A protein kinase, interacting with two calcineurin B-like proteins, regulates K+ transporter AKT1 in Arabidopsis

Ligand-dependent reduction in the membrane mobility of FLAGELLIN SENSITIVE2, an arabidopsis receptor-like kinase

Arabidopsis Flagellin sensitive2 (FLS2) is a transmembrane leucine-rich repeat receptor-like kinase, which recognizes a conserved 22 amino acid peptide (flg22) of bacterial flagellin and activates downstream defense signaling pathways resulting in enhanced resistance against plant pathogens. The underlying mechanisms for the activation of FLS2 in the cell membrane, however, are not fully understood. Using fluorescence recovery after photobleaching (FRAP), we demonstrate that approximately 75% of the FLS2 in the plasma membrane diffuses laterally with a diffusion coefficient of 0.34 microm(2) s(-1), indicating that it moves rapidly. Further, we show that FLS2 is less mobile in the presence of flg22, suggesting its ligand-dependent confinement to microdomains or transient interaction with other less mobile membrane proteins. Using an in vivo bimolecular fluorescence complementation (BiFC) system and fluorescence resonance energy transfer (FRET), which reveals in vivo protein-protein interactions, we show that FLS2 does not homodimerize either constitutively or in the presence of flg22. Our data suggest that the reduced mobility of FLS2 after binding flg22 and its existence in monomeric form are important mechanistic features of FLS2 early signaling.

The calcium sensor CBL10 mediates salt tolerance by regulating ion homeostasis 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 protein kinases (CIPKs). We report here that the CBL protein CBL10 functions as a crucial regulator of salt tolerance in Arabidopsis. Cbl10 mutant plants exhibited significant growth defects and showed hypersensitive cell death in leaf tissues under high-salt conditions. Interestingly, the Na(+) content of the cbl10 mutant, unlike other salt-sensitive mutants identified thus far, was significantly lower than in the wild type under either normal or high-salt conditions, suggesting that CBL10 mediates a novel Ca(2+)-signaling pathway for salt tolerance. Indeed, the CBL10 protein physically interacts with the salt-tolerance factor CIPK24 (SOS2), and the CBL10-CIPK24 (SOS2) complex is associated with the vacuolar compartments that are responsible for salt storage and detoxification in plant cells. These findings suggest that CBL10 and CIPK24 (SOS2) constitute a novel salt-tolerance pathway that regulates the sequestration/compartmentalization of Na(+) in plant cells. Because CIPK24 (SOS2) also interacts with CBL4 (SOS3) and regulates salt export across the plasma membrane, our study identifies CIPK24 (SOS2) as a multi-functional protein kinase that regulates different aspects of salt tolerance by interacting with distinct CBL calcium sensors.

A protein phosphorylation/dephosphorylation network regulates a plant potassium channel

Potassium (K(+)) is an essential nutrient for plant growth and development. Plants often adapt to low K(+) conditions by increasing their K(+) uptake capability. Recent studies have led to the identification of a calcium signaling pathway that enables plants to act in this capacity. Calcium is linked to two calcineurin B-like calcium sensors (CBLs) and a target kinase (CBL-interacting protein kinase 23 or CIPK23) that, in turn, appears to phosphorylate and activate the potassium channel, Arabidopsis K(+) transporter 1 (AKT1), responsible for K(+) uptake in roots. Here, we report evidence that this regulatory mechanism is more elaborate than earlier envisaged. The recently described pathway is part of an extensive network whereby several CBLs interact with multiple CIPKs in the activation of the potassium channel, AKT1. The physical interactions among the CBL, CIPK, and AKT1 components provide a mechanism for specifying the members of the CBL and CIPK families functional in AKT1 regulation. The interaction between the CIPKs and AKT1 was found to involve the kinase domain of the CIPK component and the ankyrin repeat domain of the channel. Furthermore, we identified a 2C-type protein phosphatase that physically interacts and inactivates the AKT1 channel. These findings provide evidence that the calcium-sensitive CBL and CIPK families together with 2C-type protein phosphatases form a protein phoshporylation/dephosphorylation network that regulates the AKT1 channel for K(+) transport in plants.

Two calcineurin B-like calcium sensors, interacting with protein kinase CIPK23, regulate leaf transpiration and root potassium uptake in Arabidopsis

Calcium signalling involves sensor proteins that decode temporal and spatial changes in cellular Ca2+ concentration. Calcineurin B-like proteins (CBLs) represent a unique family of plant calcium sensors that relay signals by interacting with a family of protein kinases, designated as CBL-interacting protein kinases (CIPKs). In a reverse genetic screen for altered drought tolerance, we identified a loss-of-function allele of CIPK23 as exhibiting a drought-tolerant phenotype. In the cipk23 mutant, reduced transpirational water loss from leaves coincides with enhanced ABA sensitivity of guard cells during opening as well as closing reactions, without noticeable alterations in ABA content in the plant. We identified the calcium sensors CBL1 and CBL9 as CIPK23-interacting proteins that targeted CIPK23 to the plasma membrane in vivo. Expression analysis of the CIPK23, CBL1 and CBL9 genes suggested that they may function together in diverse tissues, including guard cells and root hairs. In addition, expression of the CIPK23 gene was induced by low-potassium conditions, implicating a function of this gene product in potassium nutrition. Indeed, cipk23 mutants displayed severe growth impairment on media with low concentrations of potassium. This phenotype correlates with a reduced efficiency of K+ uptake into the roots. In support of the conclusion that CBL1 and CBL9 interact with and synergistically serve as upstream regulators of CIPK23, the cbl1 cbl9 double mutant, but not the cbl1 or cbl9 single mutants, exhibit altered phenotypes for stomatal responses and low-potassium sensitivity. Together with the recent identification of the potassium channel AKT1 as a target of CIPK23, these results imply that plasma membrane-localized CBL1- and CBL9-CIPK23 complexes simultaneously regulate K+ transport processes in roots and in stomatal guard cells.

ERF protein JERF1 that transcriptionally modulates the expression of abscisic acid biosynthesis-related gene enhances the tolerance under salinity and cold in tobacco

Increasing evidences indicate that ethylene responsive factor (ERF) proteins regulate a variety of biotic and abiotic stress responses, and plant development as well. Previously we demonstrated that JERF1, encoding an ERF transcriptional activator, is inducible by ethylene, MeJA, ABA, and NaCl, suggesting its possible regulation in multiple stress responses. In the present paper, we report that expressing JERF1 in tobacco increases the seed germination under mannitol treatment, and enhances the tolerance to high salinity and low temperature, through accumulating sodium in vacuole of leaves and stabilizing the plasma membrane, respectively, and significantly increases the growth of tobacco roots and leaves under salinity and low temperature through an unknown mechanism. The evidence that JERF1 interacts with multiple cis-acting elements, such as GCC-box, DRE, and CE1, to activate the expression of stress-related genes, supports the possible involvement of JERF1 in multiple plant stress responses with ABA-dependent and ABA-independent manner. More importantly, we reveal that expressing JERF1 in tobacco transcriptionally regulates the expression of ABA biosynthesis-related gene NtSDR, resulting in the increase of the ABA content. Together, our results indicate that JERF1 interacts with multiple cis-acting elements and activates the expression of stress responsive and ABA biosynthesis-related genes, consequently causing ABA biosynthesis, and ultimately enhancing tobacco tolerance and growth under high salinity and low temperature.

The structure of the C-terminal domain of the protein kinase AtSOS2 bound to the calcium sensor AtSOS3

The plant SOS2 family of protein kinases and their interacting activators, the SOS3 family of calcium-binding proteins, function together in decoding calcium signals elicited by different environmental stimuli. SOS2 is activated by Ca-SOS3 and subsequently phosphorylates the ion transporter SOS1 to bring about cellular ion homeostasis under salt stress. In addition to possessing the kinase activity, members of the SOS2 family of protein kinases can bind to protein phosphatase 2Cs. The crystal structure of the binary complex of Ca-SOS3 with the C-terminal regulatory moiety of SOS2 resolves central questions regarding the dual function of SOS2 as a kinase and a phosphatase-binding protein. A comparison with the structure of unbound SOS3 reveals the basis of the molecular function of this family of kinases and their interacting calcium sensors. Furthermore, our study suggests that the structure of the phosphatase-interaction domain of SOS2 defines a scaffold module conserved from yeast to human.

An Arabidopsis quiescin-sulfhydryl oxidase regulates cation homeostasis at the root symplast-xylem interface

A genetic screen of Arabidopsis 'activation-tagging' mutant collection based on tolerance to norspermidine resulted in a dominant mutant (par1-1D) with increased expression of the QSO2 gene (At1g15020), encoding a member of the quiescin-sulfhydryl oxidase (QSO) family. The par1-1D mutant and transgenic plants overexpressing QSO2 cDNA grow better than wild-type Arabidopsis in media with toxic cations (polyamines, Li(+) and Na(+)) or reduced K(+) concentrations. This correlates with a decrease in the accumulation of toxic cations and an increase in the accumulation of K(+) in xylem sap and shoots. Conversely, three independent loss-of-function mutants of QSO2 exhibit phenotypes opposite to those of par1-1D. QSO2 is mostly expressed in roots and is upregulated by K(+) starvation. A QSO2Colon, two colonsGFP fusion ectopically expressed in leaf epidermis localized at the cell wall. The recombinant QSO2 protein, produced in yeast in secreted form, exhibits disulfhydryl oxidase activity. A plausible mechanism of QSO2 action consists on the activation of root systems loading K(+) into xylem, but different from the SKOR channel, which is not required for QSO2 action. These results uncover QSOs as novel regulators of ion homeostasis.

CIPK9: a calcium sensor-interacting protein kinase required for low-potassium tolerance in Arabidopsis

Potassium is one of the major macro-nutrients essential for a number of cellular processes in plants. Low potassium level in the soil represents a limiting factor for crop production. Recent studies have identified potassium transporters that are involved in potassium acquisition, and some of them are critical for potassium nutrition under low potassium conditions. However, little is understood on the molecular components involved in low potassium signaling and responses. We report here the identification of a calcineurin B-like protein-interacting protein kinase (CIPK9) as a critical regulator of low potassium response in Arabidopsis. The CIPK9 gene was responsive to abiotic stress conditions, and its transcript was inducible in both roots and shoots by potassium deprivation. Disruption of CIPK9 function rendered the mutant plants hypersensitive to low potassium media. Further analysis indicated that K(+) uptake and content were not affected in the mutant plants, implying CIPK9 in the regulation of potassium utilization or sensing processes.

Plant cells must pass a K+ threshold to re-enter the cell cycle

Potassium is an inevitable component of plant life, and potassium channels play a pivotal role in plant growth and development. The role of potassium and of K(+) channels in plant cell division and cell-cycle progression, however, has not been determined so far. K(+) channel blocker studies with synchronized tobacco BY-2 cells revealed that K(+) uptake is required for proper cell-cycle progression during the transition from G(1) to S phase. Electrophysiological studies (patch-clamp and voltage-clamp techniques) showed a cell-cycle dependency of K(+) channel activities and reduced driving force for K(+) uptake in dividing cells. Among the four Shaker-like K(+) channel genes expressed in BY-2 cells, NKT1 represents an inwardly rectifying K(+) channel that mediates K(+) uptake. NKT1 is transcriptionally induced during G(1) phase, while transcripts of the outward-rectifier NTORK1 dominate S phase. Elongating BY-2 cells appeared hyperpolarized (-101 +/- 11 mV), and had elevated osmotic pressure and approximately twice the turgor pressure when compared with depolarized (-64 +/- 8 mV) dividing cells. This indicates that cells have to gain a threshold K(+) level to re-enter the cell cycle. Based on these findings, turgor regulation through modulation of K(+) channel density in plant cell division and cell-cycle progression is discussed.

K+ channel activity in plants: genes, regulations and functions

Potassium (K(+)) is the most abundant cation in the cytosol, and plant growth requires that large amounts of K(+) are transported from the soil to the growing organs. K(+) uptake and fluxes within the plant are mediated by several families of transporters and channels. Here, we describe the different families of K(+)-selective channels that have been identified in plants, the so-called Shaker, TPK and Kir-like channels, and what is known so far on their regulations and physiological functions in the plant.

Antisense expression of a gene encoding a calcium-binding protein in transgenic tobacco leads to altered morphology and enhanced chlorophyll

Entamoeba histolytica contains a novel calcium-binding protein like calmodulin,which was discovered earlier,and we have reported the presence of its homologue(s)and a dependent protein kinase in plants.To understand the functions of these in plants,a cDNA encoding a calcium-binding protein isolated from Entamoeba histolytica (EhCaBP)was cloned into vector pBI121 in antisense orientation and transgenic tobacco plants were raised.These plants showed variation in several phenotypic characters,of which two distinct features,more greenness and leaf thickness,were inherited in subsequent generations.The increase in the level of total chlorophyll in different plants ranged from 60% to 70%.There was no major change in chloroplast structure and in the protein level of D1,D2,LHCP and RuBP carboxylase.These morphological changes were not seen in antisense calmodulin transgenic tobacco plants,nor was the calmodulin level altered in EhCaBP antisense plants.

Escherichia coli enzyme IIANtr regulates the K+ transporter TrkA

The maintenance of ionic homeostasis in response to changes in the environment is essential for all living cells. Although there are still many important questions concerning the role of the major monovalent cation K(+), cytoplasmic K(+) in bacteria is required for diverse processes. Here, we show that enzyme IIA(Ntr) (EIIA(Ntr)) of the nitrogen-metabolic phosphotransferase system interacts with and regulates the Escherichia coli K(+) transporter TrkA. Previously we reported that an E. coli K-12 mutant in the ptsN gene encoding EIIA(Ntr) was extremely sensitive to growth inhibition by leucine or leucine-containing peptides (LCPs). This sensitivity was due to the requirement of the dephosphorylated form of EIIA(Ntr) for the derepression of ilvBN expression. Whereas the ptsN mutant is extremely sensitive to LCPs, a ptsN trkA double mutant is as resistant as WT. Furthermore, the sensitivity of the ptsN mutant to LCPs decreases as the K(+) level in culture media is lowered. We demonstrate that dephosphorylated EIIA(Ntr), but not its phosphorylated form, forms a tight complex with TrkA that inhibits the accumulation of high intracellular concentrations of K(+). High cellular K(+) levels in a ptsN mutant promote the sensitivity of E. coli K-12 to leucine or LCPs by inhibiting both the expression of ilvBN and the activity of its gene products. Here, we delineate the similarity of regulatory mechanisms for the paralogous carbon and nitrogen phosphotransferase systems. Dephosphorylated EIIA(Glc) regulates a variety of transport systems for carbon sources, whereas dephosphorylated EIIA(Ntr) regulates the transport system for K(+), which has global effects related to nitrogen metabolism.

Nutrient sensing and signaling: NPKS

Plants often grow in soils that contain very low concentrations of the macronutrients nitrogen, phosphorus, potassium, and sulfur. To adapt and grow in nutrient-deprived environments plants must sense changes in external and internal mineral nutrient concentrations and adjust growth to match resource availability. The sensing and signal transduction networks that control plant responses to nutrient deprivation are not well characterized for nitrogen, potassium, and sulfur deprivation. One branch of the signal transduction cascade related to phosphorus-deprivation response has been defined through the identification of a transcription factor that is regulated by sumoylation. Two different microRNAs play roles in regulating gene expression under phosphorus and sulfur deprivation. Reactive oxygen species increase rapidly after mineral nutrient deprivation and may be one upstream mediator of nutrient signaling. A number of molecular analyses suggest that both short-term and longer-term responses will be important in understanding the progression of signaling events when the external, then internal, supplies of nutrients become depleted.

Ca2+-dependent and -independent abscisic acid activation of plasma membrane anion channels in guard cells of Nicotiana tabacum

Drought induces stomatal closure, a response that is associated with the activation of plasma membrane anion channels in guard cells, by the phytohormone abscisic acid (ABA). In several species, this response is associated with changes in the cytoplasmic free Ca(2+) concentration. In Vicia faba, however, guard cell anion channels activate in a Ca(2+)-independent manner. Because of potential differences between species, Nicotiana tabacum guard cells were studied in intact plants, with simultaneous recordings of the plasma membrane conductance and the cytoplasmic free Ca(2+) concentration. ABA triggered transient rises in cytoplasmic Ca(2+) in the majority of the guard cells (14 out of 19). In seven out of 14 guard cells, the change in cytoplasmic free Ca(2+) closely matched the activation of anion channels, while the Ca(2+) rise was delayed in seven other cells. In the remaining five cells, ABA stimulated anion channels without a change in the cytoplasmic Ca(2+) level. Even though ABA could activate anion channels in N. tabacum guard cells independent of a rise in the cytoplasmic Ca(2+) concentration, patch clamp experiments showed that anion channels in these cells are stimulated by elevated Ca(2+) in an ATP-dependent manner. Guard cells thus seem to have evolved both Ca(2+)-independent and -dependent ABA signaling pathways. Guard cells of N. tabacum apparently utilize both pathways, while ABA signaling in V. faba seems to be restricted to the Ca(2+)-independent pathway.