A protein phosphorylation/dephosphorylation network regulates a plant potassium channel

Regulatory interplay of the Sub1A and CIPK15 pathways in the regulation of α-amylase production in flooded rice plants

Rice (Oryza sativa L.) can successfully germinate and grow even when flooded. Rice varieties possessing the submergence 1A (Sub1A) gene display a distinct flooding-tolerant phenotype, associated with lower carbohydrate consumption and restriction of the fast-elongation phenotype typical of flooding-intolerant rice varieties. Calcineurin B-like interacting protein kinase 15 (CIPK15) was recently indicated as a key regulator of α-amylases under oxygen deprivation, linked to both rice germination and flooding tolerance in adult plants. It is still unknown whether the Sub1A- and CIPK15-mediated pathways act as complementary processes for rice survival under O(2) deprivation. In adult plants Sub1A and CIPK15 may perhaps play an antagonistic role in terms of carbohydrate consumption, with Sub1A acting as a starch degradation repressor and CIPK15 as an activator. In this study, we analysed sugar metabolism in the stem of rice plants under water submergence by selecting cultivars with different traits associated with flooding survival. The relation between the Sub1A and the CIPK15 pathways was investigated. The results show that under O(2) deprivation, the CIPK15 pathway is repressed in the tolerant, Sub1A-containing, FR13A variety. CIPK15 is likely to play a role in the up-regulation of Ramy3D in flooding-intolerant rice varieties that display fast elongation under flooding and that do not possess Sub1A.

Calcium-dependent modulation and plasma membrane targeting of the AKT2 potassium channel by the CBL4/CIPK6 calcium sensor/protein kinase complex

Potassium (K(+)) channel function is fundamental to many physiological processes. However, components and mechanisms regulating the activity of plant K(+) channels remain poorly understood. Here, we show that the calcium (Ca(2+)) sensor CBL4 together with the interacting protein kinase CIPK6 modulates the activity and plasma membrane (PM) targeting of the K(+) channel AKT2 from Arabidopsis thaliana by mediating translocation of AKT2 to the PM in plant cells and enhancing AKT2 activity in oocytes. Accordingly, akt2, cbl4 and cipk6 mutants share similar developmental and delayed flowering phenotypes. Moreover, the isolated regulatory C-terminal domain of CIPK6 is sufficient for mediating CBL4- and Ca(2+)-dependent channel translocation from the endoplasmic reticulum membrane to the PM by a novel targeting pathway that is dependent on dual lipid modifications of CBL4 by myristoylation and palmitoylation. Thus, we describe a critical mechanism of ion-channel regulation where a Ca(2+) sensor modulates K(+) channel activity by promoting a kinase interaction-dependent but phosphorylation-independent translocation of the channel to the PM.

TPC1-SV channels gain shape

The most prominent ion channel localized in plant vacuoles is the slow activating SV type. Slow vacuolar (SV) channels were discovered by patch clamp studies as early as 1986. In the following two decades, numerous studies revealed that these calcium- and voltage-activated, nonselective cation channels are expressed in the vacuoles of all plants and every plant tissue. The voltage-dependent properties of the SV channel are susceptible to modulation by calcium, pH, redox state, as well as regulatory proteins. In Arabidopsis, the SV channel is encoded by the AtTPC1 gene, and even though its gene product represents the by far largest conductance of the vacuolar membrane, tpc1-loss-of-function mutants appeared not to be impaired in major physiological functions such as growth, development, and reproduction. In contrast, the fou2 gain-of-function point mutation D454N within TPC1 leads to a pronounced growth phenotype and increased synthesis of the stress hormone jasmonate. Since the TPC1 gene is present in all land plants, it likely encodes a very general function. In this review, we will discuss major SV channel properties and their impact on plant cell physiology.

Mechanistic analysis of AKT1 regulation by the CBL-CIPK-PP2CA interactions

Arabidopsis K+ transporter 1 (AKT1) participates in K+ uptake in roots, especially under low-K conditions. We recently identified a Ca²⁺ signaling pathway consisting of multiple calcineurin B-like calcium sensors (CBLs) and multiple target kinases (CBL-interacting protein kinases or CIPKs) that phosphorylate and activate AKT1, whereas a specific PP2C-type phosphatase inactivates CIPK-dependent AKT1 activity. In this study, we analyzed the interactions between PP2Cs and the CBL-CIPK pathway and found previously unsuspected mechanisms underlying the CBL-CIPK-PP2C signaling processes. The interaction between the CIPKs and PP2Cs involves the kinase domain of the CIPK component, in addition to the protein phosphatase interacting motif (PPI) in the regulatory domain. Furthermore, specific CBLs physically interact with and inactivate PP2C phosphatases to recover the CIPK-dependent AKT1 channel activity. These findings provide further insights into the signaling network consisting of CBL-CIPK-PP2C interactions in the activation of the AKT1 channel.

Calcium-dependent modulation and plasma membrane targeting of the AKT2 potassium channel by the CBL4/CIPK6 calcium sensor/protein kinase complex

Potassium (K(+)) channel function is fundamental to many physiological processes. However, components and mechanisms regulating the activity of plant K(+) channels remain poorly understood. Here, we show that the calcium (Ca(2+)) sensor CBL4 together with the interacting protein kinase CIPK6 modulates the activity and plasma membrane (PM) targeting of the K(+) channel AKT2 from Arabidopsis thaliana by mediating translocation of AKT2 to the PM in plant cells and enhancing AKT2 activity in oocytes. Accordingly, akt2, cbl4 and cipk6 mutants share similar developmental and delayed flowering phenotypes. Moreover, the isolated regulatory C-terminal domain of CIPK6 is sufficient for mediating CBL4- and Ca(2+)-dependent channel translocation from the endoplasmic reticulum membrane to the PM by a novel targeting pathway that is dependent on dual lipid modifications of CBL4 by myristoylation and palmitoylation. Thus, we describe a critical mechanism of ion-channel regulation where a Ca(2+) sensor modulates K(+) channel activity by promoting a kinase interaction-dependent but phosphorylation-independent translocation of the channel to the PM.

ANK6, a mitochondrial ankyrin repeat protein, is required for male-female gamete recognition in Arabidopsis thaliana

Double fertilization in angiosperms involves several successive steps, including guidance and reception of the pollen tube and male-female gamete recognition. Each step entails extensive communication and interaction between two different reproductive cell or tissue types. Extensive research has focused on the pollen tube, namely, its interaction with the stigma and reception by maternal cells. Little is known, however, about the mechanism by which the gametes recognize each other and interact to form a zygote. We report that an ankyrin repeat protein (ANK6) is essential for fertilization, specifically for gamete recognition. ANK6 (At5g61230) was highly expressed in the male and female gametophytes before and during but not after fertilization. Genetic analysis of a T-DNA insertional mutant suggested that loss of function of ANK6 results in embryonic lethality. Moreover, male-female gamete recognition was found to be impaired only when an ank6 male gamete reached an ank6 female gamete, thereby preventing formation of homozygous zygotes. ANK6 was localized to the mitochondria, where it interacted with SIG5, a transcription initiation factor previously found to be essential for fertility. These results show that ANK6 plays a central role in male-female gamete recognition, possibly by regulating mitochondrial gene expression.

Crystallization and preliminary crystallographic analysis of a calcineurin B-like protein 1 (CBL1) mutant from Ammopiptanthus mongolicus

Calcineurin B-like protein 1 (CBL1) is a calcium sensor in plants. It transmits the calcium signal through the downstream protein CBL-interacting protein kinase (CIPK). CBL1 and CIPK play crucial roles in the response to environmental stresses such as low K+, osmotic shock, high salt, cold and drought. Recombinant CBL1 from Ammopiptanthus mongolicus (AmCBL1) was overexpressed, purified and crystallized. However, the crystal did not diffract well. A mutant prepared using the surface-entropy method and crystallized using the hanging-drop method at 298 K with PEG 2000 MME as a precipitant diffracted to 2.90 Å resolution. The crystal belonged to space group P2(1)2(1)2, with unit-cell parameters a=99.87, b=114.42, c=63.80 Å, α=β=γ=90.00° and three molecules per asymmetric unit.

Systems involved in K+ uptake from diluted solutions in pepper plants as revealed by the use of specific inhibitors

Here, the contribution of the HAK1 transporter, the AKT1 channel and a putative AtCHX13 homolog to K(+) uptake in the high-affinity range of concentrations in pepper plants was examined. The limited development of molecular tools in pepper plants precluded a reverse genetics study in this species. By contrast, in the model plant Arabidopsis thaliana, these type of studies have shown that NH(4)(+) and Ba(2+) may be used as specific inhibitors of the two K(+) uptake systems to dissect their contribution in species in which, as in pepper, specific mutant lines are not available. By using these inhibitors together with Na(+) and Cs(+), the relative contributions of CaHAK1, CaAKT1 and a putative AtCHX13 homolog to K(+) acquisition from diluted solutions under different regimens of K(+) supply were studied. The results showed that, in plants completely starved of K(+), the gene encoding CaHAK1 was highly expressed and this system is a major contributor to K(+) uptake. However, K(+) concentrations as low as 50μM reduced CaHAK1 expression and the CaAKT1 channel came into play, participating together with CaHAK1 in K(+) absorption. The contribution of a putative AtCHX13 homolog seemed to be low under this low K(+) supply, but it cannot be ruled out that at higher K(+) concentrations this system participates in K(+) uptake. Studies of this type allow extension of the tools developed in model plants to understand nutrition in important crops.

Engineering the K+ uptake regulatory pathway by MultiRound Gateway

In a previous study, we described improved versions of MultiRound Gateway vectors. Here, we engineered a calcineurin B-like (CBL) pathway for potassium (K+) nutrition to demonstrate their effectiveness. Using the two improved entry vectors pL12R34H-Ap and pL34R12-Cm, and through 2-4 rounds of Gateway recombination reactions, we generated five pMDC99-derived binary vectors [pK21 (CIPK23+CBL1), pK29 (CIPK23+CBL9), pK31 (CIPK23+CBL1+AKT1), pK39 (CIPK23+CBL9+AKT1), and pK4 (CIPK23+CBL1+AKT1+CBL9)], in which all four genes have the same pSuper promoter and tNos terminator. pK31, pK39 and pK4 were transformed into Arabidopsis. PCR analysis confirmed that all transgenes usually co-existed in the K31, K39 or K4 transgenic plants, and qRT-PCR analysis indicated that the transgenes were expressed at reasonably high levels. The eight overexpression lines, except K31-1, displayed significantly tolerant phenotypes to low-K+ and low-K+ combined with low-Ca2+ compared to the wild type. Significant differences between the K31, K39 and K4 lines were not observed. These results indicate that the improved MultiRound Gateway vectors efficiently assembled multiple transgenes, which were stably inherited and expressed in transformed plants, even with the same promoter and terminator.

Shaker-like potassium channels in Populus, regulated by the CBL-CIPK signal transduction pathway, increase tolerance to low-K+ stress

Shaker-like potassium channels in plants play an important role in potassium absorption and transport. Here, we characterized 11 genes encoding shaker-like channels from Populus trichocarpa. Furthermore, two homologs from this family were isolated from Populus euphratica and named PeKC1 and PeKC2. Subcellular localization analysis of them in Nicotiana benthamiana revealed that they are located in the cell membrane. Yeast two-hybrid assays showed that they not only interacted strongly with PeCIPK24, a homolog of AtCIPK23, but also interacted with several other CIPK members, including PeCIPK10 and PeCIPK17. To further analyze their function, we over-expressed PeKC1 or PeKC2 in akt1 mutant, the results show that the transgenic plant can recover the mutant phonotype sensitive to low-K(+) stress. This means PeKC1 or PeKC2 can complement the function of AKT1 in akt1 mutant, involved in the CBL1-CIPK23 signal transduction pathway and play an important role under low-K(+) stress.

Biochemical characterization of in vitro phosphorylation and dephosphorylation of the plasma membrane H+-ATPase

Stomatal opening, which is mediated by blue light receptor phototropins, is driven by activation of the plasma membrane H(+)-ATPase via phosphorylation of the penultimate threonine in the C-terminus and subsequent binding of a 14-3-3 protein. However, the biochemical properties of the protein kinase and protein phosphatase for H(+)-ATPase are largely unknown. We therefore investigated in vitro phosphorylation and dephosphorylation of H(+)-ATPase. H(+)-ATPase was phosphorylated in vitro on the penultimate threonine in the C-terminus in isolated microsomes from guard cell protoplasts of Vicia faba. Phosphorylated H(+)-ATPase was dephosphorylated in vitro, and the dephosphorylation was inhibited by EDTA, a divalent cation chelator, but not by calyculin A, an inhibitor of type 1 and 2A protein phosphatases. Essentially the same results were obtained in purified plasma membranes from etiolated Arabidopsis seedlings, indicating that a similar protein kinase and phosphatase are involved in plant cells. Further analyses revealed that phosphorylation of the H(+)-ATPase is insensitive to K-252a, a potent inhibitor of protein kinase, and is hypersensitive to Triton X-100, a non-ionic detergent. Moreover, dephosphorylation required Mg(2+) but not Ca(2+), and protein phosphatase was localized in the 1% Triton X-100-insoluble fraction. These results demonstrate that a protein kinase-phosphatase pair, K-252a-insensitive protein kinase and Mg(2+)-dependent type 2C protein phosphatase, co-localizes at least in part with the H(+)-ATPase in the plasma membrane and regulates the phosphorylation status of the penultimate threonine of the H(+)-ATPase.

Regulation of microbe-associated molecular pattern-induced hypersensitive cell death, phytoalexin production, and defense gene expression by calcineurin B-like protein-interacting protein kinases, OsCIPK14/15, in rice cultured cells

Although cytosolic free Ca(2+) mobilization induced by microbe/pathogen-associated molecular patterns is postulated to play a pivotal role in innate immunity in plants, the molecular links between Ca(2+) and downstream defense responses still remain largely unknown. Calcineurin B-like proteins (CBLs) act as Ca(2+) sensors to activate specific protein kinases, CBL-interacting protein kinases (CIPKs). We here identified two CIPKs, OsCIPK14 and OsCIPK15, rapidly induced by microbe-associated molecular patterns, including chitooligosaccharides and xylanase (Trichoderma viride/ethylene-inducing xylanase [TvX/EIX]), in rice (Oryza sativa). Although they are located on different chromosomes, they have over 95% nucleotide sequence identity, including the surrounding genomic region, suggesting that they are duplicated genes. OsCIPK14/15 interacted with several OsCBLs through the FISL/NAF motif in yeast cells and showed the strongest interaction with OsCBL4. The recombinant OsCIPK14/15 proteins showed Mn(2+)-dependent protein kinase activity, which was enhanced both by deletion of their FISL/NAF motifs and by combination with OsCBL4. OsCIPK14/15-RNAi transgenic cell lines showed reduced sensitivity to TvX/EIX for the induction of a wide range of defense responses, including hypersensitive cell death, mitochondrial dysfunction, phytoalexin biosynthesis, and pathogenesis-related gene expression. On the other hand, TvX/EIX-induced cell death was enhanced in OsCIPK15-overexpressing lines. Our results suggest that OsCIPK14/15 play a crucial role in the microbe-associated molecular pattern-induced defense signaling pathway in rice cultured cells.

Studies on Arabidopsis athak5, atakt1 double mutants disclose the range of concentrations at which AtHAK5, AtAKT1 and unknown systems mediate K uptake

The high-affinity K(+) transporter AtHAK5 and the inward-rectifier K(+) channel AtAKT1 have been described to contribute to K(+) uptake in Arabidopsis thaliana. Studies with T-DNA insertion lines showed that both systems participate in the high-affinity range of concentrations and only AtAKT1 in the low-affinity range. However the contribution of other systems could not be excluded with the information and plant material available. The results presented here with a double knock-out athak5, atakt1 mutant show that AtHAK5 is the only system mediating K(+) uptake at concentrations below 0.01 mM. In the range between 0.01 and 0.05 mM K(+) AtHAK5 and AtAKT1 are the only contributors to K(+) acquisition. At higher K(+) concentrations, unknown systems come into operation and participate together with AtAKT1 in low-affinity K(+) uptake. These systems can supply sufficient K(+) to promote plant growth even in the absence of AtAKT1 or in the presence of 10 mM K(+) where AtAKT1 is not essential.

[Plant calcium sensors in osmotic signaling]

Calcium is an essential second messenger that mediates plant responses to developmental and environmental clues. Specific calcium signatures are sensed and decoded by diverse Ca(2+) sensors to induce appropriate downstream responses. Calmodulin is the most important and conserved Ca(2+) transducer in all eukaryotes. Additional plant-specific sensors are encoded by multigene families, i.e. calcineurin B-like and Ca(2+)-dependent protein kinases. Calcium binding induces structural conformational changes in Ca(2+) sensors, resulting in the modification of protein interaction or enzymatic activity. Activated Ca(2+) sensors subsequently regulate downstream targets which can be involved in signal transduction, like protein kinases and transcription factors, or in direct cell protection from stress damages, like ion transporters or detoxification enzymes. Ca(2+) plays an important role in osmotic signaling triggered by cold, drought and salinity. The multiplicity of plant calcium sensors associated with diverse cellular targets constitute a tightly regulated signaling network that induces specific stress responses to improve plant survival under unfavourable conditions.

The Arabidopsis thaliana HAK5 K+ transporter is required for plant growth and K+ acquisition from low K+ solutions under saline conditions

K(+) uptake in the high-affinity range of concentrations and its components have been widely studied. In Arabidposis thaliana, the AtHAK5 transporter and the AtAKT1 channel have been shown to be the main transport proteins involved in this process. Here, we study the role of these two systems under two important stress conditions: low K(+) supply or the presence of salinity. T-DNA insertion lines disrupting AtHAK5 and AtAKT1 are employed for long-term experiments that allow physiological characterization of the mutant lines. We found that AtHAK5 is required for K(+) absorption necessary to sustain plant growth at low K(+) in the absence as well as in the presence of salinity. Salinity greatly reduced AtHAK5 transcript levels and promoted AtAKT1-mediated K(+) efflux, resulting in an important impairment of K(+) nutrition. Although having a limited capacity, AtHAK5 plays a major role for K(+) acquisition from low K(+) concentrations in the presence of salinity.

Expressional analysis and role of calcium regulated kinases in abiotic stress signaling

Perception of stimuli and activation of a signaling cascade is an intrinsic characteristic feature of all living organisms. Till date, several signaling pathways have been elucidated that are involved in multiple facets of growth and development of an organism. Exposure to unfavorable stimuli or stress condition activates different signaling cascades in both plants and animal. Being sessile, plants cannot move away from an unfavorable condition, and hence activate the molecular machinery to cope up or adjust against that particular stress condition. In plants, role of calcium as second messenger has been studied in detail in both abiotic and biotic stress signaling. Several calcium sensor proteins such as calmodulin (CaM), calcium dependent protein kinases (CDPK) and calcinuerin B-like (CBL) were discovered to play a crucial role in abiotic stress signaling in plants. Unlike CDPK, CBL and CaM are calcium-binding proteins, which do not have any protein kinase enzyme activity and interact with a target protein kinase termed as CBL-interacting protein kinase (CIPK) and CaM kinases respectively. Genome sequence analysis of Arabidopsis and rice has led to the identification of multigene familes of these calcium signaling protein kinases. Individual and global gene expression analysis of these protein kinase family members has been analyzed under several developmental and different abiotic stress conditions. In this review, we are trying to overview and emphasize the expressional analysis of calcium signaling protein kinases under different abiotic stress and developmental stages, and linking the expression to possible function for these kinases.

Calcium signals: the lead currency of plant information processing

Ca(2+) signals are core transducers and regulators in many adaptation and developmental processes of plants. Ca(2+) signals are represented by stimulus-specific signatures that result from the concerted action of channels, pumps, and carriers that shape temporally and spatially defined Ca(2+) elevations. Cellular Ca(2+) signals are decoded and transmitted by a toolkit of Ca(2+) binding proteins that relay this information into downstream responses. Major transduction routes of Ca(2+) signaling involve Ca(2+)-regulated kinases mediating phosphorylation events that orchestrate downstream responses or comprise regulation of gene expression via Ca(2+)-regulated transcription factors and Ca(2+)-responsive promoter elements. Here, we review some of the remarkable progress that has been made in recent years, especially in identifying critical components functioning in Ca(2+) signal transduction, both at the single-cell and multicellular level. Despite impressive progress in our understanding of the processing of Ca(2+) signals during the past years, the elucidation of the exact mechanistic principles that underlie the specific recognition and conversion of the cellular Ca(2+) currency into defined changes in protein-protein interaction, protein phosphorylation, and gene expression and thereby establish the specificity in stimulus response coupling remain to be explored.

Plant sensing and signaling in response to K+-deficiency

Potassium (K(+)) is one of the essential macronutrients for plant growth and development. However, K(+) content in soils is usually limited so that the crop yields are restricted. Plants may adapt to K(+)-deficient environment by adjusting their physiological and morphological status, indicating that plants may have evolved their sensing and signaling mechanisms in response to K(+)-deficiency. This short review particularly discusses some components as possible sensors or signal transducers involved in plant sensing and signaling in response to K(+)-deficiency, such as K(+) channels and transporters, H(+)-ATPase, some cytoplasmic enzymes, etc. Possible involvement of Ca²(+) and ROS signals in plant responses to K(+)-deficiency is also discussed.

Constitutive overexpression of the calcium sensor CBL5 confers osmotic or drought stress tolerance in Arabidopsis

Calcium serves as a critical messenger in many adaptation and developmental processes. Cellular calcium signals are detected and transmitted by sensor molecules such as calcium-binding proteins. In plants, the calcineurin B-like protein (CBL) family represents a unique group of calcium sensors and plays a key role in decoding calcium transients by specifically interacting with and regulating a family of CBL-interacting protein kinases (CIPKs). In this study, we report the role of Arabidopsis CBL5 gene in high salt or drought tolerance. CBL5 gene is expressed significantly in green tissues, but not in roots. CBL5 was not induced by abiotic stress conditions such as high salt, drought or low temperature. To determine whether the CBL5 gene plays a role in stress response pathways, we ectopically expressed the CBL5 protein in transgenic Arabidopsis plants (35S-CBL5) and examined plant responses to abiotic stresses. CBL5-overexpressing plants displayed enhanced tolerance to high salt or drought stress. CBL5 overexpression also rendered plants more resistant to high salt or hyperosmotic stress during early development (i.e., seed germination) but did not alter their response to abiscisic acid (ABA). Furthermore, overexpression of CBL5 alters the gene expression of stress gene markers, such as RD29A, RD29B and Kin1 etc. These results suggest that CBL5 may function as a positive regulator of salt or drought responses in plants.

A grapevine Shaker inward K(+) channel activated by the calcineurin B-like calcium sensor 1-protein kinase CIPK23 network is expressed in grape berries under drought stress conditions

Grapevine (Vitis vinifera), the genome sequence of which has recently been reported, is considered as a model species to study fleshy fruit development and acid fruit physiology. Grape berry acidity is quantitatively and qualitatively affected upon increased K(+) accumulation, resulting in deleterious effects on fruit (and wine) quality. Aiming at identifying molecular determinants of K(+) transport in grapevine, we have identified a K(+) channel, named VvK1.1, from the Shaker family. In silico analyses indicated that VvK1.1 is the grapevine counterpart of the Arabidopsis AKT1 channel, known to dominate the plasma membrane inward conductance to K(+) in root periphery cells, and to play a major role in K(+) uptake from the soil solution. VvK1.1 shares common functional properties with AKT1, such as inward rectification (resulting from voltage sensitivity) or regulation by calcineurin B-like (CBL)-interacting protein kinase (CIPK) and Ca(2+)-sensing CBL partners (shown upon heterologous expression in Xenopus oocytes). It also displays distinctive features such as activation at much more negative membrane voltages or expression strongly sensitive to drought stress and ABA (upregulation in aerial parts, downregulation in roots). In roots, VvK1.1 is mainly expressed in cortical cells, like AKT1. In aerial parts, VvK1.1 transcripts were detected in most organs, with expression levels being the highest in the berries. VvK1.1 expression in the berry is localized in the phloem vasculature and pip teguments, and displays strong upregulation upon drought stress, by about 10-fold.VvK1.1 could thus play a major role in K(+) loading into berry tissues, especially upon drought stress.

The language of calcium signaling

Ca(2+) signals are a core regulator of plant cell physiology and cellular responses to the environment. The channels, pumps, and carriers that underlie Ca(2+) homeostasis provide the mechanistic basis for generation of Ca(2+) signals by regulating movement of Ca(2+) ions between subcellular compartments and between the cell and its extracellular environment. The information encoded within the Ca(2+) transients is decoded and transmitted by a toolkit of Ca(2+)-binding proteins that regulate transcription via Ca(2+)-responsive promoter elements and that regulate protein phosphorylation. Ca(2+)-signaling networks have architectural structures comparable to scale-free networks and bow tie networks in computing, and these similarities help explain such properties of Ca(2+)-signaling networks as robustness, evolvability, and the ability to process multiple signals simultaneously.

A protein kinase-phosphatase pair interacts with an ion channel to regulate ABA signaling in plant guard cells

The plant hormone abscisic acid (ABA) serves as a physiological monitor to assess the water status of plants and, under drought conditions, induces stomatal pore closure by activating specific ion channels, such as a slow-anion channel (SLAC1) that, in turn, mediate ion efflux from the guard cells. Earlier genetic analyses uncovered a protein kinase (OST1) and several 2C-type phosphatases, as respective positive and negative regulators of ABA-induced stomatal closure. Here we show that the OST1 kinase interacts with the SLAC1 anion channel, leading to its activation via phosphorylation. PP2CA, one of the PP2C phosphatase family members acts in an opposing manner and inhibits the activity of SLAC1 by two mechanisms: (1) direct interaction with SLAC1 itself, and (2) physical interaction with OSTI leading to inhibition of the kinase independently of phosphatase activity. The results suggest that ABA signaling is mediated by a physical interaction chain consisting of several components, including a PP2C member, SnRK2-type kinase (OST1), and an ion channel, SLAC1, to regulate stomatal movements. The findings are in keeping with a paradigm in which a protein kinase-phosphatase pair interacts physically with a target protein to couple a signal with a specific response.

The CBL-CIPK Ca(2+)-decoding signaling network: function and perspectives

Calcium serves as a versatile messenger in many adaptation and developmental processes in plants. Cellular calcium signals are detected and transmitted by calcium-binding proteins functioning as sensor molecules. The family of calcineurin B-like (CBL) proteins represents a unique group of calcium sensors and contributes to the decoding of calcium transients by interacting with and regulating the family of CBL-interacting protein kinases (CIPKs). In higher plants, CBL proteins and CIPKs form a complex signaling network that allows for flexible but specific signal-response coupling during environmental adaptation reactions. This review presents novel findings concerning the evolution of this signaling network and key insights into the physiological function of CBL-CIPK complexes. These aspects will be presented and discussed in the context of emerging functional principles governing efficient and specific information processing in this signaling system.

Feedback inhibition of ammonium uptake by a phospho-dependent allosteric mechanism in Arabidopsis

The acquisition of nutrients requires tight regulation to ensure optimal supply while preventing accumulation to toxic levels. Ammonium transporter/methylamine permease/rhesus (AMT/Mep/Rh) transporters are responsible for ammonium acquisition in bacteria, fungi, and plants. The ammonium transporter AMT1;1 from Arabidopsis thaliana uses a novel regulatory mechanism requiring the productive interaction between a trimer of subunits for function. Allosteric regulation is mediated by a cytosolic C-terminal trans-activation domain, which carries a conserved Thr (T460) in a critical position in the hinge region of the C terminus. When expressed in yeast, mutation of T460 leads to inactivation of the trimeric complex. This study shows that phosphorylation of T460 is triggered by ammonium in a time- and concentration-dependent manner. Neither Gln nor l-methionine sulfoximine-induced ammonium accumulation were effective in inducing phosphorylation, suggesting that roots use either the ammonium transporter itself or another extracellular sensor to measure ammonium concentrations in the rhizosphere. Phosphorylation of T460 in response to an increase in external ammonium correlates with inhibition of ammonium uptake into Arabidopsis roots. Thus, phosphorylation appears to function in a feedback loop restricting ammonium uptake. This novel autoregulatory mechanism is capable of tuning uptake capacity over a wide range of supply levels using an extracellular sensory system, potentially mediated by a transceptor (i.e., transporter and receptor).

Coordinated responses to oxygen and sugar deficiency allow rice seedlings to tolerate flooding

Flooding is a widespread natural disaster that leads to oxygen (O(2)) and energy deficiency in terrestrial plants, thereby reducing their productivity. Rice is unusually tolerant to flooding, but the underlying mechanism for this tolerance has remained elusive. Here, we show that protein kinase CIPK15 [calcineurin B-like (CBL)-interacting protein kinase] plays a key role in O(2)-deficiency tolerance in rice. CIPK15 regulates the plant global energy and stress sensor SnRK1A (Snf1-related protein kinase 1) and links O(2)-deficiency signals to the SnRK1-dependent sugar-sensing cascade to regulate sugar and energy production and to enable rice growth under floodwater. Our studies contribute to understanding how rice grows under the conditions of O(2) deficiency necessary for growing rice in irrigated lowlands.

Heteromeric AtKC1{middle dot}AKT1 channels in Arabidopsis roots facilitate growth under K+-limiting conditions

Plant growth and development is driven by osmotic processes. Potassium represents the major osmotically active cation in plants cells. The uptake of this inorganic osmolyte from the soil in Arabidopsis involves a root K(+) uptake module consisting of the two K(+) channel alpha-subunits, AKT1 and AtKC1. AKT1-mediated potassium absorption from K(+)-depleted soil was shown to depend on the calcium-sensing proteins CBL1/9 and their interacting kinase CIPK23. Here we show that upon activation by the CBL.CIPK complex in low external potassium homomeric AKT1 channels open at voltages positive of E(K), a condition resulting in cellular K(+) leakage. Although at submillimolar external potassium an intrinsic K(+) sensor reduces AKT1 channel cord conductance, loss of cytosolic potassium is not completely abolished under these conditions. Depending on channel activity and the actual potassium gradients, this channel-mediated K(+) loss results in impaired plant growth in the atkc1 mutant. Incorporation of the AtKC1 subunit into the channel complex, however, modulates the properties of the K(+) uptake module to prevent K(+) loss. Upon assembly of AKT1 and AtKC1, the activation threshold of the root inward rectifier voltage gate is shifted negative by approximately -70 mV. Additionally, the channel conductance gains a hypersensitive K(+) dependence. Together, these two processes appear to represent a safety strategy preventing K(+) loss through the uptake channels under physiological conditions. Similar growth retardation phenotypes of akt1 and atkc1 loss-of-function mutants in response to limiting K(+) supply further support such functional interdependence of AKT1 and AtKC1. Taken together, these findings suggest an essential role of AtKC1-like subunits for root K(+) uptake and K(+) homeostasis when plants experience conditions of K(+) limitation.

Potassium nutrition, sodium toxicity, and calcium signaling: connections through the CBL-CIPK network

Plant roots take up numerous minerals from the soil. Some minerals (e.g., K(+)) are essential nutrients and others (e.g., Na(+)) are toxic for plant growth and development. In addition to the absolute level, the balance among the minerals is critical for their physiological functions. For instance, [K(+)]/[Na(+)] ratio and homeostasis often determine plant growth rate. Either low-K or high-Na in the soil represents a stress condition that severely affects plant life and agricultural production. Earlier observations indicated that higher soil Ca2(+) improve plants growth under low-K or high-Na condition, implying functional interaction among the three cations. Recent studies have begun to delineate the signaling mechanisms underlying such interactions. Either low-K(+) or high-Na(+) can trigger cellular Ca2(+) changes that lead to activation of complex signaling networks. One such network consists of Ca2(+) sensor proteins (e.g., CBLs) interacting with their target kinases (CIPKs). The CBL-CIPK signaling modules interact with and regulate the activity of a number of transporting proteins involved in the uptake and translocation of K(+) and Na(+), maintaining the "balance" of these cations in plants under stress conditions.

CIPK6, a CBL-interacting protein kinase is required for development and salt tolerance in plants

Calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CIPK) mediate plant responses to a variety of external stresses. Here we report that Arabidopsis CIPK6 is also required for the growth and development of plants. Phenotype of tobacco plants ectopically expressing a homologous gene (CaCIPK6) from the leguminous plant chickpea (Cicer arietinum) indicated its functional conservation. A lesion inAtCIPK6 significantly reduced shoot-to-root and root basipetal auxin transport, and the plants exhibited developmental defects such as fused cotyledons, swollen hypocotyls and compromised lateral root formation, in conjunction with reduced expression of a number of genes involved in auxin transport and abiotic stress response. The Arabidopsis mutant was more sensitive to salt stress compared to wild-type, while overexpression of a constitutively active mutant of CaCIPK6 promoted salt tolerance in transgenic tobacco. Furthermore, tobacco seedlings expressing the constitutively active mutant of CaCIPK6 showed a developed root system, increased basipetal auxin transport and hypersensitivity to auxin. Our results provide evidence for involvement of a CIPK in auxin transport and consequently in root development, as well as in the salt-stress response, by regulating the expression of genes.

STOP1 regulates multiple genes that protect arabidopsis from proton and aluminum toxicities

The Arabidopsis (Arabidopsis thaliana) mutant stop1 (for sensitive to proton rhizotoxicity1) carries a missense mutation at an essential domain of the histidine-2-cysteine-2 zinc finger protein STOP1. Transcriptome analyses revealed that various genes were down-regulated in the mutant, indicating that STOP1 is involved in signal transduction pathways regulating aluminum (Al)- and H(+)-responsive gene expression. The Al hypersensitivity of the mutant could be caused by down-regulation of AtALMT1 (for Arabidopsis ALUMINUM-ACTIVATED MALATE TRANSPORTER1) and ALS3 (ALUMINUM-SENSITIVE3). This hypothesis was supported by comparison of Al tolerance among T-DNA insertion lines and a transgenic stop mutant carrying cauliflower mosaic virus 35SAtALMT1. All T-DNA insertion lines of STOP1, AtALMT1, and ALS3 were sensitive to Al, but introduction of cauliflower mosaic virus 35SAtALMT1 did not completely restore the Al tolerance of the stop1 mutant. Down-regulation of various genes involved in ion homeostasis and pH-regulating metabolism in the mutant was also identified by microarray analyses. CBL-INTERACTING PROTEIN KINASE23, regulating a major K(+) transporter, and a sulfate transporter, SULT3;5, were down-regulated in the mutant. In addition, integral profiling of the metabolites and transcripts revealed that pH-regulating metabolic pathways, such as the gamma-aminobutyric acid shunt and biochemical pH stat pathways, are down-regulated in the mutant. These changes could explain the H(+) hypersensitivity of the mutant and would make the mutant more susceptible in acid soil stress than other Al-hypersensitive T-DNA insertion lines. Finally, we showed that STOP1 is localized to the nucleus, suggesting that the protein regulates the expression of multiple genes that protect Arabidopsis from Al and H(+) toxicities, possibly as a transcription factor.

K+ transport in plants: physiology and molecular biology

Potassium (K(+)) is an essential nutrient and the most abundant cation in plant cells. Plants have a wide variety of transport systems for K(+) acquisition, catalyzing K(+) uptake across a wide spectrum of external concentrations, and mediating K(+) movement within the plant as well as its efflux into the environment. K(+) transport responds to variations in external K(+) supply, to the presence of other ions in the root environment, and to a range of plant stresses, via Ca(2+) signaling cascades and regulatory proteins. This review will summarize the molecular identities of known K(+) transporters, and examine how this information supports physiological investigations of K(+) transport and studies of plant stress responses in a changing environment.

Ethylene mediates response and tolerance to potassium deprivation in Arabidopsis

Potassium deprivation leads to large reductions in plant growth and yields. How plants sense and transduce the stress signals initiated by potassium deprivation is poorly understood. Both ethylene production and the transcription of genes involved in ethylene biosynthesis increase when plants are deprived of potassium. To elucidate the role of ethylene in low potassium signaling pathways, we used both genetic and chemical approaches. Our results showed that ethylene is important in tolerance to low potassium and for changes in both root hair and primary root growth in Arabidopsis thaliana. We show that ethylene acts upstream of reactive oxygen species in response to potassium deprivation. The expression of High-Affinity K(+) Transporter5 was used as a marker of potassium deprivation and was found to be dependent on ethylene signaling. In the ethylene insensitive2-1 (ein2-1) mutant, the ethylene-mediated low potassium responses were not completely eliminated, suggesting that some potassium deprivation-induced responses are either ethylene independent or EIN2 independent. Ethylene signaling is a component of the plant's response to low potassium that stimulates the production of reactive oxygen species and is important for changes in root morphology and whole plant tolerance to low potassium conditions.

Plant ion channels: gene families, physiology, and functional genomics analyses

Distinct potassium, anion, and calcium channels in the plasma membrane and vacuolar membrane of plant cells have been identified and characterized by patch clamping. Primarily owing to advances in Arabidopsis genetics and genomics, and yeast functional complementation, many of the corresponding genes have been identified. Recent advances in our understanding of ion channel genes that mediate signal transduction and ion transport are discussed here. Some plant ion channels, for example, ALMT and SLAC anion channel subunits, are unique. The majority of plant ion channel families exhibit homology to animal genes; such families include both hyperpolarization- and depolarization-activated Shaker-type potassium channels, CLC chloride transporters/channels, cyclic nucleotide-gated channels, and ionotropic glutamate receptor homologs. These plant ion channels offer unique opportunities to analyze the structural mechanisms and functions of ion channels. Here we review gene families of selected plant ion channel classes and discuss unique structure-function aspects and their physiological roles in plant cell signaling and transport.

Learning from evolution: Thellungiella generates new knowledge on essential and critical components of abiotic stress tolerance in plants

Thellungiella salsuginea (halophila) is a close relative of Arabidopsis thaliana but, unlike A. thaliana, it grows well in extreme conditions of cold, salt, and drought as well as nitrogen limitation. Over the last decade, many laboratories have started to use Thellungiella to investigate the physiological, metabolic, and molecular mechanisms of abiotic stress tolerance in plants, and new knowledge has been gained in particular with respect to ion transport and gene expression. The advantage of Thellungiella over other extremophile model plants is that it can be directly compared with Arabidopsis, and therefore generate information on both essential and critical components of stress tolerance. Thellungiella research is supported by a growing body of technical resources comprising physiological and molecular protocols, ecotype collections, expressed sequence tags, cDNA-libraries, microarrays, and a pending genome sequence. This review summarizes the current state of knowledge on Thellungiella and re-evaluates its usefulness as a model for research into plant stress tolerance.

Regulation of macronutrient transport

In addition to light, water and CO(2), plants require a number of mineral nutrients, in particular the macronutrients nitrogen, sulphur, phosphorus, magnesium, calcium and potassium. After uptake from the soil by the root system they are either immediately assimilated into organic compounds or distributed within the plant for usage in different tissues. A good understanding of how the transport of macronutrients into and between plant cells is adjusted to different environmental conditions is essential to achieve an increase of nutrient usage efficiency and nutritional value in crops. Here, we review the current state of knowledge regarding the regulation of macronutrient transport, taking both a physiological and a mechanistic approach. We first describe how nutrient transport is linked to environmental and internal cues such as nutrient, carbon and water availability via hormonal, metabolic and physical signals. We then present information on the molecular mechanisms for regulation of transport proteins, including voltage gating, auto-inhibition, interaction with other proteins, oligomerization and trafficking. Combining of evidence for different nutrients, signals and regulatory levels creates an opportunity for making new connections within a large body of data, and thus contributes to an integrative understanding of nutrient transport.

The CBL-CIPK network in plant calcium signaling

Calcium (Ca2+) is a ubiquitous second messenger in all eukaryotes. An outstanding question is how this cation serves as a messenger for numerous signals and confers specific cellular responses. Recent studies have established a concept termed 'Ca2+ signature' that specifies Ca2+ changes triggered by each signal. How do cells recognize these signatures (codes) and translate them into the correct cellular responses? The initial step in this 'decoding' process involves sensor proteins that bind Ca2+ and activate the downstream targets, thereby regulating the specific biochemical processes. Here, I review and discuss a set of Ca2+ sensors (calcineurin B-like proteins [CBLs]) and their targets (CBL-interacting protein kinases [CIPKs]) as an emerging paradigm for Ca2+ decoding in plants. The principles governing the action of the CBL-CIPK signaling network could be generally applicable to many other signaling networks in plants and other organisms.

Relative contribution of AtHAK5 and AtAKT1 to K+ uptake in the high-affinity range of concentrations

The relative contribution of the high-affinity K(+) transporter AtHAK5 and the inward rectifier K(+) channel AtAKT1 to K(+) uptake in the high-affinity range of concentrations was studied in Arabidopsis thaliana ecotype Columbia (Col-0). The results obtained with wild-type lines, with T-DNA insertion in both genes and specific uptake inhibitors, show that AtHAK5 and AtAKT1 mediate the NH4+-sensitive and the Ba(2+)-sensitive components of uptake, respectively, and that they are the two major contributors to uptake in the high-affinity range of Rb(+) concentrations. Using Rb(+) as a K(+) analogue, it was shown that AtHAK5 mediates absorption at lower Rb(+) concentrations than AtAKT1 and depletes external Rb(+) to values around 1 muM. Factors such as the presence of K(+) or NH4+ during plant growth determine the relative contribution of each system. The presence of NH4+ in the growth solution inhibits the induction of AtHAK5 by K(+) starvation. In K(+)-starved plants grown without NH4+, both systems are operative, but when NH4+ is present in the growth solution, AtAKT1 is probably the only system mediating Rb(+) absorption, and the capacity of the roots to deplete Rb(+) is reduced.

Emergence of a novel calcium signaling pathway in plants: CBL-CIPK signaling network

In the environment, plants are exposed to plethora of adverse stimuli such as abiotic and biotic stresses. Abiotic stresses including dehydration, salinity and low temperature poses a major threat for crop productivity. Plant responds to these stresses by activating a number of signaling pathways which enable them to defend or adjust against these stresses. To understand the mechanisms by which plants perceive environmental signals and transmit these signals to cellular machinery to activate adaptive responses is of fundamental importance to biology. Calcium plays a pivotal role in plant responses to a number of stimuli including pathogens, abiotic stresses, and hormones. However, the molecular mechanisms underlying calcium functions are poorly understood. It is hypothesized that calcium serves as second messenger and, in many cases, requires intracellular protein sensors to transduce the signal further downstream in the pathways. Recently a novel calcium signaling pathway which consist of calcineurin B-like protein (CBL) calcium sensor and CBL-interacting protein kinase (CIPK) network as a newly emerging signaling system mediating a complex array of environmental stimuli. This review focuses on the overview of functional aspects of CBL and CIPK in plants. In addition, an attempt has also been made to categorize the functions of this CBL-CIPK pair in major signaling pathways in plants.