Search

To obtain insights into the dynamics of nutrient exchange in arbuscular mycorrhizal (AM) symbiosis, we modelled mathematically the two‐membrane system at the plant–fungus interface and simulated its dynamics. In computational cell biology experiments, the full range of nutrient transport pathways was tested for their ability to exchange phosphorus (P)/carbon (C)/nitrogen (N) sources. As a result, we obtained a thermodynamically justified, independent and comprehensive model of the dynamics of the nutrient exchange at the plant–fungus contact zone. The predicted optimal transporter network coincides with the transporter set independently confirmed in wet‐laboratory experiments previously, indicating that all essential transporter types have been discovered. The thermodynamic analyses suggest that phosphate is released from the fungus via proton‐coupled phosphate transporters rather than anion channels. Optimal transport pathways, such as cation channels or proton‐coupled symporters, shuttle nutrients together with a positive charge across the membranes. Only in exceptional cases does electroneutral transport via diffusion facilitators appear to be plausible. The thermodynamic models presented here can be generalized and adapted to other forms of mycorrhiza and open the door for future studies combining wet‐laboratory experiments with computational simulations to obtain a deeper understanding of the investigated phenomena.

HKT channels mediate sodium uniport or sodium and potassium symport in plants. Monocotyledons express a higher number of HKT proteins than dicotyledons, and it is only within this clade of HKT channels that cation symport mechanisms are found. The prevailing ion composition in the extracellular medium affects the transport abilities of various HKT channels by changing their selectivity or ion transport rates. How this mutual effect is achieved at the molecular level is still unknown. Here, we built a homology model of the monocotyledonous OsHKT2;2, which shows sodium and potassium symport activity, and performed molecular dynamics simulations in the presence of sodium and potassium ions. By analyzing ion-protein interactions, we identified a cation binding pocket on the extracellular protein surface, which is formed by residues P71, D75, D501 and K504. Proline and the two aspartate residues coordinate cations, while K504 forms salt bridges with D75 and D501 and may be involved in the forwarding of cations towards the pore entrance. Functional validation via electrophysiological experiments confirmed the biological relevance of the predicted ion binding pocket and identified K504 as a central key residue. Mutation of the cation coordinating residues affected the functionality of HKT only slightly. Additional in silico mutants and simulations of K504 supported experimental results.

No abstract available.

Two-pore domain potassium (K2P) channels are membrane proteins widely identified in mammals, plants, and other organisms. A functional channel is a dimer with each subunit comprising two pore-forming loops and four transmembrane domains. The genome of the model plant Arabidopsis thaliana harbors five genes coding for K2P channels. Homologs of Arabidopsis K2P channels have been found in all higher plants sequenced so far. As with the K2P channels in mammals, plant K2P channels are targets of external and internal stimuli, which fine-tune the electrical properties of the membrane for specialized transport and/or signaling tasks. Plant K2P channels are modulated by signaling molecules such as intracellular H+ and calcium and physical factors like temperature and pressure. In this review, we ask the following: What are the similarities and differences between K2P channels in plants and animals in terms of their physiology? What is the nature of the last common ancestor (LCA) of these two groups of proteins? To answer these questions, we present physiological, structural, and phylogenetic evidence that discards the hypothesis proposing that the duplication and fusion that gave rise to the K2P channels occurred in a prokaryote LCA. Conversely, we argue that the K2P LCA was most likely a eukaryote organism. Consideration of plant and animal K2P channels in the same study is novel and likely to stimulate further exchange of ideas between students of these fields.

The uptake of potassium ions (K+) accompanied by an acidification of the apoplasm is a prerequisite for stomatal opening. The acidification (approximately 2–2.5 pH units) is perceived by voltage-gated inward potassium channels (Kin) that then can open their pores with lower energy cost. The sensory units for extracellular pH in stomatal Kin channels are proposed to be histidines exposed to the apoplasm. However, in the Arabidopsis thaliana stomatal Kin channel KAT1, mutations in the unique histidine exposed to the solvent (His267) do not affect the pH dependency. We demonstrate in the present study that His267 of the KAT1 channel cannot sense pH changes since the neighbouring residue Phe266 shifts its pKa to undetectable values through a cation–π interaction. Instead, we show that Glu240 placed in the extracellular loop between transmembrane segments S5 and S6 is involved in the extracellular acid activation mechanism. Based on structural models we propose that this region may serve as a molecular link between the pH- and the voltage-sensor. Like Glu240, several other titratable residues could contribute to the pH-sensor of KAT1, interact with each other and even connect such residues far away from the voltage-sensor with the gating machinery of the channel.

Reactive oxygen species (ROS) are essential for development and stress signalling in plants. They contribute to plant defense against pathogens, regulate stomatal transpiration and influence nutrient uptake and partitioning. Although both Ca2+ and K+ channels of plants are known to be affected, virtually nothing is known of the targets for ROS at a molecular level. Here we report that a single cysteine (Cys) residue within the Kv-like SKOR K+ channel of Arabidopsis thaliana is essential for channel sensitivity to the ROS H2O2. We show that H2O2 rapidly enhanced current amplitude and activation kinetics of heterologouslyexpressed SKOR, and the effects were reversed by the reducing agent dithiothreitol (DTT). Both H2O2 and DTT were active at the outer face of the membrane and current enhancement was strongly dependent on membrane depolarization, consistent with a H2O2-sensitive site on the SKOR protein which is exposed to the outside when the channel is in the open conformation. Cys substitutions identified a single residue, C168 located within the S3 α-helix of the voltage sensor complex, to be essential for sensitivity to H2O2. The same Cys residue was a primary determinant for current block by covalent Cys S-methioylation with aqueous methanethiosulfonates. These, and additional data identify C168 as a critical target for H2O2, and implicate ROSmediated control of the K+ channel in regulating mineral nutrient partitioning within the plant. Fil: Garcia-Mata, Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigaciones Biológicas; Argentina Fil: Wan, Jianwen. Soochow University; China Fil: Gajdanowicz, Pawel. Universität Potsdam; Alemania Fil: Gonzalez, Wendy. Universidad de Talca; Chile Fil: Hills, Adrian. Fil: Donald, Naomi. Fil: Riedelsberger, Janin. Fil: Amtmann, Anna. Universität Potsdam; Alemania Fil: Dreyer, Ingo. Universität Potsdam; Alemania Fil: Blatt, Michael R..

The family of voltage-gated (Shaker-like) potassium channels in plants includes both inward-rectifying (Kin) channels that allow plant cells to accumulate K+ and outward-rectifying (Kout) channels that mediate K+ efflux. Despite their close structural similarities, Kin and Kout channels differ in their gating sensitivity towards voltage and the extracellular K+ concentration. We have carried out a systematic program of domain swapping between the Kout channel SKOR and the Kin channel KAT1 to examine the impacts on gating of the pore regions, the S4, S5, and the S6 helices. We found that, in particular, the N-terminal part of the S5 played a critical role in KAT1 and SKOR gating. Our findings were supported by molecular dynamics of KAT1 and SKOR homology models. In silico analysis revealed that during channel opening and closing, displacement of certain residues, especially in the S5 and S6 segments, is more pronounced in KAT1 than in SKOR. From our analysis of the S4–S6 region, we conclude that gating (and K+-sensing in SKOR) depend on a number of structural elements that are dispersed over this ∼145-residue sequence and that these place additional constraints on configurational rearrangement of the channels during gating. Fil: Riedelsberger, Janin . Universität Potsdam; Alemania. Max-Planck Institute of Molecular Plant Physiology; Alemania Fil: Sharma, Tripti . Universität Potsdam; Alemania. Max-Planck Institute of Molecular Plant Physiology; Alemania Fil: Gonzalez, Wendy . Universidad de Talca; Chile Fil: Gajdanowicz, Pawel . Universität Potsdam; Alemania Fil: Morales Navarro, Samuel Elías . Universidad de Talca; Chile Fil: Garcia-Mata, Carlos. University Of Glasgow; Reino Unido. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Mueller Roeber, Bernd. Max-Planck Institute of Molecular Plant Physiology; Alemania. Universität Potsdam; Alemania Fil: González Nilo, Fernando Danilo . Universidad de Talca; Chile Fil: Blatt, Michael R. . University Of Glasgow; Reino Unido Fil: Dreyer, Ingo . Universität Potsdam; Alemania