Land plants evolved from charophytic algae, among which Charophyceae possess the most complex body plans. We present the genome of Chara braunii; comparison of the genome to those of land plants identified evolutionary novelties for plant terrestrialization and land plant heritage genes. C. braunii employs unique xylan synthases for cell wall biosynthesis, a phragmoplast (cell separation) mechanism similar to that of land plants, and many phytohormones. C. braunii plastids are controlled via landplant-like retrograde signaling, and transcriptional regulation is more elaborate than in other algae. The morphological complexity of this organism may result from expanded gene families, with three cases of particular note: genes effecting tolerance to reactive oxygen species (ROS), LysM receptor-like kinases, and transcription factors (TFs). Transcriptomic analysis of sexual reproductive structures reveals intricate control by TFs, activity of the ROS gene network, and the ancestral use of plant-like storage and stress protection proteins in the zygote.

Plasma membrane-borne pattern recognition receptors, which recognize microbe-associated molecular patterns and endogenous damage-associated molecular patterns, provide the first line of defense in innate immunity. In plants, leucine-rich repeat receptor kinases fulfill this role, as exemplified by FLS2 and EFR, the receptors for the microbe-associated molecular patterns flagellin and elongation factor Tu. Here we examined the perception of the damage-associated molecular pattern peptide 1 (AtPep1), an endogenous peptide of Arabidopsis identified earlier and shown to be perceived by the leucine-rich repeat protein kinase PEPR1. Using seedling growth inhibition, elicitation of an oxidative burst and induction of ethylene biosynthesis, we show that wild type plants and the pepr1 and pepr2 mutants, affected in PEPR1 and in its homologue PEPR2, are sensitive to AtPep1, but that the double mutant pepr1/pepr2 is completely insensitive. As a central body of our study, we provide electrophysiological evidence that at the level of the plasma membrane, AtPep1 triggers a receptor-dependent transient depolarization through activation of plasma membrane anion channels, and that this effect is absent in the double mutant pepr1/pepr2. The double mutant also fails to respond to AtPep2 and AtPep3, two distant homologues of AtPep1 on the basis of homology screening, implying that the PEPR1 and PEPR2 are responsible for their perception too. Our findings provide a basic framework to study the biological role of AtPep1-related danger signals and their cognate receptors.In plant immunity, a first line of defense is based on the perception of a group of conserved, pathogen-derived molecules, called microbe-associated molecular patterns (MAMPs) 4 by pattern recognition receptors, which cause the expression of defense genes as well as metabolic rearrangements, and ultimately activate basal resistance to potential pathogens (1, 2). In Arabidopsis, the best studied MAMPs are the bacterial flagellin (active epitope flg22) and elongation factor Tu (active EF-Tu epitopes elf13, elf18, and elf26), which are recognized by their cognate leucine rich repeat-receptor-like kinases FLS2 (flagellin-sensitive 2) and EFR (EF-Tu receptor), respectively (reviewed in Ref. 1). Perception of these MAMPs leads to a set of responses that can be used to monitor the recognition process, including the triggering of ion fluxes, the generation of reactive oxygen species (ROS), accumulation of ethylene, and finally up-regulation of defense-related genes; investment into increased resistance against bacterial pathogens negatively feeds back on plant growth (3). In addition to these MAMP/ pattern recognition receptor systems, another class of surveillance system recognizes plant-derived molecules previously known as "endogenous elicitors" and now as DAMPs (damageassociated molecular patterns): DAMPs are endogenous molecules that newly appear in the intercellular space in response to the damage caused by a pathogen attack, e.g. cell wall fragments or effectors deri...

Content Summary414I.Introduction415II.Ca2+ importer and exporter in plants415III.The Ca2+ decoding toolkit in plants415IV.Mechanisms of Ca2+ signal decoding417V.Immediate Ca2+ signaling in the regulation of ion transport418VI.Ca2+ signal integration into long‐term ABA responses419VIIIntegration of Ca2+ and hormone signaling through dynamic complex modulation of the CCaMK/CYCLOPS complex420VIIICa2+ signaling in mitochondria and chloroplasts422IXA view beyond recent advances in Ca2+ imaging423XModeling approaches in Ca2+ signaling424XIConclusions: Ca2+ signaling a still young blooming field of plant research424Acknowledgements425ORCID425References425 Summary Temporally and spatially defined changes in Ca2+ concentration in distinct compartments of cells represent a universal information code in plants. Recently, it has become evident that Ca2+ signals not only govern intracellular regulation but also appear to contribute to long distance or even organismic signal propagation and physiological response regulation. Ca2+ signals are shaped by an intimate interplay of channels and transporters, and during past years important contributing individual components have been identified and characterized. Ca2+ signals are translated by an elaborate toolkit of Ca2+‐binding proteins, many of which function as Ca2+ sensors, into defined downstream responses. Intriguing progress has been achieved in identifying specific modules that interconnect Ca2+ decoding proteins and protein kinases with downstream target effectors, and in characterizing molecular details of these processes. In this review, we reflect on recent major advances in our understanding of Ca2+ signaling and cover emerging concepts and existing open questions that should be informative also for scientists that are currently entering this field of ever‐increasing breath and impact.

Auxin-induced growth of coleoptiles depends on the presence of potassium and is suppressed by K ؉ channel blockers. To evaluate the role of K ؉ channels in auxin-mediated growth, we isolated and functionally expressed ZMK1 and ZMK2 (Zea mays K ؉ channel 1 and 2), two potassium channels from maize coleoptiles. In growth experiments, the time course of auxin-induced expression of ZMK1 coincided with the kinetics of coleoptile elongation. Upon gravistimulation of maize seedlings, ZMK1 expression followed the gravitropic-induced auxin redistribution. K ؉ channel expression increased even before a bending of the coleoptile was observed. The transcript level of ZMK2, expressed in vascular tissue, was not affected by auxin. In patch-clamp studies on coleoptile protoplasts, auxin increased K ؉ channel density while leaving channel properties unaffected. Thus, we conclude that coleoptile growth depends on the transcriptional up-regulation of ZMK1, an inwardly rectifying K ؉ channel expressed in the nonvascular tissue of this organ.

Here we report on the molecular identification, guard cell expression and functional characterization of AtGORK, an Arabidopsis thaliana guard cell outward rectifying K + channel. GORK represents a new member of the plant Shaker K + channel superfamily. When heterologously expressed in Xenopus oocytes the gene product of GORK mediated depolarization-activated K + currents. In agreement with the delayed outward rectifier in intact guard cells and protoplasts thereof, GORK is activated in a voltage-and potassium-dependent manner. Furthermore, the single channel conductance and regulation of GORK in response to pH changes resembles the biophysical properties of the guard cell delayed outward rectifier. Thus GORK very likely represents the molecular entity for depolarization-induced potassium release from guard cells. ß 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.

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Ion channels in roots allow the plant to gain access to nutrients. The composition of the individual ion channels and the functional contribution of different ␣-subunits is largely unknown. Focusing on K ؉ -selective ion channels, we have characterized AtKC1, a new ␣-subunit from the Arabidopsis shaker-like ion channel family. Promoter-␤-glucuronidase (GUS) studies identified AtKC1 expression predominantly in root hairs and root endodermis. Specific antibodies recognized AtKC1 at the plasma membrane. To analyze further the abundance and the functional contribution of the different K ؉ channels ␣-subunits in root cells, we performed real-time reverse transcription-PCR and patch-clamp experiments on isolated root hair protoplasts. Studying all shaker-like ion channel ␣-subunits, we only found the K ؉ inward rectifier AtKC1 and AKT1 and the K ؉ outward rectifier GORK to be expressed in this cell type. Akt1 knockout plants essentially lacked inward rectifying K ؉ currents. In contrast, inward rectifying K ؉ currents were present in AtKC1 knockout plants, but fundamentally altered with respect to gating and cation sensitivity. This indicates that the AtKC1 ␣-subunit represents an integral component of functional root hair K ؉ uptake channels.

SUMMARYThe perception of microbes by plants involves highly conserved molecular signatures that are absent from the host and that are collectively referred to as microbe-associated molecular patterns (MAMPs). The Arabidopsis pattern recognition receptors FLAGELLIN-SENSING 2 (FLS2) and EF-Tu receptor (EFR) represent genetically well studied paradigms that mediate defense against bacterial pathogens. Stimulation of these receptors through their cognate ligands, bacterial flagellin or bacterial elongation factor Tu, leads to a defense response and ultimately to increased resistance. However, little is known about the early signaling pathway of these receptors. Here, we characterize this early response in situ, using an electrophysiological approach. In line with a release of negatively charged molecules, voltage recordings of microelectrode-impaled mesophyll cells and root hairs of Col-0 Arabidopsis plants revealed rapid, dose-dependent membrane potential depolarizations in response to either flg22 or elf18. Using ion-selective microelectrodes, pronounced anion currents were recorded upon application of flg22 and elf18, indicating that the signaling cascades initiated by each of the two receptors converge on the same plasma membrane ion channels. Combined calcium imaging and electrophysiological measurements revealed that the depolarization was superimposed by an increase in cytosolic calcium that was indispensable for depolarization. NADPH oxidase mutants were still depolarized upon elicitor stimulation, suggesting a reactive oxygen species-independent membrane potential response. Furthermore, electrical signaling in response to either flg22 or elf 18 critically depends on the activity of the FLS2-associated receptor-like kinase BAK1, suggesting that activation of FLS2 and EFR lead to BAK1-dependent, calcium-associated plasma membrane anion channel opening as an initial step in the pathogen defense pathway.