Higher plants efficiently orchestrate rapid systemic responses to diverse environmental stimuli through electric signaling. This review explores the mechanisms underlying two main types of electric ...signals in plants, action potentials (APs) and slow wave potentials (SWPs), and how new discoveries challenge conventional neurophysiological paradigms traditionally forming their theoretical foundations. Animal APs are biophysically well-defined, whereas plant APs are often classified based on their shape, lacking thorough characterization. SWPs are depolarizing electric signals deviating from this shape, leading to an oversimplified classification of plant electric signals. Indeed, investigating the generation and propagation of plant APs and SWPs showcases a complex interplay of mechanisms that sustain self-propagating signals and internally propagating stimuli, resulting in membrane depolarization, cytosolic calcium increase, and alterations in reactive oxygen species and pH. A holistic understanding of plant electric signaling will rely on unraveling the network of ion-conducting proteins, signaling molecules, and mechanisms for signal generation and propagation.
The phytohormone ethylene has numerous effects on plant growth and development. Its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), is a non-proteinogenic amino acid produced by ACC ...SYNTHASE (ACS). ACC is often used to induce ethylene responses. Here, we demonstrate that ACC exhibits ethylene-independent signaling in Arabidopsis thaliana reproduction. By analyzing an acs octuple mutant with reduced seed set, we find that ACC signaling in ovular sporophytic tissue is involved in pollen tube attraction, and promotes secretion of the pollen tube chemoattractant LURE1.2. ACC activates Ca
-containing ion currents via GLUTAMATE RECEPTOR-LIKE (GLR) channels in root protoplasts. In COS-7 cells expressing moss PpGLR1, ACC induces the highest cytosolic Ca
elevation compared to all twenty proteinogenic amino acids. In ovules, ACC stimulates transient Ca
elevation, and Ca
influx in octuple mutant ovules rescues LURE1.2 secretion. These findings uncover a novel ACC function and provide insights for unraveling new physiological implications of ACC in plants.
Though sharing overall sequence and structural homology, plant and animal glutamate receptors display several differences on the functional and regulatory level.
Abstract
Animal ionotropic glutamate ...receptors (iGluRs) are ligand-gated channels whose evolution is intimately linked to that of the nervous system, where the agonist glutamate and co-agonists glycine/d-serine act as neurotransmitters or neuromodulators. While iGluRs are specialized in neuronal communication, plant glutamate receptor-like (GLR) homologs have evolved many plant-specific physiological functions, such as sperm signaling in moss, pollen tube growth, root meristem proliferation, innate immune, and wound responses. GLRs have been associated with Ca2+ signaling by directly channeling its extracellular influx into the cytosol. Nevertheless, very limited information on functional properties of GLRs is available, and we mostly rely on structure/function data obtained for animal iGluRs to interpret experimental results obtained for plant GLRs. Yet, a deeper characterization and better understanding of plant GLRs is progressively unveiling original and different functions when compared with their mammalian counterparts. Here, we review the function of plant GLRs comparing their predicted structure and physiological roles with those of the well-documented roles of iGluRs. We conclude that interpreting GLR function based on comparison with their animal counterparts calls for caution, especially when presuming physiological roles and the mode of action for plant GLRs, and when comparing iGluRs in neuronal tissues with those in peripheral, non-neuronal tissues.
The leaf vasculature plays a key role in solute translocation. Veins consist of at least seven distinct cell types, with specific roles in transport, metabolism, and signaling. Little is known about ...leaf vascular cells, in particular the phloem parenchyma (PP). PP effluxes sucrose into the apoplasm as a basis for phloem loading, yet PP has been characterized only microscopically. Here, we enriched vascular cells from Arabidopsis leaves to generate a single-cell transcriptome atlas of leaf vasculature. We identified at least 19 cell clusters, encompassing epidermis, guard cells, hydathodes, mesophyll, and all vascular cell types, and used metabolic pathway analysis to define their roles. Clusters comprising PP cells were enriched for transporters, including SWEET11 and SWEET12 sucrose and UmamiT amino acid efflux carriers. We provide evidence that PP development occurs independently from APL, a transcription factor required for phloem differentiation. PP cells have a unique pattern of amino acid metabolism activity distinct from companion cells (CC), explaining differential distribution/metabolism of amino acids in veins. The kinship relation of the vascular clusters is strikingly similar to the vein morphology, except for a clear separation of CC from the other vascular cells including PP. In summary, our scRNA-seq analysis provides a wide range of information into the leaf vasculature and the role and relationship of the leaf cell types.
Multiple, diverse, and complexCalcium currents characterize the developing pollen tube in the small mustard plant Arabidopsis and correlate with growth at the tip of the pollen tube. This system ...constitutes a practical model for screening for Ca2+-signaling mechanisms in plants. Wudick et al. analyzed multiple variants of glutamate receptor–like (GLR) channels and discovered that some work alone and others work in pairs or trios. Subcellular localization of GLRs is a complex response to CORNICHON sorting proteins, which leave some GLRs at the plasma membrane and ferry others to internal calcium reservoirs. The calcium current at the tip of the growing pollen tube apparently integrates multiple intracellular currents.Science, this issue p. 533Compared to animals, evolution of plant calcium (Ca2+) physiology has led to a loss of proteins for influx and small ligand–operated control of cytosolic Ca2+, leaving many Ca2+ mechanisms unaccounted for. Here, we show a mechanism for sorting and activation of glutamate receptor–like channels (GLRs) by CORNICHON HOMOLOG (CNIH) proteins. Single mutants of pollen-expressed Arabidopsis thaliana GLRs (AtGLRs) showed growth and Ca2+ flux phenotypes expected for plasma membrane Ca2+ channels. However, higher-order mutants of AtGLR3.3 revealed phenotypes contradicting this assumption. These discrepancies could be explained by subcellular AtGLR localization, and we explored the implication of AtCNIHs in this sorting. We found that AtGLRs interact with AtCNIH pairs, yielding specific intracellular localizations. AtCNIHs further trigger AtGLR activity in mammalian cells without any ligand. These results reveal a regulatory mechanism underlying Ca2+ homeostasis by sorting and activation of AtGLRs by AtCNIHs.
Glutamate receptor-like channels (GLRs) play vital roles in various physiological processes in plants, such as wound response, stomatal aperture control, seed germination, root development, innate ...immune response, pollen tube growth, and morphogenesis. Despite the importance of GLRs, knowledge about their molecular organization is limited. Here we use X-ray crystallography and single-particle cryo-EM to solve structures of the Arabidopsis thaliana GLR3.4. Our structures reveal the tetrameric assembly of GLR3.4 subunits into a three-layer domain architecture, reminiscent of animal ionotropic glutamate receptors (iGluRs). However, the non-swapped arrangement between layers of GLR3.4 domains, binding of glutathione through S-glutathionylation of cysteine C205 inside the amino-terminal domain clamshell, unique symmetry, inter-domain interfaces, and ligand specificity distinguish GLR3.4 from representatives of the iGluR family and suggest distinct features of the GLR gating mechanism. Our work elaborates on the principles of GLR architecture and symmetry and provides a molecular template for deciphering GLR-dependent signaling mechanisms in plants.
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•Cryo-EM structure of Arabidopsis thaliana glutamate receptor-like channel GLR3.4•Glutathione regulates channel activity by binding to C205 in amino-terminal domain•Crystal structures of GLR3.4 ligand-binding domain illustrate ligand promiscuity•Amino-terminal and ligand-binding domain layers show non-swapped domain arrangement
Green et al. solve structures of Arabidopsis thaliana glutamate receptor-like channel GLR3.4 that shows tetrameric subunit assembly with three-layer architecture, similar to its mammalian homologs, ionotropic glutamate receptors, but with distinct symmetry, inter-domain interfaces, ligand specificity, and non-swapped domain arrangement between layers of ligand-binding and glutathione-bound amino-terminal domains.
Aquaporins are water channel proteins that mediate the fine-tuning of cell membrane water permeability during development or in response to environmental stresses. The present work focuses on the ...oxidative stress-induced redistribution of plasma membrane intrinsic protein (PIP) aquaporins from the plasma membrane (PM) to intracellular membranes. This process was investigated in the Arabidopsis root. Sucrose density gradient centrifugation showed that exposure of roots to 0.5 mM H2O2 induces significant depletion in PM fractions of several abundant PIP homologs after 15 min. Analyses by single-particle tracking and fluorescence correlative spectroscopy showed that, in the PM of epidermal cells, H2O2 treatment induces an increase in lateral motion and a reduction in the density of a fluorescently tagged form of the prototypal AtPIP2;1 isoform, respectively. Co-expression analyses of AtPIP2;1 with endomembrane markers revealed that H2O2 triggers AtPIP2;1 accumulation in the late endosomal compartments. Life-time analyses established that the high stability of PIPs was maintained under oxidative stress conditions, suggesting that H2O2 triggers a mechanism for intracellular sequestration of PM aquaporins without further degradation. In addition to information on cellular regulation of aquaporins, this study provides novel and complementary insights into the dynamic remodeling of plant internal membranes during oxidative stress responses.
H2O2 triggers aquaporin AtPIP2;1 accumulation in the late endosomal compartments. We also observed that high stability of PIPs is maintained under oxidative stress conditions.
Calcium has long been recognized as a preeminent signaling molecule in plants with staggeringly diverse functions. The central mystery has therefore been how a single ion species can fulfill distinct ...functions while maintaining specificity and fidelity. Part of the answer lies in calcium being the most heavily controlled element in the cytosol, with dedicated transporters for sequestration into the apoplasm and intracellular stores. Controlled release of calcium into the cytosol by ion channels is the initiating step in signal transduction. Calcium-permeable ion channels are therefore important research targets. Recent studies have identified previously unknown channels, revealed atomic structures, and pinpointed locations of channels to specific cells and membranes. Here, we highlight key findings, transformative technologies, and pathways for further discovery.
The understanding of signaling and metabolic processes in multicellular organisms requires knowledge of the spatial dynamics of small molecules and the activities of enzymes, transporters, and other ...proteins in vivo, as well as biophysical parameters inside cells and across tissues. The cellular distribution of receptors, ligands, and activation state must be integrated with information about the cellular distribution of metabolites in relation to metabolic fluxes and signaling dynamics in order to achieve the promise of in vivo biochemistry. Genetically encoded sensors are engineered fluorescent proteins that have been developed for a wide range of small molecules, such as ions and metabolites, or to report biophysical processes, such as transmembrane voltage or tension. First steps have been taken to monitor the activity of transporters in vivo. Advancements in imaging technologies and specimen handling and stimulation have enabled researchers in plant sciences to implement sensor technologies in intact plants. Here, we provide a brief history of the development of genetically encoded sensors and an overview of the types of sensors available for quantifying and visualizing ion and metabolite distribution and dynamics. We further discuss the pros and cons of specific sensor designs, imaging systems, and sample manipulations, provide advice on the choice of technology, and give an outlook into future developments.
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