR), buthionine sulfoximine (BSO, a specific inhibitors of GSH biosynthesis), and N-ethylmaleimide (NEM, GSH scavenger), respectively. In addition, MG increased the activities of glyoxalase I (Gly I) and glyoxalase II (Gly II) in Cd-treated seedlings, followed by declining an increase in endogenous MG as comparision to Cd-stressed seedlings alone. On the contrary, the increased glyoxalase activity and decreased endogenous MG level were reversed by NAC and specific inhibitors of Gly I (isoascorbate, IAS; squaric acid, SA). Furthermore, MG alleviated an increase in hydrogen peroxide (H2O2) and malondialdehyde (MDA) in Cd-treated wheat seedlings. These results indicated that MG could alleviate Cd toxicity and improve the growth of Cd-stressed wheat seedlings by a coordinated induction of glutathione pool and glyoxalase system.