Structure of the Arabidopsis thaliana glutamate receptor-like channel GLR3.4

A Glutamate Receptor-Like Gene AtGLR2.5 With Its Unusual Splice Variant Has a Role in Mediating Glutamate-Elicited Changes in Arabidopsis Root Architecture

The occurrence of external L-glutamate at the Arabidopsis root tip triggers major changes in root architecture, but the mechanism of -L-Glu sensing is unknown. Members of the family of GLUTAMATE RECEPTOR-LIKE (GLR) proteins are known to act as amino acid-gated Ca2+-permeable channels and to have signalling roles in diverse plant processes. To investigate the possible role of GLRs in the root architectural response to L-Glu, we screened a collection of mutants with T-DNA insertions in each of the 20 AtGLR genes. Reduced sensitivity of root growth to L-Glu was found in mutants of one gene, GLR2.5. Interestingly, GLR2.5 was found to apparently produce four transcript variants encoding hypothetical proteins of 169-720 amino acids. One of these transcripts, GLR2.5c, encodes a truncated GLR protein lacking both the conserved amino-terminal domain and part of the ligand-binding domain. When a glr2.5 mutant was transformed with a construct constitutively expressing GLR2.5c, both L-Glu sensitivity of root growth and L-Glu-elicited Ca2+ currents in root tip protoplasts were restored. These results, along with homology modelling of the truncated ligand-binding domain of GLR2.5c, suggest that GLR2.5c has a regulatory or scaffolding role in heteromeric GLR complex(es) that may involve triggering the root architectural response to L-Glu.

Glutathione triggers leaf-to-leaf, calcium-based plant defense signaling

Animals rely on nervous systems to cope with environmental variability, whereas plants are characterized by lack of nervous system but still have evolved systemic communication systems through signaling molecules that trigger long-distance defense signaling events when encountered with environmental challenges. Here, our genetic screening of the previously constructed hairpin RNA-based Arabidopsis library identifies a glutathione (GSH)-deficient mutant that has high accumulation of glutamate (Glu), a previously defined wound signal essential for activating plant defense, but disharmoniously exhibits attenuation of defense signaling events. We further uncover GSH as a critical signaling molecule that relies on GLUTAMATE RECEPTOR-LIKE 3.3 (GLR3.3) to trigger long-distance calcium-based defense signaling events in plants. Our findings offer new insights into highly sophisticated systemic defense systems evolved by plants to defend against herbivory and pathogen invasion.

Shared signals, different fates: Calcium and ROS in plant PRR and NLR immunity

Lacking an adaptive immune system, plants rely on innate immunity comprising two main layers: PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI), both utilizing Ca2+ influx and reactive oxygen species (ROS) for signaling. PTI, mediated by pattern-recognition receptors (PRRs), responds to conserved pathogen- or damage-associated molecular patterns. Some pathogens evade PTI using effectors, triggering plants to activate ETI. At the heart of ETI are nucleotide-binding leucine-rich repeat receptors (NLRs), which detect specific pathogen effectors and initiate a robust immune response. NLRs, equipped with a nucleotide-binding domain and leucine-rich repeats, drive a potent immune reaction starting with pronounced, prolonged cytosolic Ca2+ influx, followed by increased ROS levels. This sequence of events triggers the hypersensitive response-a localized cell death designed to limit pathogen spread. This intricate use of Ca2+ and ROS highlights the crucial role of NLRs in supplementing the absence of an adaptive immune system in plant innate immunity.

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.

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.

Structure and function of GluN1-3A NMDA receptor excitatory glycine receptor channel

N-methyl-d-aspartate receptors (NMDARs) and other ionotropic glutamate receptors (iGluRs) mediate most of the excitatory signaling in the mammalian brains in response to the neurotransmitter glutamate. Uniquely, NMDARs composed of GluN1 and GluN3 are activated exclusively by glycine, the neurotransmitter conventionally mediating inhibitory signaling when it binds to pentameric glycine receptors. The GluN1-3 NMDARs are vital for regulating neuronal excitability, circuit function, and specific behaviors, yet our understanding of their functional mechanism at the molecular level has remained limited. Here, we present cryo-electron microscopy structures of GluN1-3A NMDARs bound to an antagonist, CNQX, and an agonist, glycine. The structures show a 1-3-1-3 subunit heterotetrameric arrangement and an unprecedented pattern of GluN3A subunit orientation shift between the glycine-bound and CNQX-bound structures. Site-directed disruption of the unique subunit interface in the glycine-bound structure mitigated desensitization. Our study provides a foundation for understanding the distinct structural dynamics of GluN3 that are linked to the unique function of GluN1-3 NMDARs.

UMAMIT44 is a key player in glutamate export from Arabidopsis chloroplasts

Selective partitioning of amino acids among organelles, cells, tissues, and organs is essential for cellular metabolism and plant growth. Nitrogen assimilation into glutamine and glutamate and de novo biosynthesis of most protein amino acids occur in chloroplasts; therefore, various transport mechanisms must exist to accommodate their directional efflux from the stroma to the cytosol and feed the amino acids into the extraplastidial metabolic and long-distance transport pathways. Yet, Arabidopsis (Arabidopsis thaliana) transporters functioning in plastidial export of amino acids remained undiscovered. Here, USUALLY MULTIPLE ACIDS MOVE IN AND OUT TRANSPORTER 44 (UMAMIT44) was identified and shown to function in glutamate export from Arabidopsis chloroplasts. UMAMIT44 controls glutamate homeostasis within and outside of chloroplasts and influences nitrogen partitioning from leaves to sinks. Glutamate imbalances in chloroplasts and leaves of umamit44 mutants impact cellular redox state, nitrogen and carbon metabolism, and amino acid (AA) and sucrose supply of growing sinks, leading to negative effects on plant growth. Nonetheless, the mutant lines adjust to some extent by upregulating alternative pathways for glutamate synthesis outside the plastids and by mitigating oxidative stress through the production of other amino acids and antioxidants. Overall, this study establishes that the role of UMAMIT44 in glutamate export from chloroplasts is vital for controlling nitrogen availability within source leaf cells and for sink nutrition, with an impact on growth and seed yield.

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.

Loss of activation by GABA in vertebrate delta ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) mediate excitatory signals between cells by binding neurotransmitters and conducting cations across the cell membrane. In the mammalian brain, most of these signals are mediated by two types of iGluRs: AMPA and NMDA (i.e. iGluRs sensitive to 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid and N-methyl-D-aspartic acid, respectively). Delta-type iGluRs of mammals also form neurotransmitter-binding channels in the cell membrane, but in contrast, their channel is not activated by neurotransmitter binding, raising biophysical questions about iGluR activation and biological questions about the role of delta iGluRs. We therefore investigated the divergence of delta iGluRs from their iGluR cousins using molecular phylogenetics, electrophysiology, and site-directed mutagenesis. We find that delta iGluRs are found in numerous bilaterian animals (e.g., worms, starfish, and vertebrates) and are closely related to AMPA receptors, both genetically and functionally. Surprisingly, we observe that many iGluRs of the delta family are activated by the classical inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Finally, we identify nine amino acid substitutions that likely gave rise to the inactivity of today's mammalian delta iGluRs, and these mutations abolish activity when engineered into active invertebrate delta iGluRs, partly by inducing receptor desensitization. These results offer biophysical insight into iGluR activity and point to a role for GABA in excitatory signaling in invertebrates.

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.

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).

Functional characterization and allelic mining of OsGLR genes for potential uses in rice improvement

Glutamate-like receptor (GLR) genes are a group of regulatory genes involved in many physiological processes of plants. With 26 members in the rice genome, the functionalities of most rice GLR genes remain unknown. To facilitate their potential uses in rice improvement, an integrated strategy involving CRISPR-Cas9 mediated knockouts, deep mining and analyses of transcriptomic responses to different abiotic stresses/hormone treatments and gene CDS haplotype (gcHap) diversity in 3,010 rice genomes was taken to understand the functionalities of the 26 rice GLR genes, which led us to two conclusions. First, the expansion of rice GLR genes into a large gene family during evolution had gone through repeated gene duplication events occurred primarily in two large GLR gene clusters on rice chromosomes 9 and 6, which was accompanied with considerable functional differentiation. Secondly, except for two extremely conserved ones (OsGLR6.2 and OsGLR6.3), rich gcHap diversity exists at the remaining GLR genes which played important roles in rice population differentiation and rice improvement, evidenced by their very strong sub-specific and population differentiation, by their differentiated responses to day-length and different abiotic stresses, by the large phenotypic effects of five GLR gene knockout mutants on rice yield traits, by the significant association of major gcHaps at most GLR loci with yield traits, and by the strong genetic bottleneck effects and artificial selection on the gcHap diversity in populations Xian (indica) and Geng (japonica) during modern breeding. Our results suggest the potential values of the natural variation at most rice GLR loci for improving the productivity and tolerances to abiotic stresses. Additional efforts are needed to determine the phenotypic effects of major gcHaps at these GLR loci in order to identify 'favorable' alleles at specific GLR loci specific target traits in specific environments to facilitate their application to rice improvement in future.

Differences between the GluD1 and GluD2 receptors revealed by GluD1 X-ray crystallography, binding studies and molecular dynamics

Ionotropic glutamate receptors are ligand-gated ion channels essential for fast excitatory neurotransmission in the brain. In contrast to most other members of the iGluR family, the subfamily of delta receptors, GluD1 and GluD2, does not bind glutamate but glycine/D-serine. GluD1 is widely expressed in the brain and the inner ear, where it is required for high-frequency hearing. Furthermore, it has been associated with schizophrenia, autism and depression. X-ray structures of the ligand-binding domain (LBD) of GluD2 have been published; however, no high-resolution structure is available for the ligand-binding domain of GluD1 (GluD1-LBD). Here, we report the X-ray crystal structure of the GluD1-LBD in its apo form at 2.57 Å resolution. Using isothermal titration calorimetry, we show that D-serine binds to the GluD1-LBD in an exothermic manner with a K_d of 160 μm, which is approximately five-fold greater than at GluD2. Furthermore, we identify Glu822 as a critical determinant of receptor activation in GluD1 A654T. In contrast to studies on the GluD2 lurcher mutant A654T, we did not observe any effect of 1 mm D-serine on the spontaneous currents at mouse GluD1 A654T by electrophysiological recordings of Xenopus laevis oocytes as previously also reported by others. These results point towards differences in the structure and dynamics between GluD1 and GluD2. Molecular dynamics simulations were employed to address this observation, suggesting that the apo structure of GluD1 is less flexible than the apo structure of GluD2 and that Pro725 in GluD1 may affect the interlobe closure of the ligand-binding domain of GluD1.

Two types of GLR channels cooperate differently in light and dark growth of Arabidopsis seedlings

BACKGROUND

GLutamate Receptor-like (GLR) channels are multimeric, ionotropic, ligand-gated plant transmembrane receptors. They are homologous to mammalian glutamate receptors, iGLuRs, which are critical to neuronal function. GLRs have been reported several times to play a role in photomorphogenesis. However, to date, no study has looked at the mechanism of their involvement in this process. Here we focused on examining the impact of GLRs on the regulation of early seedling growth in blue light, red light, and in the dark.

RESULTS

Wild type and six photoreceptor mutant seedlings were grown on media supplemented with known iGLuR/GLR channel antagonists: MK-801, which non-competitively blocks NMDA channels in mammalian cells, and CNQX, known for competitive blocking of AMPA channels in mammalian cells. The lengths of hypocotyls and roots were measured in seedlings of phyA, phyB, phot1, phot2, cry1, and cry2 mutants after 7 days of in vitro culture. Changes in growth parameters, both in light and in darkness upon application of chemical antagonists, show that both types of GLR channels, NMDA-like and AMPA-like, are involved in the regulation of seedling growth irrespective of light conditions. Analysis of seedling growth of photoreceptor mutants indicates that the channels are influenced by signaling from phot1, phot2, and cry1. To extend our analysis, we also evaluated the elicitation of a calcium wave, which is likely to be partially driven by GLRs, in Arabidopsis seedlings. The changes in cellobiose-induced calcium waves observed after applying GLR inhibitors suggest that both types of channels likely cooperate in shaping Arabidopsis seedling growth and development.

CONCLUSIONS

Our work provides the first experimental evidence that two types of GLR channels function in plants: NMDA-like and AMPA-like. We also demonstrate that the channels are involved in seedling growth and development, at least partially through modulation of calcium signaling, but they are unlikely to play a major role in photomorphogenesis.

GluD receptors are functional ion channels

In this article, we review contemporary evidence that GluD receptors are functional ion channels whose depolarizing currents contribute to their biological functions, akin to all other members of the ionotropic glutamate receptor (iGluR) family.

Cadmium pollution leads to selectivity loss of glutamate receptor channels for permeation of Ca2+/Mn2+/Fe2+/Zn2+ over Cd2+ in rice plant

The selective permeation of glutamate receptor channels (GLRs) for essential and toxic elements in plant cells is poorly understood. The present study found that the ratios between cadmium (Cd) and 7 essential elements (i.e., K, Mg, Ca, Mn, Fe, Zn and Cu) in grains and vegetative organs increased significantly with the increase of soil Cd levels. Accumulation of Cd resulted in the significant increase of Ca, Mn, Fe and Zn content and the expression levels of Ca channel genes (OsCNGC1,2 and OsOSCA1.1,2.4), while remarkable reduction of glutamate content and expression levels of GLR3.1-3.4 in rice. When planted in the same Cd-polluted soil, mutant fc8 displayed significantly higher content of Ca, Fe, Zn and expression levels of GLR3.1-3.4 than its wild type NPB. On the contrary, the ratios between Cd and essential elements in fc8 were significantly lower than that in NPB. These results indicate that Cd pollution may damage the structural integrity of GLRs by inhibiting glutamate synthesis and expression levels of GLR3.1-3.4, which leads to the increase of ion influx but the decrease of preferential selectivity for Ca²⁺/ Mn²⁺/ Fe²⁺/ Zn²⁺ over Cd²⁺ through GLRs in rice cells.

Merging Signaling with Structure: Functions and Mechanisms of Plant Glutamate Receptor Ion Channels

Plant glutamate receptor-like (GLR) genes encode ion channels with demonstrated roles in electrical and calcium (Ca2+) signaling. The expansion of the GLR family along the lineage of land plants, culminating in the appearance of a multiclade system among flowering plants, has been a topic of interest since their discovery nearly 25 years ago. GLRs are involved in many physiological processes, from wound signaling to transcriptional regulation to sexual reproduction. Emerging evidence supports the notion that their fundamental functions are conserved among different groups of plants as well. In this review, we update the physiological and genetic evidence for GLRs, establishing their role in signaling and cell-cell communication. Special emphasis is given to the recent discussion of GLRs' atomic structures. Along with functional assays, a structural view of GLRs' molecular organization presents a window for novel hypotheses regarding the molecular mechanisms underpinning signaling associated with the ionic fluxes that GLRs regulate. Newly uncovered transcriptional regulations associated with GLRs-which propose the involvement of genes from all clades ofArabidopsis thaliana in ways not previously observed-are discussed in the context of the broader impacts of GLR activity. We posit that the functions of GLRs in plant biology are probably much broader than anticipated, but describing their widespread involvement will only be possible with (a) a comprehensive understanding of the channel's properties at the molecular and structural levels, including protein-protein interactions, and (b) the design of new genetic approaches to explore stress and pathogen responses where precise transcriptional control may result in more precise testable hypotheses to overcome their apparent functional redundancies.

Interplay between membrane proteins and membrane protein-lipid pertaining to plant salinity stress

High salinity in agricultural lands is one of the predominant issues limiting agricultural yields. Plants have developed several mechanisms to withstand salinity stress, but the mechanisms are not effective enough for most crops to prevent and persist the salinity stress. Plant salt tolerance pathways involve membrane proteins that have a crucial role in sensing and mitigating salinity stress. Due to a strategic location interfacing two distinct cellular environments, membrane proteins can be considered checkpoints to the salt tolerance pathways in plants. Related membrane proteins functions include ion homeostasis, osmosensing or ion sensing, signal transduction, redox homeostasis, and small molecule transport. Therefore, modulating plant membrane proteins' function, expression, and distribution can improve plant salt tolerance. This review discusses the membrane protein-protein and protein-lipid interactions related to plant salinity stress. It will also highlight the finding of membrane protein-lipid interactions from the context of recent structural evidence. Finally, the importance of membrane protein-protein and protein-lipid interaction is discussed, and a future perspective on studying the membrane protein-protein and protein-lipid interactions to develop strategies for improving salinity tolerance is proposed.

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.

Cracking the code of plant herbivore defense

In 1916, Ricca hypothesized that plant defense mediators are transported by xylem vessels. While it was discovered that electrical waves generated at plant wounds also transmit information over great distances, the molecular nature of the so-called Ricca factor remained unclear. In this issue of Cell, Gao et al. identify thioglucoside glucohydrolases as a Ricca factor in Arabidopsis.

Osmoelectric siphon models for signal and water dispersal in wounded plants

When attacked by herbivores, plants produce electrical signals which can activate the synthesis of the defense mediator jasmonate. These wound-induced membrane potential changes can occur in response to elicitors that are released from damaged plant cells. We list plant-derived elicitors of membrane depolarization. These compounds include the amino acid l-glutamate (Glu), a potential ligand for GLUTAMATE RECEPTOR-LIKE (GLR) proteins that play roles in herbivore-activated electrical signaling. How are membrane depolarization elicitors dispersed in wounded plants? In analogy with widespread turgor-driven cell and organ movements, we propose osmoelectric siphon mechanisms for elicitor transport. These mechanisms are based on membrane depolarization leading to cell water shedding into the apoplast followed by membrane repolarization and water uptake. We discuss two related mechanisms likely to occur in response to small wounds and large wounds that trigger leaf-to-leaf electrical signal propagation. To reduce jasmonate pathway activation, a feeding insect must cut through tissues cleanly. If their mandibles become worn, the herbivore is converted into a robust plant defense activator. Our models may therefore help to explain why numerous plants produce abrasives which can blunt herbivore mouthparts. Finally, if verified, the models we propose may be generalizable for cell to cell transport of water and pathogen-derived regulators.

Calcium-dependent protein kinase CDPK16 phosphorylates serine-856 of glutamate receptor-like GLR3.6 protein leading to salt-responsive root growth in Arabidopsis. FRONTIERS IN PLANT SCIENCE 2023;14:1093472. [PMID: 36818849 PMCID: PMC9935832 DOI: 10.3389/fpls.2023.1093472] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Calcium-permeable channels in the plasma membrane play vital roles in plant growth, development, and response to environmental stimuli. Arabidopsis possesses 20 glutamate receptor-like proteins that share similarities with animal ionotropic glutamate receptors and mediate Ca²⁺ influx in plants. Calcium-dependent protein kinases (CDPKs) phosphorylate serine (Ser)-860 of glutamate receptor-like (GLR)3.7 protein, which interacts with 14-3-3ω and plays an essential role in salt and abscisic acid response in Arabidopsis by modulating Ca²⁺ signaling. However, the significance of CDPK- mediated phosphorylation status of Ser residues of GLR3.6 with regard to the functioning of GLR3.6 remains to be elucidated. In this study, we performed an in vitro kinase assay using CDPK16 and peptides containing the 14-3-3ω interacting domain of GLR3.6. We showed that Ser861/862 of GLR3.6 are required for the interaction with 14-3-3ω and that Ser856 of GLR3.6 is specifically phosphorylated by CDPK16 but not by CDPK3 and CDPK34. In addition, the expression of GLR3.6 was quickly downregulated by salt stress, and plants of glr3.6 mutants and GLR3.6-overexpression lines presented shorter and longer root lengths, respectively, under normal growth conditions than Col. Overexpression of the GLR3.6-Ser856 to Ala mutation resulted in a less sensitive phenotype in response to salt stress similar to glr3.6. Our results indicated that the Ser861/862 residues of GLR3.6 are required for interaction with 14-3-3ω. Additionally, the phosphorylation status of Ser856 residue of GLR3.6, which is mediated specifically by CDPK16, regulates root growth in normal and salt stress and conditions.

Dual localization of the carboxy-terminal tail of GLR3.3 in sieve element-companion cell complex

Glutamate receptor-like (GLR) 3.3 and 3.6 proteins are required for mediating wound-induced leaf-to-leaf electrical signaling. In the previous study, we found that the carboxy-terminal tail of GLR3.3 contains key residues that are indispensable for its action in electrical signaling. In the present work, we generated plants that expressed the truncated C-tail fraction of GLR3.3. To our expectation, the truncated C-tail itself was not functional in propagating leaf-to-leaf signals. However, we identified that the C-tail-mVENUS fusion proteins had dual localization patterns in sieve elements and companion cells. In companion cells, the fusion proteins overlapped largely with the nucleus. We speculated that a possible nuclear localization signal is present in the C-tail of GLR3.3, paralleling the C-tails of the ionotropic glutamate receptors in animal cells. Our further findings on the C-tail of GLR3.3 open up new possibilities for the regulatory roles of the C-tails to GLR proteins.

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.

The carboxy-terminal tail of GLR3.3 is essential for wound-response electrical signaling

Arabidopsis Clade 3 GLUTAMATE RECEPTOR-LIKEs (GLRs) are primary players in wound-induced systemic signaling. Previous studies focused on dissecting their ligand-activated channel properties involving extracellular and membrane-related domains. Here, we report that the carboxy-terminal tails (C-tails) of GLRs contain key elements controlling their function in wound signaling. GLR3.3 without its C-tail failed to rescue the glr3.3a mutant. We carried out a yeast two-hybrid screen to identify the C-tail interactors. We performed functional studies of the interactor by measuring electrical signals and defense responses. Then we mapped their binding sites and evaluated the impact of the sites on GLR functions. IMPAIRED SUCROSE INDUCTION 1 (ISI1) interacted with GLR3.3. Enhanced electrical activity was detected in reduced function isi1 mutants in a GLR3.3-dependent manner. isi1 mutants were slightly more resistant to insect feeding than the wild-type. Furthermore, a triresidue motif RFL in the GLR3.3 C-tail binds to ISI1 in yeast. Finally, we demonstrated that FL residues were conserved across GLRs and functionally required. Our study provides new insights into the functions of GLR C-tails, reveals parallels with the ionotropic glutamate receptor regulation in animal cells, and may enable rational design of strategies to engineer GLRs for future practical applications.

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.

Plant physiology: Anatomy of a plant action potential

The Venus flytrap possesses modified leaves that can snap shut fast enough to catch a fly. A new study identifies the major components of the toolkit that allows the flytrap to fire action potentials, illustrating how different ion channels and transporters are recruited to give rise to this unique plant behavioural response.

Integration of long-range signals in plants: A model for wound-induced Ca2+, electrical, ROS, and glutamate waves

Plants show long-range cytosolic Ca²⁺ signal transduction in response to wounding. Recent advances in in vivo imaging techniques have helped visualize spatiotemporal dynamics of the systemic Ca²⁺ signals and provided new insights into underlying molecular mechanisms, in which ion channels of the GLUTAMATE RECEPTOR-LIKE (GLR) family are critical for the sensory system. These, along with MECHANOSENSITIVE CHANNEL OF SMALL CONDUCTANCE-LIKE 10 (MSL10) and Arabidopsis H⁺-ATPase (AHA1) regulate the propagation system. In addition, membrane potential, reactive oxygen species (ROS), and glutamate waves operate in parallel to long-range signal transduction. We summarize these findings and introduce a model that integrates long-range Ca²⁺, electrical, ROS, and glutamate signals in systemic wound responses.

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.

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.

Reactive oxygen species signalling in plant stress responses

Reactive oxygen species (ROS) are key signalling molecules that enable cells to rapidly respond to different stimuli. In plants, ROS play a crucial role in abiotic and biotic stress sensing, integration of different environmental signals and activation of stress-response networks, thus contributing to the establishment of defence mechanisms and plant resilience. Recent advances in the study of ROS signalling in plants include the identification of ROS receptors and key regulatory hubs that connect ROS signalling with other important stress-response signal transduction pathways and hormones, as well as new roles for ROS in organelle-to-organelle and cell-to-cell signalling. Our understanding of how ROS are regulated in cells by balancing production, scavenging and transport has also increased. In this Review, we discuss these promising developments and how they might be used to increase plant resilience to environmental stress.

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.

GLUTAMATE RECEPTOR-like gene OsGLR3.4 is required for plant growth and systemic wound signaling in rice (Oryza sativa)

Recent studies have revealed the physiological roles of glutamate receptor-like channels (GLRs) in Arabidopsis; however, the functions of GLRs in rice remain largely unknown. Here, we show that knockout of OsGLR3.4 in rice leads to brassinosteroid (BR)-regulated growth defects and reduced BR sensitivity. Electrophoretic mobility shift assays and transient transactivation assays indicated that OsGLR3.4 is the downstream target of OsBZR1. Further, agonist profile assays showed that multiple amino acids can trigger transient Ca²⁺ influx in an OsGLR3.4-dependent manner, indicating that OsGLR3.4 is a Ca²⁺ -permeable channel. Meanwhile, the study of internode cells demonstrated that OsGLR3.4-mediated Ca²⁺ flux is required for actin filament organization and vesicle trafficking. Following root injury, the triggering of both slow wave potentials (SWPs) in leaves and the jasmonic acid (JA) response are impaired in osglr3.4 mutants, indicating that OsGLR3.4 is required for root-to-shoot systemic wound signaling in rice. Brassinosteroid treatment enhanced SWPs and OsJAZ8 expression in root-wounded plants, suggesting that BR signaling synergistically regulates the OsGLR3.4-mediated systemic wound response. In summary, this article describes a mechanism of OsGLR3.4-mediated cell elongation and long-distance systemic wound signaling in plants and provides new insights into the contribution of GLRs to plant growth and responses to mechanical wounding.

Label-free visual proteomics: Coupling MS- and EM-based approaches in structural biology

Combining diverse experimental structural and interactomic methods allows for the construction of comprehensible molecular encyclopedias of biological systems. Typically, this involves merging several independent approaches that provide complementary structural and functional information from multiple perspectives and at different resolution ranges. A particularly potent combination lies in coupling structural information from cryoelectron microscopy or tomography (cryo-EM or cryo-ET) with interactomic and structural information from mass spectrometry (MS)-based structural proteomics. Cryo-EM/ET allows for sub-nanometer visualization of biological specimens in purified and near-native states, while MS provides bioanalytical information for proteins and protein complexes without introducing additional labels. Here we highlight recent achievements in protein structure and interactome determination using cryo-EM/ET that benefit from additional MS analysis. We also give our perspective on how combining cryo-EM/ET and MS will continue bridging gaps between molecular and cellular studies by capturing and describing 3D snapshots of proteomes and interactomes.

Delta glutamate receptors are functional glycine- and ᴅ-serine-gated cation channels in situ

Delta receptors are members of the ionotropic glutamate receptor superfamily and form trans-synaptic connections by interacting with the extracellular scaffolding protein cerebellin-1 and presynaptic transmembrane protein neurexin-1β. Unlike other family members, however, direct agonist-gated ion channel activity has not been recorded in delta receptors. Here, we show that the GluD2 subtype of delta receptor forms cation-selective channels when bound to cerebellin-1 and neurexin-1β. Using fluorescence lifetime measurements and chemical cross-linking, we reveal that tight packing of the amino-terminal domains of GluD2 permits glycine- and d-serine–induced channel openings. Thus, cerebellin-1 and neurexin-1β act as biological cross-linkers to stabilize the extracellular domains of GluD2 receptors, allowing them to function as ionotropic excitatory neurotransmitter receptors in synapses.

Purification and cryo-EM structure determination of Arabidopsis thaliana GLR3.4

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that play crucial roles in the central nervous system. iGluR homologs, termed glutamate receptor-like channels (GLRs), have been found in plants. Investigating the structural and functional relationship between iGluRs and GLRs was limited by GLR protein expression, purification, and structural characterization. Here, we provide a detailed protocol for Arabidopsis thaliana GLR3.4 (AtGLR3.4) expression in a mammalian cell line and purification for structure determination by cryogenic electron microscopy (cryo-EM). For the complete details on the use and execution of this protocol, please refer to Green et al. (2021).

Calcium Signaling Mechanisms Across Kingdoms

Calcium (Ca²⁺) is a unique mineral that serves as both a nutrient and a signal in all eukaryotes. To maintain Ca²⁺ homeostasis for both nutrition and signaling purposes, the toolkit for Ca²⁺ transport has expanded across kingdoms of eukaryotes to encode specific Ca²⁺ signals referred to as Ca²⁺ signatures. In parallel, a large array of Ca²⁺-binding proteins has evolved as specific sensors to decode Ca²⁺ signatures. By comparing these coding and decoding mechanisms in fungi, animals, and plants, both unified and divergent themes have emerged, and the underlying complexity will challenge researchers for years to come. Considering the scale and breadth of the subject, instead of a literature survey, in this review we focus on a conceptual framework that aims to introduce to readers to the principles and mechanisms of Ca²⁺ signaling. We finish with several examples of Ca²⁺-signaling pathways, including polarized cell growth, immunity and symbiosis, and systemic signaling, to piece together specific coding and decoding mechanisms in plants versus animals. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.