Cold-induced calreticulin OsCRT3 conformational changes promote OsCIPK7 binding and temperature sensing in rice.

Unusually low temperatures caused by global climate change adversely affect rice production. Sensing cold to trigger signal network is a key base for improvement of chilling tolerance trait.  Here, we report that Oryza sativa Calreticulin 3 (OsCRT3) localized at the endoplasmic reticulum (ER) exhibits conformational changes under cold stress, thereby enhancing its interaction with CBL-interacting protein kinase 7 (OsCIPK7) to sense cold. Phenotypic analyses of OsCRT3 knock-out mutants and transgenic overexpression lines demonstrate that OsCRT3 is a positive regulator in chilling tolerance. OsCRT3 localizes at the ER and mediates increases in cytosolic calcium levels under cold stress. Notably, cold stress triggers secondary structural changes of OsCRT3 and enhances its binding affinity with OsCIPK7, which finally boosts its kinase activity. Moreover, Calcineurin B-like protein 7 (OsCBL7) and OsCBL8 interact with OsCIPK7 specifically on the plasma membrane. Taken together, our results thus identify a cold-sensing mechanism that simultaneously conveys cold-induced protein conformational change, enhances kinase activity, and Ca2+ signal generation to facilitate chilling tolerance in rice.

A Ca2+-sensor switch for tolerance to elevated salt stress in Arabidopsis.

Excessive Na+ in soils inhibits plant growth. Here, we report that Na+ stress triggers primary calcium signals specifically in a cell group within the root differentiation zone, thus forming a "sodium-sensing niche" in Arabidopsis. The amplitude of this primary calcium signal and the speed of the resulting Ca2+ wave dose-dependently increase with rising Na+ concentrations, thus providing quantitative information about the stress intensity encountered. We also delineate a Ca2+-sensing mechanism that measures the stress intensity in order to mount appropriate salt detoxification responses. This is mediated by a Ca2+-sensor-switch mechanism, in which the sensors SOS3/CBL4 and CBL8 are activated by distinct Ca2+-signal amplitudes. Although the SOS3/CBL4-SOS2/CIPK24-SOS1 axis confers basal salt tolerance, the CBL8-SOS2/CIPK24-SOS1 module becomes additionally activated only in response to severe salt stress. Thus, Ca2+-mediated translation of Na+ stress intensity into SOS1 Na+/H+ antiporter activity facilitates fine tuning of the sodium extrusion capacity for optimized salt-stress tolerance.

A role for calcium-dependent protein kinases in differential CO2 - and ABA-controlled stomatal closing and low CO2 -induced stomatal opening in Arabidopsis.

Low concentrations of CO2 cause stomatal opening, whereas [CO2 ] elevation leads to stomatal closure. Classical studies have suggested a role for Ca2+ and protein phosphorylation in CO2 -induced stomatal closing. Calcium-dependent protein kinases (CPKs) and calcineurin-B-like proteins (CBLs) can sense and translate cytosolic elevation of the second messenger Ca2+ into specific phosphorylation events. However, Ca2+ -binding proteins that function in the stomatal CO2 response remain unknown. Time-resolved stomatal conductance measurements using intact plants, and guard cell patch-clamp experiments were performed. We isolated cpk quintuple mutants and analyzed stomatal movements in response to CO2 , light and abscisic acid (ABA). Interestingly, we found that cpk3/5/6/11/23 quintuple mutant plants, but not other analyzed cpk quadruple/quintuple mutants, were defective in high CO2 -induced stomatal closure and, unexpectedly, also in low CO2 -induced stomatal opening. Furthermore, K+ -uptake-channel activities were reduced in cpk3/5/6/11/23 quintuple mutants, in correlation with the stomatal opening phenotype. However, light-mediated stomatal opening remained unaffected, and ABA responses showed slowing in some experiments. By contrast, CO2 -regulated stomatal movement kinetics were not clearly affected in plasma membrane-targeted cbl1/4/5/8/9 quintuple mutant plants. Our findings describe combinatorial cpk mutants that function in CO2 control of stomatal movements and support the results of classical studies showing a role for Ca2+ in this response.

Plasma membrane calcineurin B-like calcium-ion sensor proteins function in regulating primary root growth and nitrate uptake by affecting global phosphorylation patterns and microdomain protein distribution.

The collective function of calcineurin B-like (CBL) calcium ion (Ca2+ ) sensors and CBL-interacting protein kinases (CIPKs) in decoding plasma-membrane-initiated Ca2+ signals to convey developmental and adaptive responses to fluctuating nitrate availability remained to be determined. Here, we generated a cbl-quintuple mutant in Arabidopsis thaliana devoid of these Ca2+ sensors at the plasma membrane and performed comparative phenotyping, nitrate flux determination, phosphoproteome analyses, and studies of membrane domain protein distribution in response to low and high nitrate availability. We observed that CBL proteins exert multifaceted regulation of primary and lateral root growth and nitrate fluxes. Accordingly, we found that loss of plasma membrane Ca2+ sensor function simultaneously affected protein phosphorylation of numerous membrane proteins, including several nitrate transporters, proton pumps, and aquaporins, as well as their distribution within plasma membrane microdomains, and identified a specific phosphorylation and domain distribution pattern during distinct phases of low and high nitrate responses. Collectively, these analyses reveal a central and coordinative function of CBL-CIPK-mediated signaling in conveying plant adaptation to fluctuating nitrate availability and identify a crucial role of Ca2+ signaling in regulating the composition and dynamics of plasma membrane microdomains.

Analyzing the Impact of Protein Overexpression on Ca2+ Dynamics and Development in Tobacco Pollen Tubes.

Overexpression of RFP-tagged proteins in growing tobacco pollen tubes together with the genetically encoded Ca2+ sensor YC3.6 allows to analyze localization and dynamics of the protein of interest, as well as the impact of its overexpression on Ca2+ dynamics and pollen tube growth. Here, we describe a step-by-step instruction for transient transformation of N. tabacum pollen and subsequent in vitro germination and Ca2+ imaging.

CIPK11-Dependent Phosphorylation Modulates FIT Activity to Promote Arabidopsis Iron Acquisition in Response to Calcium Signaling.

Nutrient acquisition is entangled with growth and stress in sessile organisms. The bHLH transcription factor FIT is a key regulator of Arabidopsis iron (Fe) acquisition and post-translationally activated upon low Fe. We identified CBL-INTERACTING PROTEIN KINASE CIPK11 as a FIT interactor. Cytosolic Ca2+ concentration and CIPK11 expression are induced by Fe deficiency. cipk11 mutant plants display compromised root Fe mobilization and seed Fe content. Fe uptake is dependent on CBL1/CBL9. CIPK11 phosphorylates FIT at Ser272, and mutation of this target site modulates FIT nuclear accumulation, homo-dimerization, interaction with bHLH039, and transcriptional activity and affects the plant's Fe-uptake ability. We propose that Ca2+-triggered CBL1/9-mediated activation of CIPK11 and subsequent phosphorylation of FIT shifts inactive into active FIT, allowing regulatory protein interactions in the nucleus. This biochemical link between Fe deficiency and the cellular Ca2+ decoding machinery represents an environment-sensing mechanism to adjust nutrient uptake.

The FERONIA Receptor Kinase Maintains Cell-Wall Integrity during Salt Stress through Ca2+ Signaling.

Cells maintain integrity despite changes in their mechanical properties elicited during growth and environmental stress. How cells sense their physical state and compensate for cell-wall damage is poorly understood, particularly in plants. Here we report that FERONIA (FER), a plasma-membrane-localized receptor kinase from Arabidopsis, is necessary for the recovery of root growth after exposure to high salinity, a widespread soil stress. The extracellular domain of FER displays tandem regions of homology with malectin, an animal protein known to bind di-glucose in vitro and important for protein quality control in the endoplasmic reticulum. The presence of malectin-like domains in FER and related receptor kinases has led to widespread speculation that they interact with cell-wall polysaccharides and can potentially serve a wall-sensing function. Results reported here show that salinity causes softening of the cell wall and that FER is necessary to sense these defects. When this function is disrupted in the fer mutant, root cells explode dramatically during growth recovery. Similar defects are observed in the mur1 mutant, which disrupts pectin cross-linking. Furthermore, fer cell-wall integrity defects can be rescued by treatment with calcium and borate, which also facilitate pectin cross-linking. Sensing of these salinity-induced wall defects might therefore be a direct consequence of physical interaction between the extracellular domain of FER and pectin. FER-dependent signaling elicits cell-specific calcium transients that maintain cell-wall integrity during salt stress. These results reveal a novel extracellular toxicity of salinity, and identify FER as a sensor of damage to the pectin-associated wall.

Two spatially and temporally distinct Ca2+ signals convey Arabidopsis thaliana responses to K+ deficiency.

In plants, potassium (K+ ) homeostasis is tightly regulated and established against a concentration gradient to the environment. Despite the identification of Ca2+ -regulated kinases as modulators of K+ channels, the immediate signaling and adaptation mechanisms of plants to low-K+ conditions are only partially understood. To assess the occurrence and role of Ca2+ signals in Arabidopsis thaliana roots, we employed ratiometric analyses of Ca2+ dynamics in plants expressing the Ca2+ reporter YC3.6 in combination with patch-clamp analyses of root cells and two-electrode voltage clamp (TEVC) analyses in Xenopus laevis oocytes. K+ deficiency triggers two successive and distinct Ca2+ signals in roots exhibiting spatial and temporal specificity. A transient primary Ca2+ signature arose within 1 min in the postmeristematic stelar tissue of the elongation zone, while a secondary Ca2+ response occurred after several hours as sustained Ca2+ elevation in defined tissues of the elongation and root hair differentiation zones. Patch-clamp and TEVC analyses revealed Ca2+ dependence of the activation of the K+ channel AKT1 by the CBL1-CIPK23 Ca2+ sensor-kinase complex. Together, these findings identify a critical role of cell group-specific Ca2+ signaling in low K+ responses and indicate an essential and direct role of Ca2+ signals for AKT1 K+ channel activation in roots.

Calcineurin B-Like Protein-Interacting Protein Kinase CIPK21 Regulates Osmotic and Salt Stress Responses in Arabidopsis.

The role of calcium-mediated signaling has been extensively studied in plant responses to abiotic stress signals. Calcineurin B-like proteins (CBLs) and CBL-interacting protein kinases (CIPKs) constitute a complex signaling network acting in diverse plant stress responses. Osmotic stress imposed by soil salinity and drought is a major abiotic stress that impedes plant growth and development and involves calcium-signaling processes. In this study, we report the functional analysis of CIPK21, an Arabidopsis (Arabidopsis thaliana) CBL-interacting protein kinase, ubiquitously expressed in plant tissues and up-regulated under multiple abiotic stress conditions. The growth of a loss-of-function mutant of CIPK21, cipk21, was hypersensitive to high salt and osmotic stress conditions. The calcium sensors CBL2 and CBL3 were found to physically interact with CIPK21 and target this kinase to the tonoplast. Moreover, preferential localization of CIPK21 to the tonoplast was detected under salt stress condition when coexpressed with CBL2 or CBL3. These findings suggest that CIPK21 mediates responses to salt stress condition in Arabidopsis, at least in part, by regulating ion and water homeostasis across the vacuolar membranes.

Vacuolar CBL-CIPK12 Ca(2+)-sensor-kinase complexes are required for polarized pollen tube growth.

Polarized tip growth is a fundamental process of specialized eukaryotic cells like neuronal axons, fungal hyphae, and plant root hairs and pollen tubes. In pollen tubes, a tip-focused oscillating Ca(2+) gradient governs ions fluxes, vesicle transport, and cytoskeleton dynamics to ensure proper polarized cell growth [1, 2]. While a crucial role of vacuolar Ca(2+) signaling is established for cellular movements like guard cell dynamics [3-5], its contribution to polarized growth remains to be defined. Here we identified the two closely related tonoplast-localized Ca(2+)-sensor proteins CBL2 and CBL3 as crucial regulators of vacuolar dynamics and polarized pollen tube growth. Overexpression of CBL2 or CBL3 in Arabidopsis and tobacco pollen tubes affected vacuolar morphology, pollen germination, and tube growth, but did not alter actin organization, PI(4,5)P2 distribution, or tip-focused Ca(2+) oscillations. Similarly, loss of function of each single Ca(2+) sensor and cbl2/cbl3 double mutants exhibited impaired pollen tube growth in vitro and in vivo. Both Ca(2+) sensors interacted with the kinase CIPK12, which translocated from the cytoplasm to the vacuolar membrane upon this interaction. Also, overexpression of CIPK12 induced severe vacuolar phenotypes, and loss of function of CIPK12 lead to impairment of polar growth. Remarkably, co-expression of CBL2 or CBL3 with CIPK12 resulted in a phosphorylation-dependent, massively enhanced vacuolar inflation and further disruption of polar growth. Together, these findings identify an essential role of the vacuole and vacuolar Ca(2+) signaling for polarized tip growth. We propose that a faithfully balanced activity of Ca(2+)-activated CBL2/3-CIPK12 complexes fulfills fundamental functions to enable the fast growth of pollen tubes in higher plants.

Signaling in cells and organisms - calcium holds the line.

Previous research has established calcium (Ca(2+)) and reactive oxygen species (ROS) as important cellular second messengers in eukaryotes. Recently, the occurrence of cell-to-cell moving Ca(2+) and ROS waves was reported in plants. This was paralleled by the discovery of long-distance wound-activated surface potential changes (WASPs) that require the function of putatively Ca(2+)-releasing glutamate receptor-like channels (GLRs) in Arabidopsis. Although the functional interconnection of Ca(2+)-dependent phosphorylation and ROS waves via NADPH oxidase activation has been clearly established, potential further interconnections between these long-distance signaling processes are less clear. In this review we cover emerging concepts and existing open questions that interconnect cellular and global signaling via Ca(2+), ROS and WASPs.

Control of vacuolar dynamics and regulation of stomatal aperture by tonoplast potassium uptake.

Stomatal movements rely on alterations in guard cell turgor. This requires massive K(+) bidirectional fluxes across the plasma and tonoplast membranes. Surprisingly, given their physiological importance, the transporters mediating the energetically uphill transport of K(+) into the vacuole remain to be identified. Here, we report that, in Arabidopsis guard cells, the tonoplast-localized K(+)/H(+) exchangers NHX1 and NHX2 are pivotal in the vacuolar accumulation of K(+) and that nhx1 nhx2 mutant lines are dysfunctional in stomatal regulation. Hypomorphic and complete-loss-of-function double mutants exhibited significantly impaired stomatal opening and closure responses. Disruption of K(+) accumulation in guard cells correlated with more acidic vacuoles and the disappearance of the highly dynamic remodelling of vacuolar structure associated with stomatal movements. Our results show that guard cell vacuolar accumulation of K(+) is a requirement for stomatal opening and a critical component in the overall K(+) homeostasis essential for stomatal closure, and suggest that vacuolar K(+) fluxes are also of decisive importance in the regulation of vacuolar dynamics and luminal pH that underlie stomatal movements.

The calcineurin B-like Ca2+ sensors CBL1 and CBL9 function in pollen germination and pollen tube growth in Arabidopsis.

Ca(2+) has been established as an important second messenger regulating pollen germination and tube growth. However, to date, only a few signaling components have been identified to decode and relay Ca(2+) signals in growing pollen tubes. Here, we report a function for the calcineurin B-like (CBL) Ca(2+) sensor proteins CBL1 and CBL9 from Arabidopsis in pollen germination and tube growth. Both proteins are expressed in mature pollen and pollen tubes and impair pollen tube growth and morphology if transiently overexpressed in tobacco pollen. The induction of these phenotypes requires efficient plasma membrane targeting of CBL1 and is independent of Ca(2+) binding to the fourth EF-hand of CBL1. Overexpression of CBL1 or its closest homolog CBL9 in Arabidopsis renders pollen germination and tube growth hypersensitive towards high external K(+) concentrations while disruption of CBL1 and CBL9 reduces pollen tube growth under low K(+) conditions. Together, our data identify a crucial function for CBL1 and CBL9 in pollen germination and tube growth and suggest a model in which both proteins act at the plasma membrane through regulation of K(+) homeostasis.

A new ß-estradiol-inducible vector set that facilitates easy construction and efficient expression of transgenes reveals CBL3-dependent cytoplasm to tonoplast translocation of CIPK5.

Transient and stable expression of transgenes is central to many investigations in plant biology research. Chemical regulation of expression can circumvent problems of plant lethality caused by constitutive overexpression or allow inducible knock (out/down) approaches. Several chemically inducible or repressible systems have been described and successfully applied. However, cloning and application-specific modification of most available inducible expression systems have been limited and remained complicated due to restricted cloning options. Here we describe a new set of 57 vectors that enable transgene expression in transiently or stably transformed cells. All vectors harbor a synthetically optimized XVE expression cassette, allowing ß-estradiol mediated protein expression. Plasmids are equipped with the reporter genes GUS, GFP, mCherry, or with HA and StrepII epitope tags and harbor an optimized multiple cloning site for flexible and simple cloning strategies. Moreover, the vector design allows simple substitution of the driving promoter to achieve tissue-specificity or to modulate expression ranges of inducible transgene expression. We report details of the kinetics and dose-dependence of expression induction in Arabidopsis leaf mesophyll protoplasts, transiently transformed Nicotiana benthamiana leaves, and stably transformed Arabidopsis plants. Using these vectors, we investigated the influence of CBL (Calcineurin B-like) protein expression on the subcellular localization of CIPKs (Calcineurin B-like interacting protein kinases). These analyses uncovered that induced co-expression of CBL3 is fully sufficient for dynamic translocation of CIPK5 from the cytoplasm to the tonoplast. Thus, the vector system presented here facilitates a broad range of research applications.

The Calcineurin B-like calcium sensors CBL1 and CBL9 together with their interacting protein kinase CIPK26 regulate the Arabidopsis NADPH oxidase RBOHF.

Stimulus-specific accumulation of second messengers like reactive oxygen species (ROS) and Ca(2+) are central to many signaling and regulation processes in plants. However, mechanisms that govern the reciprocal interrelation of Ca(2+) and ROS signaling are only beginning to emerge. NADPH oxidases of the respiratory burst oxidase homolog (RBOH) family are critical components contributing to the generation of ROS while Calcineurin B-like (CBL) Ca(2+) sensor proteins together with their interacting kinases (CIPKs) have been shown to function in many Ca(2+)- signaling processes. In this study, we identify direct functional interactions between both signaling systems. We report that the CBL-interacting protein kinase CIPK26 specifically interacts with the N-terminal domain of RBOHF in yeast two-hybrid analyses and with the full-length RBOHF protein in plant cells. In addition, CIPK26 phosphorylates RBOHF in vitro and co-expression of either CBL1 or CBL9 with CIPK26 strongly enhances ROS production by RBOHF in HEK293T cells. Together, these findings identify a direct interconnection between CBL-CIPK-mediated Ca(2+) signaling and ROS signaling in plants and provide evidence for a synergistic activation of the NADPH oxidase RBOHF by direct Ca(2+)-binding to its EF-hands and Ca(2+)-induced phosphorylation by CBL1/9-CIPK26 complexes.

Calcium - a central regulator of pollen germination and tube growth.

Pollen tubes grow rapidly by very fast rates and reach extended lengths to bring about fertilization during plant reproduction. The pollen tube grows exclusively at its tip. Fundamental for such local, tip-focused growth are the presence of internal gradients and transmembrane fluxes of ions. Consequently, vegetative pollen tube cells are an excellent single cell model system to investigate cell biological processes of vesicle transport, cytoskeleton reorganization and regulation of ion transport. The second messenger Ca(2+) has emerged as a central and crucial modulator that not only regulates but also integrates the coordination each of these processes. In this review we reflect on recent advances in our understanding of the mechanisms of Ca(2+) function in pollen tube growth, focusing on its role in basic cellular processes such as control of cell growth, vesicular transport and intracellular signaling by localized gradients of second messengers. In particular we discuss new insights into the identity and role of Ca(2+) conductive ion channels and present experimental addressable hypotheses about their regulation. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

S-acylation-dependent association of the calcium sensor CBL2 with the vacuolar membrane is essential for proper abscisic acid responses.

Calcineurin B-like (CBL) proteins contribute to decoding calcium signals by interacting with CBL-interacting protein kinases (CIPKs). Currently, there is still very little information about the function and specific targeting mechanisms of CBL proteins that are localized at the vacuolar membrane. In this study, we focus on CBL2, an abundant vacuolar membrane-localized calcium sensor of unknown function from Arabidopsis thaliana. We show that vacuolar targeting of CBL2 is specifically brought about by S-acylation of three cysteine residues in its N-terminus and that CBL2 S-acylation and targeting occur by a Brefeldin A-insensitive pathway. Loss of CBL2 function renders plants hypersensitive to the phytohormone abscisic acid (ABA) during seed germination and only fully S-acylated and properly vacuolar-targeted CBL2 proteins can complement this mutant phenotype. These findings define an S-acylation-dependent vacuolar membrane targeting pathway for proteins and uncover a crucial role of vacuolar calcium sensors in ABA responses.