Methylglyoxal - a signaling molecule in plant abiotic stress responses

Molecular Evolution of Calcium Signaling and Transport in Plant Adaptation to Abiotic Stress

Adaptation to unfavorable abiotic stresses is one of the key processes in the evolution of plants. Calcium (Ca2+) signaling is characterized by the spatiotemporal pattern of Ca2+ distribution and the activities of multi-domain proteins in integrating environmental stimuli and cellular responses, which are crucial early events in abiotic stress responses in plants. However, a comprehensive summary and explanation for evolutionary and functional synergies in Ca2+ signaling remains elusive in green plants. We review mechanisms of Ca2+ membrane transporters and intracellular Ca2+ sensors with evolutionary imprinting and structural clues. These may provide molecular and bioinformatics insights for the functional analysis of some non-model species in the evolutionarily important green plant lineages. We summarize the chronological order, spatial location, and characteristics of Ca2+ functional proteins. Furthermore, we highlight the integral functions of calcium-signaling components in various nodes of the Ca2+ signaling pathway through conserved or variant evolutionary processes. These ultimately bridge the Ca2+ cascade reactions into regulatory networks, particularly in the hormonal signaling pathways. In summary, this review provides new perspectives towards a better understanding of the evolution, interaction and integration of Ca2+ signaling components in green plants, which is likely to benefit future research in agriculture, evolutionary biology, ecology and the environment.

Regulation of antioxidant defense and glyoxalase systems in cyanobacteria

Oxidative stress is common consequence of abiotic stress in plants as well as cyanobacteria caused by generation of reactive oxygen species (ROS), an inevitable product of respiration and photosynthetic electron transport. ROS act as signalling molecule at low concentration however, when its production exceeds the endurance capacity of antioxidative defence system, the organisms suffer oxidative stress. A highly toxic metabolite, methylglyoxal (MG) is also produced in cyanobacteria in response to various abiotic stresses which consequently augment the ensuing oxidative damage. Taking recourse to the common lineage of eukaryotic plants and cyanobacteria, it would be worthwhile to explore the regulatory role of glyoxalase system and antioxidative defense mechanism in combating abiotic stress in cyanobacteria. This review provides comprehensive information on the complete glyoxalase system (GlyI, GlyII and GlyIII) in cyanobacteria. Furthermore, it elucidates the recent understanding regarding the production of ROS and MG, noteworthy link between intracellular MG and ROS and its detoxification via synchronization of antioxidants (enzymatic and non-enzymatic) and glyoxalase systems using glutathione (GSH) as common co-factor.

Glutathione Metabolism in Plants under Stress: Beyond Reactive Oxygen Species Detoxification

Glutathione is an essential metabolite for plant life best known for its role in the control of reactive oxygen species (ROS). Glutathione is also involved in the detoxification of methylglyoxal (MG) which, much like ROS, is produced at low levels by aerobic metabolism under normal conditions. While several physiological processes depend on ROS and MG, a variety of stresses can dramatically increase their concentration leading to potentially deleterious effects. In this review, we examine the structure and the stress regulation of the pathways involved in glutathione synthesis and degradation. We provide a synthesis of the current knowledge on the glutathione-dependent glyoxalase pathway responsible for MG detoxification. We present recent developments on the organization of the glyoxalase pathway in which alternative splicing generate a number of isoforms targeted to various subcellular compartments. Stress regulation of enzymes involved in MG detoxification occurs at multiple levels. A growing number of studies show that oxidative stress promotes the covalent modification of proteins by glutathione. This post-translational modification is called S-glutathionylation. It affects the function of several target proteins and is relevant to stress adaptation. We address this regulatory function in an analysis of the enzymes and pathways targeted by S-glutathionylation.

Comparative Proteomics at the Critical Node of Vigor Loss in Wheat Seeds Differing in Storability

The critical node (CN, 85% germination) of seed viability is an important threshold for seed regeneration decisions after long-term conservation. Dependent on the germplasm, the storage period until CN is reached varies and information on the divergence of the proteomic profiles is limited. Therefore, the study aims to identify key proteins and mechanisms relevant for a long plateau phase and a late CN during artificial seed aging of wheat. Seeds of the storage-tolerant genotype (ST) TRI 23248, and the storage-sensitive genotype (SS) TRI 10230 were exposed to artificial ageing (AA) and extracted embryos of imbibed seeds were analyzed using an iTRAQ-based proteomic technique. ST and SS required AA for 24 and 18 days to reach the CN, respectively. Fifty-seven and 165 differentially abundant proteins (DAPs) were observed in the control and aged groups, respectively. Interestingly, a higher activity in metabolic processes, protein synthesis, transcription, cell growth/division, and signal transduction were already found in imbibed embryos of control ST seeds. After AA, 132 and 64 DAPs were accumulated in imbibed embryos of both aged ST and SS seeds, respectively, which were mainly associated with cell defense, rescue, and metabolism. Moreover, 78 DAPs of ST appeared before CN and were mainly enriched in biological pathways related to the maintenance of redox and carbon homeostasis and they presented a stronger protein translation ability. In contrast, in SS, only 3 DAPs appeared before CN and were enriched only in the structural constituents of the cytoskeleton. In conclusion, a longer span of plateau phase might be obtained in seeds when proteins indicate an intense stress response before CN and include the effective maintenance of cellular homeostasis, and avoidance of excess accumulation of cytotoxic compounds. Although key proteins, inherent factors and the precise regulatory mechanisms need to be further investigated, the found proteins may also have functional potential roles during long-term seed conservation.

Microbial methylglyoxal metabolism contributes towards growth promotion and stress tolerance in plants

Plant growth promotion by microbes is a cumulative phenomenon involving multiple traits, many of which are not explored yet. Hence, to unravel microbial mechanisms underlying growth promotion, we have analysed the genomes of two potential growth-promoting microbes, viz., Pseudomonas sp. CK-NBRI-02 (P2) and Bacillus marisflavi CK-NBRI-03 (P3) for the presence of plant-beneficial traits. Besides known traits, we found that microbes differ in their ability to metabolize methylglyoxal (MG), a ubiquitous cytotoxin regarded as general consequence of stress in plants. P2 exhibited greater tolerance to MG and possessed better ability to sustain plant growth under dicarbonyl stress. However, under salinity, only P3 showed a dose-dependent induction in MG detoxification activity in accordance with concomitant increase in MG levels, contributing to enhanced salt tolerance. Furthermore, salt-stressed transcriptomes of both the strains showed differences with respect to MG, ion and osmolyte homeostasis, with P3 being more responsive to stress. Importantly, application of either strain altered MG levels and subsequently MG detoxification machinery in Arabidopsis, probably to strengthen plant defence response and growth. We therefore, suggest a crucial role of microbial MG resistance in plant growth promotion and that it should be considered as a beneficial trait while screening microbes for stress mitigation in plants.

Detoxification of methylglyoxal by the glyoxalase system is required for glutathione availability and virulence activation in Listeria monocytogenes

Listeria monocytogenes is a Gram-positive, food-borne pathogen that lives a biphasic lifestyle, cycling between the environment and as a facultative intracellular pathogen of mammals. Upon entry into host cells, L. monocytogenes upregulates expression of glutathione synthase (GshF) and its product, glutathione (GSH), which is an allosteric activator of the master virulence regulator PrfA. Although gshF mutants are highly attenuated for virulence in mice and form very small plaques in host cell monolayers, these virulence defects can be fully rescued by mutations that lock PrfA in its active conformation, referred to as PrfA*. While PrfA activation can be recapitulated in vitro by the addition of reducing agents, the precise biological cue(s) experienced by L. monocytogenes that lead to PrfA activation are not known. Here we performed a genetic screen to identify additional small-plaque mutants that were rescued by PrfA* and identified gloA, which encodes glyoxalase A, a component of a GSH-dependent methylglyoxal (MG) detoxification system. MG is a toxic byproduct of metabolism produced by both the host and pathogen, which if accumulated, causes DNA damage and protein glycation. As a facultative intracellular pathogen, L. monocytogenes must protect itself from MG produced by its own metabolic processes and that of its host. We report that gloA mutants grow normally in broth, are sensitive to exogenous MG and severely attenuated upon IV infection in mice, but are fully rescued for virulence in a PrfA* background. We demonstrate that transcriptional activation of gshF increased upon MG challenge in vitro, and while this resulted in higher levels of GSH for wild-type L. monocytogenes, the glyoxalase mutants had decreased levels of GSH, presumably due to the accumulation of the GSH-MG hemithioacetal adduct. These data suggest that MG acts as a host cue that leads to GSH production and activation of PrfA.

Nitrate reductase is required for salicylic acid-induced water stress tolerance of pepper by upraising the AsA-GSH pathway and glyoxalase system

A trial was conducted to evaluate whether nitrate reductase (NR) participates in salicylic acid (SA)-improved water stress (WS) tolerance in pepper (Capsicum annuum L.) plants. Before starting WS treatment, 0.5 mM SA was applied to half of the well-watered (WW) plants as well as to WS-plants as a foliar spray once a day for a week. The soil water holding capacity was maintained at 40 and 80% of the full water storing capacity for WS and and well-watered (WW) plants, respectively. Water stress caused substantial decreases in total plant dry weight, F_v /F_m , chlorophyll a and b, relative water content, leaf water potential (ΨI) by 53, 37, 49, 21, 36 and 33%, respectively relative to control, but significant increases in malondialdehyde (MDA), hydrogen peroxide (H₂ O₂ ), electrolyte leakage (EL), methylglyoxal (MG), proline, key antioxidant enzymes' activities, NO and NR activity. The SA reduced oxidative stress, but improved antioxidant defence system, ascorbate-glutathione (AsA-GSH) cycle enzymes, glyoxalase system-related enzymes, glyoxalase I (Gly I) and glyoxalase II (Gly II), plant growth, photosynthetic traits, NO, NR and proline. SA-induced WS tolerance was further improved by supplementation of sodium nitroprusside (SNP), a donor of NO. NR inhibitor, sodium tungstate (ST) was applied in conjunction with SA and SA + SNP to the WW and WS-plants to assess whether NR contributes to SA-improved WS tolerance. ST abolished the beneficial effects of SA by reducing NO and NR activity in WS-pepper, but the application of SNP along with SA + ST reversed negative effects of ST, showing that NO and NR are jointly needed for SA-induced WS tolerance of pepper plants.

Functional characterization and expression profiling of glyoxalase III genes in date palm grown under abiotic stresses

Methylglyoxal (MG), a by-product of various metabolic processes, including glycolysis, is a highly reactive cytotoxic metabolite. The level of MG in the cell is maintained at a non-toxic level via MG detoxification pathways such as the universal glyoxalase system, including glyoxalase I/II/III enzymes. Glyoxalase III (DJ-1) can breakdown MG to d-lactate in a single step without reducing glutathione (GSH). Elucidating the function of the DJ-1 gene family may provide further knowledge about its role in plants under abiotic stresses. Here, we characterize four glyoxalase III genes (PdDJ-1B1, PdDJ-1B2, PdDJ-1C, and PdDJ-1D) encoding the conserved DJ-1 domain in the genome of the date palm, a crop with high drought and salinity tolerance. The expression level of the PdDJ-1 genes increased in date palm leaves upon salinity treatment. In addition, overexpression of PdDJ-1 genes in Escherichia coli and the complementation in yeast hsp31Δ knockout mutant cells enhanced their growth rate and reduced the accumulation of reactive oxygen species (ROS) under MG and oxidative stress conditions as shown by the flow cytometry assay. Subcellular localization using confocal microscopy revealed the accumulation of PdDJ-1B1, PdDJ-1C, and PdDJ-1D in the chloroplast, whereas PdDJ-1B2 was localized to the cytosol. Remarkably, constitutive expression of the PdDJ-1C gene in Arabidopsis thaliana Columbia (Col-0) resulted in the generation of non-viable albino plants implying that PdDJ-1C plays a critical function in chloroplast development. These findings suggest that PdDJ-1 protein has an important function in MG-detoxification and maintaining the redox balance in date palm plants under abiotic stress conditions.

Methylglyoxal Has Different Impacts on the Fungistatic Roles of Ammonia and Benzaldehyde, and Lactoylglutathione Lyase Is Necessary for the Resistance of Arthrobotrys oligospora to Soil Fungistasis

Biocontrol of root-knot nematode has attracted increasing attention over the past two decades. The inconsistent field performance of biocontrol agents, which is caused by soil fungistasis, often restricts their commercial application. There is still a lack of research on the genes involved in biocontrol fungi response to soil fungistasis, which is important for optimizing practical applications of biocontrol fungi. In this study, the lactoylglutathione lyase-encoding AOL_s00004g335 in the nematophagous fungi Arthrobotrys oligospora was knocked out, and three mutant strains were obtained. The hyphal growth of mutants on the three media was almost the same as that of the wild-type strain, but mutants had slightly higher resistance to NaCl, SDS, and H₂O₂. Methylglyoxal (MG) significantly increased the resistance of A. oligospora to ammonia, but decreased the resistance to benzaldehyde. Furthermore, the resistance of the mutants to soil fungistasis was largely weakened and MG could not increase the resistance of A. oligospora to soil fungistasis. Our results revealed that MG has different effects on the fungistatic roles of ammonia and benzaldehyde and that lactoylglutathione lyase is very important for A. oligospora to resist soil fungistasis.

Physiological and Biochemical Dissection Reveals a Trade-off Between Antioxidant Capacity and Heat Tolerance in Bread Wheat (Triticum aestivum L.)

Heat stress alters photosynthetic components and the antioxidant scavenging system, negatively affecting plant growth and development. Plants overcome heat stress damage through an integrated network involving enzymatic and non-enzymatic antioxidants. This study aimed to assess physiological and biochemical responses in contrasting thermo-tolerant wheat varieties exposed to 25 °C (control) and 35 °C (heat stress), during the seedling stage. Our results revealed a substantial decrease in the photosynthetic pigments, carotenoids, anthocyanin content, and increased membrane injury index, malondialdehyde, methylglyoxal (MG), H₂O₂ contents and lipoxygenase activity compared to non-stress wheat seedlings. The heat-tolerant variety BARI Gom 26 ("BG26") maintained higher cellular homeostasis compared to the heat susceptible variety Pavon 76 ("Pavon"), perpetuated by higher accumulation of proline, glycine betaine, ascorbate-glutathione cycle associated enzymes, reduced glutathione and ascorbate concentration in plant cells. Significantly lower levels of MG detoxification and antioxidant activities and ascorbate-glutathione cycle-related enzymatic activities lead to increased susceptibility in variety "Pavon". Hierarchical clustering and principal component analysis revealed that variety "BG26" possess a combination of biochemical responses tailoring antioxidant activities that induced a higher level of tolerance. Taken together, our results provide a pipeline for establishing a trade-off between antioxidant capacity and heat tolerance to facilitate functional genomics and translational research to unravel underlying mechanisms to better adapt wheat to heat stress.

Drought Stress Impacts on Plants and Different Approaches to Alleviate Its Adverse Effects

Drought stress, being the inevitable factor that exists in various environments without recognizing borders and no clear warning thereby hampering plant biomass production, quality, and energy. It is the key important environmental stress that occurs due to temperature dynamics, light intensity, and low rainfall. Despite this, its cumulative, not obvious impact and multidimensional nature severely affects the plant morphological, physiological, biochemical and molecular attributes with adverse impact on photosynthetic capacity. Coping with water scarcity, plants evolve various complex resistance and adaptation mechanisms including physiological and biochemical responses, which differ with species level. The sophisticated adaptation mechanisms and regularity network that improves the water stress tolerance and adaptation in plants are briefly discussed. Growth pattern and structural dynamics, reduction in transpiration loss through altering stomatal conductance and distribution, leaf rolling, root to shoot ratio dynamics, root length increment, accumulation of compatible solutes, enhancement in transpiration efficiency, osmotic and hormonal regulation, and delayed senescence are the strategies that are adopted by plants under water deficit. Approaches for drought stress alleviations are breeding strategies, molecular and genomics perspectives with special emphasis on the omics technology alteration i.e., metabolomics, proteomics, genomics, transcriptomics, glyomics and phenomics that improve the stress tolerance in plants. For drought stress induction, seed priming, growth hormones, osmoprotectants, silicon (Si), selenium (Se) and potassium application are worth using under drought stress conditions in plants. In addition, drought adaptation through microbes, hydrogel, nanoparticles applications and metabolic engineering techniques that regulate the antioxidant enzymes activity for adaptation to drought stress in plants, enhancing plant tolerance through maintenance in cell homeostasis and ameliorates the adverse effects of water stress are of great potential in agriculture.

Glutathione improves rice tolerance to submergence: insights into its physiological and biochemical mechanisms

Complete submergence (Sub) imposes detrimental effects on growth and survival of crop plants, including rice. Here, we investigated the beneficial effects of reduced glutathione (GSH) in mitigating Sub-induced adverse effects in two high-yielding rice cultivars BRRI dhan29 and dhan52. Both cultivars experienced growth defects, severe yellowing, necrosis and chlorosis, when they were completely immersed in water for 14 days. The poor growth performance of these cultivars was linked to biomass reduction, decreased levels of photosynthetic pigments and proline, increased levels of H₂O₂ and malondialdehyde, and declined activities of enzymatic antioxidants like superoxide dismutase, ascorbate peroxidase, peroxidase, catalase, glutathione peroxidase and glutathione S-transferase. Pretreatment with exogenous GSH led to significant growth restoration in both cultivars exposed to Sub. The elevated Sub-tolerance promoted by GSH could partly be attributed to increased levels of chlorophylls, carotenoids, soluble proteins and proline. Exogenous GSH also mitigated Sub-induced oxidative damage, as evidenced from reduced levels of H₂O₂ and malondialdehyde in accordance with the increased activities of antioxidant enzymes. Results revealed that dhan52 was more tolerant to Sub-stress than dhan29, and GSH successfully rescued both cultivars from the damage of Sub-stress. Collectively, our findings provided an insight into the GSH-mediated active recovery of rice from Sub-stress, thereby suggesting that external supply of GSH may be an effective strategy to mitigate the adverse effects of Sub in rice.

Nitric oxide and hydrogen sulfide protect plasma membrane integrity and mitigate chromium-induced methylglyoxal toxicity in maize seedlings

The present study aims to analyse the potential crosstalk between nitric oxide (NO) and hydrogen sulfide (H₂S) in triggering resilience of maize (Zea mays L.) seedlings to hexavalent chromium (Cr VI). Exogenous application of 500 μM sodium nitroprusside (SNP, as a NO donor) or sodium hydrosulfide (NaHS, as a H₂S donor) to 9-day-old maize seedlings, countered a Cr (200 μM) -elicited reduction in embryonic axis biomass. Cr caused cellular membrane injury by enhancing the levels of superoxide and hydroxyl radicals as well as methylglyoxal, and 4-hydroxy-2-nonenal. The application of SNP or NaHS considerably improved the endogenous NO and H₂S pool, decreased oxidative stress and lipid peroxidation by suppressing lipoxygenase activity and improving some antioxidant enzymes activities in radicles and epicotyls. Radicles were more affected than epicotyls by Cr-stress with enhanced electrolyte leakage and decreased proton extrusion as indicated by lesser H⁺-ATPase activity. H₂S appeared to mitigate Cr toxicity through up-regulated H⁺-ATPase and glyoxalase pathways and by maintaining optimal GSH levels as downstream effects of ROS and MG suppression. Hence, H₂S-mediated the regeneration of GSH pool is associated with the attenuation of MG toxicity by enhancing S-lactoglutathione and D-lactate production. Taken together, our results indicate complementary roles for H₂S and GSH to strengthen membrane integrity against Cr stress in maize seedlings.

Insights into the oxidative status and antioxidative responses of germinating broccoli (Brassica oleracea var. italica L.) seeds in tungstate contaminated water

The utilization of tungsten in traffic, smelting, mining, and other industrial applications allows its' accumulation in the environmental ecosystems. The present study included using a soluble form of tungsten (tungstate) at different levels (0, 1, 5, 10, 50, and 100 mg L^-1) as a water contaminant. The germinating seeds experienced tungstate at 1-50 mg L^-1 exhibited stimulation of seedling dry and fresh matter stress tolerance indices, whereas retardation of these traits at the level of 100 mg L^-1 was manifested. The stimulation of seedling growth at the levels of 1-50 mg L^-1 was associated with the regulation of reactive oxygen status, higher stability of cell membrane, and elevated level of antioxidative responses. Regarding the oxidative stress of the seedlings exposed to tungstate contaminated water, only the concentration of 100 mg L^-1 induced accumulation of hydrogen peroxide, superoxide anion, and hydroxyl radical with apparent membrane deteriorations in terms of lipid peroxidation. Furthermore, reductions of phytochelatins, reduced glutathione, ascorbate, ascorbate peroxidase, glutathione peroxidase, as well as glutathione-S-transferase were the main symptoms of tungstate phytotoxicity at the same level. The accumulation of lignin, ionic peroxidase, soluble peroxidase, and lignin-related enzymes (phenylalanine ammonia-lyase and polyphenol oxidase) were the striking reasons for restricting seedlings growth at noxious tungstate level. The results could suggest that the elevated levels of defense systems, at least in part, were accountable for raising broccoli resistance against tungstate stress at low doses. Furthermore, these plants can grow in tungsten-polluted areas by modifying their physiological processes. However, this study shed the light to the eco-toxicity of tungstate and imparts evidence for the need to establishing environmental risk management of tungstate accumulation.

Salicylic acid-induced nitric oxide enhances arsenic toxicity tolerance in maize plants by upregulating the ascorbate-glutathione cycle and glyoxalase system

The role of nitric oxide (NO) in salicylic acid (SA)-induced tolerance to arsenic (As) stress in maize plants is not reported in the literature. Before starting As stress (AsS) treatments, SA (0.5 mM) was sprayed to the foliage of maize plants. Thereafter, AsV (0.1 mM as sodium hydrogen arsenate heptahydrate) stress (AsS) was initiated and during the stress period, sodium nitroprusside (SNP 0.1 mM), a NO donor, was sprayed individually or in combination with SA. Furthermore, cPTIO (0.1 mM) was also applied as a NO scavenger during the stress period. Arsenic stress led to significant reductions in plant growth, photosynthesis, water relation parameters and endogenous NO content, but it increased hydrogen peroxide, malondialdehyde, electrolyte leakage, methylglyoxal, proline, the activities of major antioxidant enzymes, and leaf and root As content. The combined treatment of SA+SNP was more effective to reverse oxidative stress related parameters and reduce the As content in both leaves and roots, with a concomitant increase in antioxidant defense system, the ascorbate-glutathione (AsA-GSH) cycle-related enzymes, glyoxalase system enzymes, plant growth, and photosynthetic traits. The beneficial effects of SA were completely abolished with cPTIO supply by blocking the NO synthesis in AsS-maize plants, indicating that NO effectively participated in SA-improved tolerance to AsS in maize plants.

Functional characterization of the Glyoxalase-I (PdGLX1) gene family in date palm under abiotic stresses

Methylglyoxal (MG), a cytotoxic oxygenated short aldehyde, is a by-product of various metabolic reactions in plants, including glycolysis. The basal level of MG in plants is low, whereby it acts as an essential signaling molecule regulating multiple cellular processes. However, hyperaccumulation of MG under stress conditions is detrimental for plants as it inhibits multiple developmental processes, including seed germination, photosynthesis, and root growth. The evolutionarily conserved glyoxalase system is critical for MG detoxification, and it comprises of two-enzymes, the glyoxalase-I and glyoxalase-II. Here, we report the functional characterization of six putative glyoxalase-I genes from date palm (Phoenix dactylifera L.) (PdGLX1), by studying their gene expression under various environmental stress conditions and investigating their function in bacteria (Escherichia coli) and yeast (Saccharomyces cerevisiae) mutant cells. The putative PdGLX1 genes were initially identified using computational methods and cloned using molecular tools. The PdGLX1 gene expression analysis using quantitative PCR (qPCR) revealed differential expression under various stress conditions such as salinity, oxidative stress, and exogenous MG stress in a tissue-specific manner. Further, in vivo functional characterization indicated that overexpression of the putative PdGLX1 genes in E. coli enhanced their growth and MG detoxification ability. The putative PdGLX1 genes were also able to complement the loss-of-function MG hypersensitive GLO1 (YML004C) yeast mutants and promote growth by enhancing MG detoxification and reducing the accumulation of reactive oxygen species (ROS) under stress conditions as indicated by flow cytometry. These findings denote the potential importance of PdGLX1 genes in MG detoxification under stress conditions in the date palm.

Quantification of Methylglyoxal Levels in Cowpea Leaves in Response to Cowpea Aphid Infestation

Aphids are a serious pest of crops across the world. Aphids feed by inserting their flexible hypodermal needlelike mouthparts, or stylets, into their host plant tissues. They navigate their way to the phloem where they feed on its sap causing little mechanical damage to the plant. Additionally, while feeding, aphids secrete proteinaceous effectors in their saliva to alter plant metabolism and disrupt plant defenses to gain an advantage over the plant. Even with these arsenals to overcome plant responses, plants have evolved ways to detect and counter with defense responses to curtail aphid infestation. One of such response of cowpea to cowpea aphid infestation, is accumulation of the metabolite methylglyoxal. Methylglyoxal is an α,β-dicarbonyl ketoaldehyde that is toxic at high concentrations. Methylglyoxal levels increase modestly after exposure to a number of different abiotic and biotic stresses and has been shown to act as an emerging defense signaling molecule at low levels. Here we describe a protocol to measure methylglyoxal in cowpea leaves after cowpea aphid infestation, by utilizing a perchloric acid extraction process. The extracted supernatant was neutralized with potassium carbonate, and methylglyoxal was quantified through its reaction with N-acetyl-L-cysteine to form N-α-acetyl-S-(1-hydroxy-2-oxo-prop-1-yl)cysteine, a product that is quantified spectrophotometrically.

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.

Exogenous application of hydrogen sulfide reduces chromium toxicity in maize seedlings by suppressing NADPH oxidase activities and methylglyoxal accumulation

Chromium (Cr) represents an important source of metallic stress in plants. Working with maize (Zea mays) seedlings, we characterize the suppressive effects of exogenously applied NaHS (a hydrogen sulfide; [H₂S] donor) on the toxic effects of Cr (VI). Heavy metal treatment reduced radicle and epicotyl lengths and fresh weights in seedlings at 6 and 9 days following germination. The negative Cr (200 μM) effect was countered by application with NaHS (500 μM) but this countering was reduced with the co-application of the H₂S generation inhibitor hydroxylamine (HA) or the H₂S scavenger hypotaurine (HT). The Cr-elicited H₂O₂ production was suppressed by NaHS and also by an inhibitor of the reactive oxygen species (ROS) generating NADPH oxidase (NOX). These effects were correlated with relative changes in carbomyl (-CO) and thiol (-SH) groups. Nitric oxide (NO) production increased by NaHS application with associated increase in S-nitrosoglutathione (GSNO) level, but low S-nitrosoglutathione reductase (GSNOR) activities indicating an elevated S-nitrosylation. Assessment of the role of the ascorbate-glutathione antioxidant cycle indicated that whilst ascorbate played at a best minor role, glutathione was more prominent. Methylglyoxal (MG) production was increased by Cr but reduced by NaHS through a mechanism which could be based on glutathione-S-transferase (GST) detoxification. Taken together data suggest that H₂S acts to counter Cr effect in maize by NOX suppression, mostly likely by the well-characterised S-nitrosylation mechanism, as well as a reduction of MG accumulation.

Methylglyoxal Detoxification Revisited: Role of Glutathione Transferase in Model Cyanobacterium Synechocystis sp. Strain PCC 6803

In most organisms, methylglyoxal (MG), a toxic metabolite by-product that causes diabetes in humans, is predominantly detoxified by the glyoxalase enzymes. This process begins with the so-called “spontaneous” conjugation of MG with the cytoprotectant metabolite glutathione (GSH). In this study, we unravel a logical, but as yet unsuspected, link between MG detoxification and a (prokaryotic) representative of the ubiquitous glutathione transferase (GST) enzymes. We show that a GST of a model cyanobacterium plays a prominent role in the detoxification of MG in catalyzing its conjugation with GSH. This finding is important because this reaction, always regarded as nonenzymatic, could exist in plants and/or human and thus have an impact on agriculture and/or human health.

Methylglyoxal (MG) is a detrimental metabolic by-product that threatens most organisms (in humans MG causes diabetes). MG is predominantly detoxified by the glyoxalase pathway. This process begins with the conjugation of MG with glutathione (GSH), yielding a hemithioacetal product that is subsequently transformed by the glyoxalase enzymes into d-lactate and GSH. MG has been overlooked in photosynthetic organisms, although they inevitably produce it not only by the catabolism of sugars, lipids, and amino acids, as do heterotrophic organisms, but also by their active photoautotrophic metabolism. This is especially true for cyanobacteria that are regarded as having developed photosynthesis and GSH-dependent enzymes to detoxify the reactive oxygen species produced by their photosynthesis (CO₂ assimilation) and respiration (glucose catabolism), which they perform in the same cell compartment. In this study, we used a combination of in vivo and in vitro approaches to characterize a logical, but as yet never described, link between MG detoxification and a (prokaryotic) representative of the evolutionarily conserved glutathione transferase (GST) detoxification enzymes. We show that the Sll0067 GST of the model cyanobacterium Synechocystis sp. strain PCC 6803 plays a prominent role in MG tolerance and detoxification, unlike the other five GSTs of this organism. Sll0067 catalyzes the conjugation of MG with GSH to initiate its elimination driven by glyoxalases. These results are novel because the conjugation of MG with GSH is always described as nonenzymatic. They will certainly stimulate the analysis of Sll0067 orthologs from other organisms with possible impacts on human health (development of biomarkers or drugs) and/or agriculture.

Reassessing plant glyoxalases: large family and expanding functions

Methylglyoxal (MG), a reactive carbonyl compound, is generated during metabolism in living systems. However, under stress, its levels increase rapidly leading to cellular toxicity. Although the generation of MG is spontaneous in a cell, its detoxification is essentially catalyzed by the glyoxalase enzymes. In plants, modulation of MG content via glyoxalases influences diverse physiological functions ranging from regulating growth and development to conferring stress tolerance. Interestingly, there has been a preferred expansion in the number of isoforms of these enzymes in plants, giving them high plasticity in their actions for accomplishing diverse roles. Future studies need to focus on unraveling the interplay of these multiple isoforms of glyoxalases possibly contributing towards the unique adaptability of plants to diverse environments.

Salicylic acid antagonizes selenium phytotoxicity in rice: selenium homeostasis, oxidative stress metabolism and methylglyoxal detoxification

We investigated the mechanistic consequences of selenium (Se)-toxicity, and its possible mitigation using salicylic acid (SA) in rice. In comparison with control, sodium selenate-exposed 'Se1' (0.5 mM) and 'Se2' (1.0 mM) plants showed accumulation of Se by 190.63 and 288.00 % in roots, 2359.42 and 2054.35 % in leaf sheaths, and 7869.91 and 9063.72 % in leaves, respectively, resulting in severe toxicity symptoms, such as growth inhibition, chlorosis, burning of leaves, and oxidative stress. In contrast, SA addition to Se-stressed plants significantly alleviated the Se-toxicity symptoms, and radically improved shoot height (28.88 %), dry biomass (34.00 %), total chlorophyll (37.51 %), soluble sugar (17.31 %) and leaf water contents (22.31 %) in 'SA + Se2' plants over 'Se2' plants. Notably, SA maintained Se-homeostasis, and decreased 'Se2'-induced oxidative stress by enhancing ascorbate level (67.75 %) and the activities of antioxidant enzymes like superoxide dismutase (20.99 %), catalase (40.97 %), glutathione peroxidase (12.26 %), and glutathione reductase (32.58 %) relative to that in 'Se2' plants. Additionally, SA protected rice plants from the deleterious effects of methylglyoxal by stimulating the activities of glyoxalase enzymes. Furthermore, SA upregulated several genes associated with reactive oxygen species (e.g. OsCuZnSOD1, OsCATB, OsGPX1 and OsAPX2) and methylglyoxal (e.g. OsGLYI-1) detoxifications. These findings unravel a decisive role of SA in alleviating Se-phytotoxicity in rice.

Involvement of L-Cysteine Desulfhydrase and Hydrogen Sulfide in Glutathione-Induced Tolerance to Salinity by Accelerating Ascorbate-Glutathione Cycle and Glyoxalase System in Capsicum

The aim of this study is to assess the role of l-cysteine desulfhydrase (l-DES) and endogenous hydrogen sulfide (H₂S) in glutathione (GSH)-induced tolerance to salinity stress (SS) in sweet pepper (Capsicum annuum L.). Two weeks after germination, before initiating SS, half of the pepper seedlings were retained for 12 h in a liquid solution containing H₂S scavenger, hypotaurine (HT), or the l-DES inhibitor dl-propargylglycine (PAG). The seedlings were then exposed for three weeks to control or SS (100 mmol L⁻¹ NaCl) and supplemented with or without GSH or GSH+NaHS (sodium hydrosulfide, H₂S donor). Salinity suppressed dry biomass, leaf water potential, chlorophyll contents, maximum quantum efficiency, ascorbate, and the activities of dehydroascorbate reductase, monodehydroascorbate reductase, and glyoxalase II in plants. Contrarily, it enhanced the accumulation of hydrogen peroxide, malondialdehyde, methylglyoxal, electrolyte leakage, proline, GSH, the activities of glutathione reductase, peroxidase, catalase, superoxide dismutase, ascorbate peroxidase, glyoxalase I, and l-DES, as well as endogenous H₂S content. Salinity enhanced leaf Na⁺ but reduced K⁺; however, the reverse was true with GSH application. Overall, the treatments, GSH and GSH+NaHS, effectively reversed the oxidative stress and upregulated salt tolerance in pepper plants by controlling the activities of the AsA-GSH and glyoxalase-system-related enzymes as well as the levels of osmolytes.

Transcriptomic and proteomic choreography in response to light quality variation reveals key adaption mechanisms in marine Nannochloropsis oceanica

Photosynthetic organisms need to respond frequently to the fluctuation of light quality and light quantity in their habitat. In response to the fluctuation of different single wavelength lights, these organisms have to adjust and optimize the employment of light energy by redistributing excitation energy and remodeling photosystem stoichiometry or light complex structure. However, the response of whole cellular processes to fluctuations in single wavelength light is mostly unknown. Here, we report the transcriptomic and proteomic dynamics and metabolic adaptation mechanisms of Nannochloropsis oceanica to blue and red light. Preferential exposure to different light spectra induces massive reprogramming of the Nannochloropsis transcriptome and proteome. Combined with physiological and biochemical investigation, the rewiring of many cellular processes was observed, including carbon/nitrogen assimilation, photosynthesis, chlorophyll and cartenoid biosynthesis, reactive oxygen species (ROS) scavenging systems, and chromatin state regulation. A strong and rapid regulation of genes or proteins related to nitrogen metabolism, photosynthesis, chlorophyll synthesis, ROS scavenging system, and carotenoid metabolism were observed during 12 h and 24 h of exposure under red light. Additionally, two light harvesting complex proteins induced by blue light and one by red light were observed. The differential responses of N. oceanica to red and blue irradiation reveal how marine microalgae adapt to change in light quality and can be exploited for biofuel feedstock development.

AcDCXR Is a Cowpea Aphid Effector With Putative Roles in Altering Host Immunity and Physiology

Cowpea, Vigna unguiculata, is a crop that is essential to semiarid areas of the world like Sub-Sahara Africa. Cowpea is highly susceptible to cowpea aphid, Aphis craccivora, infestation that can lead to major yield losses. Aphids feed on their host plant by inserting their hypodermal needlelike flexible stylets into the plant to reach the phloem sap. During feeding, aphids secrete saliva, containing effector proteins, into the plant to disrupt plant immune responses and alter the physiology of the plant to their own advantage. Liquid chromatography tandem mass spectrometry (LC-MS/MS) was used to identify the salivary proteome of the cowpea aphid. About 150 candidate proteins were identified including diacetyl/L-xylulose reductase (DCXR), a novel enzyme previously unidentified in aphid saliva. DCXR is a member of short-chain dehydrogenases/reductases with dual enzymatic functions in carbohydrate and dicarbonyl metabolism. To assess whether cowpea aphid DCXR (AcDCXR) has similar functions, recombinant AcDCXR was purified and assayed enzymatically. For carbohydrate metabolism, the oxidation of xylitol to xylulose was tested. The dicarbonyl reaction involved the reduction of methylglyoxal, an α-β-dicarbonyl ketoaldehyde, known as an abiotic and biotic stress response molecule causing cytotoxicity at high concentrations. To assess whether cowpea aphids induce methylglyoxal in plants, we measured methylglyoxal levels in both cowpea and pea (Pisum sativum) plants and found them elevated transiently after aphid infestation. Agrobacterium-mediated transient overexpression of AcDCXR in pea resulted in an increase of cowpea aphid fecundity. Taken together, our results indicate that AcDCXR is an effector with a putative ability to generate additional sources of energy to the aphid and to alter plant defense responses. In addition, this work identified methylglyoxal as a potential novel aphid defense metabolite adding to the known repertoire of plant defenses against aphid pests.

Selenite antagonizes the phytotoxicity of Cd in the cattail Typha angustifolia

The Phytotoxicity of and mechanism underlying selenite-mediated tolerance to Cd stress in Typha angustifolia were studied hydroponically with respect to metal uptake and translocation, photosynthesis-related parameters, contents of proline and O₂^•-, products of lipid peroxidation, cell viability, enzymatic and non-enzymatic antioxidants, glyoxalases and phytochelatins. T. angustifolia were exposed to 25, 50 and 100 μM of Cd alone and in conjunction with 5 mg L^-1 of selenite in full-strength Hoagland's nutrient solution for 30 days. Results showed that Cd contents in T. angustifolia leaves and roots increased in a dose-dependent manner and were higher in roots, but those of BAC, BCF and TF changed in a contrary pattern. Addition of selenite to Cd-containing treatments further reduced Cd levels in T. angustifolia leaves and roots, as well as BAC, BCF and TF. A diphasic effect was found in T. angustifolia for the contents of total chlorophyll, GSH, PC and GSSG, as well as activities of CAT, POD, SOD and GR, in response to Cd stress alone and in conjunction with selenite supplementation, but the same effect was not observed for Pn, Cond, Tr, Ci, Fv/Fm and ϕPSII. In contrast, exogenous selenite supplementation enhanced the contents of total chlorophyll and the non-enzymatic antioxidants, as well as activities of enzymatic antioxidants, while the values of photosynthetic fluorescence parameters were rescued. Selenite addition decreased Cd-induced cell death. Proline contents and Gly I activities in T. angustifolia leaves kept increasing in a dose-dependent manner of Cd concentrations in the growth media and selenite addition further enhanced both parameters. Addition of selenite could quench Cd-mediated generation of MDA, O₂^•- and MG in T. angustifolia leaves and reduce Cd-induced Gly II activity. A U-shaped GSH/GSSG ratio in T. angustifolia leaves suggests a possible trade-off between PC synthesis and GR activity since both share the same substrate GSH. Therefore, confined BAC, BCF and TF were a mechanism that confers T. angustifolia tolerance to Cd stress, and that exogenous selenite supplementation could depress Cd-induced stress in T. angustifolia by rescuing the photosynthetic fluorescence, enhancing non-enzymatic and enzymatic antioxidants that scavenge O₂^•- and MG, and potentiating PC synthesis that chelates Cd.

Interactive Effects of Salicylic Acid and Nitric Oxide in Enhancing Rice Tolerance to Cadmium Stress

Cadmium (Cd) is one of the prominent environmental hazards, affecting plant productivity and posing human health risks worldwide. Although salicylic acid (SA) and nitric oxide (NO) are known to have stress mitigating roles, little was explored on how they work together against Cd-toxicity in rice. This study evaluated the individual and combined effects of SA and sodium nitroprusside (SNP), a precursor of NO, on Cd-stress tolerance in rice. Results revealed that Cd at toxic concentrations caused rice biomass reduction, which was linked to enhanced accumulation of Cd in roots and leaves, reduced photosynthetic pigment contents, and decreased leaf water status. Cd also potentiated its phytotoxicity by triggering reactive oxygen species (ROS) generation and depleting several non-enzymatic and enzymatic components in rice leaves. In contrast, SA and/or SNP supplementation with Cd resulted in growth recovery, as evidenced by greater biomass content, improved leaf water content, and protection of photosynthetic pigments. These signaling molecules were particularly effective in restricting Cd uptake and accumulation, with the highest effect being observed in “SA + SNP + Cd” plants. SA and/or SNP alleviated Cd-induced oxidative damage by reducing ROS accumulation and malondialdehyde production through the maintenance of ascorbate and glutathione levels, and redox status, as well as the better activities of antioxidant enzymes superoxide dismutase, catalase, glutathione S-transferase, and monodehydroascorbate reductase. Combined effects of SA and SNP were observed to be more prominent in Cd-stress mitigation than the individual effects of SA followed by that of SNP, suggesting that SA and NO in combination more efficiently boosted physiological and biochemical responses to alleviate Cd-toxicity than either SA or NO alone. This finding signifies a cooperative action of SA and NO in mitigating Cd-induced adverse effects in rice, and perhaps in other crop plants.

PpAKR1A, a Novel Aldo-Keto Reductase from Physcomitrella Patens, Plays a Positive Role in Salt Stress

The moss Physcomitrella patens is tolerant of highly saline environments. In plants, salinity stress may induce the production of toxic reactive carbonyl species (RCS) and oxidative damage. Aldo-keto reductases (AKRs) are a large group of NADP-dependent oxidoreductases involved in RCS detoxification. However, many members in this superfamily remain uncharacterized. In this study, we cloned and characterised a putative AKR1 from P. patens, named PpAKR1A. Notably, the transcription level of PpAKR1A was induced by salt and methylglyoxal (MG) stress, and the recombinant PpAKR1A protein catalysed the reduction of toxic aldehydes. PpAKR1A knockout mutants of P. patens (ppakr1a) were sensitive to NaCl and MG treatment, as indicated by much lower concentrations of chlorophyll and much higher concentrations of MG and H₂O₂ than those in WT plants. Meanwhile, ppakr1a plants exhibited decreases in the MG-reducing activity and reactive oxygen species-scavenging ability in response to salt stress, possibly due to decreases in the activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD). Our results indicate that PpAKR1A is an aldo-keto reductase that detoxifies MG and thus plays an important role in salt stress tolerance in P. patens.

Acetic acid: a cost-effective agent for mitigation of seawater-induced salt toxicity in mung bean

The current study sought the effective mitigation measure of seawater-induced damage to mung bean plants by exploring the potential roles of acetic acid (AA). Principal component analysis (PCA) revealed that foliar application of AA under control conditions improved mung bean growth, which was interlinked to enhanced levels of photosynthetic rate and pigments, improved water status and increased uptake of K+, in comparison with water-sprayed control. Mung bean plants exposed to salinity exhibited reduced growth and biomass production, which was emphatically correlated with increased accumulations of Na+, reactive oxygen species and malondialdehyde, and impaired photosynthesis, as evidenced by PCA and heatmap clustering. AA supplementation ameliorated the toxic effects of seawater, and improved the growth performance of salinity-exposed mung bean. AA potentiated several physio-biochemical mechanisms that were connected to increased uptake of Ca2+ and Mg2+, reduced accumulation of toxic Na+, improved water use efficiency, enhanced accumulations of proline, total free amino acids and soluble sugars, increased catalase activity, and heightened levels of phenolics and flavonoids. Collectively, our results provided new insights into AA-mediated protective mechanisms against salinity in mung bean, thereby proposing AA as a potential and cost-effective chemical for the management of salt-induced toxicity in mung bean, and perhaps in other cash crops.

Salicylic acid enhances nickel stress tolerance by up-regulating antioxidant defense and glyoxalase systems in mustard plants

The present study identified inverse relationships between nickel (Ni) levels and growth, photosynthesis and physio-biochemical attributes, but increasing levels of Ni stress enhanced methylglyoxal, electrolyte leakage, hydrogen peroxide, and lipid peroxidation content. Exogenous application of salicylic acid (SA) (10^-5 M) ameliorated the ill-effects of Ni by restoring growth, photosynthesis and physio-biochemical attributes and increasing the activities of enzymes associated with antioxidant systems, especially the ascorbate-glutathione (AsA-GSH) cycle and glyoxalase system. In addition, SA application to Ni-stressed plants had an additive effect on the activities of the ascorbate and glutathione pools, and the AsA-GSH cycle enzymes (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase), superoxide dismutase, catalase, glutathione S-transferase, and osmolyte biosynthesis). This trend also follows in glyoxalase system viz. glyoxalase I and glyoxalase II enzymes. Nevertheless, exogenous SA supplementation restored mineral nutrient contents. Principal component analysis showed that growth, photosynthesis, and mineral nutrient parameters were positively correlated with each other and negatively correlated with antioxidant enzymes and oxidative stress biomarkers. Hence, SA is an alternative compound with potential application in the phytoremediation of Ni.

Dissecting Adaptation Mechanisms to Contrasting Solar Irradiance in the Mediterranean Shrub Cistus incanus

Molecular mechanisms that are the base of the strategies adopted by Mediterranean plants to cope with the challenges imposed by limited or excessive solar radiation during the summer season have received limited attention. In our study, conducted on C. incanus plants growing in the shade or in full sunlight, we performed measurements of relevant physiological traits, such as leaf water potential, gas exchange and PSII photochemistry, RNA-Seq with de-novo assembly, and the analysis of differentially expressed genes. We also identified and quantified photosynthetic pigments, abscisic acid, and flavonoids. Here, we show major mechanisms regulating light perception and signaling which, in turn, sustain the shade avoidance syndrome displayed by the 'sun loving' C. incanus. We offer clear evidence of the detrimental effects of excessive light on both the assembly and the stability of PSII, and the activation of a suite of both repair and effective antioxidant mechanisms in sun-adapted leaves. For instance, our study supports the view of major antioxidant functions of zeaxanthin in sunny plants concomitantly challenged by severe drought stress. Finally, our study confirms the multiple functions served by flavonoids, both flavonols and flavanols, in the adaptive mechanisms of plants to the environmental pressures associated to Mediterranean climate.

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.

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.

Metabolomic analysis of date palm seedlings exposed to salinity and silicon treatments

Silicon is known to promote plant growth as well as stress tolerance of plants. The current study was undertaken to assess the growth promoting effect of silicon on date palm seedling development as well as its ability to abate some of the negative effects of salinity. In this study, date palm seedlings were treated with silicon and sodium chloride salts, and the effect of these salts on some physiological parameters of the plants was determined. In addition, a global nontargeted metabolomics analysis was performed for the leaf and root tissues using liquid chromatography-mass spectrometry (LC-MS). The results showed that under non-stress conditions, silicon treatment enhanced the growth of the date palm seedlings, however, under salinity, silicon slightly mitigates the negative effects of salt stress on the date palm seedlings although it enhances the potassium accumulation under this condition. The global metabolomics analysis has identified a total of 1,101 significant differentially accumulated (p, q ≤ 0.05) metabolites in leaves and roots under silicon, salinity or their combination. A differential pairwise metabolic profile comparison revealed the accumulation of distinct metabolites in response to silicon and salinity treatments such as antioxidant compounds pyridoxine, cepharanthine, allithiamine, myristic acid and boldine; osmoregulators such as mucic acid; along with the accumulation of detoxification intermediates such as S-D-lactoylglutathione, beta-cyano-L-alanine and gamma-glutamyl-conjugates. In addition, histochemical analyses revealed that application of silicon significantly (p ≤ 0.05) enhanced the formation of the Casparian strip. Identification of the differentially accumulated metabolites could offer an insight into how silicon is able to promote growth and salinity tolerance in date palms.

Abiotic Stresses: General Defenses of Land Plants and Chances for Engineering Multistress Tolerance

Abiotic stresses, such as low or high temperature, deficient or excessive water, high salinity, heavy metals, and ultraviolet radiation, are hostile to plant growth and development, leading to great crop yield penalty worldwide. It is getting imperative to equip crops with multistress tolerance to relieve the pressure of environmental changes and to meet the demand of population growth, as different abiotic stresses usually arise together in the field. The feasibility is raised as land plants actually have established more generalized defenses against abiotic stresses, including the cuticle outside plants, together with unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solutes inside cells. In stress response, they are orchestrated by a complex regulatory network involving upstream signaling molecules including stress hormones, reactive oxygen species, gasotransmitters, polyamines, phytochromes, and calcium, as well as downstream gene regulation factors, particularly transcription factors. In this review, we aimed at presenting an overview of these defensive systems and the regulatory network, with an eye to their practical potential via genetic engineering and/or exogenous application.

Strigolactones in plant adaptation to abiotic stresses: An emerging avenue of plant research

Phytohormones play central roles in boosting plant tolerance to environmental stresses, which negatively affect plant productivity and threaten future food security. Strigolactones (SLs), a class of carotenoid-derived phytohormones, were initially discovered as an "ecological signal" for parasitic seed germination and establishment of symbiotic relationship between plants and beneficial microbes. Subsequent characterizations have described their functional roles in various developmental processes, including root development, shoot branching, reproductive development, and leaf senescence. SLs have recently drawn much attention due to their essential roles in the regulation of various physiological and molecular processes during the adaptation of plants to abiotic stresses. Reports suggest that the production of SLs in plants is strictly regulated and dependent on the type of stresses that plants confront at various stages of development. Recently, evidence for crosstalk between SLs and other phytohormones, such as abscisic acid, in responses to abiotic stresses suggests that SLs actively participate within regulatory networks of plant stress adaptation that are governed by phytohormones. Moreover, the prospective roles of SLs in the management of plant growth and development under adverse environmental conditions have been suggested. In this review, we provide a comprehensive discussion pertaining to SL-mediated plant responses and adaptation to abiotic stresses.