A unique Ni2+ -dependent and methylglyoxal-inducible rice glyoxalase I possesses a single active site and functions in abiotic stress response

Rapid detection and imaging of methylglyoxal in plant tissues by the near-infrared fluorescent probe SWJT-2

Methylglyoxal (MG) can be produced via various pathways in plants. MG is toxic for plant cells at high levels, however it acts as a signaling molecule at low levels, just as H2O2 in plants. Therefore, MG detection is very important for investigating its roles in plant cells, especially in plants under environmental stresses. The near-infrared fluorescent probe SWJT-2 is a novel probe with high sensitivity for the rapid detection of MG in human HeLa cells, but at present it is not clear whether the probe can be used to determine MG levels in plant tissues. In this present research, we tried to apply the probe in plant research. Our results showed that 40 min treatment of SWJT-2 (80 μM) can be applied to the detection and imaging of MG levels in tobacco (Nicotiana benthamiana) tissues.

Tracing the intraspecies expansion of glyoxalase genes and their expanding roles across the genus Oryza

The genus Oryza is of utmost importance to human civilization as two of its species became agronomically productive and widely cultivated, and also because wild rice is a treasure trove of beneficial alleles that can be used for crop improvement. Most of the wild rice genotypes are known for their stress tolerance several times more than the domesticated rice varieties. In this study, we aimed to carry out an exhaustive genomic survey to identify glyoxalase I (GLYI) and glyoxalase II (GLYII) genes across the 11 rice genomes sequenced so far. Notably, we found the putatively functional metal-dependent GLYI and GLYII enzymes to be conserved throughout domestication and a few homologous pairs to have undergone beneficial mutations to drive positive selection, and thus, acquire newer functions. Interestingly, we also report four newly identified GLYII members in O. sativa subsp. japonica in addition to the three previously reported GLYII genes. The presence of different types of cis-elements in the promoter region of the glyoxalase genes gives insights into their role and regulation under various developmental processes besides stress adaptation. Publicly available data suggests the role of glyoxalase genes particularly in salinity stress in both wild and cultivated rice as is also confirmed through qRT-PCR. Interestingly, we found less accumulation of MG and concurrently higher enzymatic activity of GLYI and GLYII proteins in stressed seedlings of selected wild rice genotypes indicating that glyoxalases indeed contribute to the intrinsic stress tolerance of wild rice.

Genomic identification, characterization, and stress-induced expression profiling of glyoxalase and D-lactate dehydrogenase gene families in Capsicum annuum

BACKGROUND

Capsicum annuum, a significant agricultural and nutritional crop, faces production challenges due to its sensitivity to various abiotic stresses. Glyoxalase (GLY) and D-lactate dehydrogenase (D-LDH) enzymes play vital roles in mitigating these stresses by detoxifying the stress-induced cytotoxin, methylglyoxal (MG).

METHODS

A genome-wide study was conducted to identify and characterize glyoxalase I (GLYI), glyoxalase II (GLYII), unique glyoxalase III or DJ-1 (GLYIII), and D-LDH gene candidates in Capsicum annuum. The identified members were evaluated based on their evolutionary relationships with known orthologues, as well as their gene and protein features. Their expression patterns were examined in various tissues, developmental stages, and in response to abiotic stress conditions using RNA-seq data and qRT-PCR.

RESULTS

A total of 19 GLYI, 9 GLYII, 3 DJ-1, and 11 D-LDH members were identified, each featuring characteristic domains: glyoxalase, metallo-β-lactamase, DJ-1_PfpI, and FAD_binding_4, respectively. Phylogenetic analysis revealed distinct clades depending on functional diversification. Expression profiling demonstrated significant variability under stress conditions, underscoring their potential roles in stress modulation. Notably, gene-specific responses were observed with CaGLYI-2, CaGLYI-7, CaGLYII-6, CaDJ-1 A, and CaDLDH-1 showed upregulation under salinity, drought, oxidative, heat, and cold stresses, while downregulation were shown for CaGLYI-3, CaGLYII-1, CaDJ-1B, and CaDJ-1 C. Remarkably, CaGLYI-1 presented a unique expression pattern, upregulated against drought and salinity but downregulated under oxidative, heat, and cold stress.

CONCLUSION

The identified GLY and D-LDH gene families in Capsicum annuum exhibited differential expression patterns under different abiotic stresses. Specifically, CaGLYI-2, CaGLYI-7, CaGLYII-6, CaDJ-1 A, and CaDLDH-1 were upregulated in response to all five analyzed abiotic stressors, highlighting their critical role in stress modulation amidst climate change. This study enhances our understanding of plant stress physiology and opens new avenues for developing stress-resilient crop varieties, crucial for sustainable agriculture.

Genome-Wide Analysis and Expression Profiling of Glyoxalase Gene Families Under Abiotic Stresses in Cucumber (Cucumis sativus L.)

The glyoxalase pathway, consisting of glyoxalase I (GLYI) and glyoxalase II (GLYII), is an enzymatic system that converts cytotoxic methylglyoxal to non-toxic S-D-lactoylglutathione. Although the GLY gene family has been analyzed in Arabidopsis, rice, grape, cabbage, and soybean, cucumber studies are lacking. Here, we analyzed the cucumber GLY gene family, identifying 13 CsGLYI and 2 CsGLYII genes. Furthermore, we investigated the physicochemical properties, phylogenetic relationships, chromosomal localization and colinearity, gene structure, conserved motifs, cis-regulatory elements, and protein-protein interaction networks of the CsGLY family. They were primarily localized in the cytoplasm, chloroplasts, and mitochondria, with a minor presence in the nucleus. The classification of CsGLYI and CsGLYII genes into five classes closely resembled the homologous genes in Arabidopsis and soybean. Additionally, hormone-responsive elements dominated the promoter region of GLY genes, alongside light- and stress-responsive elements. The predicted interaction proteins of CsGLYIs and CsGLYIIs exerted a significant role in cellular respiration, amino acid synthesis, and metabolism, as well as methylglyoxal catabolism. In addition, the expression profiles of GLY genes were distinct in different tissues of cucumber as well as under diverse abiotic stresses. This study is conducive to the further exploration of the functional diversity among glyoxalase genes and the mechanisms of stress responses in cucumber.

Profiling of Key Hub Genes Using a Two-State Weighted Gene Co-Expression Network of 'Jao Khao' Rice under Soil Salinity Stress Based on Time-Series Transcriptome Data

RNA-sequencing enables the comprehensive detection of gene expression levels at specific time points and facilitates the identification of stress-related genes through co-expression network analysis. Understanding the molecular mechanisms and identifying key genes associated with salt tolerance is crucial for developing rice varieties that can thrive in saline environments, particularly in regions affected by soil salinization. In this study, we conducted an RNA-sequencing-based time-course transcriptome analysis of 'Jao Khao', a salt-tolerant Thai rice variety, grown under normal or saline (160 mM NaCl) soil conditions. Leaf samples were collected at 0, 3, 6, 12, 24, and 48 h. In total, 36 RNA libraries were sequenced. 'Jao Khao' was found to be highly salt-tolerant, as indicated by the non-significant differences in relative water content, cell membrane stability, leaf greenness, and chlorophyll fluorescence over a 9-day period under saline conditions. Plant growth was slightly retarded during days 3-6 but recovered by day 9. Based on time-series transcriptome data, we conducted differential gene expression and weighted gene co-expression network analyses. Through centrality change from normal to salinity network, 111 key hub genes were identified among 1,950 highly variable genes. Enriched genes were involved in ATP-driven transport, light reactions and response to light, ATP synthesis and carbon fixation, disease resistance and proteinase inhibitor activity. These genes were upregulated early during salt stress and RT-qPCR showed that 'Jao Khao' exhibited an early upregulation trend of two important genes in energy metabolism: RuBisCo (LOC_Os10g21268) and ATP synthase (LOC_Os10g21264). Our findings highlight the importance of managing energy requirements in the initial phase of the plant salt-stress response. Therefore, manipulation of the energy metabolism should be the focus in plant resistance breeding and the genes identified in this work can serve as potentially effective candidates.

The Maize Gene ZmGLYI-8 Confers Salt and Drought Tolerance in Transgenic Arabidopsis Plants

Methylglyoxal (MG), a highly reactive and cytotoxic α-oxoaldehyde compound, can over-accumulate under abiotic stress, consequently injuring plants or even causing death. Glyoxalase I (GLYI), the first enzyme of the glyoxalase pathway, plays multiple roles in the detoxification of MG and in abiotic stress responses. However, the GLY1 gene in maize has been little studied in response to abiotic stress. In this study, we screened a glyoxalase I gene (ZmGLYI-8) and overexpressed in Arabidopsis. This gene was localized in the cytoplasm and can be induced in maize seedlings under multiple stress treatments, including salt, drought, MG, ABA, H2O2 and high temperature stress. Phenotypic analysis revealed that after MG, salt and drought stress treatments, overexpression of ZmGLYI-8 increased the tolerance of transgenic Arabidopsis to MG, salt and drought stress. Furthermore, we demonstrated that the overexpression of ZmGLYI-8 scavenges accumulated reactive oxygen species, detoxifies MG and enhances the activity of antioxidant enzymes to improve the resistance of transgenic Arabidopsis plants to salt and drought stress. In summary, this study preliminarily elucidates the molecular mechanism of the maize ZmGLYI-8 gene in transgenic Arabidopsis and provides new insight into the breeding of salt- and drought-tolerant maize varieties.

OsLdh7, a rice lactate dehydrogenase, confers stress resilience in rice under cadmium stress through NAD+/NADH regulation

Lactate dehydrogenase (Ldh, EC 1.1.1.27), an oxidoreductase enzyme catalyses the interconversion of pyruvate to L-lactate and vice-versa with concomitant oxidation and reduction of NADH and NAD+. The enzyme functions as a ROS sensor and mitigates stress response by maintaining NAD+/NADH homeostasis. In this study, we delineated the role of the Ldh enzyme in imparting cadmium stress tolerance in rice. Previously, we identified a putatively active Ldh in rice (OsLdh7) through insilico modelling. Biochemical characterization of the OsLdh7 enzyme revealed it to be optimally active at pH 6.6 in the forward direction and pH 9 in the reverse direction. Overexpression of OsLdh7 in rice cv. IR64, increased tolerance of the transgenic lines to cadmium stress compared to the wild type (WT) at both seedling and reproductive stages. The transgenic lines showed increased enzyme activity in the reverse direction under cadmium stress, attributed to elevated cytosolic pH resulting from increased calcium concentration. This increased NADH content is highly essential for functioning of the ROS scavenging enzymes, RbohD and MPK6. qPCR analysis revealed that the overexpression lines had increased transcript abundance of these genes indicating an effective ROS scavenging mechanism. Additionally, the overexpression lines showed an efficient cadmium sequestration mechanism compared to the WT by increasing the transcript levels of the vacuolar transporters of cadmium as well as total phytochelatin content. Thus, our findings indicated OsLdh7 imparts cadmium stress tolerance in rice through a two-pronged approach by mitigating ROS and sequestering cadmium ions, highlighting its potential for crop improvement programs.

Methylglyoxal detoxifying gene families in tomato: Genome-wide identification, evolution, functional prediction, and transcript profiling

Methylglyoxal (MG) is a highly cytotoxic molecule produced in all biological systems, which could be converted into non-toxic D-lactate by an evolutionarily conserved glyoxalase pathway. Glutathione-dependent glyoxalase I (GLYI) and glyoxalase II (GLYII) are responsible for the detoxification of MG into D-lactate in sequential reactions, while DJ-1 domain containing glyoxalase III (GLYIII) catalyzes the same reaction in a single step without glutathione dependency. Afterwards, D-lactate dehydrogenase (D-LDH) converts D-lactate into pyruvate, a metabolically usable intermediate. In the study, a comprehensive genome-wide investigation has been performed in one of the important vegetable plants, tomato to identify 13 putative GLYI, 4 GLYII, 3 GLYIII (DJ-1), and 4 D-LDH genes. Expression pattern analysis using microarray data confirmed their ubiquitous presence in different tissues and developmental stages. Moreover, stress treatment of tomato seedlings and subsequent qRT-PCR demonstrated upregulation of SlGLYI-2, SlGLYI-3, SlGLYI-6A, SlGLYII-1A, SlGLYII-3B, SlDJ-1A, SlDLDH-1 and SlDLDH-4 in response to different abiotic stresses, whereas SlGLYI-6B, SlGLYII-1B, SlGLYII-3A, SlDJ-1D and SlDLDH-2 were downregulated. Expression data also revealed SlGLYII-1B, SlGLYI-1A, SlGLYI-2, SlDJ-1D, and SlDLDH-4 were upregulated in response to various pathogenic infections, indicating the role of MG detoxifying enzymes in both plant defence and stress modulation. The functional characterization of each of these members could lay the foundation for the development of stress and disease-resistant plants promoting sustainable agriculture and production.

Linking the key physiological functions of essential micronutrients to their deficiency symptoms in plants

In this review, we untangle the physiological key functions of the essential micronutrients and link them to the deficiency responses in plants. Knowledge of these responses at the mechanistic level, and the resulting deficiency symptoms, have improved over the last decade and it appears timely to review recent insights for each of them. A proper understanding of the links between function and symptom is indispensable for an accurate and timely identification of nutritional disorders, thereby informing the design and development of sustainable fertilization strategies. Similarly, improved knowledge of the molecular and physiological functions of micronutrients will be important for breeding programmes aiming to develop new crop genotypes with improved nutrient-use efficiency and resilience in the face of changing soil and climate conditions.

Decrypting Molecular Mechanisms Involved in Counteracting Copper and Nickel Toxicity in Jack Pine (Pinus banksiana) Based on Transcriptomic Analysis

The remediation of copper and nickel-afflicted sites is challenged by the different physiological effects imposed by each metal on a given plant system. Pinus banksiana is resilient against copper and nickel, providing an opportunity to build a valuable resource to investigate the responding gene expression toward each metal. The objectives of this study were to (1) extend the analysis of the Pinus banksiana transcriptome exposed to nickel and copper, (2) assess the differential gene expression in nickel-resistant compared to copper-resistant genotypes, and (3) identify mechanisms specific to each metal. The Illumina platform was used to sequence RNA that was extracted from seedlings treated with each of the metals. There were 449 differentially expressed genes (DEGs) between copper-resistant genotypes (RGs) and nickel-resistant genotypes (RGs) at a high stringency cut-off, indicating a distinct pattern of gene expression toward each metal. For biological processes, 19.8% of DEGs were associated with the DNA metabolic process, followed by the response to stress (13.15%) and the response to chemicals (8.59%). For metabolic function, 27.9% of DEGs were associated with nuclease activity, followed by nucleotide binding (27.64%) and kinase activity (10.16%). Overall, 21.49% of DEGs were localized to the plasma membrane, followed by the cytosol (16.26%) and chloroplast (12.43%). Annotation of the top upregulated genes in copper RG compared to nickel RG identified genes and mechanisms that were specific to copper and not to nickel. NtPDR, AtHIPP10, and YSL1 were identified as genes associated with copper resistance. Various genes related to cell wall metabolism were identified, and they included genes encoding for HCT, CslE6, MPG, and polygalacturonase. Annotation of the top downregulated genes in copper RG compared to nickel RG revealed genes and mechanisms that were specific to nickel and not copper. Various regulatory and signaling-related genes associated with the stress response were identified. They included UGT, TIFY, ACC, dirigent protein, peroxidase, and glyoxyalase I. Additional research is needed to determine the specific functions of signaling and stress response mechanisms in nickel-resistant plants.

Role of methylglyoxal and glyoxalase in the regulation of plant response to heavy metal stress

KEY MESSAGE

Methylglyoxal and glyoxalase function a significant role in plant response to heavy metal stress. We update and discuss the most recent developments of methylglyoxal and glyoxalase in regulating plant response to heavy metal stress. Methylglyoxal (MG), a by-product of several metabolic processes, is created by both enzymatic and non-enzymatic mechanisms. It plays an important role in plant growth and development, signal transduction, and response to heavy metal stress (HMS). Changes in MG content and glyoxalase (GLY) activity under HMS imply that they may be potential biomarkers of plant stress resistance. In this review, we summarize recent advances in research on the mechanisms of MG and GLY in the regulation of plant responses to HMS. It has been discovered that appropriate concentrations of MG assist plants in maintaining a balance between growth and development and survival defense, therefore shielding them from heavy metal harm. MG and GLY regulate plant physiological processes by remodeling cellular redox homeostasis, regulating stomatal movement, and crosstalking with other signaling molecules (including abscisic acid, gibberellic acid, jasmonic acid, cytokinin, salicylic acid, melatonin, ethylene, hydrogen sulfide, and nitric oxide). We also discuss the involvement of MG and GLY in the regulation of plant responses to HMS at the transcriptional, translational, and metabolic levels. Lastly, considering the current state of research, we present a perspective on the future direction of MG research to elucidate the MG anti-stress mechanism and offer a theoretical foundation and useful advice for the remediation of heavy metal-contaminated environments in the future.

A glutathione-independent DJ-1/Pfp1 domain containing glyoxalase III, OsDJ-1C, functions in abiotic stress adaptation in rice

MAIN CONCLUSION

Overexpression of OsDJ-1C in rice improves root architecture, photosynthesis, yield and abiotic stress tolerance through modulating methylglyoxal levels, antioxidant defense, and redox homeostasis. Exposure to abiotic stresses leads to elevated methylglyoxal (MG) levels in plants, impacting seed germination and root growth. In response, the activation of NADPH-dependent aldo-keto reductase and glutathione (GSH)-dependent glyoxalase enzymes helps to regulate MG levels and reduce its toxic effects. However, detoxification may not be carried out effectively due to the limitation of GSH and NADPH in plants under stress. Recently, a novel enzyme called glyoxalase III (GLY III) has been discovered which can detoxify MG in a single step without needing GSH. To understand the physiological importance of this pathway in rice, we overexpressed the gene encoding GLYIII enzyme (OsDJ-1C) in rice. It was observed that OsDJ-1C overexpression in rice regulated MG levels under stress conditions thus, linked well with plants' abiotic stress tolerance potential. The OsDJ-1C overexpression lines displayed better root architecture, improved photosynthesis, and reduced yield penalty compared to the WT plants under salinity, and drought stress conditions. These plants demonstrated an improved GSH/GSSG ratio, reduced level of reactive oxygen species, increased antioxidant capacity, and higher anti-glycation activity thereby indicating that the GLYIII mediated MG detoxification plays a significant role in plants' ability to reduce the impact of abiotic stress. Furthermore, these findings imply the potential of OsDJ-1C in crop improvement programs.

Pathogen-induced methylglyoxal negatively regulates rice bacterial blight resistance by inhibiting OsCDR1 protease activity

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight (BB), a globally devastating disease of rice (Oryza sativa) that is responsible for significant crop loss. Sugars and sugar metabolites are important for pathogen infection, providing energy and regulating events associated with defense responses; however, the mechanisms by which they regulate such events in BB are unclear. As an inevitable sugar metabolite, methylglyoxal (MG) is involved in plant growth and responses to various abiotic stresses, but the underlying mechanisms remain enigmatic. Whether and how MG functions in plant biotic stress responses is almost completely unknown. Here, we report that the Xoo strain PXO99 induces OsWRKY62.1 to repress transcription of OsGLY II genes by directly binding to their promoters, resulting in overaccumulation of MG. MG negatively regulates rice resistance against PXO99: osglyII2 mutants with higher MG levels are more susceptible to the pathogen, whereas OsGLYII2-overexpressing plants with lower MG content show greater resistance than the wild type. Overexpression of OsGLYII2 to prevent excessive MG accumulation confers broad-spectrum resistance against the biotrophic bacterial pathogens Xoo and Xanthomonas oryzae pv. oryzicola and the necrotrophic fungal pathogen Rhizoctonia solani, which causes rice sheath blight. Further evidence shows that MG reduces rice resistance against PXO99 through CONSTITUTIVE DISEASE RESISTANCE 1 (OsCDR1). MG modifies the Arg97 residue of OsCDR1 to inhibit its aspartic protease activity, which is essential for OsCDR1-enhanced immunity. Taken together, these findings illustrate how Xoo promotes infection by hijacking a sugar metabolite in the host plant.

Pyramiding D-lactate dehydrogenase with the glyoxalase pathway enhances abiotic stress tolerance in plants

Methylglyoxal is a common cytotoxic metabolite produced in plants during multiple biotic and abiotic stress. To mitigate the toxicity of MG, plants utilize the glyoxalase pathway comprising glyoxalase I (GLYI), glyoxalase II (GLYII), or glyoxalase III (GLYIII). GLYI and GLYII are the key enzymes of glyoxalase pathways that play an important role in abiotic stress tolerance. Earlier research showed that MG level is lower when both GLYI and GLYII are overexpressed together, compared to GLYI or GLYII single gene overexpressed transgenic plants. D-lactate dehydrogenase (D-LDH) is an integral part of MG detoxification which metabolizes the end product (D-lactate) of the glyoxalase pathway. In this study, two Arabidopsis transgenic lines were constructed using gene pyramiding technique: GLYI and GLYII overexpressed (G-I + II), and GLYI, GLYII, and D-LDH overexpressed (G-I + II + D) plants. G-I + II + D exhibits lower MG and D-lactate levels and enhanced abiotic stress tolerance than the G-I + II and wild-type plants. Further study explores the stress tolerance mechanism of G-I + II + D plants through the interplay of different regulators and plant hormones. This, in turn, modulates the expression of ABA-dependent stress-responsive genes like RAB18, RD22, and RD29B to generate adaptive responses during stress. Therefore, there might be a potential correlation between ABA and MG detoxification pathways. Furthermore, higher STY46, GPX3, and CAMTA1 transcripts were observed in G-I + II + D plants during abiotic stress. Thus, our findings suggest that G-I + II + D has significantly improved MG detoxification, reduced oxidative stress-induced damage, and provided a better protective mechanism against abiotic stresses than G-I + II or wild-type plants.

Proteome-wide identification of methylglyoxalated proteins in rapeseed (Brassica napus L.)

Methylglyoxal (MG), a highly reactive cellular metabolite, is crucial for plant growth and environmental responses. MG may function by modifying its target proteins, but little is known about MG-modified proteins in plants. Here, MG-modified proteins were pulled down by an antibody against methylglyoxalated proteins and detected using liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. We identified 543 candidate proteins which are involved in multiple enzymatic activities and metabolic processes. A great number of candidate proteins were predicted to localize to cytoplasm, chloroplast, and nucleus, consistent with the known subcellular compartmentalization of MG. By further analyzing the raw LC-MS/MS data, we obtained 42 methylglyoxalated peptides in 35 proteins and identified 10 methylglyoxalated lysine residues in a myrosinase-binding protein (BnaC06G0061400ZS). In addition, we demonstrated that MG modifies the glycolate oxidase and β-glucosidase to enhance and inhibit the enzymatic activity, respectively. Together, our study contributes to the investigation of the MG-modified proteins and their potential roles in rapeseed.

MeGLYI-13, a Glyoxalase I Gene in Cassava, Enhances the Tolerance of Yeast and Arabidopsis to Zinc and Copper Stresses

Although zinc and copper are the two essential nutrients necessary for plant growth, their excessive accumulation in soil not only causes environmental pollution but also seriously threatens human health and inhibits plant growth. The breeding of plants with novel zinc or copper toxicity tolerance capacities represents one strategy to address this problem. Glyoxalase I (GLYI) family genes have previously been suggested to be involved in the resistance to a wide range of abiotic stresses, including those invoked by heavy metals. Here, a MeGLYI-13 gene cloned from a cassava SC8 cultivar was characterized with regard to its potential ability in resistance to zinc or copper stresses. Sequence alignment indicated that MeGLYI-13 exhibits sequence differences between genotypes. Transient expression analysis revealed the nuclear localization of MeGLYI-13. A nuclear localization signal (NLS) was found in its C-terminal region. There are 12 Zn2+ binding sites and 14 Cu2+ binding sites predicted by the MIB tool, of which six binding sites were shared by Zn2+ and Cu2+. The overexpression of MeGLYI-13 enhanced both the zinc and copper toxicity tolerances of transformed yeast cells and Arabidopsis seedlings. Taken together, our study shows the ability of the MeGLYI-13 gene to resist zinc and copper toxicity, which provides genetic resources for the future breeding of plants resistant to zinc and copper and potentially other heavy metals.

Nickel (Ni) phytotoxicity and detoxification mechanisms: A review

Scientists studying the environment, physiology, and biology have been particularly interested in nickel (Ni) because of its dual effects (essentiality and toxicity) on terrestrial biota. It has been reported in some studies that without an adequate supply of Ni, plants are unable to finish their life cycle. The safest Ni limit for plants is 1.5 μg g^-1, while the limit for soil is between 75 and 150 μg g^-1. Ni at lethal levels harms plants by interfering with a variety of physiological functions, including enzyme activity, root development, photosynthesis, and mineral uptake. This review focuses on the occurrence and phytotoxicity of Ni with respect to growth, physiological and biochemical aspects. It also delves into advanced Ni detoxification mechanisms such as cellular modifications, organic acids, and chelation of Ni by plant roots, and emphasizes the role of genes involved in Ni detoxification. The discussion has been carried out on the current state of using soil amendments and plant-microbe interactions to successfully remediate Ni from contaminated sites. This review has identified potential drawbacks and difficulties of various strategies for Ni remediation, discussed the importance of these findings for environmental authorities and decision-makers, and concluded by noting the sustainability concerns and future research needs regarding Ni remediation.

Salt stress-induced chloroplastic hydrogen peroxide stimulates pdTPI sulfenylation and methylglyoxal accumulation

High salinity, an adverse environmental factor affecting about 20% of irrigated arable land worldwide, inhibits plant growth and development by causing oxidative stress, damaging cellular components, and disturbing global metabolism. However, whether and how reactive oxygen species disturb the metabolism of salt-stressed plants remain elusive. Here, we report that salt-induced hydrogen peroxide (H2O2) inhibits the activity of plastid triose phosphate isomerase (pdTPI) to promote methylglyoxal (MG) accumulation and stimulates the sulfenylation of pdTPI at cysteine 74. We also show that MG is a key factor limiting the plant growth, as a decrease in MG levels completely rescued the stunted growth and repressed salt stress tolerance of the pdtpi mutant. Furthermore, targeting CATALASE 2 into chloroplasts to prevent salt-induced overaccumulation of H2O2 conferred salt stress tolerance, revealing a role for chloroplastic H2O2 in salt-caused plant damage. In addition, we demonstrate that the H2O2-mediated accumulation of MG in turn induces H2O2 production, thus forming a regulatory loop that further inhibits the pdTPI activity in salt-stressed plants. Our findings, therefore, illustrate how salt stress induces MG production to inhibit the plant growth.

Overexpression of OsGF14C enhances salinity tolerance but reduces blast resistance in rice

High-salinity and blast disease are two major stresses that cause dramatic yield loss in rice production. GF14 (14-3-3) genes have been reported to play important role in biotic and abiotic stresses in plants. However, the roles of OsGF14C remain unknown. To understand the functions and regulatory mechanisms of OsGF14C in regulating salinity tolerance and blast resistance in rice, we have conducted OsGF14C-overexpressing transgenic experiments in the present study. Our results showed that overexpression of OsGF14C enhanced salinity tolerance but reduced blast resistance in rice. The enhanced salinity tolerance is related to the reduction of methylglyoxal and Na⁺ uptake instead of exclusion or compartmentation and the negative role of OsGF14C in blast resistance is associated with the suppression of OsGF14E, OsGF14F and PR genes. Our results together with the results from the previous studies suggest that the lipoxygenase gene LOX2 which is regulated by OsGF14C may play roles in coordinating salinity tolerance and blast resistance in rice. The current study for the first time revealed the possible roles of OsGF14C in regulating salinity tolerance and blast resistance in rice, and laid down a foundation for further functional study and crosstalk regulation between salinity and blast resistance in rice.

Exogenous hydrogen sulfide and methylglyoxal alleviate cadmium-induced oxidative stress in Salix matsudana Koidz by regulating glutathione metabolism

BACKGROUND

Cadmium (Cd) is a highly toxic element for plant growth. In plants, hydrogen sulfide (H₂S) and methylglyoxal (MG) have emerged as vital signaling molecules that regulate plant growth processes under Cd stress. However, the effects of sodium hydrosulfide (NaHS, a donor of H₂S) and MG on Cd uptake, physiological responses, and gene expression patterns of Salix to Cd toxicity have been poorly understood. Here, Salix matsudana Koidz. seedlings were planted in plastic pot with applications of MG (108 mg kg^- 1) and NaHS (50 mg kg^- 1) under Cd (150 mg kg^- 1) stress.

RESULTS

Cd treatment significantly increased the reactive oxygen species (ROS) levels and malondialdehyde (MDA) content, but decreased the growth parameters in S. matsudana. However, NaHS and MG supplementation significantly decreased Cd concentration, ROS levels, and MDA content, and finally enhanced the growth parameters. Cd stress accelerated the activities of antioxidative enzymes and the relative expression levels of stress-related genes, which were further improved by NaHS and MG supplementation. However, the activities of monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR) were sharply decreased under Cd stress. Conversely, NaHS and MG applications restored the MDHAR and DHAR activities compared with Cd-treated seedlings. Furthermore, Cd stress decreased the ratios of GSH/GSSG and AsA/DHA but considerably increased the H₂S and MG levels and glyoxalase I-II system in S. matsudana, while the applications of MG and NaHS restored the redox status of AsA and GSH and further improved glyoxalase II activity. In addition, compared with AsA, GSH showed a more sensitive response to exogenous applications of MG and NaHS and plays more important role in the detoxification of Cd.

CONCLUSIONS

The present study illustrated the crucial roles of H₂S and MG in reducing ROS-mediated oxidative damage to S. matsudana and revealed the vital role of GSH metabolism in regulating Cd-induced stress.

A glutathione-independent DJ-1/PfpI domain-containing tomato glyoxalaseIII2, SlGLYIII2, confers enhanced tolerance under salt and osmotic stresses

In plants, glyoxalase enzymes are activated under stress conditions to mitigate the toxic effects of hyperaccumulated methylglyoxal (MG), a highly reactive carbonyl compound. Until recently, a glutathione-dependent bi-enzymatic pathway involving glyoxalase I (GLYI) and glyoxalase II (GLYII) was considered the primary MG-detoxification system. Recently, a new glutathione-independent glyoxalase III (GLYIII) mediated direct route was also reported in plants. However, the physiological significance of this new pathway remains to be elucidated across plant species. This study identified the full complement of 22 glyoxalases in tomato. Based on their strong induction under multiple abiotic stresses, SlGLYI4, SlGLYII2 and SlGLYIII2 were selected candidates for further functional characterisation. Stress-inducible overexpression of both glutathione-dependent (SlGLYI4 + SlGLYII2) and independent (SlGLYIII2) pathways led to enhanced tolerance in both sets of transgenic plants under abiotic stresses. However, SlGLYIII2 overexpression (OE) plants outperformed the SlGLYI4 + SlGLYII2 OE counterparts for their stress tolerance under abiotic stresses. Further, knockdown of SlGLYIII2 resulted in plants with exacerbated stress responses than those silenced for both SlGLYI4 and SlGLYII2. The superior performance of SlGLYIII2 OE tomato plants for better growth and yield under salt and osmotic treatments could be attributed to better GSH/GSSG ratio, lower reactive oxygen species levels, and enhanced antioxidant potential, indicating a prominent role of GLYIII MG-detoxification pathway in abiotic stress mitigation in this species.

OsGLYI3, a glyoxalase gene expressed in rice seed, contributes to seed longevity and salt stress tolerance

The glyoxalase pathway plays a vital role in the chemical detoxification of methylglyoxal (MG) in biological systems. Our previous study suggested that OsGLYI3 may be effective in seed natural aging. In this study, the rice OsGLYI3 gene was cloned and characterized as specifically expressed in the seed. The accelerated aging (AA) treatment results indicated significant roles of OsGLYI3 in seed longevity and vigor, as the seeds of the transgenic lines with overexpressed and knocked-out OsGLYI3 exhibited higher and lower germination, respectively. The AA treatment also increased the superoxide dismutase (SOD) activity in the overexpressed transgenic seeds compared to the wild-type seeds yet lowered the SOD activity in the CRISPR/Cas9-derived transgenic rice lines. Rice OsGLYI3 was markedly upregulated in response to NaCl induced stress conditions. Compared to wild-type plants, overexpressed transgenic rice lines exhibited increased GLYI activity, decreased MG levels and improved salt stress tolerance, while CRISPR/Cas9 knockout transgenic rice lines showed decreased glyoxalase I activity, increased MG levels, and greater sensitivity to stress treatments with NaCl. Collectively, our results confirmed for the first time that OsGLYI3 is specifically expressed in rice seeds and contributes to seed longevity and salt stress tolerance.

Genome-Wide Expression Analysis of Glyoxalase I Genes Under Hyperosmotic Stress and Existence of a Stress-Responsive Mitochondrial Glyoxalase I Activity in Durum Wheat (Triticum durum Desf.)

Glyoxalase I (GLYI) catalyzes the rate-limiting step of the glyoxalase pathway that, in the presence of GSH, detoxifies the cytotoxic molecule methylglyoxal (MG) into the non-toxic D-lactate. In plants, MG levels rise under various abiotic stresses, so GLYI may play a crucial role in providing stress tolerance. In this study, a comprehensive genome database analysis was performed in durum wheat (Triticum durum Desf.), identifying 27 candidate GLYI genes (TdGLYI). However, further analyses of phylogenetic relationships and conserved GLYI binding sites indicated that only nine genes encode for putative functionally active TdGLYI enzymes, whose distribution was predicted in three different subcellular compartments, namely cytoplasm, plastids and mitochondria. Expression profile by qRT-PCR analysis revealed that most of the putative active TdGLYI genes were up-regulated by salt and osmotic stress in roots and shoots from 4-day-old seedlings, although a different behavior was observed between the two types of stress and tissue. Accordingly, in the same tissues, hyperosmotic stress induced an increase (up to about 40%) of both GLYI activity and MG content as well as a decrease of GSH (up to about -60%) and an increase of GSSG content (up to about 7-fold) with a consequent strong decrease of the GSH/GSSG ratio (up to about -95%). Interestingly, in this study, we reported the first demonstration of the existence of GLYI activity in highly purified mitochondrial fraction. In particular, GLYI activity was measured in mitochondria from durum wheat (DWM), showing hyperbolic kinetics with Km and Vmax values equal to 92 ± 0.2 μM and 0.519 ± 0.004 μmol min-1 mg-1 of proteins, respectively. DWM-GLYI resulted inhibited in a competitive manner by GSH (Ki = 6.5 ± 0.7 mM), activated by Zn2+ and increased, up to about 35 and 55%, under salt and osmotic stress, respectively. In the whole, this study provides basis about the physiological significance of GLYI in durum wheat, by highlighting the role of this enzyme in the early response of seedlings to hyperosmotic stress. Finally, our results strongly suggest the existence of a complete mitochondrial GLYI pathway in durum wheat actively involved in MG detoxification under hyperosmotic stress.

Glyoxalase III enhances salinity tolerance through reactive oxygen species scavenging and reduced glycation

Methylglyoxal (MG) is a metabolically generated highly cytotoxic compound that accumulates in all living organisms, from Escherichia coli to humans, under stress conditions. To detoxify MG, nature has evolved reduced glutathione (GSH)-dependent glyoxalase and NADPH-dependent aldo-keto reductase systems. But both GSH and NADPH have been reported to be limiting in plants under stress conditions, and thus detoxification might not be performed efficiently. Recently, glyoxalase III (GLY III)-like enzyme activity has been reported from various species, which can detoxify MG without any cofactor. In the present study, we have tested whether an E. coli gene, hchA, encoding a functional GLY III, could provide abiotic stress tolerance to living systems. Overexpression of this gene showed improved tolerance in E. coli and Saccharomyces cerevisiae cells against salinity, dicarbonyl, and oxidative stresses. Ectopic expression of the E. coli GLY III gene (EcGLY-III) in transgenic tobacco plants confers tolerance against salinity at both seedling and reproductive stages as indicated by their height, weight, membrane stability index, and total yield potential. Transgenic plants showed significantly increased glyoxalase and antioxidant enzyme activity that resisted the accumulation of excess MG and reactive oxygen species (ROS) during stress. Moreover, transgenic plants showed more anti-glycation activity to inhibit the formation of advanced glycation end product (AGE) that might prevent transgenic plants from stress-induced senescence. Taken together, all these observations indicate that overexpression of EcGLYIII confers salinity stress tolerance in plants and should be explored further for the generation of stress-tolerant plants.

Physiological defense and metabolic strategy of Pistia stratiotes in response to zinc-cadmium co-pollution

Pistia stratiotes is a cadmium (Cd) hyperaccumulating plant with strong bioaccumulation and translocation capacity for Cd. A hydroponic experiment was used to evaluate the combined effect of Zinc (Zn) and Cd at different concentrations on leaf growth and metabolism of P. stratiotes. This study revealed the physiological defense and metabolic strategy of responses to Zn-Cd co-pollution. With the Zn₅₀Cd₁, Zn₅₀Cd₁₀, Zn₁₀₀Cd_1, and Zn₁₀₀Cd₁₀ treatments for 9 d, the relative crown diameter, relative leave number, and ramet number of the plant had no significant difference with the control. Under the compound treatments containing Zn₅₀Cd₅₀ and Zn₁₀₀Cd₅₀, the activity of the glyoxalase system and amino acid metabolism in the leaves were inhibited. The leaf photosynthetic apparatus increased heat dissipation to reduce the damage to the photosystem II (PS II) reaction center caused by excess excitation energy under Zn-Cd stress. This safeguarded the balance between the absorption and utilization of light energy. Compared to the control, the Zn and Cd co-pollution for 9 d had no effect on the reduced glutathione (GSH) and oxidized glutathione (GSSG) contents. There was no effect on the dehydroascorbate reductase (DHAR) and glutathione reductase (GR) activities, but there was increased ascorbate peroxidase (APX) activity and oxidized ascorbic acid (DHA) content. These increased the antioxidant capacity of the ascorbate-glutathione (AsA-GSH) cycle. The treated plants also had increased levels of carnosol and substances related to lipid metabolism including 9, 10-Dihydroxystearate, Prostaglandin G2, Sphingosine, and 13-L-Hydroperoxylinoleic acid, maintaining the cell stability and resistance to the Zn-Cd stress.

Photosynthesizing while hyperaccumulating nickel: Insights from the genus Odontarrhena (Brassicaceae)

Nickel-induced changes in photosynthetic activity were investigated in three Ni-hyperaccumulating Odontarrhena species with increasing Ni tolerance and accumulation capacity, O. muralis, O. moravensis, and O. chalcidica. Plantlets were grown in hydroponics at increasing NiSO₄ concentrations (0, 0.25, and 1 mM) for one week, and the effects of Ni on growth, metal accumulation, photosynthesis, and nitrogen (N) allocation to components of the photosynthetic apparatus were analysed. Nickel treatments in O. chalcidica, and O. moravensis to a lesser extent, increased not only the photochemical efficiency of photosystem II (PSII) and the CO₂ assimilation rate, but also CO₂ diffusion from the atmosphere to the carboxylation sites. These two species displayed a specific increase and/or rearrangement of the photosynthetic pigments and a higher leaf N allocation to the photosynthetic components in the presence of the metal. Odontarrhena muralis displayed a decrease in photosynthetic performance at the lowest Ni concentration due to a combination of both stomatal and non-stomatal limitations. Our data represent the first complete investigation of the effects of Ni on the photosynthetic machinery in Ni hyperaccumulating plants. Our findings clearly indicate a stimulatory, hormetic-like, effect of the metal on both biophysics and biochemistry of photosynthesis in the species with the highest hyperaccumulation capacity.

A review on adsorptive separation of toxic metals from aquatic system using biochar produced from agro-waste

Water is a basic and significant asset for living beings. Water assets are progressively diminishing due to huge populace development, industrial activities, urbanization and rural exercises. Few heavy metals include zinc, copper, lead, nickel, cadmium and so forth can easily transfer into the water system either direct or indirect activities of electroplating, mining, tannery, painting, fertilizer industries and so forth. The different treatment techniques have been utilized to eliminate the heavy metals from aquatic system, which includes coagulation/flocculation, precipitation, membrane filtration, oxidation, flotation, ion exchange, photo catalysis and adsorption. The adsorption technique is a better option than other techniques because it can eliminate heavy metals even at lower metal ions concentration, simplicity and better regeneration behavior. Agricultural wastes are low-cost biosorbent and typically containing cellulose have the ability to absorb a variety of contaminants. It is important to note that almost all agro wastes are no longer used in their original form but are instead processed in a variety of techniques to improve the adsorption capacity of the substance. The wide range of adsorption capacities for agro waste materials were observed and almost more than 99% removal of toxic pollutants from aquatic systems were achieved using modified agro-waste materials. The present review aims at the water pollution due to heavy metals, as well as various heavy metal removal treatment procedures. The primary objectives of this research is to include an overview of adsorption and various agriculture based adsorbents and its comparison in heavy metal removal.

Methylglyoxal-glyoxalase system as a possible selection module for raising marker-safe plants in rice

Methylglyoxal (MG) is ubiquitously produced in all living organisms as a byproduct of glycolysis, higher levels of which are cytotoxic, leading to oxidative stress and apoptosis in the living systems. Though its generation is spontaneous but its detoxification involves glyoxalase pathway genes. Based on this understanding, the present study describes the possible role of MG as a novel non-antibiotic-based selection agent in rice. Further, by metabolizing MG, the glyoxalase pathway genes viz. glyoxalase I (GLYI) and glyoxalase II (GLYII), may serve as selection markers. Therefore, herein, transgenic rice harboring GLYI-GLYII genes (as selection markers) were developed and the effect of MG as a selection agent was assessed. The 3 mM MG concentration was observed as optimum for the selection of transformed calli, allowing efficient callus induction and proliferation along with high regeneration frequency (55 ± 2%) of the transgenic calli. Since the transformed calli exhibited constitutively higher activity of GLYI and GLYII enzymes compared to the wild type calli, the rise in MG levels was restricted even upon exogenous addition of MG during the selection process, resulting in efficient selection of the transformed calli. Therefore, MG-based selection method is a useful and efficient system for selection of transformed plants without significantly compromising the transformation efficiency. Further, this MG-based selection system is bio-safe and can pave way towards better public acceptance of transgenic plants.

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.

What signals the glyoxalase pathway in plants?

Glyoxalase (GLY) system, comprising of GLYI and GLYII enzymes, has emerged as one of the primary methylglyoxal (MG) detoxification pathways with an indispensable role during abiotic and biotic stresses. MG homeostasis is indeed very closely guarded by the cell as its higher levels are cytotoxic for the organism. The dynamic responsiveness of MG-metabolizing GLY pathway to both endogenous cues such as, phytohormones, nutrient status, etc., as well as external environmental fluctuations (abiotic and biotic stresses) indicates that a tight regulation occurs in the cell to maintain physiological levels of MG in the system. Interestingly, GLY pathway is also manipulated by its substrates and reaction products. Hence, an investigation of signalling and regulatory aspects of GLY pathway would be worthwhile. Herein, we have attempted to converge all known factors acting as signals or directly regulating GLYI/II enzymes in plants. Further, we also discuss how crosstalk between these different signal molecules might facilitate the regulation of glyoxalase pathway. We believe that MG detoxification is controlled by intricate mechanisms involving a plethora of signal molecules.

Overexpression of Glyoxalase III gene in transgenic sugarcane confers enhanced performance under salinity stress

The glyoxalase pathway is a check point to monitor the elevation of methylglyoxal (MG) level in plants and is mediated by glyoxalase I (Gly I) and glyoxalase II (Gly II) enzymes in the presence of glutathione. Recent studies established the presence of unique DJ-1/PfpI domain containing protein named glyoxalase III (Gly III) in prokaryotes, involved in the detoxification of MG into D-lactic acid through a single step process. In the present study, eleven transgenic sugarcane events overexpressing EaGly III were assessed for salinity stress (100 mM and 200 mM NaCl) tolerance. Lipid peroxidation as well as cell membrane injury remained very minimal in all the transgenic events indicating reduced oxidative damage. Transgenic events exhibited significantly higher plant water status, gas exchange parameters, chlorophyll, carotenoid, and proline content, total soluble sugars, SOD and POD activity compared to wild type (WT) under salinity stress. Histological studies by taking the cross section showed a highly stable root system in transgenic events upon exposure to salinity stress. Results of the present study indicate that transgenic sugarcane events overexpressing EaGly III performed well and exhibited improved salinity stress tolerance.

Synergistic anti-oxidative effects of Pongamia pinnata against nickel mediated by Rhizobium pisi and Ochrobacterium pseudogrignonense

Nickel is widely spread by different anthropogenic activities and shows toxicity for plant growth and development. Whether rhizobia symbiotically fix nitrogen can eliminate or reduce nickel toxic effect on plant or not is still unknown. This study was aimed to investigate the effect of different rhizobia genus inoculation on growth, nitrogen fixing ability, metal accumulation and enzymatic antioxidative balance of Pongamia pinnnaa. Inoculation with Rhizobium pisi and Ochrobacterium pseudogrignonense increased the all the growth parameters both in 0 and 40 mg/kg nickel as comparison with control. Only shoot length increased in presence of nitrogen as compared with no supply of nitrogen. Nitrogen content also increased both in rhizobia inoculation as compared to no nitrogen supply and non-inoculation control, respectively. Nickel uptake was higher in shoots and leaves but lower in roots in case of inoculation as compared to non-inoculation control. Rhizobia inoculation improved the plant antioxidant capacity by increasing the activity of enzymatic scavengers catalase (CAT), superoxide dismutase (SOD), peroxidase (POD) and ascorbate (GR). However, 40 mg/kg of nickel adding showed mostly effect on the activity CAT, SOD, POD in leaves. All the enzymatic activity showed a significant increase in absence of nitrogen supply as compared nitrogen supply. Our results suggested that rhizobia inoculation effectively mediated nickel stress for legume plants by increasing nitrogen supplement and inducing antioxidant capacity.

Transcriptome Profile Analysis of Arabidopsis Reveals the Drought Stress-Induced Long Non-coding RNAs Associated With Photosynthesis, Chlorophyll Synthesis, Fatty Acid Synthesis and Degradation

Long non-coding RNAs (lncRNAs) play an important role in the response of plants to drought stress. The previous studies have reported that overexpression of LEA3 and VOC could enhance drought tolerance and improve the oil content in Brassica napus and Arabidopsis thaliana, and most of the efforts have been invested in the gene function analysis, there is little understanding of how genes that involved in these important pathways are regulated. In the present study, the transcriptomic results of LEA3 and VOC over-expressed (OE) lines were compared with the RNAi lines, mutant lines and control lines under long-term and short-term drought treatment, a series of differentially expressed lncRNAs were identified, and their regulation patterns in mRNA were also investigated in above mentioned materials. The regulation of the target genes of differentially expressed lncRNAs on plant biological functions was studied. It was revealed that the mutant lines had less drought-response related lncRNAs than that of the OE lines. Functional analysis demonstrated that multiple genes were involved in the carbon-fixing and chlorophyll metabolism, such as CDR1, CHLM, and CH1, were regulated by the upregulated lncRNA in OE lines. In LEA-OE, AT4G13180 that promotes the fatty acid synthesis was regulated by five lncRNAs that were upregulated under both long-term and short-term drought treatments. The key genes, including of SHM1, GOX2, and GS2, in the methylglyoxal synthesis pathway were all regulated by a number of down-regulated lncRNAs in OE lines, thereby reducing the content of such harmful compounds produced under stress in plants. This study identified a series of lncRNAs related to the pathways that affect photosynthesis, chlorophyll synthesis, fatty acid synthesis, degradation, and other important effects on drought resistance and oil content. The present study provided a series of lncRNAs for further improvement of crop varieties, especially drought resistant and oil content traits.

Exergy efficiency of thermochemical syngas-to-ethanol production plants

Abstract

This work presents exergy analyses applied in four different conceptual second-generation ethanol production processes through a thermochemical route using catalysts based on Molybdenum (P-1), Copper (P-2), and Rhodium (P-3 and P-4), aiming to assess their exergetic efficiencies. The results show that the conceptual processes have satisfactory exergy efficiencies in both cases, when compared among themselves and when compared with other processes reported in literature. The processes’ efficiency for P-1, P-2, P-3 and P-4 were, respectively, 52.4%, 41.4%, 43.7% and 48.9%. The reactors were the sections in which exergy destruction was more significant, due to the exothermic reactions and mixing points (where streams with different temperatures were mixed). Such results show the potential of thermochemical ethanol production, besides opening the possibilities of process improvement.

Graphic abstract

Modulation of Abiotic Stress Responses in Rice by E3-Ubiquitin Ligases: A Promising Way to Develop Stress-Tolerant Crops

Plants are unable to physically escape environmental constraints and have, therefore, evolved a range of molecular and physiological mechanisms to maximize survival in an ever-changing environment. Among these, the post-translational modification of ubiquitination has emerged as an important mechanism to understand and improve the stress response. The ubiquitination of a given protein can change its abundance (through degradation), alter its localization, or even modulate its activity. Hence, ubiquitination increases the plasticity of the plant proteome in response to different environmental cues and can contribute to improve stress tolerance. Although ubiquitination is mediated by different enzymes, in this review, we focus on the importance of E3-ubiquitin ligases, which interact with the target proteins and are, therefore, highly associated with the mechanism specificity. We discuss their involvement in abiotic stress response and place them as putative candidates for ubiquitination-based development of stress-tolerant crops. This review covers recent developments in this field using rice as a reference for crops, highlighting the questions still unanswered.

The metabolite methylglyoxal-mediated gene expression is associated with histone methylglyoxalation

Methylglyoxal (MG) is a byproduct of glycolysis that functions in diverse mammalian developmental processes and diseases and in plant responses to various stresses, including salt stress. However, it is unknown whether MG-regulated gene expression is associated with an epigenetic modification. Here we report that MG methylglyoxalates H3 including H3K4 and increases chromatin accessibility, consistent with the result that H3 methylglyoxalation positively correlates with gene expression. Salt stress also increases H3 methylglyoxalation at salt stress responsive genes correlated to their higher expression. Following exposure to salt stress, salt stress responsive genes were expressed at higher levels in the Arabidopsis glyI2 mutant than in wild-type plants, but at lower levels in 35S::GLYI2 35S::GLYII4 plants, consistent with the higher and lower MG accumulation and H3 methylglyoxalation of target genes in glyI2 and 35S::GLYI2 35S::GLYII4, respectively. Further, ABI3 and MYC2, regulators of salt stress responsive genes, affect the distribution of H3 methylglyoxalation at salt stress responsive genes. Thus, MG functions as a histone-modifying group associated with gene expression that links glucose metabolism and epigenetic regulation.

Ectopic expression of DJ-1/PfpI domain containing Erianthus arundinaceus Glyoxalase III (EaGly III) enhances drought tolerance in sugarcane

Sugarcane transgenic overexpressing EaGly III from Erianthus arundinaceus showed enhanced water deficit stress tolerance. Methylglyoxal (MG), an α-ketoaldehyde formed from either glycolysis or TCA cycle, is capable of causing total cellular damage via the generation of reactive oxygen species (ROS), advanced glycation end products (AGEs) and nucleic acid degradation. Glyoxalase pathway is a ubiquitous pathway known for detoxification of MG, involving key enzymes glyoxalase I (Gly I) and glyoxalase II (Gly II). Recently, a novel and an additional enzyme in glyoxalase pathway, viz., glyoxalase III (Gly III), has been discovered which possesses DJ-1/PfpI domain recognized for detoxifying MG in a single step process without requirement of any coenzyme. In the present study, a Gly III gene isolated from Erianthus arundinaceus, a wild relative of sugarcane, overexpressed in commercially cultivated sugarcane hybrid Co 86032 was assessed for drought tolerance. Morphometric observations revealed that transgenic sugarcane overexpressing EaGly III acquired drought tolerance trait. Oxidative damage caused by triggering generation of ROS has been determined to be low in transgenic plants as compared to wild type (WT). Transgenics resulted in higher relative water content, chlorophyll content, gas exchange parameters, photosynthetic efficiency, proline content and soluble sugars upon water deficit stress. In addition, higher and stable level of superoxide dismutase and peroxidase activities were observed along with minimal lipid peroxidation during drought stress signifying the tolerance mechanism exhibited by transgenic events. There was no significant structural change observed in the root anatomy of transgenic plants. Altogether, EaGly III gene could be considered as a potential candidate for conferring water deficit stress tolerance for sugarcane and other agricultural crops.

Genome-Wide Investigation of 2,4-Diacetylphloroglucinol Protection Genes in Arabidopsis thaliana

The compound 2,4-diacetylphloroglucinol (DAPG) is a well-known secondary metabolite produced by Pseudomonas spp. that are used as biocontrol agents. DAPG displays a remarkably broad spectrum of toxic activity against pathogens of plants. Yet high concentrations of DAPG may also have negative effect on plants, but the phytotoxicity of DAPG is not clearly understood. Here, we used genome-wide activation, tagging Arabidopsis plants as the model plant to investigate the plant response to DAPG. A total of 15 lines were selected as DAPG-tolerant plants from among 62,000 lines investigated. The DAPG-responsible genes were then identified via thermal asymmetric interlaced PCR and quantitative reverse transcription PCR, and the gene ontology analysis showed the distribution of these genes having different biological processes, cellular regulations, and molecular functional properties. Collectively, these findings suggest that plants may rely on several pathways to prevent DAPG phytotoxicity.

Carbonyl Cytotoxicity Affects Plant Cellular Processes and Detoxifying Enzymes Scavenge These Compounds to Improve Stress Tolerance

Oxidative stress is ubiquitous in environmental stresses and prevails over the cellular metabolic and phenotypic responses in plants. Reactive oxygen species (ROS) generated under stress affect macromolecules to form another group of toxic compounds called reactive carbonyl compounds (RCCs). These molecules have a longer half-life than ROS and cause carbonyl stress that affects cellular metabolism, cellular homeostasis, and crop productivity. The later effect of oxidative stress in terms of the generation of RCCs and glycation products and their effects on plant processes have not been explored well in plant biology. Therefore, how these molecules are produced and a few important effects of RCCs on plants have been discussed in this review article. Further, the plant adaptive detoxification mechanisms of RCCs have been discussed. The enzymes that were identified in plants to detoxify these cytotoxic compounds have broad substrate specificity and the potential for use in breeding programs. The review should provide a comprehensive understanding of the cytotoxic compounds beyond ROS and subsequently their mitigation strategies for crop improvement programs.

Oryza sativa drought-, heat-, and salt-induced RING finger protein 1 (OsDHSRP1) negatively regulates abiotic stress-responsive gene expression

Plants are sessile and unable to avoid environmental stresses, such as drought, high temperature, and high salinity, which often limit the overall plant growth. Plants have evolved many complex mechanisms to survive these abiotic stresses via post-translational modifications. Recent evidence suggests that ubiquitination plays a crucial role in regulating abiotic stress responses in plants by regulating their substrate proteins. Here, we reported the molecular function of a RING finger E3 ligase, Oryza sativa Drought, Heat and Salt-induced RING finger protein 1 (OsDHSRP1), involved in regulating plant abiotic stress tolerance via the Ub/26S proteasome system. The OsDHSRP1 gene transcripts were highly expressed under various abiotic stresses such as NaCl, drought, and heat and the phytohormone abscisic acid (ABA). In addition, in vitro ubiquitination assays demonstrated that the OsDHSRP1 protein possesses a RING-H2 type domain that confers ligase functionality. The results of yeast two-hybrid (Y2H), in vitro pull-down, and bimolecular fluorescence complementation assays support that OsDHSRP1 is able to regulate two substrates, O. sativa glyoxalase (OsGLYI-11.2) and O. sativa abiotic stress-induced cysteine proteinase 1 (OsACP1). We further confirmed that these two substrate proteins were ubiquitinated by OsDHSRP1 E3 ligase and caused protein degradation via the Ub/26S proteasome system. The Arabidopsis plants overexpressing OsDHSRP1 exhibited hypersensitivity to drought, heat, and NaCl stress and a decrease in their germination rates and root lengths compared to the control plants because the degradation of the OsGLYI-11.2 protein maintained lower glyoxalase levels, which increased the methylglyoxal amount in transgenic Arabidopsis plants. However, the OsDHSRP1-overexpressing plants showed no significant difference when treated with ABA. Our finding supports the hypothesis that the OsDHSRP1 E3 ligase acts as a negative regulator, and the degradation of its substrate proteins via ubiquitination plays important roles in regulating various abiotic stress responses via an ABA-independent pathway.

Revealing the complexity of protein abundance in chickpea root under drought-stress using a comparative proteomics approach

Global warming has reached an alarming situation, which led to a dangerous climatic condition. The irregular rainfalls and land degradation are the significant consequences of these climatic changes causing a decrease in crop productivity. The effect of drought and its tolerance mechanism, a comparative roots proteomic analysis of chickpea seedlings grown under hydroponic conditions for three weeks, performed at different time points using 2-Dimensional gel electrophoresis (2-DE). After PD-Quest analysis, 110 differentially expressed spots subjected to MALDI-TOF/TOF and 75 spots identified with a significant score. These identified proteins classified into eight categories based on their functional annotation. Proteins involved in carbon and energy metabolism comprised 23% of total identified proteins include mainly glyceraldehyde-3-phosphate dehydrogenase, malate dehydrogenase, transaldolase, and isocitrate dehydrogenase. Proteins related to stress response (heat-shock protein, CS domain protein, and chitinase 2-like) contributed 16% of total protein spots followed by 13% involved in protein metabolism (adenosine kinase 2, and protein disulfide isomerase). ROS metabolism contributed 13% (glutathione S-transferase, ascorbate peroxidase, and thioredoxin), and 9% for signal transduction (actin-101, and 14-3-3-like protein B). Five percent protein identified for secondary metabolism (cinnamoyl-CoA reductase-1 and chalcone-flavononeisomerase 2) and 7% for nitrogen (N) and amino acid metabolism (glutamine synthetase and homocysteine methyltransferase). The abundance of some proteins validated by using Western blotting and Real-Time-PCR. The detailed information for drought-responsive root protein(s) through comparative proteomics analysis can be utilized in the future for genetic improvement programs to develop drought-tolerant chickpea lines.

Zn2+ dependent glyoxalase I plays the major role in methylglyoxal detoxification and salinity stress tolerance in plants

Glyoxalase pathway is the major pathway of methylglyoxal detoxification and is ubiquitously present in all organisms ranging from prokaryotes to eukaryotes. Glyoxalase I (GLYI) and Glyoxalase II (GLYII), the two core enzymes of this pathway work together to neutralize methylglyoxal (MG), a dicarbonyl molecule with detrimental cytotoxicity at higher concentrations. The first step towards the detoxification of MG is catalyzed by GLYI, a metalloenzyme that requires divalent metal ions (either Zn²⁺ as seen in eukaryotes or Ni²⁺ as in prokaryotes). However, both Zn²⁺ and Ni²⁺ dependent GLYIs have been shown to co-exist in a higher eukaryote i.e. Arabidopsis thaliana. In the present study, we determine the role of both Zn²⁺ dependent (AtGLYI2) and Ni²⁺ dependent (AtGLYI3, AtGLYI6) GLYIs from Arabidopsis in salinity stress tolerance. AtGLYI2 overexpressing Arabidopsis plants showed better growth rate while maintaining lower levels of MG under high saline conditions. They were taller with more number of silique formation with respect to their Ni²⁺ dependent counterparts. Further, lack in germination of Arabidopsis AtGLYI2 mutants in presence of exogenous MG indicates the direct involvement of Zn²⁺ dependent GLYI in MG detoxification, suggesting Zn²⁺ dependent GLYI as the main enzyme responsible for MG detoxification and salinity stress tolerance.

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.

From methylglyoxal to pyruvate: a genome-wide study for the identification of glyoxalases and D-lactate dehydrogenases in Sorghum bicolor

Background

The glyoxalase pathway is evolutionarily conserved and involved in the glutathione-dependent detoxification of methylglyoxal (MG), a cytotoxic by-product of glycolysis. It acts via two metallo-enzymes, glyoxalase I (GLYI) and glyoxalase II (GLYII), to convert MG into D-lactate, which is further metabolized to pyruvate by D-lactate dehydrogenases (D-LDH). Since D-lactate formation occurs solely by the action of glyoxalase enzymes, its metabolism may be considered as the ultimate step of MG detoxification. By maintaining steady state levels of MG and other reactive dicarbonyl compounds, the glyoxalase pathway serves as an important line of defence against glycation and oxidative stress in living organisms. Therefore, considering the general role of glyoxalases in stress adaptation and the ability of Sorghum bicolor to withstand prolonged drought, the sorghum glyoxalase pathway warrants an in-depth investigation with regard to the presence, regulation and distribution of glyoxalase and D-LDH genes.

Result

Through this study, we have identified 15 GLYI and 6 GLYII genes in sorghum. In addition, 4 D-LDH genes were also identified, forming the first ever report on genome-wide identification of any plant D-LDH family. Our in silico analysis indicates homology of putatively active SbGLYI, SbGLYII and SbDLDH proteins to several functionally characterised glyoxalases and D-LDHs from Arabidopsis and rice. Further, these three gene families exhibit development and tissue-specific variations in their expression patterns. Importantly, we could predict the distribution of putatively active SbGLYI, SbGLYII and SbDLDH proteins in at least four different sub-cellular compartments namely, cytoplasm, chloroplast, nucleus and mitochondria. Most of the members of the sorghum glyoxalase and D-LDH gene families are indeed found to be highly stress responsive.

Conclusion

This study emphasizes the role of glyoxalases as well as that of D-LDH in the complete detoxification of MG in sorghum. In particular, we propose that D-LDH which metabolizes the specific end product of glyoxalases pathway is essential for complete MG detoxification. By proposing a cellular model for detoxification of MG via glyoxalase pathway in sorghum, we suggest that different sub-cellular organelles are actively involved in MG metabolism in plants.

Drought-responsive genes, late embryogenesis abundant group3 (LEA3) and vicinal oxygen chelate, function in lipid accumulation in Brassica napus and Arabidopsis mainly via enhancing photosynthetic efficiency and reducing ROS

Drought is an abiotic stress that affects plant growth, and lipids are the main economic factor in the agricultural production of oil crops. However, the molecular mechanisms of drought response function in lipid metabolism remain little known. In this study, overexpression (OE) of different copies of the drought response genes LEA3 and VOC enhanced both drought tolerance and oil content in Brassica napus and Arabidopsis. Meanwhile, seed size, membrane stability and seed weight were also improved in OE lines. In contrast, oil content and drought tolerance were decreased in the AtLEA3 mutant (atlea3) and AtVOC-RNAi of Arabidopsis and in both BnLEA-RNAi and BnVOC-RNAi B. napus RNAi lines. Hybrids between two lines with increased or reduced expression (LEA3-OE with VOC-OE, atlea3 with AtVOC-RNAi) showed corresponding stronger trends in drought tolerance and lipid metabolism. Comparative transcriptomic analysis revealed the mechanisms of drought response gene function in lipid accumulation and drought tolerance. Gene networks involved in fatty acid (FA) synthesis and FA degradation were up- and down-regulated in OE lines, respectively. Key genes in the photosynthetic system and reactive oxygen species (ROS) metabolism were up-regulated in OE lines and down-regulated in atlea3 and AtVOC-RNAi lines, including LACS9, LIPASE1, PSAN, LOX2 and SOD1. Further analysis of photosynthetic and ROS enzymatic activities confirmed that the drought response genes LEA3 and VOC altered lipid accumulation mainly via enhancing photosynthetic efficiency and reducing ROS. The present study provides a novel way to improve lipid accumulation in plants, especially in oil production crops.

GLYI4 Plays A Role in Methylglyoxal Detoxification and Jasmonate-Mediated Stress Responses in Arabidopsis thaliana

Plant hormones play a central role in various physiological functions and in mediating defense responses against (a)biotic stresses. In response to primary metabolism alteration, plants can produce also small molecules such as methylglyoxal (MG), a cytotoxic aldehyde. MG is mostly detoxified by the combined actions of the enzymes glyoxalase I (GLYI) and glyoxalase II (GLYII) that make up the glyoxalase system. Recently, by a genome-wide association study performed in Arabidopsis, we identified GLYI4 as a novel player in the crosstalk between jasmonate (JA) and salicylic acid (SA) hormone pathways. Here, we investigated the impact of GLYI4 knock-down on MG scavenging and on JA pathway. In glyI4 mutant plants, we observed a general stress phenotype, characterized by compromised MG scavenging, accumulation of reactive oxygen species (ROS), stomatal closure, and reduced fitness. Accumulation of MG in glyI4 plants led to lower efficiency of the JA pathway, as highlighted by the increased susceptibility of the plants to the pathogenic fungus Plectospherella cucumerina. Moreover, MG accumulation brought about a localization of GLYI4 to the plasma membrane, while MeJA stimulus induced a translocation of the protein into the cytoplasmic compartment. Collectively, the results are consistent with the hypothesis that GLYI4 is a hub in the MG and JA pathways.

Alr2321, a multiple stress inducible glyoxalase I of Anabaena sp. PCC7120 detoxifies methylglyoxal and reactive species oxygen

Abiotic stresses enhance the cellular level of reactive oxygen species (ROS) which consequently leads to toxic methylglyoxal (MG) production. Glyoxalases (GlyI & GlyII) catalyze the conversion of toxic MG into non-toxic lactic acid but their properties and functions have been overlooked in cyanobacteria. This is the first attempt to conduct a genome-wide analysis of GlyI protein (PF00903) from Anabaena sp. PCC7120. Out of total nine GlyI domain possessing proteins, only three (Alr2321, Alr4469, All1022) harbour conserve His/Glu/His/Glu metal binding site at their homologous position and are deficient in conserved region specific for Zn²⁺ dependent members. Their biochemical, structural and functional characterization revealed that only Alr2321 is a homodimeric Ni²⁺ dependent active GlyI with catalytic efficiency 11.7 × 10⁶ M^-1 s^-1. It has also been found that Alr2321 is activated by various divalent metal ions and has maximum GlyI activity with Ni²⁺ followed by Co²⁺ > Mn²⁺ > Cu²⁺ and no activity with Zn^2+. Moreover, the expression of alr2321 was found to be maximally up-regulated under heat (19 fold) followed by cadmium, desiccation, arsenic, salinity and UV-B stresses. BL21/pGEX-5X2-alr2321 showed improved growth under various abiotic stresses as compared to BL21/pGEX-5X2 by increased scavenging of intracellular MG and ROS levels. Taken together, these results suggest noteworthy links between intracellular MG and ROS, its detoxification by Alr2321, a member of GlyI family of Anabaena sp. PCC7120, in relation to abiotic stress.

Nutrient enhancement of chickpea grown with plant growth promoting bacteria in local soil of Bathinda, Northwestern India

Plant growth promoting bacteria (PGPB) enhance crop productivity as part of green technology to reduce the use of chemical fertilizers. They also have the capability to enhance macro- and micronutrient content of plants. In the present study, PGPB isolates belonging to Pseudomonas citronellis (PC), Pseudomonas sp. RA6, Serratia sp. S2, Serratia marcescens CDP13, and Frateuria aurantia (Symbion-K) were tested on two chickpea varieties, PBG1 and PBG5 grown for 30 days in local soil from Bathinda region in Northwestern India. PC and CDP13 were found to be better chickpea growth stimulators compared to the commercial Symbion-K based on shoot length and biomass. Most PGPB enhanced macro- and micronutrients in shoots to varying degrees compared to the control. PBG5 gave better response compared to PBG1 with reference to plant growth attributes and enhancement of the macronutrients, calcium, nitrogen and phosphorus and micronutrients, boron, copper, iron, and zinc. PBG5 is a high yielding variety with better resistance compared to PBG1. Overall, PGPB isolated from the local soil and PGPB from other parts of India were shown to be useful for enhancement of nutrient content and plant growth.

Preliminary Characterization of a Ni2+-Activated and Mycothiol-Dependent Glyoxalase I Enzyme from Streptomyces coelicolor

The glyoxalase system consists of two enzymes, glyoxalase I (Glo1) and glyoxalase II (Glo2), and converts a hemithioacetal substrate formed between a cytotoxic alpha-ketoaldehyde, such as methylglyoxal (MG), and an intracellular thiol, such as glutathione, to a non-toxic alpha-hydroxy acid, such as d-lactate, and the regenerated thiol. Two classes of Glo1 have been identified. The first is a Zn2+-activated class and is exemplified by the Homo sapiens Glo1. The second class is a Ni2+-activated enzyme and is exemplified by the Escherichia coli Glo1. Glutathione is the intracellular thiol employed by Glo1 from both these sources. However, many organisms employ other intracellular thiols. These include trypanothione, bacillithiol, and mycothiol. The trypanothione-dependent Glo1 from Leishmania major has been shown to be Ni2+-activated. Genetic studies on Bacillus subtilis and Corynebacterium glutamicum focused on MG resistance have indicated the likely existence of Glo1 enzymes employing bacillithiol or mycothiol respectively, although no protein characterizations have been reported. The current investigation provides a preliminary characterization of an isolated mycothiol-dependent Glo1 from Streptomyces coelicolor. The enzyme has been determined to display a Ni2+-activation profile and indicates that Ni2+-activated Glo1 are indeed widespread in nature regardless of the intracellular thiol employed by an organism.

Identification of physiological and biochemical markers for salt (NaCl) stress in the seedlings of mungbean [Vigna radiata (L.) Wilczek] genotypes

Salt stress, which is dominant among environmental stresses, poses challenges to global agriculture. We studied the role of exogenous application of sodium chloride (NaCl) in three arid and three semi-arid genotypes of mungbean [Vigna radiata (L.) Wilczek] by examining some physiological and biochemical stress indicators. Ten-day old seedlings were subjected to salt stress (00-250 mM) by split application along with the half strength Hoagland's medium. The salt stress caused a decline in the fresh weight, dry weight, relative water content, photosynthetic pigments (chlorophyll and carotenoids) and glutathione content of the seedlings. On the other hand, it increased the electrolyte leakage, lipoxygenase activity, and the proline, protein and total soluble sugar contents. Osmolyte accumulation was relatively higher in the arid genotypes revealing that they are more tolerant to NaCl stress. The physiological and biochemical screening provides a basic platform for selecting the stress-tolerant genotypes in the absence of suitable salt-tolerance markers in mungbean.