Glutamate functions in stomatal closure in Arabidopsis and fava bean

Involvement of GLR-mediated nitric oxide effects on ROS metabolism in Arabidopsis plants under salt stress

Plant glutamate receptor-like channels (GLRs) play important roles in plant development, immune response, defense signaling and Nitric oxide (NO) production. However, their involvement in abiotic stress responses, particularly in regulating Reactive Oxygen Species (ROS), is not well understood. This study aimed to investigate GLR-mediated NO production on ROS regulation in salt-stressed cells. To achieve this, Arabidopsis thaliana Columbia (Col-0) were treated with NaCl, glutamate antagonists [(DNQX (6,7-dinitroquinoxaline-2,3-dione and AP-5(D-2-amino-5-phosphono pentanoic acid)], and NO scavenger [cPTIO (2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt)]. Salt-stressed plants in combination with DNQX and AP-5 have exhibited higher increase in lipid peroxidation (TBARS), hydrogen peroxide (H2O2) and superoxide radical (O-2) contents as compared to solely NaCl-treated plants. Furthermore, NO and total glutathione contents, and S-nitrosoglutathione reductase (GSNOR) activity decreased with these treatments. AP-5 and DNQX increased the activities of NADPH oxidase (NOX), catalase (CAT), peroxidase (POX), cell wall peroxidase (CWPOX) in salt-stressed Arabidopsis leaves. However, their activities (except NOX) were significantly inhibited by cPTIO. Conversely, the combination of NaCl and GLR antagonists, NO scavenger decreased the activities of ascorbate peroxidase (APX), superoxide dismutase (SOD), glutathione reductase (GR), dehydroascorbate reductase (DHAR) and monodehydroascorbate reductase (MDHAR) resulting in elevated GSSG levels, a low GSH/GSSG ratio, impaired ROS scavenging, excessive ROS accumulation and cell membrane damage. The findings of this study provide evidence that GLR-mediated NO plays a crucial role in improvement of the tolerance of Arabidopsis plants to salt-induced oxidative stress. It helps to maintain cellular redox homeostasis by reducing ROS accumulation and increasing the activity of SOD, GSNOR, and the ASC-GSH cycle enzymes.

The GABA shunt contributes to ROS homeostasis in guard cells of Arabidopsis

γ-Aminobutyric acid (GABA) accumulates rapidly under stress via the GABA shunt pathway, which has been implicated in reducing the accumulation of stress-induced reactive oxygen species (ROS) in plants. γ-Aminobutyric acid has been demonstrated to act as a guard-cell signal in Arabidopsis thaliana, modulating stomatal opening. Knockout of the major GABA synthesis enzyme Glutamate Decarboxylase 2 (GAD2) increases the aperture of gad2 mutants, which results in greater stomatal conductance and reduces water-use efficiency compared with wild-type plants. Here, we found that the additional loss of GAD1, GAD4, and GAD5 in gad2 leaves increased GABA deficiency but abolished the more open stomatal pore phenotype of gad2, which we link to increased cytosolic calcium (Ca2+ ) and ROS accumulation in gad1/2/4/5 guard cells. Compared with wild-type and gad2 plants, glutamate was ineffective in closing gad1/2/4/5 stomatal pores, whereas lowering apoplastic calcium, applying ROS inhibitors or complementation with GAD2 reduced gad1/2/4/5 guard-cell ROS, restored the gad2-like greater stomatal apertures of gad1/2/4/5 beyond that of wild-type. We conclude that GADs are important contributors to ROS homeostasis in guard cells likely via a Ca2+ -mediated pathway. As such, this study reveals greater complexity in GABA's role as a guard-cell signal and the interactions it has with other established signals.

Calcium Signaling and the Response to Heat Shock in Crop Plants

Climate change and the increasing frequency of high temperature (HT) events are significant threats to global crop yields. To address this, a comprehensive understanding of how plants respond to heat shock (HS) is essential. Signaling pathways involving calcium (Ca2+), a versatile second messenger in plants, encode information through temporal and spatial variations in ion concentration. Ca2+ is detected by Ca2+-sensing effectors, including channels and binding proteins, which trigger specific cellular responses. At elevated temperatures, the cytosolic concentration of Ca2+ in plant cells increases rapidly, making Ca2+ signals the earliest response to HS. In this review, we discuss the crucial role of Ca2+ signaling in raising plant thermotolerance, and we explore its multifaceted contributions to various aspects of the plant HS response (HSR).

Widely targeted metabolic profiling provides insights into variations in bioactive compounds and antioxidant activity of sesame, soybean, peanut, and perilla

Oilseeds are important sources of diversified nutraceuticals with marked health attributes. Thus, a better understanding of metabolome differences between common oilseeds will be conducive to the food pharmacy. This study aimed to compare the metabolite profiles and antioxidant activity of sesame, soybean, peanut, and perilla seeds and reveal the variation in bioactive compounds. LC-MS-based widely targeted metabolic profiling identified a total of 975 metabolites, of which 753 were common to the four crops. Multivariate analyses unveiled a crop-specific accumulation of metabolites, with 298-388 DAMs (differentially accumulated metabolites) identified. Amino acid metabolism, phenylpropanoid biosynthesis, flavonoid biosynthesis, and lipid metabolism were the most differentially regulated pathways. Furthermore, we revealed the variation in the relative content of 48, 20, 18, 9, 18, 11, and 6 differentially accumulated bioactive flavonoids, phenolic acids, amino acids, vitamins, terpenoids, alkaloids, and coumarins, respectively. Most of the flavonoids accumulated highly in soybean, followed by perilla. Sesame exhibited a better amino acid profile than other oilseeds. DPPH and FRAP assays showed that the antioxidant activity of perilla seed extracts was the highest, followed by soybean, peanut, and sesame. Our results provide data support for the comprehensive use of sesame, perilla, soybean, and peanut seeds in food, and pharmaceutical industries.

Effect of Glutamic Acid and 6-benzylaminopurine on Flower Bud Biostimulation, Fruit Quality and Antioxidant Activity in Blueberry

Blueberry is a highly demanded and consumed fruit due to its beneficial effects on human health, because of its bioactive compounds with a high antioxidant capacity. The interest in increasing the yield and quality of blueberries has led to the application of some innovative techniques such as biostimulation. The objective of this research was to assess the effect of the exogenous application of glutamic acid (GLU) and 6-benzylaminopurine (6-BAP) as biostimulants on flower bud sprouting, fruit quality, and antioxidant compounds in blueberry cv. Biloxi. The application of GLU and 6-BAP positively affected bud sprouting, fruit quality, and antioxidant content. The application of 500 and 10 mg L^-1 GLU and 6-BAP, respectively, increased the number of flower buds, while 500 and 20 mg L^-1 generated fruits with higher content of flavonoids, vitamin C, and anthocyanins and higher enzymatic activity of catalase and ascorbate peroxidase enzymes. Hence, the application of these biostimulants is an effective way to enhance the yield and fruit quality of blueberries.

Bioengineering Techniques to Improve Nitrogen Transformation and Utilization: Implications for Nitrogen Use Efficiency and Future Sustainable Crop Production

Nitrogen (N) is crucial for plant growth and development, especially in physiological and biochemical processes such as component of different proteins, enzymes, nucleic acids, and plant growth regulators. Six categories, such as transporters, nitrate absorption, signal molecules, amino acid biosynthesis, transcription factors, and miscellaneous genes, broadly encompass the genes regulating NUE in various cereal crops. Herein, we outline detailed research on bioengineering modifications of N metabolism to improve the different crop yields and biomass. We emphasize effective and precise molecular approaches and technologies, including N transporters, transgenics, omics, etc., which are opening up fascinating opportunities for a complete analysis of the molecular elements that contribute to NUE. Moreover, the detection of various types of N compounds and associated signaling pathways within plant organs have been discussed. Finally, we highlight the broader impacts of increasing NUE in crops, crucial for better agricultural yield and in the greater context of global climate change.

Genome-wide association analysis for drought tolerance and associated traits in faba bean (Vicia faba L.)

Faba bean (Vicia faba L.) is an important high protein legume adapted to diverse climatic conditions with multiple benefits for the overall sustainability of the cropping systems. Plant-based protein demand is being expanded and faba bean is a good candidate to cover this need. However, the crop is very sensitive to abiotic stresses, especially drought, which severely affects faba bean yield and development worldwide. Therefore, identifying genes associated with drought stress tolerance is a major challenge in faba bean breeding. Although the faba bean response to drought stress has been widely studied, the molecular approaches to improve drought tolerance in this crop are still limited. Here we built on recent genomic advances such as the development of the first high-density SNP genotyping array, to conduct a genome-wide association study (GWAS) using thousands of genetic polymorphisms throughout the entire faba bean genome. A worldwide collection of 100 faba bean accessions was grown under control and drought conditions and 10 morphological, phenological and physiological traits were evaluated to identify single nucleotide polymorphism (SNP) markers associated with drought tolerance. We identified 29 SNP markers significantly correlated with these traits under drought stress conditions. The flanking sequences were blasted to the Medicago truncatula reference genomes in order to annotate potential candidate genes underlying the causal variants. Three of the SNPs for chlorophyll content after the stress, correspond to uncharacterized proteins indicating the presence of novel genes associated with drought tolerance in faba bean. The significance of stress-inducible signal transducers provides valuable information on the possible mechanisms underlying the faba bean response to drought stress, thus providing a foundation for future marker-assisted breeding in the crop.

l-Glutamate activates salicylic acid signaling to promote stomatal closure and PR1 expression in Arabidopsis

Glutamate (l-Glu), an animal neurotransmitter, plays an essential role in plant signaling and regulates various plant physiological responses. We previously showed that l-Glu regulates stomatal closure in Arabidopsis via the glutamate receptor-like 3.5 gene (GLR3.5). Here, we showed that l-Glu activates salicylic acid (SA) signaling in Arabidopsis. l-Glu not only promoted stomatal closure but also triggered the expression of the PR1 gene via GLR3.5. These l-Glu-dependent actions were strongly suppressed in SA-insensitive npr1-1 and SA-deficient sid2-2 mutants, indicating that SA is involved in l-Glu signaling. A loss-of-function mutant of the gene encoding the SRK2E/OST1 kinase, which plays a pivotal role in abscisic acid signaling, was insensitive to both l-Glu-induced stomatal closure and PR1 expression. The glr3.5 mutants did not alleviate SA-induced stomatal closure, indicating that SA may function downstream of GLR3.5. These results indicate that l-Glu activates SA signaling, and that SRK2E/OST1 may play pivotal roles in such signaling.

Comprehensive Analysis of Glutamate Receptor-like Genes in Rice (Oryza sativa L.): Genome-Wide Identification, Characteristics, Evolution, Chromatin Accessibility, gcHap Diversity, Population Variation and Expression Analysis

Glutamate receptors (GLR) are widely present in animals and plants, playing essential roles in regulating plant growth, development and stress response. At present, most studies of GLRs in plants are focused on Arabidopsis thaliana, while there have been few studies on rice. In this study, we identified 26 OsGLR genes in rice (Oryza sativa L.). Then, we analyzed the chromosomal location, physical and chemical properties, subcellular location, transmembrane (TM) helices, signal peptides, three-dimensional (3D) structure, cis-acting elements, evolution, chromatin accessibility, population variation, gene-coding sequence haplotype (gcHap) and gene expression under multiple abiotic stress and hormone treatments. The results showed that out of the 26 OsGLR genes, ten genes had the TM domain, signal peptides and similar 3D structures. Most OsGLRs exhibited high tissue specificity in expression under drought stress. In addition, several OsGLR genes were specifically responsive to certain hormones. The favorable gcHap of many OsGLR genes in modern varieties showed obvious differentiation between Xian/indica and Geng/japonica subspecies. This study, for the first time, comprehensively analyzes the OsGLR genes in rice, and provides an important reference for further research on their molecular function.

Structure, Function, and Applications of Soybean Calcium Transporters

Glycine max is a calcium-loving crop. The external application of calcium fertilizer is beneficial to the increase of soybean yield. Indeed, calcium is a vital nutrient in plant growth and development. As a core metal ion in signaling transduction, calcium content is maintained in dynamic balance under normal circumstances. Now, eight transporters were found to control the uptake and efflux of calcium. Though these calcium transporters have been identified through genome-wide analysis, only a few of them were functionally verified. Therefore, in this study, we summarized the current knowledge of soybean calcium transporters in structural features, expression characteristics, roles in stress response, and prospects. The above results will be helpful in understanding the function of cellular calcium transport and provide a theoretical basis for elevating soybean yield.

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

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

Extracellular malate induces stomatal closure via direct activation of guard-cell anion channel SLAC1 and stimulation of Ca2+ signalling

Plants secrete malate from guard cells to apoplast under stress conditions and exogenous malate induces stomatal closure. Malate is considered an extracellular chemical signal of stomatal closure. However, the molecular mechanism of malate-induced stomatal closure is not fully elucidated. We investigated responses of stomatal aperture, ion channels, and cytosolic Ca²⁺ to malate. A treatment with malate induced stomatal closure in Arabidopsis thaliana wild-type plants, but not in the mutants deficient in the slow (S-type) anion channel gene SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1). The treatment with malate increased S-type anion currents in guard-cell protoplasts of wild-type plants but not in the slac1 mutant. In addition, extracellular rather than intracellular application of malate increased the S-type currents of constitutively active mutants of SLAC1, which have kinase-independent activities, in a heterologous expression system using Xenopus oocytes. The treatment with malate transiently increased cytosolic Ca²⁺ concentration in the wild-type Arabidopsis guard cells and the malate-induced stomatal closure was inhibited by the Ca²⁺ channel blocker and the Ca²⁺ chelator. These results indicate that extracellular malate directly activates SLAC1 and simultaneously stimulates Ca²⁺ signalling in guard cells, resulting in steady and solid activation of SLAC1 for stomatal closure.

An Insight into Animal Glutamate Receptors Homolog of Arabidopsis thaliana and Their Potential Applications-A Review

Most excitatory impulses received by neurons are mediated by ionotropic glutamate receptors (iGluRs). These receptors are located at the apex and play an important role in memory, neuronal development, and synaptic plasticity. These receptors are ligand-dependent ion channels that allow a wide range of cations to pass through. Glutamate, a neurotransmitter, activates three central ionotropic receptors: N-methyl-D-aspartic acid (NMDA), -amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), and kainic acid (KA). According to the available research, excessive glutamate release causes neuronal cell death and promotes neurodegenerative disorders. Arabidopsis thaliana contains 20 glutamate receptor genes (AtGluR) comparable to the human ionotropic glutamate (iGluRs) receptor. Many studies have proved that AtGL-rec genes are involved in a number of plant growth and physiological activities, such as in the germination of seeds, roots, abiotic and biotic stress, and cell signaling, which clarify the place of these genes in plant biology. In spite of these, the iGluRs, Arabidopsis glutamate receptors (AtGluR), is associated with the ligand binding activity, which confirms the evolutionary relationship between animal and plant glutamate receptors. Along with the above activities, the impact of mammalian agonists and antagonists on Arabidopsis suggests a correlation between plant and animal glutamate receptors. In addition, these glutamate receptors (plant/animal) are being utilized for the early detection of neurogenerative diseases using the fluorescence resonance energy transfer (FRET) approach. However, a number of scientific laboratories and institutes are consistently working on glutamate receptors with different aspects. Currently, we are also focusing on Arabidopsis glutamate receptors. The current review is focused on updating knowledge on AtGluR genes, their evolution, functions, and expression, and as well as in comparison with iGluRs. Furthermore, a high throughput approach based on FRET nanosensors developed for understanding neurotransmitter signaling in animals and plants via glutamate receptors has been discussed. The updated information will aid in the future comprehension of the complex molecular dynamics of glutamate receptors and the exploration of new facts in plant/animal biology.

Deciphering genome-wide transcriptomic changes in grapevines heavily infested by spotted lanternflies

The spotted lanternfly, a newly invasive insect in the U.S. that is a great concern for the grapevine industry, produces damage on its host plants through aggressive feeding, using a piercing and sucking method to feed on the phloem of plants. In the eastern US, adult SLF can invade vineyards through fruit ripening until the end of the growing season; however, it is still unclear how prolonged late-season SLF feeding can affect the health of grapevines, as well as the host responses to this extensive damage. Thus, we have performed a comprehensive genome-wide transcriptome analysis in grapevines heavily infested by the spotted lanternfly, as it occurs in Pennsylvania vineyards, and compared it to other relevant transcriptomes in grapes with different degrees to susceptibility to similar pests. Among a variety of plant responses, we highlight here a subset of relevant biological pathways that distinguish or are common to the spotted lanternfly and other phloem feeders in grapevine. The molecular interaction between spotted lanternfly and the vine begins with activation of signal transduction cascades mediated mainly by protein kinase genes. It also induces the expression of transcription factors in the nucleus, of other signaling molecules like phytohormones and secondary metabolites, and their downstream target genes responsible for defense and physiological functions, such as detoxification and photosynthesis. Grapevine responses furthermore include the activation of genes for cell wall strengthening via biosynthesis of major structural components. With this study, we hope to provide the regulatory network to explain effects that the invasive spotted lanternfly has on grapevine health with the goal to improve its susceptibility.

Signaling by plant glutamate receptor-like channels: What else!

Plant glutamate receptor-like channels (GLRs) are transmembrane proteins that allow the movement of several ions across membranes. In the model plant Arabidopsis, there are 20 GLR isoforms grouped in three clades and, since their discovery, it was hypothesized that GLRs were mainly involved in signaling processes. Indeed, in the last years, several pieces of evidence demonstrate different signaling roles played by GLRs, related to pollen development, sexual reproduction, chemotaxis, root development, regulation of stomatal aperture, and response to pathogens. Recently, GLRs have gained attention for their role in long-distance electric and calcium signaling. In this review, we resume the evidence about the role of GLRs in signaling processes. This role is mostly linked to the GLRs involvement in the regulation of ion fluxes across membranes and, in particular, of calcium, which represents a key second messenger in plant cell responses to both endogenous and exogenous stimuli.

Glutamate: A multifunctional amino acid in plants

Glutamate (Glu) is a versatile metabolite and a signaling molecule in plants. Glu biosynthesis is associated with the primary nitrogen assimilation pathway. The conversion between Glu and 2-oxoglutarate connects Glu metabolism to the tricarboxylic acid cycle, carbon metabolism, and energy production. Glu is the predominant amino donor for transamination reactions in the cell. In addition to protein synthesis, Glu is a building block for tetrapyrroles, glutathione, and folate. Glu is the precursor of γ-aminobutyric acid that plays an important role in balancing carbon/nitrogen metabolism and various cellular processes. Glu can conjugate to the major auxin indole 3-acetic acid (IAA), and IAA-Glu is destined for oxidative degradation. Glu also conjugates with isochorismate for the production of salicylic acid. Accumulating evidence indicates that Glu functions as a signaling molecule to regulate plant growth, development, and defense responses. The ligand-gated Glu receptor-like proteins (GLRs) mediate some of these responses. However, many of the Glu signaling events are GLR-independent. The receptor perceiving extracellular Glu as a danger signal is still unknown. In addition to GLRs, Glu may act on receptor-like kinases or receptor-like proteins to trigger immune responses. Glu metabolism and Glu signaling may entwine to regulate growth, development, and defense responses in plants.

Evolution of glutamatergic signaling and synapses

Glutamate (Glu) is the primary excitatory transmitter in the mammalian brain. But, we know little about the evolutionary history of this adaptation, including the selection of l-glutamate as a signaling molecule in the first place. Here, we used comparative metabolomics and genomic data to reconstruct the genealogy of glutamatergic signaling. The origin of Glu-mediated communications might be traced to primordial nitrogen and carbon metabolic pathways. The versatile chemistry of L-Glu placed this molecule at the crossroad of cellular biochemistry as one of the most abundant metabolites. From there, innovations multiplied. Many stress factors or injuries could increase extracellular glutamate concentration, which led to the development of modular molecular systems for its rapid sensing in bacteria and archaea. More than 20 evolutionarily distinct families of ionotropic glutamate receptors (iGluRs) have been identified in eukaryotes. The domain compositions of iGluRs correlate with the origins of multicellularity in eukaryotes. Although L-Glu was recruited as a neuro-muscular transmitter in the early-branching metazoans, it was predominantly a non-neuronal messenger, with a possibility that glutamatergic synapses evolved more than once. Furthermore, the molecular secretory complexity of glutamatergic synapses in invertebrates (e.g., Aplysia) can exceed their vertebrate counterparts. Comparative genomics also revealed 15+ subfamilies of iGluRs across Metazoa. However, most of this ancestral diversity had been lost in the vertebrate lineage, preserving AMPA, Kainate, Delta, and NMDA receptors. The widespread expansion of glutamate synapses in the cortical areas might be associated with the enhanced metabolic demands of the complex brain and compartmentalization of Glu signaling within modular neuronal ensembles.

Recent Advances in the Physiology of Ion Channels in Plants

Our knowledge of plant ion channels was significantly enhanced by the first application of the patch-clamp technique to isolated guard cell protoplasts over 35 years ago. Since then, research has demonstrated the importance of ion channels in the control of gas exchange in guard cells, their role in nutrient uptake in roots, and the participation of calcium-permeable cation channels in the regulation of cell signaling affected by the intracellular concentrations of this second messenger. In recent years, through the employment of reverse genetics, mutant proteins, and heterologous expression systems, research on ion channels has identified mechanisms that modify their activity through protein-protein interactions or that result in activation and/or deactivation of ion channels through posttranslational modifications. Additional and confirmatory information on ion channel functioning has been derived from the crystallization and molecular modeling of plant proteins that, together with functional analyses, have helped to increase our knowledge of the functioning of these important membrane proteins that may eventually help to improve crop yield. Here, an update on the advances obtained in plant ion channel function during the last few years is presented.

Rapid movements in plants

Plant movements are generally slow, but some plant species have evolved the ability to move very rapidly at speeds comparable to those of animals. Whereas movement in animals relies on the contraction machinery of muscles, many plant movements use turgor pressure as the primary driving force together with secondarily generated elastic forces. The movement of stomata is the best-characterized model system for studying turgor-driven movement, and many gene products responsible for this movement, especially those related to ion transport, have been identified. Similar gene products were recently shown to function in the daily sleep movements of pulvini, the motor organs for macroscopic leaf movements. However, it is difficult to explain the mechanisms behind rapid multicellular movements as a simple extension of the mechanisms used for unicellular or slow movements. For example, water transport through plant tissues imposes a limit on the speed of plant movements, which becomes more severe as the size of the moving part increases. Rapidly moving traps in carnivorous plants overcome this limitation with the aid of the mechanical behaviors of their three-dimensional structures. In addition to a mechanism for rapid deformation, rapid multicellular movements also require a molecular system for rapid cell-cell communication, along with a mechanosensing system that initiates the response. Electrical activities similar to animal action potentials are found in many plant species, representing promising candidates for the rapid cell-cell signaling behind rapid movements, but the molecular entities of these electrical signals remain obscure. Here we review the current understanding of rapid plant movements with the aim of encouraging further biological studies into this fascinating, challenging topic.

Comparative Transcriptome, Metabolome, and Ionome Analysis of Two Contrasting Common Bean Genotypes in Saline Conditions

Soil salinity is a major abiotic stress factor that limits agricultural productivity worldwide, and this problem is expected to grow in the future. Common bean is an important protein source in developing countries however highly susceptible to salt stress. To understand the underlying mechanism of salt stress responses, transcriptomics, metabolomics, and ion content analysis were performed on both salt-tolerant and susceptible common bean genotypes in saline conditions. Transcriptomics has demonstrated increased photosynthesis in saline conditions for tolerant genotype while the susceptible genotype acted in contrast. Transcriptome also displayed active carbon and amino-acid metabolism for the tolerant genotype. Analysis of metabolites with GC-MS demonstrated the boosted carbohydrate metabolism in the tolerant genotype with increased sugar content as well as better amino-acid metabolism. Accumulation of lysine, valine, and isoleucine in the roots of the susceptible genotype suggested a halted stress response. According to ion content comparison, the tolerant genotype managed to block accumulation of Na⁺ in the leaves while accumulating significantly less Na⁺ in the roots compared to susceptible genotype. K⁺ levels increased in the leaves of both genotype and the roots of the susceptible one but dropped in the roots of the tolerant genotype. Additionally, Zn⁺² and Mn⁺² levels were dropped in the tolerant roots, while Mo⁺² levels were significantly higher in all tissues in both control and saline conditions for tolerant genotype. The results of the presented study have demonstrated the differences in contrasting genotypes and thus provide valuable information on the pivotal molecular mechanisms underlying salt tolerance.

Inhibition of light-induced stomatal opening by allyl isothiocyanate does not require guard cell cytosolic Ca2+ signaling

The glucosinolate-myrosinase system is a well-known defense system that has been shown to induce stomatal closure in Brassicales. Isothiocyanates are highly reactive hydrolysates of glucosinolates, and an isothiocyanate, allyl isothiocyanate (AITC), induces stomatal closure accompanied by elevation of free cytosolic Ca2+ concentration ([Ca2+]cyt) in Arabidopsis. It remains unknown whether AITC inhibits light-induced stomatal opening. This study investigated the role of Ca2+ in AITC-induced stomatal closure and inhibition of light-induced stomatal opening. AITC induced stomatal closure and inhibited light-induced stomatal opening in a dose-dependent manner. A Ca2+ channel inhibitor, La3+, a Ca2+chelator, EGTA, and an inhibitor of Ca2+ release from internal stores, nicotinamide, inhibited AITC-induced [Ca2+]cyt elevation and stomatal closure, but did not affect inhibition of light-induced stomatal opening. AITC activated non-selective Ca2+-permeable cation channels and inhibited inward-rectifying K+ (K+in) channels in a Ca2+-independent manner. AITC also inhibited stomatal opening induced by fusicoccin, a plasma membrane H+-ATPase activator, but had no significant effect on fusicoccin-induced phosphorylation of the penultimate threonine of H+-ATPase. Taken together, these results suggest that AITC induces Ca2+ influx and Ca2+ release to elevate [Ca2+]cyt, which is essential for AITC-induced stomatal closure but not for inhibition of K+in channels and light-induced stomatal opening.

Signaling Role of Glutamate in Plants

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

The structural bases for agonist diversity in an Arabidopsis thaliana glutamate receptor-like channel

Arabidopsis thaliana glutamate receptor-like (GLR) channels are amino acid-gated ion channels involved in physiological processes including wound signaling, stomatal regulation, and pollen tube growth. Here, fluorescence microscopy and genetics were used to confirm the central role of GLR3.3 in the amino acid-elicited cytosolic Ca²⁺ increase in Arabidopsis seedling roots. To elucidate the binding properties of the receptor, we biochemically reconstituted the GLR3.3 ligand-binding domain (LBD) and analyzed its selectivity profile; our binding experiments revealed the LBD preference for l-Glu but also for sulfur-containing amino acids. Furthermore, we solved the crystal structures of the GLR3.3 LBD in complex with 4 different amino acid ligands, providing a rationale for how the LBD binding site evolved to accommodate diverse amino acids, thus laying the grounds for rational mutagenesis. Last, we inspected the structures of LBDs from nonplant species and generated homology models for other GLR isoforms. Our results establish that GLR3.3 is a receptor endowed with a unique amino acid ligand profile and provide a structural framework for engineering this and other GLR isoforms to investigate their physiology.

The sucrose-to-malate ratio correlates with the faster CO2 and light stomatal responses of angiosperms compared to ferns

Stomatal responses to environmental signals differ substantially between ferns and angiosperms. However, the mechanisms that lead to such different responses remain unclear. Here we investigated the extent to which leaf metabolism contributes to coordinate the differential stomatal behaviour among ferns and angiosperms. Stomata from all species were responsive to light and CO₂ transitions. However, fern stomatal responses were slower and minor in both absolute and relative terms. Angiosperms have higher stomatal density, but this is not correlated with speed of stomatal closure. The metabolic responses throughout the diel course and under different CO₂ conditions differ substantially among ferns and angiosperms. Higher sucrose content and an increased sucrose-to-malate ratio during high CO₂ -induced stomatal closure was observed in angiosperms compared to ferns. Furthermore, the speed of stomatal closure was positively and negatively correlated with sugars and organic acids, respectively, suggesting that the balance between sugars and organic acids aids in explaining the faster stomatal responses of angiosperms. Our results suggest that mesophyll-derived metabolic signals, especially those associated with sucrose and malate, may also be important to modulate the differential stomatal behaviour between ferns and angiosperms, providing important new information that helps in understanding the metabolism-mediated mechanisms regulating stomatal movements across land plant evolution.

Nitrite and nitric oxide are important in the adjustment of primary metabolism during the hypersensitive response in tobacco

Nitrate and ammonia deferentially modulate primary metabolism during the hypersensitive response in tobacco. In this study, tobacco RNAi lines with low nitrite reductase (NiRr) levels were used to investigate the roles of nitrite and nitric oxide (NO) in this process. The lines accumulate NO2-, with increased NO generation, but allow sufficient reduction to NH4+ to maintain plant viability. For wild-type (WT) and NiRr plants grown with NO3-, inoculation with the non-host biotrophic pathogen Pseudomonas syringae pv. phaseolicola induced an accumulation of nitrite and NO, together with a hypersensitive response (HR) that resulted in decreased bacterial growth, increased electrolyte leakage, and enhanced pathogen resistance gene expression. These responses were greater with increases in NO or NO2- levels in NiRr plants than in the WT under NO3- nutrition. In contrast, WT and NiRr plants grown with NH4+ exhibited compromised resistance. A metabolomic analysis detected 141 metabolites whose abundance was differentially changed as a result of exposure to the pathogen and in response to accumulation of NO or NO2-. Of these, 13 were involved in primary metabolism and most were linked to amino acid and energy metabolism. HR-associated changes in metabolism that are often linked with primary nitrate assimilation may therefore be influenced by nitrite and NO production.

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

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

Asparagine alters action potential parameters in single plant cell

Effect of amino acid L-asparagine on electrical signalling of single Nitellopsis obtusa (Characeaen) cell was investigated using glass-microelectrode technique in current-clamp and voltage-clamp modes. Cell exposure for 30 min to 0.1 mM and 1 mM of asparagine resulted in changes of electrically stimulated action potential (AP) parameters in comparison to standard conditions. Results indicate that asparagine acts in dose-dependent manner: increases AP amplitude by hyperpolarizing AP threshold potential (E_th), prolongs action potential repolarization, increases maximum Cl^- efflux amplitude along with the increase of activation and inactivation durations. Presented findings provide new aspects of exogenous amino acids' effect on plants' electrical signalling with emphasis on separate single plant cell excitability and AP characteristics.

Insights into ABA-mediated regulation of guard cell primary metabolism revealed by systems biology approaches

Despite the fact that guard cell abscisic acid (ABA) signalling pathway is well documented, our understanding concerning how and to which extent ABA regulates guard cell metabolism remains fragmentary. Here we have adopted different systems approaches to investigate how ABA modulates guard cell central metabolism by providing genes that are possibly ABA-regulated. By using previous published Arabidopsis guard cell transcript profiling data, we carried out an extensive co-expression network analysis using ABA-related genes and those related to the metabolism and transport of sugars, starch and organic acids. Next, we investigated the presence of ABA responsive elements (ABRE) in the promoter of genes that are highly expressed in guard cells, responsive to ABA and co-expressed with ABA-related genes. Together, these analyses indicated that 44 genes are likely regulated by ABA and 8 of them are highly expressed in guard cells in both the presence and absence of ABA, including genes of the tricarboxylic acid cycle and those related to sucrose and hexose transport and metabolism. It seems likely that ABA may modulate both sucrose transport through guard cell plasma membrane and sucrose metabolism within guard cells. In this context, genes associated with sucrose synthase, sucrose phosphate synthase, trehalose-6-phosphate, invertase, UDP-glucose epimerase/pyrophosphorylase and different sugar transporters contain ABRE in their promoter and are thus possibly ABA regulated. Although validation experiments are required, our study highlights the importance of systems biology approaches to drive new hypothesis and to unravel genes and pathways that are regulated by ABA in guard cells.

Metabolomics of Early Stage Plant Cell-Microbe Interaction Using Stable Isotope Labeling

Metabolomics has been used in unraveling metabolites that play essential roles in plant-microbe (including pathogen) interactions. However, the problem of profiling a plant metabolome with potential contaminating metabolites from the coexisting microbes has been largely ignored. To address this problem, we implemented an effective stable isotope labeling approach, where the metabolome of a plant bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000 was labeled with heavy isotopes. The labeled bacterial cells were incubated with Arabidopsis thaliana epidermal peels (EPs) with guard cells, and excessive bacterial cells were subsequently removed from the plant tissues by washing. The plant metabolites were characterized by liquid chromatography mass spectrometry using multiple reactions monitoring, which can differentiate plant and bacterial metabolites. Targeted metabolomic analysis suggested that Pst DC3000 infection may modulate stomatal movement by reprograming plant signaling and primary metabolic pathways. This proof-of-concept study demonstrates the utility of this strategy in differentiation of the plant and microbe metabolomes, and it has broad applications in studying metabolic interactions between microbes and other organisms.

Metabolism within the specialized guard cells of plants

Contents 1018 I. 1018 II. 1019 III. 1022 IV. 1025 V. 1026 VI. 1029 1030 References 1030 SUMMARY: Stomata are leaf epidermal structures consisting of two guard cells surrounding a pore. Changes in the aperture of this pore regulate plant water-use efficiency, defined as gain of C by photosynthesis per leaf water transpired. Stomatal aperture is actively regulated by reversible changes in guard cell osmolyte content. Despite the fact that guard cells can photosynthesize on their own, the accumulation of mesophyll-derived metabolites can seemingly act as signals which contribute to the regulation of stomatal movement. It has been shown that malate can act as a signalling molecule and a counter-ion of potassium, a well-established osmolyte that accumulates in the vacuole of guard cells during stomatal opening. By contrast, their efflux from guard cells is an important mechanism during stomatal closure. It has been hypothesized that the breakdown of starch, sucrose and lipids is an important mechanism during stomatal opening, which may be related to ATP production through glycolysis and mitochondrial metabolism, and/or accumulation of osmolytes such as sugars and malate. However, experimental evidence supporting this theory is lacking. Here we highlight the particularities of guard cell metabolism and discuss this in the context of the guard cells themselves and their interaction with the mesophyll cells.

How do plants sense their nitrogen status?

The primary processes that contribute to the efficient capture of soil nitrate are the development of a root system that effectively explores the soil and the expression of high-affinity nitrate uptake systems in those roots. Both these processes are highly regulated to take into account the availability and distribution of external nitrate pools and the endogenous N status of the plant. While significant progress has been made in elucidating the early steps in sensing and responding to external nitrate, there is much less clarity about how the plant monitors its N status. This review specifically addresses the questions of what N compounds are sensed and in which part of the plant, as well as the identity of the signalling pathways responsible for their detection. Candidates that are considered for the role of N sensory systems include the target of rapamycin (TOR) signalling pathway, the general control non-derepressible 2 (GCN2) pathway, the plastidic PII-dependent pathway, and the family of glutamate-like receptors (GLRs). However, despite significant recent progress in elucidating the function and mode of action of these signalling systems, there is still much uncertainty about the extent to which they contribute to the process by which plants monitor their N status. The possibility is discussed that the large GLR family of Ca2+ channels, which are gated by a wide range of different amino acids and expressed throughout the plant, could act as amino acid sensors upstream of a Ca2+-regulated signalling pathway, such as the TOR pathway, to regulate the plant's response to changes in N status.

Evolutionary insight into the ionotropic glutamate receptor superfamily of photosynthetic organisms

Photosynthetic eukaryotes have a complex evolutionary history shaped by multiple endosymbiosis events that required a tight coordination between the organelles and the rest of the cell. Plant ionotropic glutamate receptors (iGLRs) form a large superfamily of proteins with a predicted or proven non-selective cation channel activity regulated by a broad range of amino acids. They are involved in different physiological processes such as C/N sensing, resistance against fungal infection, root and pollen tube growth and response to wounding and pathogens. Most of the present knowledge is limited to iGLRs located in plasma membranes. However, recent studies localized different iGLR isoforms to mitochondria and/or chloroplasts, suggesting the possibility that they play a specific role in bioenergetic processes. In this work, we performed a comparative analysis of GLR sequences from bacteria and various photosynthetic eukaryotes. In particular, novel types of selectivity filters of bacteria are reported adding new examples of the great diversity of the GLR superfamily. The highest variability in GLR sequences was found among the algal sequences (cryptophytes, diatoms, brown and green algae). GLRs of land plants are not closely related to the GLRs of green algae analyzed in this work. The GLR family underwent a great expansion in vascular plants. Among plant GLRs, Clade III includes sequences from Physcomitrella patens, Marchantia polymorpha and gymnosperms and can be considered the most ancient, while other clades likely emerged later. In silico analysis allowed the identification of sequences with a putative target to organelles. Sequences with a predicted localization to mitochondria and chloroplasts are randomly distributed among different type of GLRs, suggesting that no compartment-related specific function has been maintained across the species.