Cyclic adenosine monophosphate regulates calcium channels in the plasma membrane of Arabidopsis leaf guard and mesophyll cells

Nyctinastic movement in legumes: Developmental mechanisms, factors and biological significance

In legumes, a common phenomenon known as nyctinastic movement is observed. This movement involves the horizontal expansion of leaves during the day and relative vertical closure at night. Nyctinastic movement is driven by the pulvinus, which consists of flexor and extensor motor cells. The turgor pressure difference between these two cell types generates a driving force for the bending and deformation of the pulvinus. This review focuses on the developmental mechanisms of the pulvinus, the factors affecting nyctinastic movement, and the biological significance of this phenomenon in legumes, thus providing a reference for further research on nyctinastic movement.

Twin cyclic mononucleotide cyclase and phosphodiesterase domain architecture as a common feature in complex plant proteins

The majority of proteins in both prokaryote and eukaryote proteomes consist of two or more functional centers, which allows for intramolecular tuning of protein functions. Such architecture, as opposed to animal orthologs, applies to the plant cyclases (CNC) and phosphodiesterases (PDEs), the vast majority of which are part of larger multifunctional proteins. In plants, until recently, only two cases of combinations of CNC-PDE in one protein were reported. Here we propose that in plants, multifunctional proteins in which the PDE motif has been identified, the presence of the additional CNC center is common. Searching the Arabidopsis thaliana proteome with a combined PDE-CNC motif allowed the creation of a database of proteins with both activities. One such example is methylenetetrahydrofolate dehydrogenase, in which we determined the activities of adenylate cyclase (AC) and PDE. Based on biochemical and mutagenesis analyses we assessed the impact of the AC and PDE catalytic centers on the dehydrogenase activity. This allowed us to propose additional regulatory mechanism that govern folate metabolism by cAMP. It is therefore conceivable that the combined CNC-PDE architecture is a common regulatory configuration, where control of the level of cyclic nucleotides (cNMP) influences other catalytic activities of the protein.

Two triphosphate tunnel metalloenzymes from apple exhibit adenylyl cyclase activity

Adenylyl cyclase (AC) is the key catalytic enzyme for the synthesis of 3',5'-cyclic adenosine monophosphate. Various ACs have been identified in microorganisms and mammals, but studies on plant ACs are still limited. No AC in woody plants has been reported until now. Based on the information on HpAC1, three enzymes were screened out from the woody fruit tree apple, and two of them (MdTTM1 and MdTTM2) were verified and confirmed to display AC activity. Interestingly, in the apple genome, these two genes were annotated as triphosphate tunnel metalloenzymes (TTMs) which were widely found in three superkingdoms of life with multiple substrate specificities and enzymatic activities, especially triphosphate hydrolase. In addition, the predicted structures of these two proteins were parallel, especially of the catalytic tunnel, including conserved domains, motifs, and folded structures. Their tertiary structures exhibited classic TTM properties, like the characteristic EXEXK motif and β-stranded anti-parallel tunnel capable of coordinating divalent cations. Moreover, MdTTM2 and HpAC1 displayed powerful hydrolase activity to triphosphate and restricted AC activity. All of these findings showed that MdTTMs had hydrolysis and AC activity, which could provide new solid evidence for AC distribution in woody plants as well as insights into the relationship between ACs and TTMs.

CYCLIC NUCLEOTIDE-GATED ION CHANNEL 2 modulates auxin homeostasis and signaling

Cyclic nucleotide-gated ion channels (CNGCs) have been firmly established as Ca2+-conducting ion channels that regulate a wide variety of physiological responses in plants. CNGC2 has been implicated in plant immunity and Ca2+ signaling due to the autoimmune phenotypes exhibited by null mutants of CNGC2 in Arabidopsis thaliana. However, cngc2 mutants display additional phenotypes that are unique among autoimmune mutants, suggesting that CNGC2 has functions beyond defense and generates distinct Ca2+ signals in response to different triggers. In this study, we found that cngc2 mutants showed reduced gravitropism, consistent with a defect in auxin signaling. This was mirrored in the diminished auxin response detected by the auxin reporters DR5::GUS and DII-VENUS and in a strongly impaired auxin-induced Ca2+ response. Moreover, the cngc2 mutant exhibits higher levels of the endogenous auxin indole-3-acetic acid, indicating that excess auxin in the cngc2 mutant causes its pleiotropic phenotypes. These auxin signaling defects and the autoimmunity syndrome of the cngc2 mutant could be suppressed by loss-of-function mutations in the auxin biosynthesis gene YUCCA6 (YUC6), as determined by identification of the cngc2 suppressor mutant repressor of cngc2 (rdd1) as an allele of YUC6. A loss-of-function mutation in the upstream auxin biosynthesis gene TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA1, WEAK ETHYLENE INSENSITIVE8) also suppressed the cngc2 phenotypes, further supporting the tight relationship between CNGC2 and the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS-YUCCA -dependent auxin biosynthesis pathway. Taking these results together, we propose that the Ca2+ signal generated by CNGC2 is a part of the negative feedback regulation of auxin homeostasis in which CNGC2 balances cellular auxin perception by influencing auxin biosynthesis.

Functional Crypto-Adenylate Cyclases Operate in Complex Plant Proteins

Adenylyl cyclases (ACs) and their catalytic product cAMP are regulatory components of many plant responses. Here, we show that an amino acid search motif based on annotated adenylate cyclases (ACs) identifies 12 unique Arabidopsis thaliana candidate ACs, four of which have a role in the biosynthesis of the stress hormone abscisic acid (ABA). One of these, the 9-cis-epoxycarotenoid dioxygenase (NCED3 and At3g14440), was identified by sequence and structural analysis as a putative AC and then tested experimentally with two different methods. Given that the in vitro activity is low (fmoles cAMP pmol-1 protein min-1), but highly reproducible, we term the enzyme a crypto-AC. Our results are consistent with a role for ACs with low activities in multi-domain moonlighting proteins that have at least one other distinct molecular function, such as catalysis or ion channel activation. We propose that crypto-ACs be examined from the perspective that considers their low activities as an innate feature of regulatory ACs embedded within multi-domain moonlighting proteins. It is therefore conceivable that crypto-ACs form integral components of complex plant proteins participating in intra-molecular regulatory mechanisms, and in this case, potentially linking cAMP to ABA synthesis.

Calcium Signaling in Plant Programmed Cell Death

Programmed cell death (PCD) is a process intended for the maintenance of cellular homeostasis by eliminating old, damaged, or unwanted cells. In plants, PCD takes place during developmental processes and in response to biotic and abiotic stresses. In contrast to the field of animal studies, PCD is not well understood in plants. Calcium (Ca²⁺) is a universal cell signaling entity and regulates numerous physiological activities across all the kingdoms of life. The cytosolic increase in Ca²⁺ is a prerequisite for the induction of PCD in plants. Although over the past years, we have witnessed significant progress in understanding the role of Ca²⁺ in the regulation of PCD, it is still unclear how the upstream stress perception leads to the Ca²⁺ elevation and how the signal is further propagated to result in the onset of PCD. In this review article, we discuss recent advancements in the field, and compare the role of Ca²⁺ signaling in PCD in biotic and abiotic stresses. Moreover, we discuss the upstream and downstream components of Ca²⁺ signaling and its crosstalk with other signaling pathways in PCD. The review is expected to provide new insights into the role of Ca²⁺ signaling in PCD and to identify gaps for future research efforts.

Cross Talk Between Cyclic Nucleotides and Calcium Signaling Pathways in Plants-Achievements and Prospects

A variety of plant cellular activities are regulated through mechanisms controlling the level of signal molecules, such as cyclic nucleotides (cNMPs, e.g., cyclic adenosine 3':5'-monophosphate, cAMP, and cyclic guanosine 3':5'- monophosphate, cGMP) and calcium ions (Ca2⁺). The mechanism regulating cNMP levels affects their synthesis, degradation, efflux and cellular distribution. Many transporters and the spatiotemporal pattern of calcium signals, which are transduced by multiple, tunable and often strategically positioned Ca2⁺-sensing elements, play roles in calcium homeostasis. Earlier studies have demonstrated that while cNMPs and Ca2⁺ can act separately in independent transduction pathways, they can interact and function together. Regardless of the context, the balance between Ca2⁺ and cNMP is the most important consideration. This balance seems to be crucial for effectors, such as phosphodiesterases, cyclic nucleotide gated channels and cyclase activity. Currently, a wide range of molecular biology techniques enable thorough analyses of cellular cross talk. In recent years, data have indicated relationships between calcium ions and cyclic nucleotides in mechanisms regulating specific signaling pathways. The purpose of this study is to summarize the current knowledge on nucleotide-calcium cross talk in plants.

A new adenylyl cyclase, putative disease-resistance RPP13-like protein 3, participates in abscisic acid-mediated resistance to heat stress in maize

In plants, 3´,5´-cyclic adenosine monophosphate (cAMP) is an important second messenger with varied functions; however, only a few adenylyl cyclases (ACs) that synthesize cAMP have been identified. Moreover, the biological roles of ACs/cAMP in response to stress remain largely unclear. In this study, we used quantitative proteomics techniques to identify a maize heat-induced putative disease-resistance RPP13-like protein 3 (ZmRPP13-LK3), which has three conserved catalytic AC centres. The AC activity of ZmRPP13-LK3 was confirmed by in vitro enzyme activity analysis, in vivo RNAi experiments, and functional complementation in the E. coli cyaA mutant. ZmRPP13-LK3 is located in the mitochondria. The results of in vitro and in vivo experiments indicated that ZmRPP13-LK3 interacts with ZmABC2, a possible cAMP exporter. Under heat stress, the concentrations of ZmRPP13-LK3 and cAMP in the ABA-deficient mutant vp5 were significantly less than those in the wild-type, and treatment with ABA and an ABA inhibitor affected ZmRPP13-LK3 expression in the wild-type. Application of 8-Br-cAMP, a cAMP analogue, increased heat-induced expression of heat-shock proteins in wild-type plants and alleviated heat-activated oxidative stress. Taken together, our results indicate that ZmRPP13-LK3, a new AC, can catalyse ATP for the production of cAMP and may be involved in ABA-regulated heat resistance.

Moonlighting Proteins Shine New Light on Molecular Signaling Niches

Plants as sessile organisms face daily environmental challenges and have developed highly nuanced signaling systems to enable suitable growth, development, defense, or stalling responses. Moonlighting proteins have multiple tasks and contribute to cellular signaling cascades where they produce additional variables adding to the complexity or fuzziness of biological systems. Here we examine roles of moonlighting kinases that also generate 3',5'-cyclic guanosine monophosphate (cGMP) in plants. These proteins include receptor like kinases and lipid kinases. Their guanylate cyclase activity potentiates the development of localized cGMP-enriched nanodomains or niches surrounding the kinase and its interactome. These nanodomains contribute to allosteric regulation of kinase and other molecules in the immediate complex directly or indirectly modulating signal cascades. Effects include downregulation of kinase activity, modulation of other members of the protein complexes such as cyclic nucleotide gated channels and potential triggering of cGMP-dependent degradation cascades terminating signaling. The additional layers of information provided by the moonlighting kinases are discussed in terms of how they may be used to provide a layer of fuzziness to effectively modulate cellular signaling cascades.

A tandem motif-based and structural approach can identify hidden functional phosphodiesterases

Cyclic nucleotide monophosphates (cNMPs) are increasingly recognized as essential signaling molecules governing many physiological and developmental processes in prokaryotes and eukaryotes. Degradation of cNMPs is as important as their generation because it offers the capability for transient and dynamic cellular level regulation but unlike their generating enzymes, the degrading enzymes, cyclic nucleotide phosphodiesterases (PDEs) are somewhat elusive in higher plants. Based on sequence analysis and structural properties of canonical PDE catalytic centers, we have developed a consensus sequence search motif and used it to identify candidate PDEs. One of these is an Arabidopsis thaliana K⁺-Uptake Permease (AtKUP5). Structural and molecular docking analysis revealed that the identified PDE domain occupies the C-terminal of this protein forming a solvent-exposed distinctive pocket that can spatially accommodate the cyclic adenosine monophosphate (cAMP) substrate and importantly, cAMP assumes a binding pose that is favorable for interactions with the key amino acids in the consensus motif. PDE activity was confirmed by the sensitive liquid chromatography tandem mass spectrometry (LC-MS/MS) method. Notably, this activity was stimulated by the Ca²⁺/CaM complex, the binding of which to the PDE center was confirmed by surface plasmon resonance (SPR). Since AtKUP5 also has adenylate cyclase (AC) activity that is essential for K⁺ transport, we propose that this dual moonlighting AC-PDE architecture, offers modulatory roles that afford intricate intramolecular regulation of cAMP levels thereby enabling fine-tuning of cAMP signaling in K⁺ homeostasis.

The Complex Story of Plant Cyclic Nucleotide-Gated Channels

Plant cyclic nucleotide-gated channels (CNGCs) are tetrameric cation channels which may be activated by the cyclic nucleotides (cNMPs) adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP). The genome of Arabidopsis thaliana encodes 20 CNGC subunits associated with aspects of development, stress response and immunity. Recently, it has been demonstrated that CNGC subunits form heterotetrameric complexes which behave differently from the homotetramers produced by their constituent subunits. These findings have widespread implications for future signalling research and may help explain how specificity can be achieved by CNGCs that are known to act in disparate pathways. Regulation of complex formation may involve cyclic nucleotide-gated channel-like proteins.

Cyclic AMP: A Polyhedral Signalling Molecule in Plants

The cyclic nucleotide cAMP (3',5'-cyclic adenosine monophosphate) is nowadays recognised as an important signalling molecule in plants, involved in many molecular processes, including sensing and response to biotic and abiotic environmental stresses. The validation of a functional cAMP-dependent signalling system in higher plants has spurred a great scientific interest on the polyhedral role of cAMP, as it actively participates in plant adaptation to external stimuli, in addition to the regulation of physiological processes. The complex architecture of cAMP-dependent pathways is far from being fully understood, because the actors of these pathways and their downstream target proteins remain largely unidentified. Recently, a genetic strategy was effectively used to lower cAMP cytosolic levels and hence shed light on the consequences of cAMP deficiency in plant cells. This review aims to provide an integrated overview of the current state of knowledge on cAMP's role in plant growth and response to environmental stress. Current knowledge of the molecular components and the mechanisms of cAMP signalling events is summarised.

Mg2+ Is a Missing Link in Plant Cell Ca2+ Signalling and Homeostasis-A Study on Vicia faba Guard Cells

Hyperpolarization-activated calcium channels (HACCs) are found in the plasma membrane and tonoplast of many plant cell types, where they have an important role in Ca²⁺-dependent signalling. The unusual gating properties of HACCs in plants, i.e., activation by membrane hyperpolarization rather than depolarization, dictates that HACCs are normally open in the physiological hyperpolarized resting membrane potential state (the so-called pump or P-state); thus, if not regulated, they would continuously leak Ca²⁺ into cells. HACCs are permeable to Ca²⁺, Ba²⁺, and Mg²⁺; activated by H₂O₂ and the plant hormone abscisic acid (ABA); and their activity in guard cells is greatly reduced by increasing amounts of free cytosolic Ca²⁺ ([Ca²⁺]_Cyt), and hence closes during [Ca²⁺]_Cyt surges. Here, we demonstrate that the presence of the commonly used Mg-ATP inside the guard cell greatly reduces HACC activity, especially at voltages ≤ −200 mV, and that Mg²⁺ causes this block. Therefore, we firstly conclude that physiological cytosolic Mg²⁺ levels affect HACC gating and that channel opening requires either high negative voltages (≥−200 mV) or displacement of Mg²⁺ away from the immediate vicinity of the channel. Secondly, based on structural comparisons with a Mg²⁺-sensitive animal inward-rectifying K⁺ channel, we propose that the likely candidate HACCs described here are cyclic nucleotide gated channels (CNGCs), many of which also contain a conserved diacidic Mg²⁺ binding motif within their pores. This conclusion is consistent with the electrophysiological data. Finally, we propose that Mg²⁺, much like in animal cells, is an important component in Ca²⁺ signalling and homeostasis in plants.

Coordinated Transport of Nitrate, Potassium, and Sodium

Potassium (K+) and nitrogen (N) are essential nutrients, and their absorption and distribution within the plant must be coordinated for optimal growth and development. Potassium is involved in charge balance of inorganic and organic anions and macromolecules, control of membrane electrical potential, pH homeostasis and the regulation of cell osmotic pressure, whereas nitrogen is an essential component of amino acids, proteins, and nucleic acids. Nitrate (NO3 -) is often the primary nitrogen source, but it also serves as a signaling molecule to the plant. Nitrate regulates root architecture, stimulates shoot growth, delays flowering, regulates abscisic acid-independent stomata opening, and relieves seed dormancy. Plants can sense K+/NO3 - levels in soils and adjust accordingly the uptake and root-to-shoot transport to balance the distribution of these ions between organs. On the other hand, in small amounts sodium (Na+) is categorized as a "beneficial element" for plants, mainly as a "cheap" osmolyte. However, at high concentrations in the soil, Na+ can inhibit various physiological processes impairing plant growth. Hence, plants have developed specific mechanisms to transport, sense, and respond to a variety of Na+ conditions. Sodium is taken up by many K+ transporters, and a large proportion of Na+ ions accumulated in shoots appear to be loaded into the xylem by systems that show nitrate dependence. Thus, an adequate supply of mineral nutrients is paramount to reduce the noxious effects of salts and to sustain crop productivity under salt stress. In this review, we will focus on recent research unraveling the mechanisms that coordinate the K+-NO3 -; Na+-NO3 -, and K+-Na+ transports, and the regulators controlling their uptake and allocation.

Arabidopsis cyclic nucleotide-gated channel 6 is negatively modulated by multiple calmodulin isoforms during heat shock

An increased concentration of cytosolic Ca2+ is an early response of plant cells to heat shock. Arabidopsis cyclic nucleotide-gated ion channel 6 (CNGC6) mediates heat-induced Ca2+ influx and is activated by cAMP. However, it remains unclear how the Ca2+ conductivity of CNGC6 is negatively regulated under the elevated cytosolic Ca2+ concentration. In this study, Arabidopsis calmodulin isoforms CaM1/4, CaM2/3/5, CaM6, and CaM7 were found to bind to CNGC6 to varying degrees, and this binding was dependent on the presence of Ca2+ and IQ6, an atypical isoleucine-glutamine motif in CNGC6. Knockout of CaM2, CaM3, CaM5, and CaM7 genes led to a marked increase in plasma membrane inward Ca2+ current under heat shock conditions; however, knockout of CaM1, CaM4, and CaM6 genes had no significant effect on plasma membrane Ca2+ current. Moreover, the deletion of IQ6 from CNGC6 led to a marked increase in plasma membrane Ca2+ current under heat shock conditions. Taken together, the data suggest that CNGC6-mediated Ca2+ influx is likely to be negatively regulated by CaM2/3/5 and CaM7 isoforms under heat shock conditions, and that IQ6 plays an important role in CaM binding and the feedback regulation of the channel.

Cyclic nucleotide gated channels (CNGCs) in plant signalling-Current knowledge and perspectives

Cell signaling is an evolutionarily conserved mechanism that responds and adapts to various internal and external factors. Generally, a signal is mediated by various signaling molecules and is transferred to a cascade of effector proteins. To date, there is significant evidence that cyclic nucleotides (cNMPs), e.g., adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP), may represent important elements of many signaling pathways in plants. However, in contrast to the impressive progress made in understanding cyclic nucleotide signaling in mammalian hosts, only few studies have investigated this topic in plants. Existing evidence indicates that cNMPs participate in growth and developmental processes, as well as the response to various stresses. Once synthesized by adenylyl or guanylyl cyclases, these signals are transduced by acting through a number of cellular effectors. The regulatory effects of cNMPs in eukaryotes can be mediated via various downstream effector proteins, such as protein kinases, Exchange Protein directly Activated by cAMP (EPAC), and Cyclic Nucleotide-Gated ion Channels (CNGC). These proteins sense changes in intracellular cNMP levels and regulate numerous cellular responses. Moreover, the amplitude of cNMP levels and the duration of its signal in the cell is also governed by phosphodiesterases (PDEs), enzymes that are responsible for the breakdown of cNMPs. Data collected in recent years strongly suggest that cyclic nucleotide gated channels are the main cNMP effectors in plant cells. These channels are important cellular switches that transduce changes in intracellular concentrations of cyclic nucleotides into changes in membrane potential and ion concentrations. Structurally, these channels belong to the superfamily of pore-loop cation channels. In this review, we provide an overview of the molecular properties of CNGC structure, regulation and ion selectivity, and subcellular localization, as well as describing the signal transduction pathways in which these channels are involved. We will also summarize recent insights into the role of CNGC proteins in plant growth, development and response to stressors.

An Arabidopsis thaliana leucine-rich repeat protein harbors an adenylyl cyclase catalytic center and affects responses to pathogens

Adenylyl cyclases (ACs) catalyze the formation of the second messenger cAMP from ATP. Here we report the characterization of an Arabidopsis thaliana leucine-rich repeat (LRR) protein (At3g14460; AtLRRAC1) as an adenylyl cyclase. Using an AC-specific search motif supported by computational assessments of protein models we identify an AC catalytic center within the N-terminus and demonstrate that AtLRRAC1 can generate cAMP in vitro. Knock-out mutants of AtLRRAC1 have compromised immune responses to the biotrophic fungus Golovinomyces orontii and the hemibiotrophic bacteria Pseudomonas syringae, but not against the necrotrophic fungus Botrytis cinerea. These findings are consistent with a role of cAMP-dependent pathways in the defense against biotrophic and hemibiotrophic plant pathogens.

The Arabidopsis thaliana K+-Uptake Permease 5 (AtKUP5) Contains a Functional Cytosolic Adenylate Cyclase Essential for K+ Transport

Potassium (K⁺) is the most abundant cation in plants, and its uptake and transport are key to growth, development and responses to the environment. Here, we report that Arabidopsis thaliana K⁺ uptake permease 5 (AtKUP5) contains an adenylate cyclase (AC) catalytic center embedded in its N-terminal cytosolic domain. The purified recombinant AC domain generates cAMP in vitro; and when expressed in Escherichia coli, increases cAMP levels in vivo. Both the AC domain and full length AtKUP5 rescue an AC-deficient E. coli mutant, cyaA, and together these data provide evidence that AtKUP5 functions as an AC. Furthermore, full length AtKUP5 complements the Saccharomyces cerevisiae K⁺ transport impaired mutant, trk1 trk2, demonstrating its function as a K⁺ transporter. Surprisingly, a point mutation in the AC center that impairs AC activity, also abolishes complementation of trk1 trk2, suggesting that a functional catalytic AC domain is essential for K⁺ uptake. AtKUP5-mediated K⁺ uptake is not affected by cAMP, the catalytic product of the AC, but, interestingly, causes cytosolic cAMP accumulation. These findings are consistent with a role for AtKUP5 as K⁺ flux sensor, where the flux-dependent cAMP increases modulate downstream components essential for K⁺ homeostasis, such as cyclic nucleotide gated channels.

A Resource for Inactivation of MicroRNAs Using Short Tandem Target Mimic Technology in Model and Crop Plants

microRNAs (miRNAs) are endogenous small non-coding RNAs that bind to mRNAs and target them for cleavage and/or translational repression, leading to gene silencing. We previously developed short tandem target mimic (STTM) technology to deactivate endogenous miRNAs in Arabidopsis. Here, we created hundreds of STTMs that target both conserved and species-specific miRNAs in Arabidopsis, tomato, rice, and maize, providing a resource for the functional interrogation of miRNAs. We not only revealed the functions of several miRNAs in plant development, but also demonstrated that tissue-specific inactivation of a few miRNAs in rice leads to an increase in grain size without adversely affecting overall plant growth and development. RNA-seq and small RNA-seq analyses of STTM156/157 and STTM165/166 transgenic plants revealed the roles of these miRNAs in plant hormone biosynthesis and activation, secondary metabolism, and ion-channel activity-associated electrophysiology, demonstrating that STTM technology is an effective approach for studying miRNA functions. To facilitate the study and application of STTM transgenic plants and to provide a useful platform for storing and sharing of information about miRNA-regulated gene networks, we have established an online Genome Browser (https://blossom.ffr.mtu.edu/designindex2.php) to display the transcriptomic and miRNAomic changes in STTM-induced miRNA knockdown plants.

Genome-wide identification and analysis of the CNGC gene family in maize

As one of the non-selective cation channel gene families, the cyclic nucleotide-gated channel (CNGC) gene family plays a vital role in plant physiological processes that are related to signal pathways, plant development, and environmental stresses. However, genome-wide identification and analysis of the CNGC gene family in maize has not yet been undertaken. In the present study, twelve ZmCNGC genes were identified in the maize genome, which were unevenly distributed on chromosomes 1, 2, 4, 5, 6, 7, and 8. They were classified into five major groups: Groups I, II, III, IVa, and IVb. Phylogenetic analysis showed that gramineous plant CNGC genes expanded unequally during evolution. Group IV CNGC genes emerged first, whereas Groups I and II appeared later. Prediction analysis of cis-acting regulatory elements showed that 137 putative cis-elements were related to hormone-response, abiotic stress, and organ development. Furthermore, 120 protein pairs were predicted to interact with the 12 ZmCNGC proteins and other maize proteins. The expression profiles of the ZmCNGC genes were expressed in tissue-specific patterns. These results provide important information that will increase our understanding of the CNGC gene family in maize and other plants.

A Cyclic Nucleotide-Gated Channel, HvCNGC2-3, Is Activated by the Co-Presence of Na⁺ and K⁺ and Permeable to Na⁺ and K⁺ Non-Selectively

Cyclic nucleotide-gated channels (CNGCs) have been postulated to contribute significantly in plant development and stress resistance. However, their electrophysiological properties remain poorly understood. Here, we characterized barley CNGC2-3 (HvCNGC2-3) by the two-electrode voltage-clamp technique in the Xenopus laevis oocyte heterologous expression system. Current was not observed in X. laevis oocytes injected with HvCNGC2-3 complementary RNA (cRNA) in a bathing solution containing either Na⁺ or K⁺ solely, even in the presence of 8-bromoadenosine 3′,5′-cyclic monophosphate (8Br-cAMP) or 8-bromoguanosine 3′,5′-cyclic monophosphate (8Br-cGMP). A weakly voltage-dependent slow hyperpolarization-activated ion current was observed in the co-presence of Na⁺ and K⁺ in the bathing solution and in the presence of 10 µM 8Br-cAMP, but not 8Br-cGMP. Permeability ratios of HvCNGC2-3 to K⁺, Na⁺ and Cl⁻ were determined as 1:0.63:0.03 according to reversal-potential analyses. Amino-acid replacement of the unique ion-selective motif of HvCNGC2-3, AQGL, with the canonical motif, GQGL, resulted in the abolition of the current. This study reports a unique two-ion-dependent activation characteristic of the barley CNGC, HvCNGC2-3.

An Arabidopsis Clathrin Assembly Protein with a Predicted Role in Plant Defense Can Function as an Adenylate Cyclase

Adenylate cyclases (ACs), much like guanylate cyclases (GCs), are increasingly recognized as essential parts of many plant processes including biotic and abiotic stress responses. In order to identify novel ACs, we have applied a search motif derived from experimentally tested GCs and identified a number of Arabidopsis thaliana candidates including a clathrin assembly protein (AT1G68110; AtClAP). AtClAP contains a catalytic centre that can complement the AC-deficient mutant cyaA in E. coli, and a recombinant AtClAP fragment (AtClAP^261–379) can produce cyclic adenosine 3′,5′ monophosphate (cAMP) from adenosine triphosphate (ATP) in vitro. Furthermore, an integrated analysis of gene expression and expression correlation implicate cAMP in pathogen defense and in actin cytoskeletal remodeling during endocytic internalization.

Discovery of Novel Functional Centers With Rationally Designed Amino Acid Motifs

Plants are constantly exposed to environmental stresses and in part due to their sessile nature, they have evolved signal perception and adaptive strategies that are distinct from those of other eukaryotes. This is reflected at the cellular level where receptors and signalling molecules cannot be identified using standard homology-based searches querying with proteins from prokaryotes and other eukaryotes. One of the reasons for this is the complex domain architecture of receptor molecules. In order to discover hidden plant signalling molecules, we have developed a motif-based approach designed specifically for the identification of functional centers in plant molecules. This has made possible the discovery of novel components involved in signalling and stimulus-response pathways; the molecules include cyclic nucleotide cyclases, a nitric oxide sensor and a novel target for the hormone abscisic acid. Here, we describe the major steps of the method and illustrate it with recent and experimentally confirmed molecules as examples. We foresee that carefully curated search motifs supported by structural and bioinformatic assessments will uncover many more structural and functional aspects, particularly of signalling molecules.

Plant ion channels and transporters in herbivory-induced signalling

In contrast to many biotic stresses that plants face, feeding by herbivores produces unique mechanical and chemical signatures. Plants have evolved effective systems to recognise these mechanical stimuli and chemical elicitors at the plasma membrane (PM), where this recognition generates ion fluxes, including an influx of Ca2+ that elicits cellular Ca2+ signalling, production of reactive oxygen species (ROS), and variation in transmembrane potential. These signalling events also function in propagation of long-distance signals (Ca2+ waves, ROS waves, and electrical signals), which contribute to rapid, systemic induction of defence responses. Recent studies have identified several candidate channels or transporters that likely produce these ion fluxes at the PM. Here, we describe the important roles of these channels/transporters in transduction or transmission of herbivory-induced early signalling events, long-distance signals, and jasmonic acid and green leaf volatile signalling in plants.

A mechanosensitive Ca2+ channel activity is dependent on the developmental regulator DEK1

Responses of cells to mechanical stress are thought to be critical in coordinating growth and development. Consistent with this idea, mechanically activated channels play important roles in animal development. For example, the PIEZO1 channel controls cell division and epithelial-layer integrity and is necessary for vascular development in mammals. In plants, the actual contribution of mechanoperception to development remains questionable because very few putative mechanosensors have been identified and the phenotypes of the corresponding mutants are rather mild. Here, we show that the Arabidopsis Defective Kernel 1 (DEK1) protein, which is essential for development beyond early embryogenesis, is associated with a mechanically activated Ca2+ current in planta, suggesting that perception of mechanical stress plays a critical role in plant development.

Cyclic Nucleotide Monophosphates and Their Cyclases in Plant Signaling

The cyclic nucleotide monophosphates (cNMPs), and notably 3',5'-cyclic guanosine monophosphate (cGMP) and 3',5'-cyclic adenosine monophosphate (cAMP) are now accepted as key signaling molecules in many processes in plants including growth and differentiation, photosynthesis, and biotic and abiotic defense. At the single molecule level, we are now beginning to understand how cNMPs modify specific target molecules such as cyclic nucleotide-gated channels, while at the systems level, a recent study of the Arabidopsis cNMP interactome has identified novel target molecules with specific cNMP-binding domains. A major advance came with the discovery and characterization of a steadily increasing number of guanylate cyclases (GCs) and adenylate cyclases (ACs). Several of the GCs are receptor kinases and include the brassinosteroid receptor, the phytosulfokine receptor, the Pep receptor, the plant natriuretic peptide receptor as well as a nitric oxide sensor. We foresee that in the near future many more molecular mechanisms and biological roles of GCs and ACs and their catalytic products will be discovered and further establish cNMPs as a key component of plant responses to the environment.

Structured plant metabolomics for the simultaneous exploration of multiple factors

Multiple factors act simultaneously on plants to establish complex interaction networks involving nutrients, elicitors and metabolites. Metabolomics offers a better understanding of complex biological systems, but evaluating the simultaneous impact of different parameters on metabolic pathways that have many components is a challenging task. We therefore developed a novel approach that combines experimental design, untargeted metabolic profiling based on multiple chromatography systems and ionization modes, and multiblock data analysis, facilitating the systematic analysis of metabolic changes in plants caused by different factors acting at the same time. Using this method, target geraniol compounds produced in transgenic tobacco cell cultures were grouped into clusters based on their response to different factors. We hypothesized that our novel approach may provide more robust data for process optimization in plant cell cultures producing any target secondary metabolite, based on the simultaneous exploration of multiple factors rather than varying one factor each time. The suitability of our approach was verified by confirming several previously reported examples of elicitor-metabolite crosstalk. However, unravelling all factor-metabolite networks remains challenging because it requires the identification of all biochemically significant metabolites in the metabolomics dataset.

Changes in the Arabidopsis thaliana Proteome Implicate cAMP in Biotic and Abiotic Stress Responses and Changes in Energy Metabolism

The second messenger 3',5'-cyclic adenosine monophosphate (cAMP) is increasingly recognized as having many different roles in plant responses to environmental stimuli. To gain further insights into these roles, Arabidopsis thaliana cell suspension culture was treated with 100 nM of cell permeant 8-bromo-cAMP for 5 or 10 min. Here, applying mass spectrometry and comparative proteomics, 20 proteins were identified as differentially expressed and we noted a specific bias in proteins with a role in abiotic stress, particularly cold and salinity, biotic stress as well as proteins with a role in glycolysis. These findings suggest that cAMP is sufficient to elicit specific stress responses that may in turn induce complex changes to cellular energy homeostasis.

The arabidopsis cyclic nucleotide interactome

BACKGROUND

Cyclic nucleotides have been shown to play important signaling roles in many physiological processes in plants including photosynthesis and defence. Despite this, little is known about cyclic nucleotide-dependent signaling mechanisms in plants since the downstream target proteins remain unknown. This is largely due to the fact that bioinformatics searches fail to identify plant homologs of protein kinases and phosphodiesterases that are the main targets of cyclic nucleotides in animals.

METHODS

An affinity purification technique was used to identify cyclic nucleotide binding proteins in Arabidopsis thaliana. The identified proteins were subjected to a computational analysis that included a sequence, transcriptional co-expression and functional annotation analysis in order to assess their potential role in plant cyclic nucleotide signaling.

RESULTS

A total of twelve cyclic nucleotide binding proteins were identified experimentally including key enzymes in the Calvin cycle and photorespiration pathway. Importantly, eight of the twelve proteins were shown to contain putative cyclic nucleotide binding domains. Moreover, the identified proteins are post-translationally modified by nitric oxide, transcriptionally co-expressed and annotated to function in hydrogen peroxide signaling and the defence response. The activity of one of these proteins, GLYGOLATE OXIDASE 1, a photorespiratory enzyme that produces hydrogen peroxide in response to Pseudomonas, was shown to be repressed by a combination of cGMP and nitric oxide treatment.

CONCLUSIONS

We propose that the identified proteins function together as points of cross-talk between cyclic nucleotide, nitric oxide and reactive oxygen species signaling during the defence response.

Cyclic AMP deficiency negatively affects cell growth and enhances stress-related responses in tobacco Bright Yellow-2 cells

Cyclic adenosine 3',5'-monophosphate (cAMP) is a recognized second messenger; however, knowledge of cAMP involvement in plant physiological processes originates primarily from pharmacological studies. To obtain direct evidence for cAMP function in plants, tobacco Bright Yellow-2 (BY-2) cells were transformed with the cAMP sponge, which is a genetically encoded tool that reduces cAMP availability. BY-2 cells expressing the cAMP sponge (cAS cells), showed low levels of free cAMP and exhibited growth inhibition that was not proportional to the cAMP sponge transcript level. Growth inhibition in cAS cells was closely related to the precocious inhibition of mitosis due to a delay in cell cycle progression. The cAMP deficiency also enhanced antioxidant systems. Remarkable changes occurred in the cAS proteomic profile compared with that of wild-type (WT) cells. Proteins involved in translation, cytoskeletal organization, and cell proliferation were down-regulated, whereas stress-related proteins were up-regulated in cAS cells. These results support the hypothesis that BY-2 cells sense cAMP deficiency as a stress condition. Finally, many proteasome subunits were differentially expressed in cAS cells compared with WT cells, indicating that cAMP signaling broadly affects protein degradation via the ubiquitin/proteasome pathway.

The CLAVATA signaling pathway mediating stem cell fate in shoot meristems requires Ca(2+) as a secondary cytosolic messenger

CLAVATA1 (CLV1) is a receptor protein expressed in the shoot apical meristem (SAM) that translates perception of a non-cell-autonomous CLAVATA3 (CLV3) peptide signal into altered stem cell fate. CLV3 reduces expression of WUSCHEL (WUS) and FANTASTIC FOUR 2 (FAF2) in the SAM. Expression of WUS and FAF2 leads to maintenance of undifferentiated stem cells in the SAM. CLV3 binding to CLV1 inhibits expression of these genes and controls stem cell fate in the SAM through an unidentified signaling pathway. Cytosolic Ca(2+) elevations, cyclic nucleotide (cGMP)-activated Ca(2+) channels, and cGMP have been linked to signaling downstream of receptors similar to CLV1. Hence, we hypothesized that cytosolic Ca(2+) elevation mediates the CLV3 ligand/CLV1 receptor signaling that controls meristem stem cell fate. CLV3 application to Arabidopsis seedlings results in elevation of cytosolic Ca(2+) and cGMP. CLV3 control of WUS was prevented in a genotype lacking a functional cGMP-activated Ca(2+) channel. In wild-type plants, CLV3 inhibition of WUS and FAF2 expression was impaired by treatment with either a Ca(2+) channel blocker or a guanylyl cyclase inhibitor. When CLV3-dependent repression of WUS is blocked, altered control of stem cell fate leads to an increase in SAM size; we observed a larger SAM size in seedlings treated with the Ca(2+) channel blocker. These results suggest that the CLV3 ligand/CLV1 receptor system initiates a signaling cascade that elevates cytosolic Ca(2+), and that this cytosolic secondary messenger is involved in the signal transduction cascade linking CLV3/CLV1 to control of gene expression and stem cell fate in the SAM.

AtCNGC2 is involved in jasmonic acid-induced calcium mobilization

Calcium (Ca(2+)) mobilization is a central theme in various plant signal transduction pathways. We demonstrate that Arabidopsis thaliana cyclic nucleotide-gated channel 2 (AtCNGC2) is involved in jasmonic acid (JA)-induced apoplastic Ca(2+) influx in Arabidopsis epidermal cells. Ca(2+) imaging results showed that JA can induce an elevation in the cytosolic cAMP concentration ([cAMP]cyt), reaching a maximum within 3 min. Dibutyryl cAMP (db-cAMP), a cell membrane-permeable analogue of cAMP, induced an increase in the cytosolic Ca(2+) concentration ([Ca(2+)]cyt), with a peak at 4 min. This [Ca(2+)]cyt increase was triggered by the JA-induced increase in [cAMP]cyt. W-7[N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], an antagonist of calmodulin, positively modulated the JA-induced increase in [Ca(2+)]cyt, while W-5[N-(6-aminohexyl)-1-naphthalenesulfonamide], an inactive antagonist of calmodulin, had no apparent effect. db-cAMP and JA positively induced the expression of primary (i.e. JAZ1 and MYC2) and secondary (i.e. VSP1) response genes in the JA signalling pathway in wild-type Arabidopsis thaliana, whereas they had no significant effect in the AtCNGC2 mutant 'defense, no death (dnd1) plants. These data provide evidence that JA first induces the elevation of cAMP, and cAMP, as an activating ligand, activates the AtCNGC2 channel, resulting in apoplastic Ca(2+) influx through AtCNGC2.

The Arabidopsis thaliana K(+)-uptake permease 7 (AtKUP7) contains a functional cytosolic adenylate cyclase catalytic centre

Adenylate cyclases (ACs) catalyse the formation of the second messenger cyclic adenosine 3',5'-monophosphate (cAMP) from adenosine 5'-triphosphate (ATP). Although cAMP is increasingly recognised as an important signalling molecule in higher plants, ACs have remained somewhat elusive. Here we used a search motif derived from experimentally tested guanylyl cyclases (GCs), substituted the residues essential for substrate specificity and identified the Arabidopsis thaliana K(+)-uptake permease 7 (AtKUP7) as one of several candidate ACs. Firstly, we show that a recombinant N-terminal, cytosolic domain of AtKUP7(1-100) is able to complement the AC-deficient mutant cyaA in Escherichia coli and thus restoring the fermentation of lactose, and secondly, we demonstrate with both enzyme immunoassays and mass spectrometry that a recombinant AtKUP7(1-100) generates cAMP in vitro.

The role of plasma membrane H(+) -ATPase in jasmonate-induced ion fluxes and stomatal closure in Arabidopsis thaliana

Methyl jasmonate (MeJA) elicits stomatal closure in many plant species. Stomatal closure is accompanied by large ion fluxes across the plasma membrane (PM). Here, we recorded the transmembrane ion fluxes of H(+) , Ca(2+) and K(+) in guard cells of wild-type (Col-0) Arabidopsis, the CORONATINE INSENSITIVE1 (COI1) mutant coi1-1 and the PM H(+) -ATPase mutants aha1-6 and aha1-7, using a non-invasive micro-test technique. We showed that MeJA induced transmembrane H(+) efflux, Ca(2+) influx and K(+) efflux across the PM of Col-0 guard cells. However, this ion transport was abolished in coi1-1 guard cells, suggesting that MeJA-induced transmembrane ion flux requires COI1. Furthermore, the H(+) efflux and Ca(2+) influx in Col-0 guard cells was impaired by vanadate pre-treatment or PM H(+) -ATPase mutation, suggesting that the rapid H(+) efflux mediated by PM H(+) -ATPases could function upstream of the Ca(2+) flux. After the rapid H(+) efflux, the Col-0 guard cells had a longer oscillation period than before MeJA treatment, indicating that the activity of the PM H(+) -ATPase was reduced. Finally, the elevation of cytosolic Ca(2+) concentration and the depolarized PM drive the efflux of K(+) from the cell, resulting in loss of turgor and closure of the stomata.

There's more to the picture than meets the eye: nitric oxide cross talk with Ca2+ signaling

Calcium and nitric oxide (NO) are two important biological messengers. Increasing evidence indicates that Ca(2+) and NO work together in mediating responses to pathogenic microorganisms and microbe-associated molecular patterns. Ca(2+) fluxes were recognized to account for NO production, whereas evidence gathered from a number of studies highlights that NO is one of the key messengers mediating Ca(2+) signaling. Here, we present a concise description of the current understanding of the molecular mechanisms underlying the cross talk between Ca(2+) and NO in plant cells exposed to biotic stress. Particular attention will be given to the involvement of cyclic nucleotide-gated ion channels and Ca(2+) sensors. Notably, we provide new evidence that calmodulin might be regulated at the posttranslational level by NO through S-nitrosylation. Furthermore, we report original transcriptomic data showing that NO produced in response to oligogalacturonide regulates the expression of genes related to Ca(2+) signaling. Deeper insight into the molecules involved in the interplay between Ca(2+) and NO not only permits a better characterization of the Ca(2+) signaling system but also allows us to further understand how plants respond to pathogen attack.

Identification of cyclic GMP-activated nonselective Ca2+-permeable cation channels and associated CNGC5 and CNGC6 genes in Arabidopsis guard cells

Cytosolic Ca(2+) in guard cells plays an important role in stomatal movement responses to environmental stimuli. These cytosolic Ca(2+) increases result from Ca(2+) influx through Ca(2+)-permeable channels in the plasma membrane and Ca(2+) release from intracellular organelles in guard cells. However, the genes encoding defined plasma membrane Ca(2+)-permeable channel activity remain unknown in guard cells and, with some exceptions, largely unknown in higher plant cells. Here, we report the identification of two Arabidopsis (Arabidopsis thaliana) cation channel genes, CNGC5 and CNGC6, that are highly expressed in guard cells. Cytosolic application of cyclic GMP (cGMP) and extracellularly applied membrane-permeable 8-Bromoguanosine 3',5'-cyclic monophosphate-cGMP both activated hyperpolarization-induced inward-conducting currents in wild-type guard cells using Mg(2+) as the main charge carrier. The cGMP-activated currents were strongly blocked by lanthanum and gadolinium and also conducted Ba(2+), Ca(2+), and Na(+) ions. cngc5 cngc6 double mutant guard cells exhibited dramatically impaired cGMP-activated currents. In contrast, mutations in CNGC1, CNGC2, and CNGC20 did not disrupt these cGMP-activated currents. The yellow fluorescent protein-CNGC5 and yellow fluorescent protein-CNGC6 proteins localize in the cell periphery. Cyclic AMP activated modest inward currents in both wild-type and cngc5cngc6 mutant guard cells. Moreover, cngc5 cngc6 double mutant guard cells exhibited functional abscisic acid (ABA)-activated hyperpolarization-dependent Ca(2+)-permeable cation channel currents, intact ABA-induced stomatal closing responses, and whole-plant stomatal conductance responses to darkness and changes in CO2 concentration. Furthermore, cGMP-activated currents remained intact in the growth controlled by abscisic acid2 and abscisic acid insensitive1 mutants. This research demonstrates that the CNGC5 and CNGC6 genes encode unique cGMP-activated nonselective Ca(2+)-permeable cation channels in the plasma membrane of Arabidopsis guard cells.

Proteomic signatures implicate cAMP in light and temperature responses in Arabidopsis thaliana

The second messenger 3'-5'-cyclic adenosine monophosphate (cAMP) and adenylyl cyclases (ACs), enzymes that catalyse the formation of cAMP from ATP, are increasingly recognized as important signaling molecules in a number of physiological responses in higher plants. Here we used proteomics to identify cAMP-dependent protein signatures in Arabidopsis thaliana and identify a number of differentially expressed proteins with a role in light- and temperature-dependent responses, notably photosystem II subunit P-1, plasma membrane associated cation-binding protein and chaperonin 60 β. Based on these proteomics results we conclude that, much like in cyanobacteria, algae and fungi, cAMP may have a role in light signaling and the regulation of photosynthesis as well as responses to temperature and we speculate that ACs could act as light and/or temperature sensors in higher plants.

BIOLOGICAL SIGNIFICANCE

This current study is significant since it presents the first proteomic response to cAMP, a novel and key second messenger in plants. It will be relevant to researchers in plant physiology and in particular those with an interest in second messengers and their role in biotic and abiotic stress responses.

Calcium imaging of the cyclic nucleotide response

Calcium (Ca(2+)) is a key component of the signalling network by which plant cells respond to developmental and environmental signals. A change in guard cell cytosolic free Ca(2+)([Ca(2+)]cyt) is an early event in the response of stomata to both opening and closing stimuli, and cyclic nucleotide-mediated Ca(2+) signalling has been implicated in the regulation of stomatal aperture. A range of techniques have been used to measure [Ca(2+)]cyt in plant cells. Here we describe a potential method for imaging cyclic nucleotide-induced changes in [Ca(2+)]cyt in guard cells using the cameleon ratiometric Ca(2+) reporter protein.

Identification of cyclic nucleotide gated channels using regular expressions

Cyclic nucleotide-gated channels (CNGCs) are nonselective cation channels found in plants, animals, and some bacteria. They have a six-transmembrane/one-pore structure, a cytosolic cyclic nucleotide-binding domain, and a cytosolic calmodulin-binding domain. Despite their functional similarities, the plant CNGC family members appear to have different conserved amino acid motifs within corresponding functional domains than animal and bacterial CNGCs do. Here we describe the development and application of methods employing plant CNGC-specific sequence motifs as diagnostic tools to identify novel candidate channels in different plants. These methods are used to evaluate the validity of annotations of putative orthologs of CNGCs from plant genomes. The methods detail how to employ regular expressions of conserved amino acids in functional domains of annotated CNGCs and together with Web tools such as PHI-BLAST and ScanProsite to identify novel candidate CNGCs in species including Physcomitrella patens.

Molecular methods for the study of signal transduction in plants

Novel and improved analytical methods have led to a rapid increase in our understanding of the molecular mechanism underlying plant signal transduction. Progress has been made both at the level of single-component analysis and in vivo imaging as well as at the systems level where transcriptomics and particularly phosphoproteomics afford a window into complex biological responses. Here we review the role of the cyclic nucleotides cAMP and cGMP in plant signal transduction as well as the discovery and biochemical and biological characterization of an increasing number of complex multi-domain nucleotide cyclases that catalyze the synthesis of cAMP and cGMP from ATP and GTP, respectively.

A type of voltage-dependent Ca2+ channel on Vicia faba guard cell plasma membrane outwardly permeates K+

The fine regulation of stomatal aperture is important for both plant photosynthesis and transpiration, while stomatal closing is an essential plant response to biotic and abiotic stresses such as drought, salinity, wounding, and pathogens. Quick stomatal closing is primarily due to rapid solute loss. Cytosolic free calcium ([Ca(2+)](cyt)) is a ubiquitous second messenger, and its elevation or oscillation plays important roles in stomatal movements, which can be triggered by the opening of Ca(2+)-permeable channels on the plasma membrane. For Ca(2+)-permeable channel recordings, Ba(2+) is preferred as a charge-carrying ion because it has higher permeability to Ca(2+) channels and blocks K(+) channel activities to facilitate current recordings; however, it prevents visualization of Ca(2+) channels' K(+) permeability. Here, we employed Ca(2+) instead of Ba(2+) in recording Ca(2+)-permeable channels on Vicia faba guard cell plasma membrane to mimic physiological solute conditions inside guard cells more accurately. Inward Ca(2+) currents could be recorded at the single-channel level, and these currents could be inhibited by micromolar Gd(3+), but their reversal potential is far away from the theoretical equilibrium potential for Ca(2+). Further experiments showed that the discrepancy of the reversal potential of the recorded Ca(2+) currents is influenced by cytosolic K(+). This suggests that voltage-dependent Ca(2+) channels also mediate K(+) efflux at depolarization voltages. In addition, a new kind of high-conductance channels with fivefold to normal Ca(2+) channel and 18-fold to normal outward K(+) conductance was found. Our data presented here suggest that plants have their own saving strategies in their rapid response to stress stimuli, and multiple kinds of hyperpolarization-activated Ca(2+)-permeable channels coexist on plasma membranes.

A heat-activated calcium-permeable channel--Arabidopsis cyclic nucleotide-gated ion channel 6--is involved in heat shock responses

An increased concentration of cytosolic calcium ions (Ca²⁺) is an early response by plant cells to heat shock. However, the molecular mechanism underlying the heat-induced initial Ca²⁺ response in plants is unclear. In this study, we identified and characterized a heat-activated Ca²⁺-permeable channel in the plasma membrane of Arabidopsis thaliana root protoplasts using reverse genetic analysis and the whole-cell patch-clamp technique. The results indicated that A. thaliana cyclic nucleotide-gated ion channel 6 (CNGC6) mediates heat-induced Ca²⁺ influx and facilitates expression of heat shock protein (HSP) genes and the acquisition of thermotolerance. GUS and GFP reporter assays showed that CNGC6 expression is ubiquitous in A. thaliana, and the protein is localized to the plasma membrane of cells. Furthermore, it was found that the level of cytosolic cAMP was increased by a mild heat shock, that CNGC6 was activated by cytosolic cAMP, and that exogenous cAMP promoted the expression of HSP genes. The results reveal the role of cAMP in transduction of heat shock signals in plants. The correlation of an increased level of cytosolic cAMP in a heat-shocked plant with activation of the Ca²⁺ channels and downstream expression of HSP genes sheds some light on how plants transduce a heat stimulus into a signal cascade that leads to a heat shock response.

Evolutionary and structural perspectives of plant cyclic nucleotide-gated cation channels

Ligand-gated cation channels are a frequent component of signaling cascades in eukaryotes. Eukaryotes contain numerous diverse gene families encoding ion channels, some of which are shared and some of which are unique to particular kingdoms. Among the many different types are cyclic nucleotide-gated channels (CNGCs). CNGCs are cation channels with varying degrees of ion conduction selectivity. They are implicated in numerous signaling pathways and permit diffusion of divalent and monovalent cations, including Ca(2+) and K(+). CNGCs are present in both plant and animal cells, typically in the plasma membrane; recent studies have also documented their presence in prokaryotes. All eukaryote CNGC polypeptides have a cyclic nucleotide-binding domain and a calmodulin binding domain as well as a six transmembrane/one pore tertiary structure. This review summarizes existing knowledge about the functional domains present in these cation-conducting channels, and considers the evidence indicating that plant and animal CNGCs evolved separately. Additionally, an amino acid motif that is only found in the phosphate binding cassette and hinge regions of plant CNGCs, and is present in all experimentally confirmed CNGCs but no other channels was identified. This CNGC-specific amino acid motif provides an additional diagnostic tool to identify plant CNGCs, and can increase confidence in the annotation of open reading frames in newly sequenced genomes as putative CNGCs. Conversely, the absence of the motif in some plant sequences currently identified as probable CNGCs may suggest that they are misannotated or protein fragments.

Roles of Ca2+ and cyclic nucleotide gated channel in plant innate immunity

The increase of cytosolic Ca(2+) is a vital event in plant pathogen signaling cascades. Molecular components linking pathogen signal perception to cytosolic Ca(2+) increase have not been well characterized. Plant cyclic nucleotide gated channels (CNGCs) play important roles in the pathogen signaling cascade, in terms of facilitating Ca(2+) uptake into the cytosol in response to pathogen and pathogen associated molecular pattern (PAMP) signals. Perception of pathogens leads to cyclic nucleotide production and the activation of CNGCs. The Ca(2+) signal is transduced through Ca(2+) sensors (Calmodulin (CaM) and CaM-like proteins (CMLs)), which regulates the production of nitric oxide (NO). In addition, roles of Ca(2+)/CaM interacting proteins such as CaM binding Protein (CBP) and CaM-binding transcription activators (CAMTAs)) have been recently identified in the plant defense signaling cascade as well. Furthermore, Ca(2+)-dependent protein kinases (CDPKs) have been found to function as components in terms of transcriptional activation in response to a pathogen (PAMP) signal. Although evidence shows that Ca(2+) is an essential signaling component upstream from many vital signaling molecules (such as NO), some work also indicates that these downstream signaling components can also regulate Ca(2+) homeostasis. NO can induce cytosolic Ca(2+) increase (through activation of plasma membrane- and intracellular membrane-localized Ca(2+) channels) during pathogen signaling cascades. Thus, much work is needed to further elucidate the complexity of the plant pathogen signaling network in the future.

cAMP activates hyperpolarization-activated Ca2+ channels in the pollen of Pyrus pyrifolia

Many signal-transduction processes in plant cells have been suggested to be triggered by signal-induced opening of calcium ion (Ca(2+)) channels in the plasma membrane. Cyclic nucleotides have been proposed to lead to an increase in cytosolic free Ca(2+) in pollen. However, direct recordings of cyclic-nucleotide-induced Ca(2+) currents in pollen have not yet been obtained. Here, we report that cyclic AMP (cAMP) activated a hyperpolarization-activated Ca(2+) channel in the Pyrus pyrifolia pollen tube using the patch-clamp technique, which resulted in a significant increase in pollen tube protoplast cytosolic-Ca(2+) concentration. Outside-out single channel configuration identified that cAMP directly increased the Ca(2+) channel open-probability without affecting channel conductance. cAMP-induced currents were composed of both Ca(2+) and K(+). However, cGMP failed to mimic the cAMP effect. Higher cytosolic free-Ca(2+) concentration significantly decreased the cAMP-induced currents. These results provide direct evidence for cAMP activation of hyperpolarization-activated Ca(2+) channels in the plasma membrane of pollen tubes, which, in turn, modulate cellular responses in regulation of pollen tube growth.

Roles of heterotrimeric G proteins in guard cell ion channel regulation

Stomata are formed by pairs of surrounding guard cells and perform important roles in photosynthesis, transpiration and innate immunity of terrestrial plants. Ionic solutes in the cytosol of guard cells are important for cell turgor and volume change. Consequently, transmembrane flux of ions such as K+, Cl-, and malate2- through K+ channels and anion channels of guard cells are a direct driving force for turgor change, while the opening of calcium permeable channels can serve as a trigger of cytosolic free calcium concentration elevations or oscillations, which play second messenger roles. In plants, heterotrimeric G proteins have fewer members than in animals, but they are well investigated and found to regulate these channels and to play fundamental roles in guard cell function. This mini-review focuses on the recent understanding of G-protein regulation of ion channels on the plasma membrane of guard cells and their participation in stomatal movements.

Ca2+ conduction by plant cyclic nucleotide gated channels and associated signaling components in pathogen defense signal transduction cascades

Ca(2+) elevation in the cytosol is an essential early event during pathogen response signaling cascades. However, the specific ion channels involved in Ca(2+) influx into plant cells, and how Ca(2+) signals are initiated and regulate downstream events during pathogen defense responses, are at present unclear. Plant cyclic nucleotide gated ion channels (CNGCs) provide a pathway for Ca(2+) conductance across the plasma membrane (PM) and facilitate cytosolic Ca(2+) elevation in response to pathogen signals. Recent studies indicate that the recognition of pathogens results in cyclic nucleotide production and the activation of CNGCs, which leads to downstream generation of pivotal signaling molecules (such as nitric oxide (NO)). Calmodulins (CaMs) and CaM-like proteins (CMLs) are also involved in this signaling, functioning as Ca(2+) sensors and mediating the synthesis of NO during the plant pathogen response signaling cascade. In this article, these and other pivotal signaling components downstream from the Ca(2+) signal, such as Ca(2+)-dependent protein kinases (CDPKs) and CaM-binding transcription activators (CAMTAs), are discussed in terms of their involvement in the pathogen response signal transduction cascade.

Cyclic nucleotide gated channel and Ca²⁺-mediated signal transduction during plant senescence signaling

Previous studies reveal that both Ca(2+) and nitric oxide (NO) play pivotal roles in the plant senescence signaling cascade. However, not much is known about the molecular identity of the Ca(2+) entry during senescence programming and its relationship to the downstream NO signal. Our recent study shows that Arabidopsis cyclic nucleotide gated channel2 (CNGC2) contributes to Ca(2+) uptake and senescence signaling. The CNGC2 loss-of-function mutant dnd1 displays reduced Ca(2+) accumulation in leaves and a series of early senescence phenotypes compared to wild type (WT). Notably, endogenous NO content in dnd1 leaves is lower than leaves of WT. Application of an NO donor can effectively rescue a number of early senescence phenotypes found in the dnd1 plants. Current evidence supports the notion that NO functions as a negative regulator in senescence signaling and our model supports this point. In this article, we expand our discussion of CNGC2 mediated Ca(2+) uptake and other related signaling components involved in the plant senescence signaling cascade.

Ca2+ signaling by plant Arabidopsis thaliana Pep peptides depends on AtPepR1, a receptor with guanylyl cyclase activity, and cGMP-activated Ca2+ channels

A family of peptide signaling molecules (AtPeps) and their plasma membrane receptor AtPepR1 are known to act in pathogen-defense signaling cascades in plants. Little is currently known about the molecular mechanisms that link these signaling peptides and their receptor, a leucine-rich repeat receptor-like kinase, to downstream pathogen-defense responses. We identify some cellular activities of these molecules that provide the context for a model for their action in signaling cascades. AtPeps activate plasma membrane inwardly conducting Ca(2+) permeable channels in mesophyll cells, resulting in cytosolic Ca(2+) elevation. This activity is dependent on their receptor as well as a cyclic nucleotide-gated channel (CNGC2). We also show that the leucine-rich repeat receptor-like kinase receptor AtPepR1 has guanylyl cyclase activity, generating cGMP from GTP, and that cGMP can activate CNGC2-dependent cytosolic Ca(2+) elevation. AtPep-dependent expression of pathogen-defense genes (PDF1.2, MPK3, and WRKY33) is mediated by the Ca(2+) signaling pathway associated with AtPep peptides and their receptor. The work presented here indicates that extracellular AtPeps, which can act as danger-associated molecular patterns, signal by interaction with their receptor, AtPepR1, a plasma membrane protein that can generate cGMP. Downstream from AtPep and AtPepR1 in a signaling cascade, the cGMP-activated channel CNGC2 is involved in AtPep- and AtPepR1-dependent inward Ca(2+) conductance and resulting cytosolic Ca(2+) elevation. The signaling cascade initiated by AtPeps leads to expression of pathogen-defense genes in a Ca(2+)-dependent manner.