The Arabidopsis calcium sensor calcineurin B-like 3 inhibits the 5'-methylthioadenosine nucleosidase in a calcium-dependent manner

A soybean EF-Tu family protein GmEF8, an interactor of GmCBL1, enhances drought and heat tolerance in transgenic Arabidopsis and soybean

A soybean elongation factor Tu family (EF-Tu) protein, GmEF8, was determined to interact with GmCBL1, and GmEF8 expression was found to be induced by various abiotic stresses such as drought and heat. An ortholog of GmEF8 was identified in Arabidopsis, a T-DNA knockout line for which exhibited hypersensitivity to drought and heat stresses. Complementation with GmEF8 rescued the sensitivity of the Arabidopsis mutant to drought and heat stresses, and GmEF8 overexpression conferred drought and heat tolerance to transgenic Arabidopsis plants. In soybean, plants with GmEF8-overexpressing hairy roots (OE-GmEF8) exhibited enhanced drought and heat tolerance and had higher proline levels compared to plants with RNAi GmEF8-knockdown hairy roots (MR-GmEF8) and control hairy roots (EV). A number of drought-responsive genes, such as GmRD22 and GmP5CS, were induced in the OE-GmEF8 line compared to MR-GmEF8 and EV under normal growth conditions. These results suggest that GmEF8 has a positive role in regulating drought and heat stresses in Arabidopsis and soybean. This study reveals a potential role of the soybean GmEF8 gene in response to abiotic stresses, providing a foundation for further investigation into the complexities of stress signal transduction pathways.

Calcium Sensor SlCBL4 Associates with SlCIPK24 Protein Kinase and Mediates Salt Tolerance in Solanum lycopersicum

Soil salinity is one of the major environmental stresses that restrict the growth and development of tomato (Solanum lycopersicum L.) worldwide. In Arabidopsis, the calcium signaling pathway mediated by calcineurin B-like protein 4 (CBL4) and CBL-interacting protein kinase 24 (CIPK24) plays a critical role in salt stress response. In this study, we identified and isolated two tomato genes similar to the Arabidopsis genes, designated as SlCBL4 and SlCIPK24, respectively. Bimolecular fluorescence complementation (BiFC) and pull-down assays indicated that SlCBL4 can physically interact with SlCIPK24 at the plasma membrane of plant cells in a Ca2+-dependent manner. Overexpression of SlCBL4 or superactive SlCIPK24 mutant (SlCIPK24M) conferred salt tolerance to transgenic tomato (cv. Moneymaker) plants. In particular, the SlCIPK24M-overexpression lines displayed dramatically enhanced tolerance to high salinity. It is notable that the transgenic plants retained higher contents of Na+ and K+ in the roots compared to the wild-type tomato under salt stress. Taken together, our findings clearly suggest that SlCBL4 and SlCIPK24 are functional orthologs of the Arabidopsis counterpart genes, which can be used or engineered to produce salt-tolerant tomato plants.

The CBL-CIPK Calcium Signaling Network: Unified Paradigm from 20 Years of Discoveries

Calcium (Ca²⁺) serves as an essential nutrient as well as a signaling agent in all eukaryotes. In plants, calcineurin B-like proteins (CBLs) are a unique group of Ca²⁺ sensors that decode Ca²⁺ signals by activating a family of plant-specific protein kinases known as CBL-interacting protein kinases (CIPKs). Interactions between CBLs and CIPKs constitute a signaling network that enables information integration and physiological coordination in response to a variety of extracellular cues such as nutrient deprivation and abiotic stresses. Studies in the past two decades have established a unified paradigm that illustrates the functions of CBL-CIPK complexes in controlling membrane transport through targeting transporters and channels in the plasma membrane and tonoplast.

Genome-wide identification and biochemical characterization of calcineurin B-like calcium sensor proteins in Chlamydomonas reinhardtii

Calcium (Ca2+) signaling is involved in the regulation of diverse biological functions through association with several proteins that enable them to respond to abiotic and biotic stresses. Though Ca2+-dependent signaling has been implicated in the regulation of several physiological processes in Chlamydomonas reinhardtii, Ca2+ sensor proteins are not characterized completely. C. reinhardtii has diverged from land plants lineage, but shares many common genes with animals, particularly those encoding proteins of the eukaryotic flagellum (or cilium) along with the basal body. Calcineurin, a Ca2+/calmodulin-dependent protein phosphatase, is an important effector of Ca2+ signaling in animals, while calcineurin B-like proteins (CBLs) play an important role in Ca2+ sensing and signaling in plants. The present study led to the identification of 13 novel CBL-like Ca2+ sensors in C. reinhardtii genome. One of the archetypical genes of the newly identified candidate, CrCBL-like1 was characterized. The ability of CrCBL-like1 protein to sense as well as bind Ca2+ were validated using two-step Ca2+-binding kinetics. The CrCBL-like1 protein localized around the plasma membrane, basal bodies and in flagella, and interacted with voltage-gated Ca2+ channel protein present abundantly in the flagella, indicating its involvement in the regulation of the Ca2+ concentration for flagellar movement. The CrCBL-like1 transcript and protein expression were also found to respond to abiotic stresses, suggesting its involvement in diverse physiological processes. Thus, the present study identifies novel Ca2+ sensors and sheds light on key players involved in Ca2+signaling in C. reinhardtii, which could further be extrapolated to understand the evolution of Ca2+ mediated signaling in other eukaryotes.

CBL–CIPK module-mediated phosphoregulation: facts and hypothesis

Calcium (Ca2+) signaling is a versatile signaling network in plant and employs very efficient signal decoders to transduce the encoded message. The CBL–CIPK module is one of the sensor-relay decoders that have probably evolved with the acclimatization of land plant. The CBLs are unique proteins with non-canonical Ca2+ sensing EF-hands, N-terminal localization motif and a C-terminal phosphorylation motif. The partner CIPKs are Ser/Thr kinases with kinase and regulatory domains. Phosphorylation plays a major role in the functioning of the module. As the module has a functional kinase to transduce signal, it employs phosphorylation as a preferred mode for modulation of targets as well as its interaction with CBL. We analyze the data on the substrate regulation by the module from the perspective of substrate phosphorylation. We have also predicted some of the probable sites in the identified substrates that may be the target of the CIPK mediated phosphorylation. In addition, phosphatases have been implicated in reversing the CIPK mediated phosphorylation of substrates. Therefore, we have also presented the role of phosphatases in the modulation of the CBL–CIPK and its targets. We present here an overview of the phosphoregulation mechanism of the CBL–CIPK module.

Ectopic Expression of SjCBL1, Calcineurin B-Like 1 Gene From Sedirea japonica, Rescues the Salt and Osmotic Stress Hypersensitivity in Arabidopsis cbl1 Mutant

Extensive studies with Arabidopsis thaliana suggested that calcineurin B-like (CBL) proteins constitute a unique family of calcium sensors in plants, which mediate a variety of abiotic stress responses. However, little is known about their function in most plants that do not have available genome sequences. In this study, we have developed a pair of universal primers that make it possible to isolate CBL1-like genes from various plants without sequence information. Using these primers, we successfully cloned a full-length cDNA of CBL1-like gene in Sedirea japonica (SjCBL1). Bimolecular fluorescence complementation (BiFC) and pull-down assays demonstrated that like Arabidopsis CBL1 (AtCBL1), SjCBL1 can interacts physically with Arabidopsis CBL-interacting protein kinase 1 (AtCIPK1) at the plasma membrane of plant cells in a Ca2+-dependent manner. In addition, overexpression of SjCBL1 in the Arabidopsis cbl1 mutant resulted in not only rescuing the hypersensitive phenotype toward salt and osmotic stresses, but also substantially enhancing the tolerance to them. Taken together, these results strongly suggest that SjCBL1 is a functional ortholog of AtCBL1 in Sedirea japonica, which can play a critical role in response to salt and osmotic stresses. Therefore, it is clear that our findings should significantly contribute to broadening and deepening our understanding of the CBL1-mediated Ca2+ signaling networks in the plant kingdom.

Natural variation in the promoter of rice calcineurin B-like protein10 (OsCBL10) affects flooding tolerance during seed germination among rice subspecies

Rice (Oryza sativa L.) has two ecotypes, upland and lowland rice, that have been observed to show different tolerance levels under flooding stress. In this study, two rice cultivars, upland (Up221, flooding-intolerant) and lowland (Low88, flooding-tolerant), were initially used to study their molecular mechanisms in response to flooding germination. We observed that variations in the OsCBL10 promoter sequences in these two cultivars might contribute to this divergence in flooding tolerance. Further analysis using another eight rice cultivars revealed that the OsCBL10 promoter could be classified as either a flooding-tolerant type (T-type) or a flooding-intolerant type (I-type). The OsCBL10 T-type promoter only existed in japonica lowland cultivars, whereas the OsCBL10 I-type promoter existed in japonica upland, indica upland and indica lowland cultivars. Flooding-tolerant rice cultivars containing the OsCBL10 T-type promoter have shown lower Ca²⁺ flow and higher α-amylase activities in comparison to those in flooding-intolerant cultivars. Furthermore, the OsCBL10 overexpression lines were sensitive to both flooding and hypoxic treatments during rice germination with enhanced Ca²⁺ flow in comparison to wild-type. Subsequent findings also indicate that OsCBL10 may affect OsCIPK15 protein abundance and its downstream pathways. In summary, our results suggest that the adaptation to flooding stress during rice germination is associated with two different OsCBL10 promoters, which in turn affect OsCBL10 expression in different cultivars and negatively affect OsCIPK15 protein accumulation and its downstream cascade.

Revisiting paradigms of Ca2+ signaling protein kinase regulation in plants

Calcium (Ca2+) serves as a universal second messenger in eukaryotic signal transduction. Understanding the Ca2+ activation kinetics of Ca2+ sensors is critical to understanding the cellular signaling mechanisms involved. In this review, we discuss the regulatory properties of two sensor classes: the Ca2+-dependent protein kinases (CPKs/CDPKs) and the calcineurin B-like (CBL) proteins that control the activity of CBL-interacting protein kinases (CIPKs) and identify emerging topics and some foundational points that are not well established experimentally. Most plant CPKs are activated by physiologically relevant Ca2+ concentrations except for those with degenerate EF hands, and new results suggest that the Ca2+-dependence of kinase activation may be modulated by both protein–protein interactions and CPK autophosphorylation. Early results indicated that activation of plant CPKs by Ca2+ occurred by relief of autoinhibition. However, recent studies of protist CDPKs suggest that intramolecular interactions between CDPK domains contribute allosteric control to CDPK activation. Further studies are required to elucidate the mechanisms regulating plant CPKs. With CBL–CIPKs, the two major activation mechanisms are thought to be (i) binding of Ca2+-bound CBL to the CIPK and (ii) phosphorylation of residues in the CIPK activation loop. However, the relative importance of these two mechanisms in regulating CIPK activity is unclear. Furthermore, information detailing activation by physiologically relevant [Ca2+] is lacking, such that the paradigm of CBLs as Ca2+ sensors still requires critical, experimental validation. Developing models of CPK and CIPK regulation is essential to understand how these kinases mediate Ca2+ signaling and to the design of experiments to test function in vivo.

The CBL and CIPK Gene Family in Grapevine (Vitis vinifera): Genome-Wide Analysis and Expression Profiles in Response to Various Abiotic Stresses

Calcium plays a central role in regulating signal transduction pathways. Calcineurin B-like proteins (CBLs), which harbor a crucial region consisting of EF hands that capture Ca²⁺, interact in a specific manner with CBL-interacting protein kinases (CIPKs). This two gene families or their interacting-complex widely respond to various environment stimuli and development processes. The genome-wide annotation and specific expression patterns of CBLs and CIPKs, however, in grapevine remain unclear. In the present study, eight CBL and 20 CIPK genes were identified in grapevine genome, and divided into four and five subfamilies, respectively, based on phylogenetic analysis, and validated by gene structure and the distribution of conserved protein motifs. Four (50%) out of eight VvCBLs and eight (40%) out of 20 VvCIPKs were found to be derived from tandem duplication, and five (25%) out of 20 VvCIPKs were derived from segmental duplication, indicating that the expansion of grapevine CBL and CIPK gene families were mainly contributed by gene duplication, and all duplication events between VvCIPK genes only detected in intron poor clade. Estimating of synonymous and non-synonymous substitution rates of both gene families suggested that VvCBL genes seems more conserved than VvCIPK genes, and were derived by positive selection pressure, whereas VvCIPK genes were mainly derived by purifying selection pressure. Expressional analyses of VvCBL and VvCIPK genes based on microarray and qRT-PCR data performed diverse expression patterns of VvCBLs and VvCIPKs in response to both various abiotic stimuli and at different development stages. Furthermore, the co-expression analysis of grapevine CBLs and CIPKs suggested that CBL-CIPK complex seems to be more responsive to abiotic stimuli than during different development stages. VvCBLs may play an important and special role in regulating low temperature stress. The protein interaction analysis suggested divergent mechanisms might exist between Arabidopsis and grapevine. Our results will facilitate the future functional characterization of individual VvCBLs and VvCIPKs.

Calcineurin B-like Protein CBL10 Directly Interacts with TOC34 (Translocon of the Outer Membrane of the Chloroplasts) and Decreases Its GTPase Activity in Arabidopsis

As calcium sensor relays in plants, calcineurin B-like (CBL) proteins provide an important contribution to decoding Ca2+ signatures elicited by a variety of abiotic stresses. Currently, it is well known that CBLs perceive and transmit the Ca2+ signals mainly to a group of serine/threonine protein kinases called CBL-interacting protein kinases (CIPKs). In this study, we report that the CBL10 member of this family has a novel interaction partner besides the CIPK proteins. Yeast two-hybrid screening with CBL10 as bait identified an Arabidopsis cDNA clone encoding a TOC34 protein, which is a member of the TOC (Translocon of the Outer membrane of the Chloroplasts) complex and possesses the GTPase activity. Further analyses showed that in addition to CBL10, CBL7 also interacts with TOC34 at much lower strength in the yeast two-hybrid system. However, the rest of the CBL family members failed to interact with TOC34. Bimolecular fluorescence complementation (BiFC) analysis verified that the CBL10-TOC34 interaction occurs at the outer membrane of chloroplasts in vivo. In addition, we also demonstrated that CBL10 physically associates with TOC34 in vitro, resulting in a significant decrease in the GTPase activity of the TOC34 protein. Taken together, our findings clearly indicate that a member of the CBL family, CBL10, can modulate not only the CIPK members but also TOC34, allowing the CBL family to relay the Ca2+ signals in more diverse ways than currently known.

Pseudomonas syringae type III effector HopAF1 suppresses plant immunity by targeting methionine recycling to block ethylene induction

HopAF1 is a type III effector protein of unknown function encoded in the genomes of several strains of Pseudomonas syringae and other plant pathogens. Structural modeling predicted that HopAF1 is closely related to deamidase proteins. Deamidation is the irreversible substitution of an amide group with a carboxylate group. Several bacterial virulence factors are deamidases that manipulate the activity of specific host protein substrates. We identified Arabidopsis methylthioadenosine nucleosidase proteins MTN1 and MTN2 as putative targets of HopAF1 deamidation. MTNs are enzymes in the Yang cycle, which is essential for the high levels of ethylene biosynthesis in Arabidopsis We hypothesized that HopAF1 inhibits the host defense response by manipulating MTN activity and consequently ethylene levels. We determined that bacterially delivered HopAF1 inhibits ethylene biosynthesis induced by pathogen-associated molecular patterns and that Arabidopsis mtn1 mtn2 mutant plants phenocopy the effect of HopAF1. Furthermore, we identified two conserved asparagines in MTN1 and MTN2 from Arabidopsis that confer loss of function phenotypes when deamidated via site-specific mutation. These residues are potential targets of HopAF1 deamidation. HopAF1-mediated manipulation of Yang cycle MTN proteins is likely an evolutionarily conserved mechanism whereby HopAF1 orthologs from multiple plant pathogens contribute to disease in a large variety of plant hosts.

Arabidopsis CALCINEURIN B-LIKE10 Functions Independently of the SOS Pathway during Reproductive Development in Saline Conditions

The accumulation of sodium in soil (saline conditions) negatively affects plant growth and development. The Salt Overly Sensitive (SOS) pathway in Arabidopsis (Arabidopsis thaliana) functions to remove sodium from the cytosol during vegetative development preventing its accumulation to toxic levels. In this pathway, the SOS3 and CALCINEURIN B-LIKE10 (CBL10) calcium sensors interact with the SOS2 protein kinase to activate sodium/proton exchange at the plasma membrane (SOS1) or vacuolar membrane. To determine if the same pathway functions during reproductive development in response to salt, fertility was analyzed in wild type and the SOS pathway mutants grown in saline conditions. In response to salt, CBL10 functions early in reproductive development before fertilization, while SOS1 functions mostly after fertilization when seed development begins. Neither SOS2 nor SOS3 function in reproductive development in response to salt. Loss of CBL10 function resulted in reduced anther dehiscence, shortened stamen filaments, and aborted pollen development. In addition, cbl10 mutant pistils could not sustain the growth of wild-type pollen tubes. These results suggest that CBL10 is critical for reproductive development in the presence of salt and that it functions in different pathways during vegetative and reproductive development.

The CBL-CIPK signaling module in plants: a mechanistic perspective

In a given environment, plants are constantly exposed to multitudes of stimuli. These stimuli are sensed and transduced to generate a diverse array of responses by several signal transduction pathways. Calcium (Ca²⁺ ) signaling is one such important pathway involved in transducing a large number of stimuli or signals in both animals and plants. Ca²⁺ engages a plethora of decoders to mediate signaling in plants. Among these groups of decoders, the sensor responder complex of calcineurin B-like protein (CBL) and CBL-interacting protein kinases (CIPKs) play a very significant role in transducing these signals. The signal transduction mechanism in most cases is phosphorylation events, but some structural role for the pair has also come to light recently. In this review, we discuss the structural nature of the sensor-responder duo; their mechanism of substrate phosphorylation and also their structural role in modulating targets. Moreover, the mechanism of complex formation and mechanistic role of protein phosphatases with CBL-CIPK module has been mentioned. A comparison of CBL-CIPK with other decoders of Ca²⁺ signaling in plants also signifies the relatedness and diversity in signaling pathways. Further an attempt has been made to compare this aspect of Ca²⁺ signaling pathways in different plant species to develop a holistic understanding of conservation of stimulus-response-coupling mediated by this Ca²⁺ -CBL-CIPK module.

Calcineurin B-like 3 calcium sensor associates with and inhibits 5'-methylthioadenosine nucleosidase 2 in Arabidopsis

Calcineurin B-like (CBL) proteins constitute a unique family of calcium sensor relays in plants. It is well known that CBLs detect the calcium signals elicited by a variety of abiotic stresses and relay the information to a group of serine/threonine protein kinases called CBL-interacting protein kinases (CIPKs). In this study, we found that a few CBL members can also target another group of enzymes 5'-methylthioadenosine nucleosidases (MTANs), which are encoded by two genes in Arabidopsis, AtMTAN1 and AtMTAN2. In the yeast two-hybrid system, AtMTAN1 interacted with multiple CBL members such as CBL2, CBL3 and CBL6, whereas AtMTAN2 associated exclusively with CBL3. We further demonstrated that the CBL3-AtMTAN2 association occurs in a calcium-dependent manner, which results in a significant decrease in the enzyme activity of the AtMTAN2 protein. Taken together, these results clearly indicate that the CBL family can target at least two distinct groups of enzymes (CIPKs and MTANs), conferring an additional level of complexity on the CBL-mediated signaling networks. In addition, our finding also provides a novel molecular mechanism by which calcium signals are transduced to alter metabolite profiles in plants.

Molecular cloning and expression analysis of the MTN gene during adventitious root development in IBA-induced tetraploid black locust

5'-Methylthioadenosine (MTA) nucleosidase (MTN) plays a key role in the methionine (Met) recycling pathway of plants. Here, we report the isolation of the 1158 bp full-length, cDNA sequence encoding tetraploid black locust (Robinia pseudoacacia L.) MTN (TrbMTN), which contains an open reading frame of 810 bp that encodes a 269 amino acid protein. The amino acid sequence of TrbMTN has more than 88% sequence identity to the MTNs from other plants, with a closer phylogenetic relationship to MTNs from legumes than to MTNs from other plants. Subcellular localization analysis revealed that the TrbMTN gene localizes mainly to the cell membrane and cytoplasm of onion epidermal cells. Indole-3-butyric acid (IBA)-treated cuttings showed higher TrbMTN transcript levels than untreated control cuttings during root primordium and adventitious root formation. TrbMTN and key Met cycle genes showed differential expression in shoots, leaves, stems, and roots, with the highest expression observed in stems. IBA-treated cuttings also showed higher TrbMTN activity than control cuttings during root primordium and adventitious root formation. These results indicate that TrbMTN gene might play an important role in the regulation of IBA-induced adventitious root development in tetraploid black locust cuttings.

Methionine salvage and S-adenosylmethionine: essential links between sulfur, ethylene and polyamine biosynthesis

Both Met (methionine) and SAM (S-adenosylmethionine), the activated form of Met, participate in a number of essential metabolic pathways in plants. The subcellular compartmentalization of Met fluxes will be discussed in the present review with respect to regulation and communication with the sulfur assimilation pathway, the network of the aspartate-derived amino acids and the demand for production of SAM. SAM enters the ethylene, nicotianamine and polyamine biosynthetic pathways and provides the methyl group for the majority of methylation reactions required for plant growth and development. The multiple essential roles of SAM require regulation of its synthesis, recycling and distribution to sustain these different pathways. A particular focus of the present review will be on the function of recently identified genes of the Met salvage cycle or Yang cycle and the importance of the Met salvage cycle in the metabolism of MTA (5'-methylthioadenosine). MTA has the potential for product inhibition of ethylene, nicotianamine and polyamine biosynthesis which provides an additional link between these pathways. Interestingly, regulation of Met cycle genes was found to differ between plant species as shown for Arabidopsis thaliana and Oryza sativa.

The Salt Overly Sensitive (SOS) pathway: established and emerging roles

Soil salinity is a growing problem around the world with special relevance in farmlands. The ability to sense and respond to environmental stimuli is among the most fundamental processes that enable plants to survive. At the cellular level, the Salt Overly Sensitive (SOS) signaling pathway that comprises SOS3, SOS2, and SOS1 has been proposed to mediate cellular signaling under salt stress, to maintain ion homeostasis. Less well known is how cellularly heterogenous organs couple the salt signals to homeostasis maintenance of different types of cells and to appropriate growth of the entire organ and plant. Recent evidence strongly indicates that different regulatory mechanisms are adopted by roots and shoots in response to salt stress. Several reports have stated that, in roots, the SOS proteins may have novel roles in addition to their functions in sodium homeostasis. SOS3 plays a critical role in plastic development of lateral roots through modulation of auxin gradients and maxima in roots under mild salt conditions. The SOS proteins also play a role in the dynamics of cytoskeleton under stress. These results imply a high complexity of the regulatory networks involved in plant response to salinity. This review focuses on the emerging complexity of the SOS signaling and SOS protein functions, and highlights recent understanding on how the SOS proteins contribute to different responses to salt stress besides ion homeostasis.

An Arabidopsis cell growth defect factor-related protein, CRS, promotes plant senescence by increasing the production of hydrogen peroxide

Arabidopsis thaliana Cell Growth Defect factor 1 (Cdf1) has been implicated in promotion of proapoptotic Bax-like cell death via the induction of reactive oxygen species (ROS). Here we report a conserved function of a chloroplast-targeting Cdf-related gene Responsive to Senescence (CRS) using CRS overexpression and loss of function in plants as well as CRS heterologous expression in yeast. CRS expression was strongly induced in senescent leaves, suggesting its main functions during plant senescence. CRS expression in yeast mitochondria increased the ROS level and led to cell death in a manner similar to Cdf1. In whole plants, overexpression of CRS caused the loss of chlorophylls (Chls) and the rapid onset of leaf senescence, while the lack of CRS led to the delay of leaf senescence in a loss-of-function mutant, crs. The higher and lower accumulation of H(2)O(2) was correlated with early and late senescence in CRS-overexpressing and crs mutant plants, respectively. Furthermore, expression of senescence-related marker genes and metacaspase genes was induced in CRS-overexpressing plants in response to dark. Our findings suggest that CRS plays a key role in the leaf senescence process that accompanies H(2)O(2) accumulation resulting in cell death promotion.

A protein kinase, calcineurin B-like protein-interacting protein Kinase9, interacts with calcium sensor calcineurin B-like Protein3 and regulates potassium homeostasis under low-potassium stress in Arabidopsis

Potassium (K⁺) is an essential macronutrient for plant growth and development. Previous studies have demonstrated that Calcineurin B-Like Protein1 (CBL1) or CBL9 and CBL-Interacting Protein Kinase23 (CIPK23) regulate K⁺ uptake in Arabidopsis (Arabidopsis thaliana) roots by modulating K⁺ channel Arabidopsis K⁺ Transporter1. In this study, we show that the protein kinase CIPK9 interacts with the calcium sensor CBL3 and plays crucial roles in K⁺ homeostasis under low-K⁺ stress in Arabidopsis. Arabidopsis wild-type plants showed leaf chlorotic symptoms when grown for 10 d on low-K⁺ (100 μM) medium. Here, we show that plants lacking CIPK9 displayed a tolerant phenotype to low-K⁺ stress, which still maintained green leaves when the wild-type plants showed typical K⁺-deficient symptoms. Overexpressing lines of CIPK9 resulted in a low-K⁺-sensitive phenotype compared with wild-type plants. Furthermore, CBL2 and CBL3 were identified as upstream regulators of CIPK9. Both CBL2- and CBL3-overexpressing lines displayed similar low-K⁺-sensitive phenotypes and K⁺ contents to CIPK9-overexpressing lines. However, only cbl3 mutant plants, but not cbl2 mutant plants, showed the low-K⁺-tolerant phenotype similar to cipk9 mutants. Taken together, these results demonstrate that CIPK9 and CBL3 work together and function in K⁺ homeostasis under low-K⁺ stress in Arabidopsis.

CBSXs are sensor relay proteins sensing adenosine-containing ligands in Arabidopsis

We recently determined that CBSX proteins, which have only one pair of cystathionine β-synthase (CBS) domains, directly regulate the activation of thioredoxins and thereby control cellular H2O2 levels and modulate both plant development and growth. The Arabidopsis genome contains six CBSXs, and these are localized to different subcellular compartments‑ CBSX1 and CBSX2 in the chloroplast, CBSX3 in the mitochondria, CBSX4 in the cytosol, and CBSX5 and CBSX6 in the endoplasmic reticulum. The CBSXs have been identified in prokaryotes and plants, but not in animals. The considerable differences in length and amino acid sequence between CBSX members may result in variations in protein structure and in their specificity to interact with ligands and/or target proteins. Here, we discuss the possibility that the CBSXs are novel sensor relay proteins that use adenosine-containing molecules as a ligand.

Ca(2+) signals: the versatile decoders of environmental cues

Plants are often subjected to various environmental stresses that lead to deleterious effects on growth, production, sustainability, etc. The information of the incoming stress is read by the plants through the mechanism of signal transduction. The plant Ca(2+) serves as secondary messenger during adaptations to stressful conditions and developmental processes. A plethora of Ca(2+) sensors and decoders functions to bring about these changes. The cellular concentrations of Ca(2+), their subcellular localization, and the specific interaction affinities of Ca(2+) decoder proteins all work together to make this process a complex but synchronized signaling network. In this review, we focus on the versatility of these sensors and decoders in the model plant Arabidopsis as well as plants of economical importance. Here, we have also thrown light on the possible mechanism of action of these important components.

Recycling of methylthioadenosine is essential for normal vascular development and reproduction in Arabidopsis

5'-Methylthioadenosine (MTA) is the common by-product of polyamine (PA), nicotianamine (NA), and ethylene biosynthesis in Arabidopsis (Arabidopsis thaliana). The methylthiol moiety of MTA is salvaged by 5'-methylthioadenosine nucleosidase (MTN) in a reaction producing methylthioribose (MTR) and adenine. The MTN double mutant, mtn1-1mtn2-1, retains approximately 14% of the MTN enzyme activity present in the wild type and displays a pleiotropic phenotype that includes altered vasculature and impaired fertility. These abnormal traits were associated with increased MTA levels, altered PA profiles, and reduced NA content. Exogenous feeding of PAs partially recovered fertility, whereas NA supplementation improved fertility and also reversed interveinal chlorosis. The analysis of PA synthase crystal structures containing bound MTA suggests that the corresponding enzyme activities are sensitive to available MTA. Mutant plants that expressed either MTN or human methylthioadenosine phosphorylase (which metabolizes MTA without producing MTR) appeared wild type, proving that the abnormal traits of the mutant are due to MTA accumulation rather than reduced MTR. Based on our results, we propose that the key targets affected by increased MTA content are thermospermine synthase activity and spermidine-dependent posttranslational modification of eukaryotic initiation factor 5A.

Cell-cycle control as a target for calcium, hormonal and developmental signals: the role of phosphorylation in the retinoblastoma-centred pathway

BACKGROUND

During the life cycle of plants, both embryogenic and post-embryogenic growth are essentially based on cell division and cell expansion that are under the control of inherited developmental programmes modified by hormonal and environmental stimuli. Considering either stimulation or inhibition of plant growth, the key role of plant hormones in the modification of cell division activities or in the initiation of differentiation is well supported by experimental data. At the same time there is only limited insight into the molecular events that provide linkage between the regulation of cell-cycle progression and hormonal and developmental control. Studies indicate that there are several alternative ways by which hormonal signalling networks can influence cell division parameters and establish functional links between regulatory pathways of cell-cycle progression and genes and protein complexes involved in organ development.

SCOPE

An overview is given here of key components in plant cell division control as acceptors of hormonal and developmental signals during organ formation and growth. Selected examples are presented to highlight the potential role of Ca(2+)-signalling, the complex actions of auxin and cytokinins, regulation by transcription factors and alteration of retinoblastoma-related proteins by phosphorylation.

CONCLUSIONS

Auxins and abscisic acid can directly influence expression of cyclin, cyclin-dependent kinase (CDK) genes and activities of CDK complexes. D-type cyclins are primary targets for cytokinins and over-expression of CyclinD3;1 can enhance auxin responses in roots. A set of auxin-activated genes (AXR1-ARGOS-ANT) controls cell number and organ size through modification of CyclinD3;1 gene expression. The SHORT ROOT (SHR) and SCARECROW (SCR) transcriptional factors determine root patterning by activation of the CYCD6;1 gene. Over-expression of the EBP1 gene (plant homologue of the ErbB-3 epidermal growth factor receptor-binding protein) increased biomass by auxin-dependent activation of both D- and B-type cyclins. The direct involvement of auxin-binding protein (ABP1) in the entry into the cell cycle and the regulation of leaf size and morphology is based on the transcriptional control of D-cyclins and retinoblastoma-related protein (RBR) interacting with inhibitory E2FC transcriptional factor. The central role of RBRs in cell-cycle progression is well documented by a variety of experimental approaches. Their function is phosphorylation-dependent and both RBR and phospho-RBR proteins are present in interphase and mitotic phase cells. Immunolocalization studies showed the presence of phospho-RBR protein in spots of interphase nuclei or granules in mitotic prophase cells. The Ca(2+)-dependent phosphorylation events can be accomplished by the calcium-dependent, calmodulin-independent or calmodulin-like domain protein kinases (CDPKs/CPKs) phosphorylating the CDK inhibitor protein (KRP). Dephosphorylation of the phospho-RBR protein by PP2A phosphatase is regulated by a Ca(2+)-binding subunit.

Inhibition of 5'-methylthioadenosine metabolism in the Yang cycle alters polyamine levels, and impairs seedling growth and reproduction in Arabidopsis

The methionine or Yang cycle recycles Met from 5'-methylthioadenosine (MTA) which is produced from S-adenosyl-L-methionine (SAM) as a by-product of ethylene, polyamines, and nicotianamine (NA) synthesis. MTA nucleosidase is encoded by two genes in Arabidopsis thaliana, MTN1 and MTN2. Analysis of T-DNA insertion mutants and of wt revealed that MTN1 provides approximately 80% of the total MTN activity. Severe knock down of MTN enzyme activity in the mtn1-1 and mtn1-2 allelic lines resulted in accumulation of SAM/dSAM (decarboxylated SAM) and of MTA in seedlings grown on MTA as sulfur source. While ethylene and NA synthesis were not altered in mtn1-1 and mtn1-2 seedlings grown on MTA, putrescine and spermine were elevated. By contrast, mtn2-1 and mtn2-2 seedlings with near wt enzyme activity had wt levels of SAM/dSAM, MTA, and polyamines. In addition to the metabolic phenotypes, mtn1-1 and mtn1-2 seedlings were growth retarded, while seedlings of wt, mtn2-1, and mtn2-2 showed normal growth on 500 microm MTA. The double knock down mutant mtn1-1/mtn2-1 was sterile. In conclusion, the data presented identify MTA as a crucial metabolite that acts as a regulatory link between the Yang cycle and polyamine biosynthesis and identifies MTA nucleosidase as a crucial enzyme of the Yang cycle.

Breaking the code: Ca2+ sensors in plant signalling

Ca2+ ions play a vital role as second messengers in plant cells during various developmental processes and in response to environmental stimuli. Plants have evolved a diversity of unique proteins that bind Ca2+ using the evolutionarily conserved EF-hand motif. The currently held hypothesis is that these proteins function as Ca2+ sensors by undergoing conformational changes in response to Ca2+-binding that facilitate their regulation of target proteins and thereby co-ordinate various signalling pathways. The three main classes of these EF-hand Ca2+sensors in plants are CaMs [calmodulins; including CMLs (CaM-like proteins)], CDPKs (calcium-dependent protein kinases) and CBLs (calcineurin B-like proteins). In the plant species examined to date, each of these classes is represented by a large family of proteins, most of which have not been characterized biochemically and whose physiological roles remain unclear. In the present review, we discuss recent advances in research on CaMs and CMLs, CDPKs and CBLs, and we attempt to integrate the current knowledge on the different sensor classes into common physiological themes.