Structure and activity of SLAC1 channels for stomatal signaling in leaves

Streamlined spatial and environmental expression signatures characterize the minimalist duckweed Wolffia australiana

Single-cell genomics permits a new resolution in the examination of molecular and cellular dynamics, allowing global, parallel assessments of cell types and cellular behaviors through development and in response to environmental circumstances, such as interaction with water and the light-dark cycle of the Earth. Here, we leverage the smallest, and possibly most structurally reduced, plant, the semiaquatic Wolffia australiana, to understand dynamics of cell expression in these contexts at the whole-plant level. We examined single-cell-resolution RNA-sequencing data and found Wolffia cells divide into four principal clusters representing the above- and below-water-situated parenchyma and epidermis. Although these tissues share transcriptomic similarity with model plants, they display distinct adaptations that Wolffia has made for the aquatic environment. Within this broad classification, discrete subspecializations are evident, with select cells showing unique transcriptomic signatures associated with developmental maturation and specialized physiologies. Assessing this simplified biological system temporally at two key time-of-day (TOD) transitions, we identify additional TOD-responsive genes previously overlooked in whole-plant transcriptomic approaches and demonstrate that the core circadian clock machinery and its downstream responses can vary in cell-specific manners, even in this simplified system. Distinctions between cell types and their responses to submergence and/or TOD are driven by expression changes of unexpectedly few genes, characterizing Wolffia as a highly streamlined organism with the majority of genes dedicated to fundamental cellular processes. Wolffia provides a unique opportunity to apply reductionist biology to elucidate signaling functions at the organismal level, for which this work provides a powerful resource.

A proteome-wide quantitative platform for nanoscale spatially resolved extraction of membrane proteins into native nanodiscs

The intricate molecular environment of the native membrane profoundly influences every aspect of membrane protein (MP) biology. Despite this, the most prevalent method of studying MPs uses detergent-like molecules that disrupt and remove this vital local membrane context. This severely impedes our ability to quantitatively decipher the local molecular context and comprehend its regulatory role in the structure, function, and biogenesis of MPs. Using a library of membrane-active polymers we have developed a platform for the high-throughput analysis of the membrane proteome. The platform enables near-complete spatially resolved extraction of target MPs directly from their endogenous membranes into native nanodiscs that maintain the local membrane context. We accompany this advancement with an open-access database that quantifies the polymer-specific extraction variability for 2065 unique mammalian MPs and provides the most optimized condition for each of them. Our method enables rapid and near-complete extraction and purification of target MPs directly from their endogenous organellar membranes at physiological expression levels while maintaining the nanoscale local membrane environment. Going beyond the plasma membrane proteome, our platform enables extraction from any target organellar membrane including the endoplasmic reticulum, mitochondria, lysosome, Golgi, and even transient organelles such as the autophagosome. To further validate this platform, we took several independent MPs and demonstrated how our resource can enable rapid extraction and purification of target MPs from different organellar membranes with high efficiency and purity. Further, taking two synaptic vesicle MPs, we show how the database can be extended to capture multiprotein complexes between overexpressed MPs. We expect these publicly available resources to empower researchers across disciplines to efficiently capture membrane 'nano-scoops' containing a target MP and interface with structural, functional, and other bioanalytical approaches. We demonstrate an example of this by combining our extraction platform with single-molecule TIRF imaging to demonstrate how it can enable rapid determination of homo-oligomeric states of target MPs in native cell membranes.

Mechanistic insights into phosphoactivation of SLAC1 in guard cell signaling

Stomata in leaves regulate gas (carbon dioxide and water vapor) exchange and water transpiration between plants and the atmosphere. SLow Anion Channel 1 (SLAC1) mediates anion efflux from guard cells and plays a crucial role in controlling stomatal aperture. It serves as a central hub for multiple signaling pathways in response to environmental stimuli, with its activity regulated through phosphorylation via various plant protein kinases. However, the molecular mechanism underlying SLAC1 phosphoactivation has remained elusive. Through a combination of protein sequence analyses, AlphaFold-based modeling and electrophysiological studies, we unveiled that the highly conserved motifs on the N- and C-terminal segments of SLAC1 form a cytosolic regulatory domain (CRD) that interacts with the transmembrane domain(TMD), thereby maintaining the channel in an autoinhibited state. Mutations in these conserved motifs destabilize the CRD, releasing autoinhibition in SLAC1 and enabling its transition into an activated state. Our further studies demonstrated that SLAC1 activation undergoes an autoinhibition-release process and subsequent structural changes in the pore helices. These findings provide mechanistic insights into the activation mechanism of SLAC1 and shed light on understanding how SLAC1 controls stomatal closure in response to environmental stimuli.

Artificial cellulosic leaf with adjustable enzymatic CO2 sequestration capability

Developing artificial leaves to address the environmental burden of CO2 is pivotal for advancing our Net Zero Future. In this study, we introduce EcoLeaf, an artificial leaf that closely mimics the characteristics of natural leaves. It harnesses visible light as its sole energy source and orchestrates the controlled expansion and contraction of stomata and the exchange of petiole materials to govern the rate of CO2 sequestration from the atmosphere. Furthermore, EcoLeaf has a cellulose composition and mechanical strength similar to those of natural leaves, allowing it to seamlessly integrate into the ecosystem during use and participate in natural degradation and nutrient cycling processes at the end of its life. We propose that the carbon sequestration pathway within EcoLeaf is adaptable and can serve as a versatile biomimetic platform for diverse biogenic carbon sequestration pathways in the future.

A charged existence: A century of transmembrane ion transport in plants

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

Predicting Mutational Effects on Ca2+-Activated Chloride Conduction of TMEM16A Based on a Simulation Study

The dysfunction and defects of ion channels are associated with many human diseases, especially for loss-of-function mutations in ion channels such as cystic fibrosis transmembrane conductance regulator mutations in cystic fibrosis. Understanding ion channels is of great current importance for both medical and fundamental purposes. Such an understanding should include the ability to predict mutational effects and describe functional and mechanistic effects. In this work, we introduce an approach to predict mutational effects based on kinetic information (including reaction barriers and transition state locations) obtained by studying the working mechanism of target proteins. Specifically, we take the Ca2+-activated chloride channel TMEM16A as an example and utilize the computational biology model to predict the mutational effects of key residues. Encouragingly, we verified our predictions through electrophysiological experiments, demonstrating a 94% prediction accuracy regarding mutational directions. The mutational strength assessed by Pearson's correlation coefficient is -0.80 between our calculations and the experimental results. These findings suggest that the proposed methodology is reliable and can provide valuable guidance for revealing functional mechanisms and identifying key residues of the TMEM16A channel. The proposed approach can be extended to a broad scope of biophysical systems.

Cryo-EM structures of the plant anion channel SLAC1 from Arabidopsis thaliana suggest a combined activation model

The anion channel SLAC1 functions as a crucial effector in the ABA signaling, leading to stomata closure. SLAC1 is activated by phosphorylation in its intracellular domains. Both a binding-activation model and an inhibition-release model for activation have been proposed based on only the closed structures of SLAC1, rendering the structure-based activation mechanism controversial. Here we report cryo-EM structures of Arabidopsis SLAC1 WT and its phosphomimetic mutants in open and closed states. Comparison of the open structure with the closed ones reveals the structural basis for opening of the conductance pore. Multiple phosphorylation of an intracellular domain (ICD) causes dissociation of ICD from the transmembrane domain. A conserved, positively-charged sequence motif in the intracellular loop 2 (ICL2) seems to be capable of sensing of the negatively charged phosphorylated ICD. Interactions between ICL2 and ICD drive drastic conformational changes, thereby widening the pore. From our results we propose that SLAC1 operates by a mechanism combining the binding-activation and inhibition-release models.

Engineering stomata for enhanced carbon capture and water-use efficiency

Stomatal pores facilitate gaseous exchange between the inner air spaces of the leaf and the atmosphere. As gatekeepers that balance CO2 entry for photosynthesis against transpirational water loss, they are a focal point for efforts to improve crop performance, especially in the efficiency of water use, within the changing global environment. Until recently, engineering strategies had focused on stomatal conductance in the steady state. These strategies are limited by the physical constraints of CO2 and water exchange such that gains in water-use efficiency (WUE) commonly come at a cost in carbon assimilation. Attention to stomatal speed and responsiveness circumvents these constraints and offers alternatives to enhancing WUE that also promise increases in carbon assimilation in the field.

WeiTsing, a pericycle-expressed ion channel, safeguards the stele to confer clubroot resistance

Plant roots encounter numerous pathogenic microbes that often cause devastating diseases. One such pathogen, Plasmodiophora brassicae (Pb), causes clubroot disease and severe yield losses on cruciferous crops worldwide. Here, we report the isolation and characterization of WeiTsing (WTS), a broad-spectrum clubroot resistance gene from Arabidopsis. WTS is transcriptionally activated in the pericycle upon Pb infection to prevent pathogen colonization in the stele. Brassica napus carrying the WTS transgene displayed strong resistance to Pb. WTS encodes a small protein localized in the endoplasmic reticulum (ER), and its expression in plants induces immune responses. The cryoelectron microscopy (cryo-EM) structure of WTS revealed a previously unknown pentameric architecture with a central pore. Electrophysiology analyses demonstrated that WTS is a calcium-permeable cation-selective channel. Structure-guided mutagenesis indicated that channel activity is strictly required for triggering defenses. The findings uncover an ion channel analogous to resistosomes that triggers immune signaling in the pericycle.

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.

Wheat ABA Receptor TaPYL5 Constitutes a Signaling Module with Its Downstream Partners TaPP2C53/TaSnRK2.1/TaABI1 to Modulate Plant Drought Response

Abscisic acid receptors (ABR) play crucial roles in transducing the ABA signaling initiated by osmotic stresses, which has a significant impact on plant acclimation to drought by modulating stress-related defensive physiological processes. We characterized TaPYL5, a member of the ABR family in wheat (Triticum aestivum), as a mediator of drought stress adaptation in plants. The signals derived from the fusion of TaPYL5-GFP suggest that the TaPYL5 protein was directed to various subcellular locations, namely stomata, plasma membrane, and nucleus. Drought stress significantly upregulated the TaPYL5 transcripts in roots and leaves. The biological roles of ABA and drought responsive cis-elements, specifically ABRE and recognition sites MYB, in mediating gene transcription under drought conditions were confirmed by histochemical GUS staining analysis for plants harbouring a truncated TaPYL5 promoter. Yeast two-hybrid and BiFC assays indicated that TaPYL5 interacted with TaPP2C53, a clade A member of phosphatase (PP2C), and the latter with TaSnRK2.1, a kinase member of the SnRK2 family, implying the formation of an ABA core signaling module TaPYL5/TaPP2C53/TaSnRK2.1. TaABI1, an ABA responsive transcription factor, proved to be a component of the ABA signaling pathway, as evidenced by its interaction with TaSnRK2.1. Transgene analysis of TaPYL5 and its module partners, as well as TaABI1, revealed that they have an effect on plant drought responses. TaPYL5 and TaSnRK2.1 positively regulated plant drought acclimation, whereas TaPP2C53 and TaABI1 negatively regulated it. This coincided with the osmotic stress-related physiology shown in their transgenic lines, such as stomata movement, osmolytes biosynthesis, and antioxidant enzyme function. TaPYL5 significantly altered the transcription of numerous genes involved in biological processes related to drought defense. Our findings suggest that TaPYL5 is one of the most important regulators in plant drought tolerance and a valuable target for engineering drought-tolerant cultivars in wheat.

The clade F PP2C phosphatase ZmPP84 negatively regulates drought tolerance by repressing stomatal closure in maize

Drought is a major environmental stress that threatens crop production. Therefore, identification of genes involved in drought stress response is of vital importance to decipher the molecular mechanism of stress signal transduction and breed drought tolerance crops, especially for maize. Clade A PP2C phosphatases are core abscisic acid (ABA) signaling components, regulating ABA signal transduction and drought response. However, the roles of other clade PP2Cs in drought resistance remain largely unknown. Here, we discovered a clade F PP2C, ZmPP84, that negatively regulates drought tolerance by screening a transgenic overexpression maize library. Quantitative RT-PCR indicates that the transcription of ZmPP84 is suppressed by drought stress. We identified that ZmMEK1, a member of the MAPKK family, interacts with ZmPP84 by immunoprecipitation and mass spectrometry analysis. Additionally, we found that ZmPP84 can dephosphorylate ZmMEK1 and repress its kinase activity on the downstream substrate kinase ZmSIMK1, while ZmSIMK1 is able to phosphorylate S-type anion channel ZmSLAC1 at S146 and T520 in vitro. Mutations of S146 and T520 to phosphomimetic aspartate could activate ZmSLAC1 currents in Xenopus oocytes. Taken together, our study suggests that ZmPP84 is a negative regulator of drought stress response that inhibits stomatal closure through dephosphorylating ZmMEK1, thereby repressing ZmMEK1-ZmSIMK1 signaling pathway.

The JASMONATE ZIM-domain-OPEN STOMATA1 cascade integrates jasmonic acid and abscisic acid signaling to regulate drought tolerance by mediating stomatal closure in poplar

Drought, which directly affects the yield of crops and trees, is a natural stress with a profound impact on the economy. Improving water use efficiency (WUE) and drought tolerance are relatively effective strategies to alleviate drought stress. OPEN STOMATA1 (OST1), at the core of abscisic acid (ABA) signaling, can improve WUE by regulating stomatal closure and photosynthesis. Methyl jasmonate (MeJA) and ABA crosstalk is considered to be involved in the response to drought stress, but the detailed molecular mechanism is insufficiently known. Here, Populus euphratica, which naturally grows in arid and semiarid regions, was selected as the species for studying MeJA and ABA crosstalk under drought. A yeast two-hybrid assay was performed using PeOST1 as bait and a nucleus-localized factor, JASMONATE ZIM-domain protein 2 (PeJAZ2), was found to participate in MeJA signaling by interacting with PeOST1. Overexpression of PeJAZ2 in poplar notably increased water deficit tolerance and WUE in both severe and mild drought stress by regulating ABA signaling rather than ABA synthesis. Furthermore, a PeJAZ2 overexpression line was shown to have greater ABA-induced stomatal closure and hydrogen peroxide (H2O2) production. Collectively, this evidence establishes a mechanism in which PeJAZ2 acts as a positive regulator in response to drought stress via ABA-induced stomatal closure caused by H2O2 production. Our study presents a new insight into the crosstalk of ABA and jasmonic acid signaling in regulating WUE and drought stress, providing a basis of the drought tolerance mechanism of P. euphratica.

Guard cell anion channel PbrSLAC1 regulates stomatal closure through PbrSnRK2.3 protein kinases

Stomatal pores on the leaf surface are the gateways for gas exchange between plants and the atmosphere, which is regulated mainly by the S-type anion channel SLAC1. However, the gene encoding the main S-type anion channel SLAC1 in pear and its genetic characteristics remain unknown. In this study, Pbr015894.1 was identified as the candidate for PbrSLAC1 in pear, and it was found to be expressed abundantly in leaves, particularly in the guard cells. Virus-induced gene silencing experiments indicated that stomatal closure was achieved by a change in cell turgor instigated by PbrSLAC1 channel transport of NO₃^- in pear leaves and induced by abscisic acid. Furthermore, the expression of PbrSLAC1 in Arabidopsis slac1-3 and slac1-4 rescued the defective NO₃^- transport seen in these mutants, pointing to its role in anion transport. Fluorescence microscopy suggested that PbrSLAC1 was localized in the plasma membrane, and a dual-luciferase assay system demonstrated an interaction between PbrSLAC1 and PbrSnRK2.3/2.8. Moreover, anion conductance mediated by PbrSLAC1 was activated by PbrSnRK2.3 in Xenopus laevis oocytes and the channel showed greater permeability for nitrate than chloride, sulfate, or malate ions. Taken together, these results demonstrate that PbrSLAC1, an anion channel regulated by PbrSnRK2.3, is involved in stomatal closure by mediating the efflux of NO₃^- in pear leaf.

Multi-crystal native-SAD phasing at 5 keV with a helium environment

De novo structure determination from single-wavelength anomalous diffraction using native sulfur or phospho-rus in biomolecules (native-SAD) is an appealing method to mitigate the labor-intensive production of heavy-atom derivatives and seleno-methio-nyl substitutions. The native-SAD method is particularly attractive for membrane proteins, which are difficult to produce and often recalcitrant to grow into decent-sized crystals. Native-SAD uses lower-energy X-rays to enhance anomalous signals from sulfur or phospho-rus. However, at lower energies, the scattering and absorption of air contribute to the background noise, reduce the signals and are thus adverse to native-SAD phasing. We have previously demonstrated native-SAD phasing at an energy of 5 keV in air at the NSLS-II FMX beamline. Here, the use of a helium path developed to reduce both the noise from background scattering and the air absorption of the diffracted X-ray beam are described. The helium path was used for collection of anomalous diffraction data at 5 keV for two proteins: thaumatin and the membrane protein TehA. Although anomalous signals from each individual crystal are very weak, robust anomalous signals are obtained from data assembled from micrometre-sized crystals. The thaumatin structure was determined from 15 microcrystals and the TehA structure from 18 microcrystals. These results demonstrate the usefulness of a helium environment in support of native-SAD phasing at 5 keV.

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

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

The Dicarboxylate Transporters from the AceTr Family and Dct-02 Oppositely Affect Succinic Acid Production in S. cerevisiae

Membrane transporters are important targets in metabolic engineering to establish and improve the production of chemicals such as succinic acid from renewable resources by microbial cell factories. We recently provided a Saccharomyces cerevisiae strain able to strongly overproduce succinic acid from glycerol and CO₂ in which the Dct-02 transporter from Aspergillus niger, assumed to be an anion channel, was used to export succinic acid from the cells. In a different study, we reported a new group of succinic acid transporters from the AceTr family, which were also described as anion channels. Here, we expressed these transporters in a succinic acid overproducing strain and compared their impact on extracellular succinic acid accumulation with that of the Dct-02 transporter. The results show that the tested transporters of the AceTr family hinder succinic acid accumulation in the extracellular medium at low pH, which is in strong contrast to Dct-02. Data suggests that the AceTr transporters prefer monovalent succinate, whereas Dct-02 prefers divalent succinate anions. In addition, the results provided deeper insights into the characteristics of Dct-02, showing its ability to act as a succinic acid importer (thus being bidirectional) and verifying its capability of exporting malate.

Detergent-Free Isolation of Membrane Proteins and Strategies to Study Them in a Near-Native Membrane Environment

Atomic-resolution structural studies of membrane-associated proteins and peptides in a membrane environment are important to fully understand their biological function and the roles played by them in the pathology of many diseases. However, the complexity of the cell membrane has severely limited the application of commonly used biophysical and biochemical techniques. Recent advancements in NMR spectroscopy and cryoEM approaches and the development of novel membrane mimetics have overcome some of the major challenges in this area. For example, the development of a variety of lipid-nanodiscs has enabled stable reconstitution and structural and functional studies of membrane proteins. In particular, the ability of synthetic amphipathic polymers to isolate membrane proteins directly from the cell membrane, along with the associated membrane components such as lipids, without the use of a detergent, has opened new avenues to study the structure and function of membrane proteins using a variety of biophysical and biological approaches. This review article is focused on covering the various polymers and approaches developed and their applications for the functional reconstitution and structural investigation of membrane proteins. The unique advantages and limitations of the use of synthetic polymers are also discussed.

The function of BK channels extracted and purified within SMALPs

Human BK channels are large voltage and Ca²⁺-activated K⁺ channels, involved in several important functions within the body. The core channel is a tetramer of α subunits, and its function is modulated by the presence of β and γ accessory subunits. BK channels composed of α subunits, as well as BK channels composed of α and β1 subunits, were successfully solubilised from HEK cells with styrene maleic acid (SMA) polymer and purified by nickel affinity chromatography. Native SMA–PAGE analysis of the purified proteins showed the α subunits were extracted as a tetramer. In the presence of β1 subunits, they were co-extracted with the α subunits as a heteromeric complex. Purified SMA lipid particles (SMALPs) containing BK channel could be inserted into planar lipid bilayers (PLB) and single channel currents recorded, showing a high conductance (≈260 pS), as expected. The open probability was increased in the presence of co-purified β1 subunits. However, voltage-dependent gating of the channel was restricted. In conclusion, we have demonstrated that SMA can be used to effectively extract and purify large, complex, human ion channels, from low expressing sources. That these large channels can be incorporated into PLB from SMALPs and display voltage-dependent channel activity. However, the SMA appears to reduce the voltage dependent gating of the channels.

Impedance Measures for Detecting Electrical Responses during Acute Injury and Exposure of Compounds to Roots of Plants

Electrical activity is widely used for assessing a plant's response to an injury or environmental stimulus. Commonly, a differential electrode recording between silver wire leads with the reference wire connected to the soil, or a part of the plant, is used. One method uses KCl-filled glass electrodes placed into the plant, similar to recording membrane/cell potentials in animal tissues. This method is more susceptible to artifacts of equipment noise and photoelectric effects than an impedance measure. An impedance measure using stainless steel wires is not as susceptible to electrically induced noises. Impedance measurements are able to detect injury in plants as well as exposure of the roots to environmental compounds (glutamate). The impedance measures were performed in 5 different plants (tomato, eggplant, pepper, liverwort, and Coleus scutellarioides), and responses to mechanical movement of the plant, as well as injury, were recorded. Monitoring electrical activity in a plant that arises in a distant plant was also demonstrated using the impedance method. The purpose of this report is to illustrate the ease in using impedance measures for monitoring electrical signals from individual plants or aggregates of plants for potentially scaling for high throughput and monitoring controlled culturing and outdoor field environments.

Structure of the Arabidopsis guard cell anion channel SLAC1 suggests activation mechanism by phosphorylation

Stomata play a critical role in the regulation of gas exchange and photosynthesis in plants. Stomatal closure participates in multiple stress responses, and is regulated by a complex network including abscisic acid (ABA) signaling and ion-flux-induced turgor changes. The slow-type anion channel SLAC1 has been identified to be a central controller of stomatal closure and phosphoactivated by several kinases. Here, we report the structure of SLAC1 in Arabidopsis thaliana (AtSLAC1) in an inactivated, closed state. The cytosolic amino (N)-terminus and carboxyl (C)-terminus of AtSLAC1 are partially resolved and form a plug-like structure which packs against the transmembrane domain (TMD). Breaking the interactions between the cytosolic plug and transmembrane domain triggers channel activation. An inhibition-release model is proposed for SLAC1 activation by phosphorylation that the cytosolic plug dissociates from the transmembrane domain upon phosphorylation, and induces conformational changes to open the pore. These findings facilitate our understanding of the regulation of SLAC1 activity and stomatal aperture in plants.

The anion channel SLAC1 controls stomatal closure upon phosphoactivation. Here via structural analysis and electrophysiology, the authors propose an inhibition-release model where phosphorylation causes dissociation of a cytosolic plug from the SLAC1 transmembrane domains to induce conformational change in the pore-forming helices.

Cryo-EM structure and electrophysiological characterization of ALMT from Glycine max reveal a previously uncharacterized class of anion channels

Aluminum-activated malate transporters (ALMTs) form an anion channel family that plays essential roles in diverse functions in plants. Arabidopsis ALMT12, also named QUAC1 (quick anion channel 1), regulates stomatal closure in response to environmental stimuli. However, the molecular basis of ALMT12/QUAC1 activity remains elusive. Here, we describe the cryo-EM structure of ALMT12/QUAC1 from Glycine max at 3.5-Å resolution. GmALMT12/QUAC1 is a symmetrical dimer, forming a single electropositive T-shaped pore across the membrane. The transmembrane and cytoplasmic domains are assembled into a twisted two-layer architecture, with their associated dimeric interfaces nearly perpendicular. GmALMT12/QUAC1-mediated currents display rapid kinetics of activation/deactivation and a bell-shaped voltage dependency, reminiscent of the rapid (R)-type anion currents. Our structural and functional analyses reveal a domain-twisting mechanism for malate-mediated activation. Together, our study uncovers the molecular basis for a previously uncharacterized class of anion channels and provides insights into the gating and modulation of the ALMT12/QUAC1 anion channel.

THESEUS1 modulates cell wall stiffness and abscisic acid production in Arabidopsis thaliana

Plant cells can be distinguished from animal cells by their cell walls and high-turgor pressure. Although changes in turgor and the stiffness of cell walls seem coordinated, we know little about the mechanism responsible for coordination. Evidence has accumulated that plants, like yeast, have a dedicated cell wall integrity maintenance mechanism. It monitors the functional integrity of the wall and maintains integrity through adaptive responses induced by cell wall damage arising during growth, development, and interactions with the environment. These adaptive responses include osmosensitive induction of phytohormone production, defense responses, as well as changes in cell wall composition and structure. Here, we investigate how the cell wall integrity maintenance mechanism coordinates changes in cell wall stiffness and turgor in Arabidopsis thaliana We show that the production of abscisic acid (ABA), the phytohormone-modulating turgor pressure, and responses to drought depend on the presence of a functional cell wall. We find that the cell wall integrity sensor THESEUS1 modulates mechanical properties of walls, turgor loss point, ABA biosynthesis, and ABA-controlled processes. We identify RECEPTOR-LIKE PROTEIN 12 as a component of cell wall integrity maintenance-controlling, cell wall damage-induced jasmonic acid (JA) production. We propose that THE1 is responsible for coordinating changes in turgor pressure and cell wall stiffness.

Structural and Functional Insights into the Role of Guard Cell Ion Channels in Abiotic Stress-Induced Stomatal Closure

A stomatal pore is formed by a pair of specialized guard cells and serves as a major gateway for water transpiration and atmospheric CO₂ influx for photosynthesis in plants. These pores must be tightly controlled, as inadequate CO₂ intake and excessive water loss are devastating for plants. When the plants are exposed to extreme weather conditions such as high CO₂ levels, O₃, low air humidity, and drought, the turgor pressure of the guard cells exhibits an appropriate response against these stresses, which leads to stomatal closure. This phenomenon involves a complex network of ion channels and their regulation. It is well-established that the turgor pressure of guard cells is regulated by ions transportation across the membrane, such as anions and potassium ions. In this review, the guard cell ion channels are discussed, highlighting the structure and functions of key ion channels; the SLAC1 anion channel and KAT1 potassium channel, and their regulatory components, emphasizing their significance in guard cell response to various stimuli.

Membrane protein extraction and purification using partially-esterified SMA polymers

Styrene maleic acid (SMA) polymers have proven to be very successful for the extraction of membrane proteins, forming SMA lipid particles (SMALPs), which maintain a lipid bilayer around the membrane protein. SMALP-encapsulated membrane proteins can be used for functional and structural studies. The SMALP approach allows retention of important protein-annular lipid interactions, exerts lateral pressure, and offers greater stability than traditional detergent solubilisation. However, SMA polymer does have some limitations, including a sensitivity to divalent cations and low pH, an absorbance spectrum that overlaps with many proteins, and possible restrictions on protein conformational change. Various modified polymers have been developed to try to overcome these challenges, but no clear solution has been found. A series of partially-esterified variants of SMA (SMA 2625, SMA 1440 and SMA 17352) has previously been shown to be highly effective for solubilisation of plant and cyanobacterial thylakoid membranes. It was hypothesised that the partial esterification of maleic acid groups would increase tolerance to divalent cations. Therefore, these partially-esterified polymers were tested for the solubilisation of lipids and membrane proteins, and their tolerance to magnesium ions. It was found that all partially esterified polymers were capable of solubilising and purifying a range of membrane proteins, but the yield of protein was lower with SMA 1440, and the degree of purity was lower for both SMA 1440 and SMA 17352. SMA 2625 performed comparably to SMA 2000. SMA 1440 also showed an increased sensitivity to divalent cations. Thus, it appears the interactions between SMA and divalent cations are more complex than proposed and require further investigation.

Current Developments in Native Nanometric Discoidal Membrane Bilayer Formed by Amphipathic Polymers

Unlike cytosolic proteins, membrane proteins (MPs) are embedded within the plasma membrane and the lipid bilayer of intracellular organelles. MPs serve in various cellular processes and account for over 65% of the current drug targets. The development of membrane mimetic systems such as bicelles, short synthetic polymers or amphipols, and membrane scaffold proteins (MSP)-based nanodiscs has facilitated the accommodation of synthetic lipids to stabilize MPs, yet the preparation of these membrane mimetics remains detergent-dependent. Bio-inspired synthetic polymers present an invaluable tool for excision and liberation of superstructures of MPs and their surrounding annular lipid bilayer in the nanometric discoidal assemblies. In this article, we discuss the significance of self-assembling process in design of biomimetic systems, review development of multiple series of amphipathic polymers and the significance of these polymeric "belts" in biomedical research in particular in unraveling the structures, dynamics and functions of several high-value membrane protein targets.

Acidosis-induced activation of anion channel SLAH3 in the flooding-related stress response of Arabidopsis

Plants, as sessile organisms, gained the ability to sense and respond to biotic and abiotic stressors to survive severe changes in their environments. The change in our climate comes with extreme dry periods but also episodes of flooding. The latter stress condition causes anaerobiosis-triggered cytosolic acidosis and impairs plant function. The molecular mechanism that enables plant cells to sense acidity and convey this signal via membrane depolarization was previously unknown. Here, we show that acidosis-induced anion efflux from Arabidopsis (Arabidopsis thaliana) roots is dependent on the S-type anion channel AtSLAH3. Heterologous expression of SLAH3 in Xenopus oocytes revealed that the anion channel is directly activated by a small, physiological drop in cytosolic pH. Acidosis-triggered activation of SLAH3 is mediated by protonation of histidine 330 and 454. Super-resolution microscopy analysis showed that the increase in cellular proton concentration switches SLAH3 from an electrically silent channel dimer into its active monomeric form. Our results show that, upon acidification, protons directly switch SLAH3 to its open configuration, bypassing kinase-dependent activation. Moreover, under flooding conditions, the stress response of Arabidopsis wild-type (WT) plants was significantly higher compared to SLAH3 loss-of-function mutants. Our genetic evidence of SLAH3 pH sensor function may guide the development of crop varieties with improved stress tolerance.

Structures and Dynamics of Native-State Transmembrane Protein Targets and Bound Lipids

Membrane proteins work within asymmetric bilayers of lipid molecules that are critical for their biological structures, dynamics and interactions. These properties are lost when detergents dislodge lipids, ligands and subunits, but are maintained in native nanodiscs formed using styrene maleic acid (SMA) and diisobutylene maleic acid (DIBMA) copolymers. These amphipathic polymers allow extraction of multicomponent complexes of post-translationally modified membrane-bound proteins directly from organ homogenates or membranes from diverse types of cells and organelles. Here, we review the structures and mechanisms of transmembrane targets and their interactions with lipids including phosphoinositides (PIs), as resolved using nanodisc systems and methods including cryo-electron microscopy (cryo-EM) and X-ray diffraction (XRD). We focus on therapeutic targets including several G protein-coupled receptors (GPCRs), as well as ion channels and transporters that are driving the development of next-generation native nanodiscs. The design of new synthetic polymers and complementary biophysical tools bodes well for the future of drug discovery and structural biology of native membrane:protein assemblies (memteins).