Glutamate receptor-like channel3.3 is involved in mediating glutathione-triggered cytosolic calcium transients, transcriptional changes, and innate immunity responses in Arabidopsis

Glutathione: a key modulator of plant defence and metabolism through multiple mechanisms

Redox reactions are fundamental to energy conversion in living cells, and also determine and tune responses to the environment. Within this context, the tripeptide glutathione plays numerous roles. As an important antioxidant, glutathione confers redox stability on the cell and also acts as an interface between signalling pathways and metabolic reactions that fuel growth and development. It also contributes to the assembly of cell components, biosynthesis of sulfur-containing metabolites, inactivation of potentially deleterious compounds, and control of hormonal signalling intensity. The multiplicity of these roles probably explains why glutathione status has been implicated in influencing plant responses to many different conditions. In particular, there is now a considerable body of evidence showing that glutathione is a crucial player in governing the outcome of biotic stresses. This review provides an overview of glutathione synthesis, transport, degradation, and redox turnover in plants. It examines the expression of genes associated with these processes during pathogen challenge and related conditions, and considers the diversity of mechanisms by which glutathione can influence protein function and gene expression.

PTI-ETI synergistic signal mechanisms in plant immunity

Plants face a relentless onslaught from a diverse array of pathogens in their natural environment, to which they have evolved a myriad of strategies that unfold across various temporal scales. Cell surface pattern recognition receptors (PRRs) detect conserved elicitors from pathogens or endogenous molecules released during pathogen invasion, initiating the first line of defence in plants, known as pattern-triggered immunity (PTI), which imparts a baseline level of disease resistance. Inside host cells, pathogen effectors are sensed by the nucleotide-binding/leucine-rich repeat (NLR) receptors, which then activate the second line of defence: effector-triggered immunity (ETI), offering a more potent and enduring defence mechanism. Moreover, PTI and ETI collaborate synergistically to bolster disease resistance and collectively trigger a cascade of downstream defence responses. This article provides a comprehensive review of plant defence responses, offering an overview of the stepwise activation of plant immunity and the interactions between PTI-ETI synergistic signal transduction.

From molecule to cell: the expanding frontiers of plant immunity

In recent years, the field of plant immunity has witnessed remarkable breakthroughs. During the co-evolution between plants and pathogens, plants have developed a wealth of intricate defense mechanisms to safeguard their survival. Newly identified immune receptors have added unexpected complexity to the surface and intracellular sensor networks, enriching our understanding of the ongoing plant-pathogen interplay. Deciphering the molecular mechanisms of resistosome shapes our understanding of these mysterious molecules in plant immunity. Moreover, technological innovations are expanding the horizon of the plant-pathogen battlefield into spatial and temporal scales. While the development provides new opportunities for untangling the complex realm of plant immunity, challenges remain in uncovering plant immunity across spatiotemporal dimensions from both molecular and cellular levels.

Phosphatidylinositol-specific phospholipase C-associated phospholipid metabolism mediates DcGLRs channel to promote calcium influx under CaCl2 treatment in shredded carrots during storage

The rapid activation of phosphatidylinositol-specific phospholipase C (PI-PLC) occurs early after the stimulation of biotic and abiotic stress in plants, which directly associated with the calcium channel-induced calcium ion (Ca2+) influx. Exogenous calcium chloride (CaCl2) mediates the calcium signaling transduction to promote the γ-aminobutyric acid accumulation and nutritional quality in shredded carrots whereas the generation mechanism remains uncertain. Therefore, the involvement of PI-PLC-associated phospholipid metabolism was investigated in present study. Our result revealed that CaCl2 treatment promoted the expression and activity of PI-PLC and increased the inositol 1,4,5-trisphosphate and hexakisphosphate content in shredded carrots. The transcripts of multi-glutamate receptor-like channels (DcGLRs), the glutamate and γ-aminobutyric acid (GABA) content, and Ca2+ influx were induced by CaCl2 treatment in shredded carrots during storage. However, PI-PLC inhibitor (U73122) treatment inhibited the activation of PI-PLC, the increase of many DcGLRs family genes expression levels, and Ca2+ influx. Moreover, the identification of DcPI-PLC4/6 and DcGLRs proteins, along with the analysis of characteristic domains such as PLCXc, PLCYc, C2 domain, transmembranous regions, and ligand binding domain, suggests their involvement in phospholipid catalysis and calcium transport in carrots. Furthermore, DcPI-PLC4/6 overexpression in tobacco leaves induced the Ca2+ influx by activating the expressions of NtGLRs and the accumulation of glutamate and GABA. These findings collectively indicate that CaCl2 treatment-induced PI-PLC activation influences DcGLRs expression levels to mediate cytosolic Ca2+ influx, thus, highlighting the "PI-PLC-GLRs-Ca2+" pathway in calcium signaling generation and GABA biosynthesis in shredded carrots.

Glutamate Receptor-like (GLR) Family in Brassica napus: Genome-Wide Identification and Functional Analysis in Resistance to Sclerotinia sclerotiorum

Plant glutamate receptor-like channels (GLRs) are homologs of animal ionotropic glutamate receptors. GLRs are critical in various plant biological functions, yet their genomic features and functions in disease resistance remain largely unknown in many crop species. Here, we report the results on a thorough genome-wide study of the GLR family in oilseed rape (Brassica napus) and their role in resistance to the fungal pathogen Sclerotinia sclerotiorum. A total of 61 GLRs were identified in oilseed rape. They comprised three groups, as in Arabidopsis thaliana. Detailed computational analyses, including prediction of domain and motifs, cellular localization, cis-acting elements, PTM sites, and amino acid ligands and their binding pockets in BnGLR proteins, unveiled a set of group-specific characteristics of the BnGLR family, which included chromosomal distribution, motif composition, intron number and size, and methylation sites. Functional dissection employing virus-induced gene silencing of BnGLRs in oilseed rape and Arabidopsis mutants of BnGLR homologs demonstrated that BnGLR35/AtGLR2.5 positively, while BnGLR12/AtGLR1.2 and BnGLR53/AtGLR3.2 negatively, regulated plant resistance to S. sclerotiorum, indicating that GLR genes were differentially involved in this resistance. Our findings reveal the complex involvement of GLRs in B. napus resistance to S. sclerotiorum and provide clues for further functional characterization of BnGLRs.

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.

In Silico Analysis of Glutamate Receptors in Capsicum chinense: Structure, Evolution, and Molecular Interactions

Plant glutamate receptors (GLRs) are integral membrane proteins that function as non-selective cation channels, involved in the regulation of developmental events crucial in plants. Knowledge of these proteins is restricted to a few species and their true agonists are still unknown in plants. Using tomato SlGLRs, a search was performed in the pepper database to identify GLR sequences in habanero pepper (Capsicum chinense Jacq.). Structural, phylogenetic, and orthology analysis of the CcGLRs, as well as molecular docking and protein interaction networks, were conducted. Seventeen CcGLRs were identified, which contained the characteristic domains of GLR. The variation of conserved residues in the M2 transmembrane domain between members suggests a difference in ion selectivity and/or conduction. Also, new conserved motifs in the ligand-binding regions are reported. Duplication events seem to drive the expansion of the species, and these were located in the evolution by using orthologs. Molecular docking analysis allowed us to identify differences in the agonist binding pocket between CcGLRs, which suggest the existence of different affinities for amino acids. The possible interaction of some CcGLRs with proteins leads to suggesting specific functions for them within the plant. These results offer important functional clues for CcGLR, probably extrapolated to other Solanaceae.

Ca2+/calmodulin-mediated desensitization of glutamate receptors shapes plant systemic wound signalling and anti-herbivore defence

Plants rely on systemic signalling mechanisms to establish whole-body defence in response to insect and nematode attacks. GLUTAMATE RECEPTOR-LIKE (GLR) genes have been implicated in long-distance transmission of wound signals to initiate the accumulation of the defence hormone jasmonate (JA) at undamaged distal sites. The systemic signalling entails the activation of Ca2+-permeable GLR channels by wound-released glutamate, triggering membrane depolarization and cytosolic Ca2+ influx throughout the whole plant. The systemic electrical and calcium signals rapidly dissipate to restore the resting state, partially due to desensitization of the GLR channels. Here we report the discovery of calmodulin-mediated, Ca2+-dependent desensitization of GLR channels, revealing a negative feedback loop in the orchestration of plant systemic wound responses. A CRISPR-engineered GLR3.3 allele with impaired desensitization showed prolonged systemic electrical signalling and Ca2+ waves, leading to enhanced plant defence against herbivores. Moreover, this Ca2+/calmodulin-mediated desensitization of GLR channels is a highly conserved mechanism in plants, providing a potential target for engineering anti-herbivore defence in crops.

Genome-wide identification of the glutamate receptor-like gene family in Vanilla planifolia and their response to Fusarium oxysporum infection

Glutamate receptor-like genes (GLRs) are essential for plant growth and development and for coping with environmental (biological and non-biological) stresses. In this study, 13 GLR members were identified in the Vanilla planifolia genome and attributed to two subgroups (Clade I and Clade III) based on their physical relationships. Cis-acting element analysis and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations indicated the GLR gene regulation's complexity and their functional diversity. Expression analysis revealed a relatively higher and more general expression pattern of Clade III members compared to the Clade I subgroup in tissues. Most GLRs showed significant differences in expression during Fusarium oxysporum infection. This suggested that GLRs play a critical role in the response of V. planifolia to pathogenic infection. These results provide helpful information for further functional research and crop improvement of VpGLRs.

Genome-wide identification and expression analysis of the glutamate receptor gene family in sweet potato and its two diploid relatives

Plant glutamate receptor (GLR) homologs are crucial calcium channels that play an important role in plant development, signal transduction, and response to biotic and abiotic stresses. However, the GLR gene family has not yet been thoroughly and systematically studied in sweet potato. In this study, a total of 37 GLR genes were identified in the cultivated hexaploid sweet potato (Ipomoea batatas), and 32 GLR genes were discovered in each of the two diploid relatives (Ipomoea trifida and Ipomoea triloba) for the first time. Based on their evolutionary relationships to those of Arabidopsis, these GLRs were split into five subgroups. We then conducted comprehensive analysis to explore their physiological properties, protein interaction networks, promoter cis-elements, chromosomal placement, gene structure, and expression patterns. The results indicate that the homologous GLRs of the cultivated hexaploid sweet potato and its two relatives are different. These variations are reflected in their functions related to plant growth, hormonal crosstalk, development of tuberous roots, resistance to root rot, and responses to abiotic stress factors, all of which are governed by specific individual GLR genes. This study offers a comprehensive analysis of GLR genes in sweet potato and its two diploid relatives. It also provides a theoretical basis for future research into their regulatory mechanisms, significantly influencing the field of molecular breeding in sweet potatoes.

Genome-Wide Association Study of Resistance to Pseudomonas syringae in the USDA Collection of Citrullus amarus

Pseudomonas leaf spot (PLS), caused by Pseudomonas syringae pv. syringae, is an emerging disease of watermelon in the United States with the potential to severely reduce yield under humid conditions. The genetic basis of resistance to this disease is not known and no resistant germplasm is available. Because Citrullus amarus is an important reservoir of resistance genes for the cultivated watermelon, C. lanatus, we screened the United States Department of Agriculture plant introduction collection of C. amarus for resistance to PLS. Accessions (n = 117) were phenotyped for their level of resistance to PLS in two separate tests. Accession means of percent leaf area affected ranged from 1.5 to 99.4%. The broad-sense heritability for the trait was 0.51. Whole-genome resequencing generated 2,126,759 single-nucleotide polymorphisms (SNPs) which were used to perform a genome-wide association study (GWAS) aimed at discovering molecular markers for resistance. Three different models-BLINK, FarmCPU, and MLM-were included in the GWAS analyses. BLINK and FarmCPU, which are multilocus models, found eight SNPs, located on chromosomes Ca01, Ca05, Ca06, Ca08, and Ca10, that were significantly associated with resistance to PLS. Two of these SNPs were found by both BLINK and FarmCPU. The MLM model did not detect any significant associations. BLINK and FarmCPU estimated an explained phenotypic variance of 43.6 and 28.5%, respectively, for SNP S6_19327000 and 25.0 and 26.0%, respectively, for SNP S1_33362258, the two most significant SNPs found. In total, 43 candidate genes with known involvement in disease resistance were discovered within the genomic intervals of seven of the eight peak SNPs. Eleven of the candidate genes that were found have been reported to be involved in resistance to P. syringae in other plant species. Two significant SNPs were within resistance genes previously documented to play important roles of plant resistance specific to P. syringae in other pathosystems. The SNPs identified in this study will be instrumental in finding causal genes involved in PLS resistance in watermelon and developing resistant germplasm through breeding.

Genome-wide identification of glutamate receptor-like gene family in soybean

Glutamate receptor-like genes (GLRs) are essential in the growth and development of plants and many physiological and biochemical processes; however, related information in soybean is lacking. In this study, 105 GLRs, including 67 Glycine soja and 38 Glycine max GLRs, were identified and divided into two clades (Clades II and III) according to their phylogenetic relationships. GLR members in the same branch had a relatively conservative motif composition and genetic structure. Furthermore, the soybean GLR family mainly experienced purification selection during evolution. Cis-acting element analysis, gene ontology, and Kyoto Encyclopedia of Genes and Genomic annotations indicated the complexity of the gene regulation and functional diversity of the soybean GLR. Moreover, transcriptome data analysis showed that these GLRs had different expression profiles in different tissues, and Clade III members had higher and more common expression patterns. Additionally, the expression profiles under jasmonic acid treatment and salt stress indicate that the GLR participated in the jasmonic acid signaling pathway and plays a role in salt treatment. This study provides information for a comprehensive understanding of the soybean GLR family and a reference for further functional research and genetic improvement.

Genome-wide identification of glutamate receptor genes in adzuki bean and the roles of these genes in light and rust fungal response

Plant glutamate receptor proteins (GLRs) are involved in plant development, biotic stress, and light-signal transduction. Vigna angularis is a traditional crop with important economic value in China, and the identification of functional genes can facilitate the breeding of stress resistant varieties. Here, we identified the members of the GLR gene family in the adzuki bean genome and investigated gene expression under light and rust fungal (Uromyces vignae) stimuli. Sixteen GLR genes were identified in V. angularis (VaGLRs), and these genes clustered in a single clade (clade III) with two groups. Evolutionary analysis showed that three VaGLRs result from tandem duplications and four result from whole genome/segmental duplications. To understand the regulation of expression of VaGLRs, cis-acting elements were analyzed in the promoter regions of the VaGLRs including cis-acting elements associated with light and stress responsiveness. Expression analysis by qRT-PCR revealed transcripts of eight and 10 VaGLRs in response to light stimuli and rust infection, respectively. For light responsiveness, the expression levels of XP_017430569.1 and XP_017425299.1 were higher under light condition than in darkness, while the expression levels of XP_017406996.1, XP_017425763.1, and XP_017423557.1 gradually recovered during dark treatment. Additionally, the relative expression levels of XP_017413816.1, XP_017436268.1, and XP_017425299.1 were significantly elevated during U. vignae infection in a resistant cultivar compared to the expression levels in a susceptible cultivar. XP_017425299.1 expression was induced both by light and rust infection, suggesting this gene may link light and disease resistance signaling pathways. Our results provide insight into how the VaGLRs contribute to adzuki bean response to light stimulus and pathogen attack. These identified VaGLRs also provide important reference to improve adzuki bean germplasm resources.

Glutamate receptor like channels: Emerging players in calcium mediated signaling in plants

Glutamate receptors like channels (GLRs) are ligand gated non-selective cation channels and are multigenic in nature. They are homologs of mammalian ionic glutamate receptors (iGLRs) that play an important role in neurotransmission. It has been more than 25 years of discovery of plant GLRs, since then, significant progress has been made to unravel their structure and function in plants. Recently, the first crystal structure of plant GLR has been resolved that suggests that, though, plant GLRs contain the conserved signature domains of iGLRs, their unique features enable agonist/antagonist-dependent change in their activity. GLRs exhibit diverse subcellular localization and undergo dynamic expression variation in response to developmental and environmental stress conditions in plants. The combined use of genetic, electrophysiology and calcium imaging using different genetically encoded calcium indicators has revealed that GLRs are involved in generating calcium (Ca²⁺) influx across the plasma membrane and are involved in shaping the Ca²⁺ signature in response to different developmental and environmental stimuli. These findings indicate that GLRs influence cytosolic Ca²⁺ dynamics, thus, highlighting "GLR-Ca²⁺-crosstalk (GCC)" in developmental and stress-responsive signaling pathways. With this background, the present review summarises the recent developments pertaining to GLR function, in the broader context of regulation of stress tolerance in plants.

Unraveling the mutualistic interaction between endophytic Curvularia lunata CSL1 and tomato to mitigate cadmium (Cd) toxicity via transcriptomic insights

In this study, endophytic fungus Curvularia lunata strain SL1 was used to explore its bioremediation potential and growth restoration of tomato (Solanum lycopersicum) under cadmium (Cd) stress. Our findings demonstrate that SL1 establishes a symbiotic relationship with tomato plants, which modulates the antioxidant system, secondary metabolites, and gene expression in tomato plants exposed to Cd stress. Under Cd stress, tomato seedling growth was significantly reduced by up to 42.8 %, although this reduction was mitigated by up to 25 % after SL1 inoculation. Similar to this, SLI inoculation inhibits Cd absorption and translocation to the upper parts of the plant. Additionally, during Cd stress, phytohormones related to stress, including jasmonic acid (JA), abscisic acid (ABA), and ethylene (ET), were elevated; however, SL1 inoculation lowered their level. RNA-Seq data revealed that the highest number of differentially expressed genes (DEGs) was detected in the comparison between control and 1 mM Cd, followed by 2 mM Cd stress. These DEGs were mostly related to oxidoreductase activity, catalytic activity, plant hormones transduction, and photosynthesis. The findings also suggested that SL1 could improve tomato tolerance to Cd stress by modulating Ca²⁺ signaling, phytohormone biosynthesis, MAPK signaling pathway, and some transcription factors.

Controlling stomatal aperture, a potential strategy for managing plant bacterial disease

Bacterial blight of crucifers caused by Pseudomonas cannabina pv. alisalensis (Pcal) inflicts great damage on crucifer production. To explore efficient and sustainable strategies for Pcal disease control, we here investigated and screened for amino acids with reduced disease development. We found that exogenous foliar application with multiple amino acids reduced disease symptoms and bacterial populations in cabbage after spray-inoculation, but not syringe-inoculation. These results indicate that these amino acids showed a protective effect before Pcal entered plants. Therefore, we observed stomatal responses, which is a main gateway for Pcal entry into the apoplast, after amino acid treatments. As a results, we found several amino acids induce stomatal closure. Moreover, our findings demonstrated that reducing stomatal aperture width can limit bacterial entry into plants, leading to reduced disease symptoms. Indeed, Cys, Glu, and Lys, which showed a protective effect on cabbage, reduced stomatal aperture width and bacterial entry. Therefore, managing stomatal aperture can be a powerful strategy for controlling bacterial disease.

Genes related to osmoregulation and antioxidation play important roles in the response of Trollius chinensis seedlings to saline-alkali stress

Saline-alkali stress is one of the main abiotic stress factors affecting plant growth and development. Trollius chinensis is a perennial herbal medicinal plant with high values for garden application. However, its response and tolerance to saline-alkali stress is unclear. In this study, we mixed four salts (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃) with a concentration ratio of 1:9:9:1, and applied low (40 and 80 mM) and high (120 and 160 mM) saline-alkali stress to analyze osmotic regulation substances, antioxidant systems and the gene expression of T. chinensis. Along with higher saline-alkali stress, the leaf relative water content (RWC) started to decrease only from high stress, while the malondialdehyde (MDA) content in leaves decreased continuously, and the contents of proline (Pro), soluble sugar (SS) and soluble protein (SP) increased compared with control. The activities of antioxidant enzymes and the contents of non-enzymatic antioxidants were increased positively with the accumulation of superoxide anion (O₂ ^•-) and hydrogen peroxide (H₂O₂). For instance, the ascorbic acid-glutathione (AsA-GSH) cycle was enhanced in T. chinensis seedling leaves subject to saline-alkali stress. Principal Component Analysis (PCA) indicates that MDA, Pro, SS, SP, H₂O₂, O₂ ^•-, and GSH are important indexes to evaluate the response and tolerance of T. chinensis to saline-alkali stress. Through RNA-Seq, a total of 474 differentially expressed genes (DEGs) were found in plant under low saline-alkaline stress (40 mM, MSA1) vs. control. Among them, 364 genes were up-regulated and 110 genes were down-regulated. DEGs were extensively enriched in carbohydrate transport, transferase activity, zeatin biosynthesis, ABC transporters, and spliceosome. The transcription factor family MYB, BZIP, WRKY, and NAC were related to its saline-alkali tolerance. In addition, some DEGs encode key enzymes in the processes of osmoregulation and antioxidation, including betaine aldehyde dehydrogenase (BADH), inositol monophosphatase (IMP), chloroperoxidase (CPO), thioredoxin (Trx), and germin-like protein (GLPs) were found. Overall, these findings provide new insights into the physiological changes and molecular mechanism of T. chinensis to saline-alkali stress and lay a foundation for application of T. chinensis in saline-alkali environment.

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.

A glutamate receptor-like gene is involved in ABA-mediated growth control in Physcomitrium (Physcomitrella) patens

Plant glutamate receptor homologs (GLRs), which function as key calcium channels, play pivotal roles in various developmental processes as well as stress responses. The moss Physcomitrium patens, a representative of the earliest land plant lineage, possess multiple pathways of hormone signaling for coordinating growth and adaptation responses. However, it is not clear whether GLRs are connected to hormone-mediated growth control in the moss. In this study, we report that one of the two GLRs in P. patens, PpGLR1, involves in abscisic acid (ABA)-mediated growth regulation. ABA represses the growth of wild-type moss, and intriguingly, the PpGLR1 transcript levels are significantly increased in response to ABA treatment, based on both gene expression and the PpGLR1pro::GUS reporter results. Furthermore, the growth of Ppglr1 knockout moss mutants is hypersensitive to ABA treatment. These results suggest that PpGLR1 plays a critical role in ABA-mediated growth regulation, which provide useful information for our further investigation of the regulatory mechanism between Ca²⁺ signal and ABA in moss growth control.

Roles of Glutamate Receptor-Like Channels (GLRs) in Plant Growth and Response to Environmental Stimuli

Plant glutamate receptor-like channels (GLRs) are the homologues of ionotropic glutamate receptors (iGluRs) that mediate neurotransmission in mammals, and they play important roles in various plant-specific physiological processes, such as pollen tube growth, sexual reproduction, root meristem proliferation, internode cell elongation, stomata aperture regulation, and innate immune and wound responses. Notably, these biological functions of GLRs have been mostly linked to the Ca²⁺-permeable channel activity as GLRs can directly channel the transmembrane flux of Ca²⁺, which acts as a key second messenger in plant cell responses to both endogenous and exogenous stimuli. Thus, it was hypothesized that GLRs are mainly involved in Ca²⁺ signaling processes in plant cells. Recently, great progress has been made in GLRs for their roles in long-distance signal transduction pathways mediated by electrical activity and Ca²⁺ signaling. Here, we review the recent progress on plant GLRs, and special attention is paid to recent insights into the roles of GLRs in response to environmental stimuli via Ca²⁺ signaling, electrical activity, ROS, as well as hormone signaling networks. Understanding the roles of GLRs in integrating internal and external signaling for plant developmental adaptations to a changing environment will definitely help to enhance abiotic stress tolerance.

Genome-Wide Identification and Expression Analysis of BraGLRs Reveal Their Potential Roles in Abiotic Stress Tolerance and Sexual Reproduction

Glutamate receptors (GLRs) are involved in multiple functions during the plant life cycle through affecting the Ca²⁺ concentration. However, GLRs in Brassica species have not yet been reported. In this study, 16 glutamate receptor-like channels (GLR) belonged to two groups were identified in the Brassica rapa (B. rapa) genome by bioinformatic analysis. Most members contain domains of ANF_receptor, Peripla_BP_6, Lig_chan, SBP_bac_3, and Lig_chan_Glu_bd that are closely related to glutamate receptor channels. This gene family contains many elements associated with drought stress, low temperature stress, methyl jasmonate (MeJA), salicylic acid (SA), and other stress resistance. Gene expression profiles showed that BraGLR genes were expressed in roots, stems, leaves, flowers, and siliques. BraGLR5 expression was elevated after drought stress in drought-sensitive plants. BraGLR1, BraGLR8, and BraGLR11 expression were significantly upregulated after salt stress. BraGLR3 expression is higher in the female sterile-line mutants than in the wild type. The expression levels of BraGLR6, BraGLR9, BraGLR12, and BraGLR13 were significantly higher in the male sterile-line mutants than in the wild type. The expression of most BraGLRs increased after self-pollination, with BraGLR9 exhibiting the greatest increase. These results suggest that BraGLRs play an important role in abiotic stress tolerance and sexual reproduction.

Amino acids and their derivatives mediating defense priming and growth tradeoff

Plant response to pathogens attacks generally comes at the expense of growth. Defense priming is widely accepted as an efficient strategy used for augmenting resistance with reduced fitness in terms of growth and yield. Plant-derived small molecules, both primary as well as secondary metabolites, can function as activators to prime plant defense. Amino acids and their derivatives regulate numerous aspects of plant growth and development, and biotic and abiotic stress responses. In this review, we discuss the recent progress in understanding the roles of amino acids and related molecules in defense priming and their link with plant growth. We also highlight some of the outstanding questions and provide an outlook on the prospects of 'engineering' the tradeoff between defense and growth in plants.

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.

Genome-Wide Identification, Characterization, and Expression Analysis of Glutamate Receptor-like Gene (GLR) Family in Sugarcane

The plant glutamate receptor-like gene (GLR) plays a vital role in development, signaling pathways, and in its response to environmental stress. However, the GLR gene family has not been comprehensively and systematically studied in sugarcane. In this work, 43 GLR genes, including 34 in Saccharum spontaneum and 9 in the Saccharum hybrid cultivar R570, were identified and characterized, which could be divided into three clades (clade I, II, and III). They had different evolutionary mechanisms, the former was mainly on the WGD/segmental duplication, while the latter mainly on the proximal duplication. Those sugarcane GLR proteins in the same clade had a similar gene structure and motif distribution. For example, 79% of the sugarcane GLR proteins contained all the motifs, which proved the evolutionary stability of the sugarcane GLR gene family. The diverse cis-acting regulatory elements indicated that the sugarcane GLRs may play a role in the growth and development, or under the phytohormonal, biotic, and abiotic stresses. In addition, GO and KEGG analyses predicted their transmembrane transport function. Based on the transcriptome data, the expression of the clade III genes was significantly higher than that of the clade I and clade II. Furthermore, qRT-PCR analysis demonstrated that the expression of the SsGLRs was induced by salicylic acid (SA) treatment, methyl jasmonic acid (MeJA) treatment, and abscisic acid (ABA) treatment, suggesting their involvement in the hormone synthesis and signaling pathway. Taken together, the present study should provide useful information on comparative genomics to improve our understanding of the GLR genes and facilitate further research on their functions.

Calcium channels and transporters: Roles in response to biotic and abiotic stresses

Calcium (Ca²⁺) serves as a ubiquitous second messenger by mediating various signaling pathways and responding to numerous environmental conditions in eukaryotes. Therefore, plant cells have developed complex mechanisms of Ca²⁺ communication across the membrane, receiving the message from their surroundings and transducing the information into cells and organelles. A wide range of biotic and abiotic stresses cause the increase in [Ca²⁺]_cyt as a result of the Ca²⁺ influx permitted by membrane-localized Ca²⁺ permeable cation channels such as CYCLIC NUCLEOTIDE-GATE CHANNELs (CNGCs), and voltage-dependent HYPERPOLARIZATION-ACTIVATED CALCIUM²⁺ PERMEABLE CHANNELs (HACCs), as well as GLUTAMATE RECEPTOR-LIKE RECEPTORs (GLRs) and TWO-PORE CHANNELs (TPCs). Recently, resistosomes formed by some NUCLEOTIDE-BINDING LEUCINE-RICH REPEAT RECEPTORs (NLRs) are also proposed as a new type of Ca²⁺ permeable cation channels. On the contrary, some Ca²⁺ transporting membrane proteins, mainly Ca²⁺-ATPase and Ca²⁺/H⁺ exchangers, are involved in Ca²⁺ efflux for removal of the excessive [Ca²⁺]_cyt in order to maintain the Ca²⁺ homeostasis in cells. The Ca²⁺ efflux mechanisms mediate the wide ranges of cellular activities responding to external and internal stimuli. In this review, we will summarize and discuss the recent discoveries of various membrane proteins involved in Ca²⁺ influx and efflux which play an essential role in fine-tuning the processing of information for plant responses to abiotic and biotic stresses.

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.

News about amino acid metabolism in plant-microbe interactions

Plants constantly come into contact with a diverse mix of pathogenic and beneficial microbes. The ability to distinguish between them and to respond appropriately is essential for plant health. Here we review recent progress in understanding the role of amino acid sensing, signaling, transport, and metabolism during plant-microbe interactions. Biochemical pathways converting individual amino acids into active compounds have recently been elucidated, and comprehensive large-scale approaches have brought amino acid sensors and transporters into focus. These findings show that plant central amino acid metabolism is closely interwoven with stress signaling and defense responses at various levels. The individual biochemical mechanisms and the interconnections between the different processes are just beginning to emerge and might serve as a foundation for new plant protection strategies.

Ca2+ signals in plant immunity

Calcium ions function as a key second messenger ion in eukaryotes. Spatially and temporally defined cytoplasmic Ca²⁺ signals are shaped through the concerted activity of ion channels, exchangers, and pumps in response to diverse stimuli; these signals are then decoded through the activity of Ca²⁺ -binding sensor proteins. In plants, Ca²⁺ signaling is central to both pattern- and effector-triggered immunity, with the generation of characteristic cytoplasmic Ca²⁺ elevations in response to potential pathogens being common to both. However, despite their importance, and a long history of scientific interest, the transport proteins that shape Ca²⁺ signals and their integration remain poorly characterized. Here, we discuss recent work that has both shed light on and deepened the mysteries of Ca²⁺ signaling in plant immunity.

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.

A tale of many families: calcium channels in plant immunity

Plants launch a concerted immune response to dampen potential infections upon sensing microbial pathogen and insect invasions. The transient and rapid elevation of the cytosolic calcium concentration [Ca2+]cyt is among the essential early cellular responses in plant immunity. The free Ca2+ concentration in the apoplast is far higher than that in the resting cytoplasm. Thus, the precise regulation of calcium channel activities upon infection is the key for an immediate and dynamic Ca2+ influx to trigger downstream signaling. Specific Ca2+ signatures in different branches of the plant immune system vary in timing, amplitude, duration, kinetics, and sources of Ca2+. Recent breakthroughs in the studies of diverse groups of classical calcium channels highlight the instrumental role of Ca2+ homeostasis in plant immunity and cell survival. Additionally, the identification of some immune receptors as noncanonical Ca2+-permeable channels opens a new view of how immune receptors initiate cell death and signaling. This review aims to provide an overview of different Ca2+-conducting channels in plant immunity and highlight their molecular and genetic mode-of-actions in facilitating immune signaling. We also discuss the regulatory mechanisms that control the stability and activity of these channels.

The Glutamate Receptor Plays a Role in Defense against Botrytis cinerea through Electrical Signaling in Tomato

Plant glutamate-like receptor genes (GLRs) are homologous to mammalian ionotropic glutamate receptors genes (iGluRs). Although GLRs have been implicated in plant defenses to biotic stress, the relationship between GLR-mediated plant immunity against fungal pathogens and electrical signals remains poorly understood. Here, we found that pretreatment with a GLR inhibitor, 6,7-dinitriquinoxaline-2,3-dione (DNQX), increased the susceptibility of tomato plants to the necrotrophic fungal pathogen Botrytis cinerea. Assessment of the glr3.3, glr3.5 and glr3.3/glr3.5 double-mutants upon B. cinerea infection showed that tomato GLR3.3 and GLR3.5 are essential for plant immunity against B. cinerea, wherein GLR3.3 plays the main role. Analysis of the membrane potential changes induced by glutamate (Glu) or glycine (Gly) revealed that amplitude was significantly reduced by knocking out GLR3.3 in tomato. While treatment with Glu or Gly significantly increased immunity against B. cinerea in wild-type plants, this effect was significantly attenuated in glr3.3 mutants. Thus, our data demonstrate that GLR3.3- and GLR3.5-mediated plant immunity against B. cinerea is associated with electrical signals in tomato plants.

Chromosome-scale assembly reveals asymmetric paleo-subgenome evolution and targets for the acceleration of fungal resistance breeding in the nut crop, pecan

Pecan (Carya illinoinensis) is a tree nut crop of worldwide economic importance that is rich in health-promoting factors. However, pecan production and nut quality are greatly challenged by environmental stresses such as the outbreak of severe fungal diseases. Here, we report a high-quality, chromosome-scale genome assembly of the controlled-cross pecan cultivar 'Pawnee' constructed by integrating Nanopore sequencing and Hi-C technologies. Phylogenetic and evolutionary analyses reveal two whole-genome duplication (WGD) events and two paleo-subgenomes in pecan and walnut. Time estimates suggest that the recent WGD event and considerable genome rearrangements in pecan and walnut account for expansions in genome size and chromosome number after the divergence from bayberry. The two paleo-subgenomes differ in size and protein-coding gene sets. They exhibit uneven ancient gene loss, asymmetrical distribution of transposable elements (especially LTR/Copia and LTR/Gypsy), and expansions in transcription factor families (such as the extreme pecan-specific expansion in the far-red impaired response 1 family), which are likely to reflect the long evolutionary history of species in the Juglandaceae. A whole-genome scan of resequencing data from 86 pecan scab-associated core accessions identified 47 chromosome regions containing 185 putative candidate genes. Significant changes were detected in the expression of candidate genes associated with the chitin response pathway under chitin treatment in the scab-resistant and scab-susceptible cultivars 'Excell' and 'Pawnee'. These findings enable us to identify key genes that may be important susceptibility factors for fungal diseases in pecan. The high-quality sequences are valuable resources for pecan breeders and will provide a foundation for the production and quality improvement of tree nut crops.

Roles of Glutathione in Mediating Abscisic Acid Signaling and Its Regulation of Seed Dormancy and Drought Tolerance

Plant growth and development and interactions with the environment are regulated by phytohormones and other signaling molecules. During their evolution, plants have developed strategies for efficient signal perception and for the activation of signal transduction cascades to maintain proper growth and development, in particular under adverse environmental conditions. Abscisic acid (ABA) is one of the phytohormones known to regulate plant developmental events and tolerance to environmental stresses. The role of ABA is mediated by both its accumulated level, which is regulated by its biosynthesis and catabolism, and signaling, all of which are influenced by complex regulatory mechanisms. Under stress conditions, plants employ enzymatic and non-enzymatic antioxidant strategies to scavenge excess reactive oxygen species (ROS) and mitigate the negative effects of oxidative stress. Glutathione (GSH) is one of the main antioxidant molecules playing a critical role in plant survival under stress conditions through the detoxification of excess ROS, maintaining cellular redox homeostasis and regulating protein functions. GSH has recently emerged as an important signaling molecule regulating ABA signal transduction and associated developmental events, and response to stressors. This review highlights the current knowledge on the interplay between ABA and GSH in regulating seed dormancy, germination, stomatal closure and tolerance to drought.

Deciphering the Role of Ion Channels in Early Defense Signaling against Herbivorous Insects

Plants and insect herbivores are in a relentless battle to outwit each other. Plants have evolved various strategies to detect herbivores and mount an effective defense system against them. These defenses include physical and structural barriers such as spines, trichomes, cuticle, or chemical compounds, including secondary metabolites such as phenolics and terpenes. Plants perceive herbivory by both mechanical and chemical means. Mechanical sensing can occur through the perception of insect biting, piercing, or chewing, while chemical signaling occurs through the perception of various herbivore-derived compounds such as oral secretions (OS) or regurgitant, insect excreta (frass), or oviposition fluids. Interestingly, ion channels or transporters are the first responders for the perception of these mechanical and chemical cues. These transmembrane pore proteins can play an important role in plant defense through the induction of early signaling components such as plasma transmembrane potential (V_m) fluctuation, intracellular calcium (Ca²⁺), and reactive oxygen species (ROS) generation, followed by defense gene expression, and, ultimately, plant defense responses. In recent years, studies on early plant defense signaling in response to herbivory have been gaining momentum with the application of genetically encoded GFP-based sensors for real-time monitoring of early signaling events and genetic tools to manipulate ion channels involved in plant-herbivore interactions. In this review, we provide an update on recent developments and advances on early signaling events in plant-herbivore interactions, with an emphasis on the role of ion channels in early plant defense signaling.

Damage-Associated Molecular Patterns (DAMPs) in Plant Innate Immunity: Applying the Danger Model and Evolutionary Perspectives

Danger signals trigger immune responses upon perception by a complex surveillance system. Such signals can originate from the infectious nonself or the damaged self, the latter termed damage-associated molecular patterns (DAMPs). Here, we apply Matzinger's danger model to plant innate immunity to discuss the adaptive advantages of DAMPs and their integration into preexisting signaling pathways. Constitutive DAMPs (cDAMPs), e.g., extracellular ATP, histones, and self-DNA, fulfill primary, conserved functions and adopt a signaling role only when cellular damage causes their fragmentation or localization to aberrant compartments. By contrast, immunomodulatory peptides (also known as phytocytokines) exclusively function as signals and, upon damage, are activated as inducible DAMPs (iDAMPs). Dynamic coevolutionary processes between the signals and their emerging receptors and shared co-receptors have likely linked danger recognition to preexisting, conserved downstream pathways.

A 'GLoRy' Battle for Cotton against Fusarium

Glutamate receptor-like (GLR) proteins are ligand-gated ion channels regulating various plant physiology. They function as calcium channels in plant immunity and mediate long-distance defense to herbivores. A recent study by Liu et al. identified a cotton GLR as a noncanonical resistance protein to Fusarium wilt, expanding the paradigm of GLR functions in plant immunity.

Screening for Arabidopsis mutants with altered Ca2+ signal response using aequorin-based Ca2+ reporter system

Environmental stimuli evoke transient increases of the cytosolic Ca²⁺ level. To identify upstream components of Ca²⁺ signaling, we have optimized two forward genetic screening systems based on Ca²⁺ reporter aequorin. AEQsig6 and AEQub plants were used for generating ethyl methanesulfonate (EMS)-mutagenized libraries. The AEQsig6 EMS-mutagenized library was preferably used to screen the mutants with reduced Ca²⁺ signal response due to its high effectiveness, while the AEQub EMS-mutagenized library was used for screening of the mutants with altered Ca²⁺ signal response.

For complete details on the use and execution of this protocol, please refer to Chen et al. (2020) and Zhu et al. (2013).

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Highly efficient systems for screening of Ca²⁺ signal mutants in Arabidopsis

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Step-by-step instructions to analyze Ca²⁺ signal response using Ca²⁺ reporter aequorin

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AEQsig6 system for identifying mutants impaired in shoot-based Ca²⁺ signaling

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FAS system for isolating tissue- or stimuli-specific Ca²⁺ responsive mutants

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.

Structure of the Arabidopsis Glutamate Receptor-like Channel GLR3.2 Ligand-Binding Domain

Glutamate receptor-like channels (GLRs) play important roles in numerous plant physiological processes. GLRs are homologous to ionotropic glutamate receptors (iGluRs) that mediate neurotransmission in vertebrates. Here we determine crystal structures of Arabidopsis thaliana GLR3.2 ligand-binding domain (LBD) in complex with glycine and methionine to 1.58- and 1.75-Å resolution, respectively. Our structures show a fold similar to that of iGluRs, but with several secondary structure elements either missing or different. The closed clamshell conformation of GLR3.2 LBD suggests that both glycine and methionine act as agonists. The mutation R133A strongly increases the constitutive activity of the channel, suggesting that the LBD mutated at the residue critical for agonist binding produces a more stable closed clamshell conformation. Furthermore, our structures explain the promiscuity of GLR activation by different amino acids, confirm evolutionary conservation of structure between GLRs and iGluRs, and predict common molecular principles of their gating mechanisms driven by bilobed clamshell-like LBDs.

Glutamate-Induced Electrical and Calcium Signals in the Moss Physcomitrella patens

The mode of transmission of signals between plant cells is an important aspect of plant physiology. The main role in the generation of long-distance signals is played by changes in the membrane potential and cytoplasm calcium concentration, but the relationship between these responses evoked by the same stimuli in the same plant remains unknown. As one of the first plants that colonized land, the moss Physcomitrella patens is a suitable model organism for studying the evolution of signaling pathways in plants. Here, by the application of glutamate as a stimulus, we demonstrated that electrical but not calcium signals can be true carriers of information in long-distance signaling in Physcomitrella. The generation of electrical signals in a form of propagating transient depolarization seems to be dependent on the opening of calcium channels since the responses were reduced or totally blocked by calcium channel inhibitors. While the microelectrode measurements demonstrated the transmission of electric signals between leaf cells and juvenile cells (protonema), the fluorescence imaging of cytoplasmic calcium changes indicated that calcium response occurs only locally-at the site of glutamate application, and only in protonema cells. This study indicates different involvement of glutamate-induced electrical and calcium signals in cell-to-cell communication in these evolutionarily old terrestrial plants.

Extracellular Ca2+ induces desensitized cytosolic Ca2+ rise sensitive to phospholipase C inhibitor which suppresses root growth with Ca2+ dependence

Calcium (Ca) is an essential element for all organisms. In animal cells, the plasma membrane-localized Ca receptor CaSR coupled to a phospholipase C (PLC)-dependent signaling cascade monitors extracellular Ca²⁺ concentrations ([Ca²⁺]_ext) and responds with increases in cytosolic calcium concentrations ([Ca²⁺]_cyt). Plant roots encounter variable soil conditions, but how they sense changes in [Ca²⁺]_ext is largely unknown. In this study, we demonstrate that increasing [Ca²⁺]_ext evokes a transient increase in [Ca²⁺] in the cytosol, mitochondria, and nuclei of Arabidopsis thaliana root cells. These increases were strongly desensitized to repeat applications of [Ca²⁺]_ext, a typical feature of receptor-mediated cellular signaling in animal and plant cells. Treatment with gadolinium (Gd³⁺), a CaSR activator in animal cells, induced concentration-dependent increases in [Ca²⁺]_cyt in roots, which showed self-desensitization and cross-desensitization to [Ca²⁺]_ext-induced increases in [Ca²⁺]_cyt (EICC). EICC was sensitive to extracellular H⁺, K⁺, Na⁺, and Mg²⁺ levels. Treatment with the PLC inhibitor neomycin suppressed EICC and Ca accumulation in roots. The inhibitory effect of neomycin on root elongation was fully rescued by increasing [Ca²⁺]_ext but not [Mg²⁺] or [K⁺] in the growth medium. These results suggest that [Ca²⁺]_ext and the movement of Ca²⁺ into the cytosol of plant roots are regulated by a receptor-mediated signaling pathway involving PLC.

Calcium spikes, waves and oscillations in plant development and biotic interactions

The calcium ion (Ca²⁺) is a universal signal in all eukaryotic cells. A fundamental question is how Ca²⁺, a simple cation, encodes complex information with high specificity. Extensive research has established a two-step process (encoding and decoding) that governs the specificity of Ca²⁺ signals. While the encoding mechanism entails a complex array of channels and transporters, the decoding process features a number of Ca²⁺ sensors and effectors that convert Ca²⁺ signals into cellular effects. Along this general paradigm, some signalling components may be highly conserved, but others are divergent among different organisms. In plant cells, Ca²⁺ participates in numerous signalling processes, and here we focus on the latest discoveries on Ca²⁺-encoding mechanisms in development and biotic interactions. In particular, we use examples such as polarized cell growth of pollen tube and root hair in which tip-focused Ca²⁺ oscillations specify the signalling events for rapid cell elongation. In plant-microbe interactions, Ca²⁺ spiking and oscillations hold the key to signalling specificity: while pathogens elicit cytoplasmic spiking, symbiotic microorganisms trigger nuclear Ca²⁺ oscillations. Herbivore attacks or mechanical wounding can trigger Ca²⁺ waves traveling a long distance to transmit and convert the local signal to a systemic defence program in the whole plant. What channels and transporters work together to carve out the spatial and temporal patterns of the Ca²⁺ fluctuations? This question has remained enigmatic for decades until recent studies uncovered Ca²⁺ channels that orchestrate specific Ca²⁺ signatures in each of these processes. Future work will further expand the toolkit for Ca²⁺-encoding mechanisms and place Ca²⁺ signalling steps into larger signalling networks.

Harnessing symbiotic plant-fungus interactions to unleash hidden forces from extreme plant ecosystems

Global climate change is arguably one of the biggest threats of modern times and has already led to a wide range of impacts on the environment, economy, and society. Owing to past emissions and climate system inertia, global climate change is predicted to continue for decades even if anthropogenic greenhouse gas emissions were to stop immediately. In many regions, such as central Europe and the Mediterranean region, the temperature is likely to rise by 2-5 °C and annual precipitation is predicted to decrease. Expected heat and drought periods followed by floods, and unpredictable growing seasons, are predicted to have detrimental effects on agricultural production systems, causing immense economic losses and food supply problems. To mitigate the risks of climate change, agricultural innovations counteracting these effects need to be embraced and accelerated. To achieve maximum improvement, the required agricultural innovations should not focus only on crops but rather pursue a holistic approach including the entire ecosystem. Over millions of years, plants have evolved in close association with other organisms, particularly soil microbes that have shaped their evolution and contemporary ecology. Many studies have already highlighted beneficial interactions among plants and the communities of microorganisms with which they coexist. Questions arising from these discoveries are whether it will be possible to decipher a common molecular pattern and the underlying biochemical framework of interspecies communication, and whether such knowledge can be used to improve agricultural performance under environmental stress conditions. In this review, we summarize the current knowledge of plant interactions with fungal endosymbionts found in extreme ecosystems. Special attention will be paid to the interaction of plants with the symbiotic root-colonizing endophytic fungus Serendipita indica, which has been developed as a model system for beneficial plant-fungus interactions.

Redox Homeostasis and Signaling in a Higher-CO2 World

Rising CO₂ concentrations and their effects on plant productivity present challenging issues. Effects on the photosynthesis/photorespiration balance and changes in primary metabolism are known, caused by the competitive interaction of CO₂ and O₂ at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase. However, impacts on stress resistance are less clear. Reactive oxygen species are key players in biotic and abiotic stress responses, but there is no consensus on whether elevated CO₂ constitutes a stress. Although high CO₂ increases yield in C₃ plants, it can also increase cellular oxidation and activate phytohormone defense pathways. Reduction-oxidation processes play key roles in acclimation to high CO₂, with specific enzymes acting in compartment-specific signaling. Traditionally, acclimation to high CO₂ has been considered in terms of altered carbon gain, but emerging evidence suggests that CO₂ is a signal as well as a substrate. Some CO₂ effects on defense are likely mediated independently of primary metabolism. Nonetheless, primary photosynthetic metabolism is highly integrated with defense and stress signaling pathways, meaning that plants will be able to acclimate to the changing environment over the coming decades.

Perception of Damaged Self in Plants

Plants use specific receptor proteins on the cell surface to detect host-derived danger signals released in response to attacks by pathogens or herbivores and activate immune responses against them.

Exogenous Treatment with Glutamate Induces Immune Responses in Arabidopsis

Plant resistance inducers (PRIs) are compounds that protect plants from diseases by activating immunity responses. Exogenous treatment with glutamate (Glu), an important amino acid for all living organisms, induces resistance against fungal pathogens in rice and tomato. To understand the molecular mechanisms of Glu-induced immunity, we used the Arabidopsis model system. We found that exogenous treatment with Glu induces resistance against pathogens in Arabidopsis. Consistent with this, transcriptome analyses of Arabidopsis seedlings showed that Glu significantly induces the expression of wound-, defense-, and stress-related genes. Interestingly, Glu activates the expression of genes induced by pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns at much later time points than the flg22 peptide, which is a bacterial-derived PAMP. The Glu receptor-like (GLR) proteins GLR3.3 and GLR3.6 are involved in the early expression of Glu-inducible genes; however, the sustained expression of these genes does not require the GLR proteins. Glu-inducible gene expression is also not affected by mutations in genes that encode PAMP receptors (EFR, FLS2, and CERK1), regulators of pattern-triggered immunity (BAK1, BKK1, BIK1, and PBL1), or a salicylic acid biosynthesis enzyme (SID2). The treatment of roots with Glu activates the expression of PAMP-, salicylic acid-, and jasmonic acid-inducible genes in leaves. Moreover, the treatment of roots with Glu primes chitin-induced responses in leaves, possibly through transcriptional activation of LYSIN-MOTIF RECEPTOR-LIKE KINASE 5 (LYK5), which encodes a chitin receptor. Because Glu treatment does not cause discernible growth retardation, Glu can be used as an effective PRI.

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.

Methionine synthase 1 provides methionine for activation of the GLR3.5 Ca2+ channel and regulation of germination in Arabidopsis

Seed germination is a developmental process regulated by numerous internal and external cues. Our previous studies have shown that calcium influx mediated by the Arabidopsis glutamate receptor homolog 3.5 (AtGLR3.5) modulates the expression of the ABSCISIC ACID INSENSITIVE 4 (ABI4) transcription factor during germination and that L-methionine (L-Met) activates AtGLR3.1/3.5 Ca2+ channels in guard cells. However, it is not known whether L-Met participates in regulation of germination and what cellular mechanism is responsible for Met production during germination. Here, we describe Arabidopsis methionine synthase 1 (AtMS1), which acts in the final step of Met biosynthesis, synthesizes the Met required for the activation of AtGLR3.5 Ca2+ channels whose expression is up-regulated during germination, leading to the regulation of seed germination. We show that exogenous L-Met promotes germination in an AtGRL3.5-dependent manner. We also demonstrate that L-Met directly regulates the AtGLR3.5-mediated increase in cytosolic Ca2+ level in seedlings. We provide pharmacological and genetic evidence that Met synthesized via AtMS1 acts upstream of the AtGLR3.5-mediated Ca2+ signal and regulates the expression of ABI4, a major regulator in the abscisic acid response in seeds. Overall, our results link AtMS1, L-Met, the AtGLR3.5 Ca2+ channel, Ca2+ signals, and ABI4, and shed light on the physiological role and molecular mechanism of L-Met in germination.

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.