Calcium signals: the lead currency of plant information processing

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.

The role of P-type IIA and P-type IIB Ca2+-ATPases in plant development and growth

As sessile organisms, plants have evolved mechanisms to adapt to variable and rapidly fluctuating environmental conditions. Calcium (Ca2+) in plant cells is a versatile intracellular second messenger that is essential for stimulating short- and long-term responses to environmental stresses through changes in its concentration in the cytosol ([Ca2+]cyt). Increases in [Ca2+]cyt direct the strength and length of these stimuli. In order to terminate them, the cells must then remove the cytosolic Ca2+ against a concentration gradient, either taking it away from the cell or storing it in organelles such as the endoplasmic reticulum (ER) and/or vacuoles. Here, we review current knowledge about the biological roles of plant P-type Ca2+-ATPases as potential actors in the regulation of this cytosolic Ca2+ efflux, with a focus the IIA ER-type Ca2+-ATPases (ECAs) and the IIB autoinhibited Ca2+-ATPases (ACAs). While ECAs are analogous proteins to animal sarcoplasmic-endoplasmic reticulum Ca2+-ATPases (SERCAs), ACAs are equivalent to animal plasma membrane-type ATPases (PMCAs). We examine their expression patterns in cells exhibiting polar growth and consider their appearance during the evolution of the plant lineage. Full details of the functions and coordination of ECAs and ACAs during plant growth and development have not yet been elucidated. Our current understanding of the regulation of fluctuations in Ca2+ gradients in the cytoplasm and organelles during growth is in its infancy, but recent technological advances in Ca2+ imaging are expected to shed light on this subject.

NADPH Oxidases: The Vital Performers and Center Hubs during Plant Growth and Signaling

NADPH oxidases (NOXs), mostly known as respiratory burst oxidase homologs (RBOHs), are the key producers of reactive oxygen species (ROS) in plants. A lot of literature has addressed ROS signaling in plant development regulation and stress responses as well as on the enzyme's structure, evolution, function, regulation and associated mechanisms, manifesting the role of NOXs/RBOHs as the vital performers and center hubs during plant growth and signaling. This review focuses on recent advances of NOXs/RBOHs on cell growth, hormone interaction, calcium signaling, abiotic stress responses, and immunity. Several primary particles, including Ca2+, CDPKs, BIK1, ROPs/RACs, CERK, FER, ANX, SnRK and SIK1-mediated regulatory mechanisms, are fully summarized to illustrate the signaling behavior of NOXs/RBOHs and their sophisticated and dexterous crosstalks. Diverse expression and activation regulation models endow NOXs/RBOHs powerful and versatile functions in plants to maintain innate immune homeostasis and development integrity. NOXs/RBOHs and their related regulatory items are the ideal targets for crop improvement in both yield and quality during agricultural practices.

Genome-Wide Characterization and Analysis of CIPK Gene Family in Two Cultivated Allopolyploid Cotton Species: Sequence Variation, Association with Seed Oil Content, and the Role of GhCIPK6

Calcineurin B-like protein-interacting protein kinases (CIPKs), as key regulators, play an important role in plant growth and development and the response to various stresses. In the present study, we identified 80 and 78 CIPK genes in the Gossypium hirsutum and G. barbadense, respectively. The phylogenetic and gene structure analysis divided the cotton CIPK genes into five groups which were classified into an exon-rich clade and an exon-poor clade. A synteny analysis showed that segmental duplication contributed to the expansion of Gossypium CIPK gene family, and purifying selection played a major role in the evolution of the gene family in cotton. Analyses of expression profiles showed that GhCIPK genes had temporal and spatial specificity and could be induced by various abiotic stresses. Fourteen GhCIPK genes were found to contain 17 non-synonymous single nucleotide polymorphisms (SNPs) and co-localized with oil or protein content quantitative trait loci (QTLs). Additionally, five SNPs from four GhCIPKs were found to be significantly associated with oil content in one of the three field tests. Although most GhCIPK genes were not associated with natural variations in cotton oil content, the overexpression of the GhCIPK6 gene reduced the oil content and increased C18:1 and C18:1+C18:1d6 in transgenic cotton as compared to wild-type plants. In addition, we predicted the potential molecular regulatory mechanisms of the GhCIPK genes. In brief, these results enhance our understanding of the roles of CIPK genes in oil synthesis and stress responses.

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.

The ThSOS3 Gene Improves the Salt Tolerance of Transgenic Tamarix hispida and Arabidopsis thaliana

The salt overly sensitive (SOS) signal transduction pathway is one of the most highly studied salt tolerance pathways in plants. However, the molecular mechanism of the salt stress response in Tamarix hispida has remained largely unclear. In this study, five SOS genes (ThSOS1-ThSOS5) from T. hispida were cloned and characterized. The expression levels of most ThSOS genes significantly changed after NaCl, PEG₆₀₀₀, and abscisic acid (ABA) treatment in at least one organ. Notably, the expression of ThSOS3 was significantly downregulated after 6 h under salt stress. To further analyze ThSOS3 function, ThSOS3 overexpression and RNAi-mediated silencing were performed using a transient transformation system. Compared with controls, ThSOS3-overexpressing transgenic T. hispida plants exhibited greater reactive oxygen species (ROS)-scavenging capability and antioxidant enzyme activity, lower malondialdehyde (MDA) and H₂O₂ levels, and lower electrolyte leakage rates under salt stress. Similar results were obtained for physiological parameters in transgenic Arabidopsis, including H₂O₂ and MDA accumulation, superoxide dismutase (SOD) and peroxidase (POD) activity, and electrolyte leakage. In addition, transgenic Arabidopsis plants overexpressing ThSOS3 displayed increased root growth and fresh weight gain under salt stress. Together, these data suggest that overexpression of ThSOS3 confers salt stress tolerance on plants by enhancing antioxidant enzyme activity, improving ROS-scavenging capability, and decreasing the MDA content and lipid peroxidation of cell membranes. These results suggest that ThSOS3 might play an important physiological role in salt tolerance in transgenic T. hispida plants. This study provides a foundation for further elucidation of salt tolerance mechanisms involving ThSOSs in T. hispida.

Calcium-Regulated Phosphorylation Systems Controlling Uptake and Balance of Plant Nutrients

Essential elements taken up from the soil and distributed throughout the whole plant play diverse roles in different tissues. Cations and anions contribute to maintenance of intracellular osmolarity and the formation of membrane potential, while nitrate, ammonium, and sulfate are incorporated into amino acids and other organic compounds. In contrast to these ion species, calcium concentrations are usually kept low in the cytosol and calcium displays unique behavior as a cytosolic signaling molecule. Various environmental stresses stimulate increases in the cytosolic calcium concentration, leading to activation of calcium-regulated protein kinases and downstream signaling pathways. In this review, we summarize the stress responsive regulation of nutrient uptake and balancing by two types of calcium-regulated phosphorylation systems: CPK and CBL-CIPK. CPK is a family of protein kinases activated by calcium. CBL is a group of calcium sensor proteins that interact with CIPK kinases, which phosphorylate their downstream targets. In Arabidopsis, quite a few ion transport systems are regulated by CPKs or CBL-CIPK complexes, including channels/transporters that mediate transport of potassium (KAT1, KAT2, GORK, AKT1, AKT2, HAK5, SPIK), sodium (SOS1), ammonium (AMT1;1, AMT1;2), nitrate and chloride (SLAC1, SLAH2, SLAH3, NRT1.1, NRT2.4, NRT2.5), and proton (AHA2, V-ATPase). CPKs and CBL-CIPKs also play a role in C/N nutrient response and in acquisition of magnesium and iron. This functional regulation by calcium-dependent phosphorylation systems ensures the growth of plants and enables them to acquire tolerance against various environmental stresses. Calcium serves as the key factor for the regulation of membrane transport systems.

Abscisic Acid Is Required for Root Elongation Associated With Ca2+ Influx in Response to Water Stress

Abscisic acid (ABA) is a critical hormone for plant survival under water stress. In this study, large-scale mutants of the Arabidopsis ecotype Columbia-0 (Col-0) were generated by ethyl methanesulfonate (EMS)-mutagenesis, and an improved root elongation under water-stress 1 (irew1) mutant showing significantly enhanced root growth was isolated under a water potential gradient assay. Then, irew1 and ABA-related mutants in Arabidopsis or tomato plants were observed under water potential gradient assay or water-deficient conditions. ABA pathway, Ca²⁺ response, and primary root (PR) elongation rate were monitored in addition to DNA- and RNA-Seq analyses. We found that based on phenotyping and transcriptional analyses, irew1 exhibited enhanced PR growth, ABA, and Ca²⁺ responses, compared to wild type subjected to water stress. Interestingly, exogenous Ca²⁺ application enhanced PR growth of irew1, ABA-biosynthesis deficient mutants in Arabidopsis, and tomato plants, in response to water potential gradients or water-deficient conditions. In combination with other ABA-related mutants and pharmacological studies, our results suggest that ABA is required for root elongation associated with Ca²⁺ influx in response to water stress.

Arabidopsis CPK6 positively regulates ABA signaling and drought tolerance through phosphorylating ABA-responsive element-binding factors

Abscisic acid (ABA) regulates numerous developmental processes and drought tolerance in plants. Calcium-dependent protein kinases (CPKs) are important Ca2+ sensors playing crucial roles in plant growth and development as well as responses to stresses. However, the molecular mechanisms of many CPKs in ABA signaling and drought tolerance remain largely unknown. Here we combined protein interaction studies, and biochemical and genetic approaches to identify and characterize substrates that were phosphorylated by CPK6 and elucidated the mechanism that underlines the role of CPK6 in ABA signaling and drought tolerance. The expression of CPK6 is induced by ABA and dehydration. Two cpk6 T-DNA insertion mutants are insensitive to ABA during seed germination and root elongation of seedlings; in contrast, overexpression of CPK6 showed the opposite phenotype. Moreover, CPK6-overexpressing lines showed enhanced drought tolerance. CPK6 interacts with and phosphorylates a subset of core ABA signaling-related transcription factors, ABA-responsive element-binding factors (ABFs/AREBs), and enhances their transcriptional activities. The phosphorylation sites in ABF3 and ABI5 were also identified through MS and mutational analyses. Taken together, we present evidence that CPK6 mediates ABA signaling and drought tolerance through phosphorylating ABFs/AREBs. This work thus uncovers a rather conserved mechanism of calcium-dependent Ser/Thr kinases in ABA signaling.

Integration of Abscisic Acid Signaling with Other Signaling Pathways in Plant Stress Responses and Development

Plants are immobile and, to overcome harsh environmental conditions such as drought, salt, and cold, they have evolved complex signaling pathways. Abscisic acid (ABA), an isoprenoid phytohormone, is a critical signaling mediator that regulates diverse biological processes in various organisms. Significant progress has been made in the determination and characterization of key ABA-mediated molecular factors involved in different stress responses, including stomatal closure and developmental processes, such as seed germination and bud dormancy. Since ABA signaling is a complex signaling network that integrates with other signaling pathways, the dissection of its intricate regulatory network is necessary to understand the function of essential regulatory genes involved in ABA signaling. In the present review, we focus on two aspects of ABA signaling. First, we examine the perception of the stress signal (abiotic and biotic) and the response network of ABA signaling components that transduce the signal to the downstream pathway to respond to stress tolerance, regulation of stomata, and ABA signaling component ubiquitination. Second, ABA signaling in plant development processes, such as lateral root growth regulation, seed germination, and flowering time regulation is investigated. Examining such diverse signal integration dynamics could enhance our understanding of the underlying genetic, biochemical, and molecular mechanisms of ABA signaling networks in plants.

Expression, Subcellular Localization, and Interactions of CPK Family Genes in Maize

Calcium-dependent protein kinase (CPKs) is a key player in the calcium signaling pathway to decode calcium signals into various physiological responses. cDNA sequences of 9 ZmCPK genes were successfully cloned from all four phylogenetic groups in maize. qRT-PCR analysis showed the expression variation of these selected genes under abscisic acid (ABA) and calcium chloride (CaCl2) treatment. Due to the presence of N-myristoylation/palmitoylation sites, the selected ZmCPK members were localized in a plasma membrane. To clarify whether ZmCPK, a key player in calcium signaling, interacts with key players of ABA, protein phosphatase 2Cs (PP2Cs) and the SNF1-related protein kinase 2s (SnRK2s) and mitogen-activated protein kinase (MAPK) signaling pathways in maize, we examined the interaction between 9 CPKs, 8 PP2Cs, 5 SnRKs, and 20 members of the MPK family in maize by using yeast two-hybrid assay. Our results showed that three ZmCPKs interact with three different members of ZmSnRKs while four ZmCPK members had a positive interaction with 13 members of ZmMPKs in different combinations. These four ZmCPK proteins are from three different groups in maize. These findings of physical interactions between ZmCPKs, ZmSnRKs, and ZmMPKs suggested that these signaling pathways do not only have indirect influence but also have direct crosstalk that may involve the defense mechanism in maize. The present study may improve the understanding of signal transduction in plants.

LORELEI-LIKE GPI-ANCHORED PROTEINS 2/3 Regulate Pollen Tube Growth as Chaperones and Coreceptors for ANXUR/BUPS Receptor Kinases in Arabidopsis

Pollen tube growth is crucial for successful fertilization. In Arabidopsis thaliana, the ANXUR (ANX)/BUPS receptor kinase complex controls and maintains pollen tube growth in response to autocrine rapid alkalinization factor 4/19 (RALF4/19) signaling; however, the molecular and cellular mechanisms underlying the ANX/BUPS-mediated regulation of pollen tube growth remain unclear. In this study, we found that pollen-specific LORELEI-like GPI-anchored proteins 2 and 3 (LLG2/3) promote pollen tube growth in vitro and in vivo. LLG2/3 interacte with ANX/BUPS in a RALF4-concentration-dependent manner, suggesting that ANX/BUPS and LLG2/3 might form a receptor-coreceptor complex for perceiving RALF peptide signals. Disruption of the ANX/BUPS-LLG2/3 interaction led to the cytoplasmic retention of ANX1/2, in either llg2/3 knockdown mutants or in anx1/2 mutants lacking the J region, which mediates the ANX/BUPS-LLG2/3 interaction. Moreover, we found that RALF4 induced the production of reactive oxygen species (ROS), which stimulate pollen tube growth and reduce pollen burst rate. ROS levels are reduced in the pollen tubes of LLG2/3 RNAi lines, and application of exogenous H₂O₂ could partially rescue the defective pollen tube growth of LLG2/3 RNAi lines. Taken together, our study identifies LLG2/3 as novel regulatory components of pollen tube growth, and shows that they chaperone ANX/BUPS for secretion to the apical plasma membrane of pollen tube and act as coreceptors of ANX/BUPS in the activation of ROS production for promoting pollen tube growth.

Transcriptome Analysis of Salt-Sensitive and Tolerant Genotypes Reveals Salt-Tolerance Metabolic Pathways in Sugar Beet

Soil salinization is a common environmental problem that seriously affects the yield and quality of crops. Sugar beet (Beta vulgaris L.), one of the main sugar crops in the world, shows a strong tolerance to salt stress. To decipher the molecular mechanism of sugar beet under salt stress, we conducted transcriptomic analyses of two contrasting sugar beet genotypes. To the best of our knowledge, this is the first comparison of salt-response transcriptomes in sugar beet with contrasting genotypes. Compared to the salt-sensitive cultivar (S710), the salt-tolerant one (T710MU) showed better growth and exhibited a higher chlorophyll content, higher antioxidant enzyme activity, and increased levels of osmotic adjustment molecules. Based on a high-throughput experimental system, 1714 differentially expressed genes were identified in the leaves of the salt-sensitive genotype, and 2912 in the salt-tolerant one. Many of the differentially expressed genes were involved in stress and defense responses, metabolic processes, signal transduction, transport processes, and cell wall synthesis. Moreover, expression patterns of several genes differed between the two cultivars in response to salt stress, and several key pathways involved in determining the salt tolerance of sugar beet, were identified. Our results revealed the mechanism of salt tolerance in sugar beet and provided potential metabolic pathways and gene markers for growing salt-tolerant cultivars.

Functional Validation of JcWRKY2, a Group III Transcription Factor Toward Mitigating Salinity Stress in Transgenic Tobacco

The plants being sessile cannot escape from the adverse environmental stresses, hence get negatively affected in terms of their growth and yield. Transcriptional control simultaneously regulate different cellular processes, minimizing the deleterious effects of these stresses. The salicylic acid (SA)-inducible WRKY family of transcription factors auto or crossregulate the stress signaling in response to abiotic and biotic stresses, facilitating enhanced stress tolerance. In this study, we characterized the group III WRKY gene, JcWRKY2 from ecological and economical valued shrub Jatropha curcas. The JcWRKY2 tobacco transgenics showed improved physiological growth parameters, elevated chlorophyll content, improved antioxidative activities, and increased endogenous SA with both salt and SA stress. Interestingly, the pretreatment with SA and hydrogen peroxide facilitated improved germination of transgenic seeds with salinity stress. The transgenics showed differential regulation of antioxidative enzymes, calcium/calmodulin, dehydrins, and phospholipase genes with salt and SA stress. The increased SA content in transgenics on stress treatments, enhanced the antioxidant capacity leading to reduced susceptibility to stresses. Thus, JcWRKY2 transgenics participate in SA-mediated, improved antioxidative status during salinity stress with reduced reactive oxygen species damage.

Transcriptome profiles of soybean leaves and roots in response to zinc deficiency

Zinc (Zn) deficiency is a widespread agricultural problem in arable soils of the whole world. However, the molecular mechanisms underlying Zn-deficiency response are largely unknown. Here, we analyzed the transcriptomic profilings of soybean leaves and roots in response to Zn deficiency through Illumina's high-throughput RNA sequencing in order to understand the molecular basis of Zn-deficiency response in the plants. A total of 614 and 1011 gene loci were found to be differentially expressed in leaves and roots, respectively, and 88 loci were commonly found in both leaves and roots. Twelve differentially expressed genes (DEGs) were randomly selected for validation by quantitative reverse transcription polymerase chain reaction, and their fold changes were similar to those of RNA-seq. Gene ontology enrichment analysis showed that ion transport, nicotianamine (NA) biosynthetic process and queuosine biosynthetic process were enriched in the upregulated genes, while oxidation-reduction process and defense response were enriched in the downregulated genes. Among the DEGs, 20 DEGs are potentially involved in Zn homeostasis, including seven ZRT, IRT-related protein (ZIP) transporter genes, three NA synthase genes, and seven metallothionein genes; 40 DEGs are possibly involved in diverse hormonal signals such as auxin, cytokinin, ethylene and gibberellin; nine DEGs are putatively involved in calcium signaling; 85 DEGs are putative transcription factor genes. Nine DEGs were found to contain zinc-deficiency-response element in their promoter regions. These results could provide comprehensive insights into the soybean response to Zn deficiency and will be helpful for further elucidation of the molecular mechanisms of Zn-deficiency response and Zn-deficiency tolerance in plants.

Adaptation of Plants to Salt Stress: Characterization of Na+ and K+ Transporters and Role of CBL Gene Family in Regulating Salt Stress Response

Salinity is one of the most serious factors limiting the productivity of agricultural crops, with adverse effects on germination, plant vigor, and crop yield. This salinity may be natural or induced by agricultural activities such as irrigation or the use of certain types of fertilizer. The most detrimental effect of salinity stress is the accumulation of Na+ and Cl− ions in tissues of plants exposed to soils with high NaCl concentrations. The entry of both Na+ and Cl− into the cells causes severe ion imbalance, and excess uptake might cause significant physiological disorder(s). High Na+ concentration inhibits the uptake of K+, which is an element for plant growth and development that results in lower productivity and may even lead to death. The genetic analyses revealed K+ and Na+ transport systems such as SOS1, which belong to the CBL gene family and play a key role in the transport of Na+ from the roots to the aerial parts in the Arabidopsis plant. In this review, we mainly discuss the roles of alkaline cations K+ and Na+, Ion homeostasis-transport determinants, and their regulation. Moreover, we tried to give a synthetic overview of soil salinity, its effects on plants, and tolerance mechanisms to withstand stress.

Harnessing the new emerging imaging technologies to uncover the role of Ca2+ signalling in plant nutrient homeostasis

Increasing crop yields by using ecofriendly practices is of high priority to tackle problems regarding food security and malnutrition worldwide. A sustainable crop production requires a limited use of fertilizer and the employment of plant varieties with improved ability to acquire nutrients from soil. To reach these goals, the scientific community aims to understand plant nutrients homeostasis by deciphering the nutrient sensing and signalling mechanisms of plants. Several lines of evidence about the involvement of Ca²⁺ as the signal of an impaired nutrient availability have been reported. Ca²⁺ signalling is a tightly regulated process that requires specific protein toolkits to perceive external stimuli and to induce the specific responses in the plant needed to survive. Here, we summarize both older and recent findings concerning the involvement of Ca²⁺ signalling in the homeostasis of nutrients. In this review, we present new emerging technologies, based on the use of genetically encoded Ca²⁺ sensors and advanced microscopy, which offer the chance to perform in planta analyses of Ca²⁺ dynamics at cellular resolution. The harnessing of these technologies with different genetic backgrounds and subjected to different nutritional stresses will provide important insights to the still little-known mechanisms of nutrient sensing in plants.

The involvements of calcium-dependent protein kinases and catechins in tea plant [Camellia sinensis (L.) O. Kuntze] cold responses

Temperature is one of the most important environmental factors limiting tea plant growth and tea production. Previously we reported that both Ca²⁺ and ROS signals play important roles in tea plant cold acclimation. Here, we identified 26 CsCPK transcripts, analyzed their phylogenetic and sequence characters, and detected their transcriptions to monitor Ca²⁺ signaling status. Tissue-specific expression profiles indicated that most CsCPK genes were constitutively expressed in tested tissues, suggesting their possible roles in development. Cold along with calcium inhibitor assays suggested that CsCPKs are important cold regulators and CsCPK30/5/4/9 maybe the key members. Moreover, LaCl₃ or EGTA pre-treatment could result in impaired Ca²⁺ signaling and compromised cold-responding network, but higher catechins accumulation revealed their potential positive roles in cold responses. Those findings indicated that catechins and other secondary metabolites in tea plant may form an alternative cold-responding network that closely correlated with Ca²⁺ signaling status.

The Glutamate Receptor-Like Protein GLR3.7 Interacts With 14-3-3ω and Participates in Salt Stress Response in Arabidopsis thaliana

Ionotropic glutamate receptors (iGluRs) are ligand-gated cation channels that mediate fast excitatory neurotransmission in the mammalian central nervous system. In the model plant Arabidopsis thaliana, a family of 20 glutamate receptor-like proteins (GLRs) shares similarities to animal iGluRs in sequence and predicted secondary structure. However, the function of GLRs in plants is little known. In the present study, a serine site (Ser-860) of AtGLR3.7 phosphorylated by a calcium-dependent protein kinase (CDPK) was identified and confirmed by an in vitro kinase assay. Using a bimolecular fluorescence complementation and quartz crystal microbalance analyses, the physical interaction between AtGLR3.7 and the 14-3-3ω protein was confirmed. The mutation of Ser-860 to alanine abolished this interaction, indicating that Ser-860 is the 14-3-3ω binding site of AtGLR3.7. Compared with wild type, seed germination of the glr3.7-2 mutant was more sensitive to salt stress. However, the primary root growth of GLR3.7-S860A overexpression lines was less sensitive to salt stress than that of the wild-type line. In addition, the increase of cytosolic calcium ion concentration by salt stress was significantly lower in the glr3.7-2 mutant line than in the wild-type line. Moreover, association of 14-3-3 proteins to microsomal fractions was less in GLR3.7-S860A overexpression lines than in GLR3.7 overexpression line under 150 mM NaCl salt stress condition. Overall, our results indicated that GLR3.7 is involved in salt stress response in A. thaliana by affecting calcium signaling.

Linkage mapping and genome-wide association reveal candidate genes conferring thermotolerance of seed-set in maize

It is predicted that high-temperature stress will increasingly affect crop yields worldwide as a result of climate change. In order to determine the genetic basis of thermotolerance of seed-set in maize under field conditions, we performed mapping of quantitative trait loci (QTLs) in a recombinant inbred line (RIL) population using a collection of 8329 specifically developed high-density single-nucleotide polymorphism (SNP) markers, combined with a genome-wide association study (GWAS) of 261 diverse maize lines using 259 973 SNPs. In total, four QTLs and 17 genes associated with 42 SNPs related to thermotolerance of seed-set were identified. Among them, four candidate genes were found in both linkage mapping and GWAS. Thermotolerance of seed-set was increased significantly in near-isogenic lines (NILs) that incorporated the four candidate genes in a susceptible parent background. The expression profiles of two of the four genes showed that they were induced by high temperatures in the maize tassel in a tolerant parent background. Our results indicate that thermotolerance of maize seed-set is regulated by multiple genes each of which has minor effects, with calcium signaling playing a central role. The genes identified may be exploited in breeding programs to improve seed-set and yield of maize under heat stress.

Calcium acetate-induced reduction of cadmium accumulation in Oryza sativa: Expression of auto-inhibited calcium-ATPase and cadmium transporters

Calcium (Ca) signalling has an essential role in regulating plant responses to various abiotic stresses. This study applied Ca in various forms (Ca acetate and CaCl₂ ) and concentrations to reduce cadmium (Cd) concentration in rice and propose a possible mechanism through which Ca acts to control the Cd concentration in rice. The results showed that supplementation of Cd-contaminated soil with Ca acetate reduced the Cd concentration in rice after exposure for 7 days in both hydroponic and soil conditions. The possible involvement of the auto-inhibited Ca²⁺ -ATPase gene (ACA) might act to control the primary signal of the Cd stress response. The messages from ACA3 and ACA13 tended to up-regulate the low-affinity cation transporter (OsLCT1) and down-regulate Cd uptake and the Cd translocation transporter, including the genes, natural resistance-associated macrophage protein 5 (Nramp5) and Zn/Cd-transporting ATPase 2 (HMA2), which resulted in a reduction in the Cd concentration in rice. After cultivation for 120 days, the application of Ca acetate into Cd-contaminated soil inhibited Cd uptake of rice. Increasing the Ca acetate concentration in the soil lowered the Cd concentration in rice shoots and grains. Moreover, Ca acetate maintained rice productivity and quality whereas both aspects decreased under Cd stress.

Rapeseed calcium-dependent protein kinase CPK6L modulates reactive oxygen species and cell death through interacting and phosphorylating RBOHD

Reactive oxygen species (ROS) play important roles in plant growth, development, responses to abiotic and biotic stresses. Hypersensitive response (HR)-like cell death is often associated with excess ROS. However, how a calcium-dependent protein kinase (CPK) modulates this process remains elusive in rapeseed (Brassica napus L.). In the present study, we identified and characterized CPK6L from rapeseed as a novel regulator of ROS and cell death. The subcellular localization of BnaCPK6L was investigated through GFP and was found to be located at the endoplasmic reticulum membrane. Overexpression of the constitutively active BnaCPK6LCA resulted in significant accumulation of ROS and HR-like cell death than the full-length. A quantitative RT-PCR survey identified that the expression levels of a few ROS, cell death and defense-related marker genes were up-regulated upon BnaCPK6LCA expression. Mating-based split ubiquitin system (mbSUS) screening revealed that BnaCPK6L interacted with BnaRBOHD (Respiratory Burst Oxidase Homolog D), which was validated by bimolecular fluorescence complementation (BiFC). An in vitro phosphorylation assay indicated that BnaCPK6L phosphorylated BnaRBOHD. Lastly, we also found that three 2C type protein phosphatases (PP2Cs) interacted with BnaCPK6L. Taken together, this study indicates that BnaCPK6L plays an important role in ROS and HR-like cell death through interacting with and phosphorylating RBOHD.

Sieve element biology provides leads for research on phytoplasma lifestyle in plant hosts

Phytoplasmas reside exclusively in sieve tubes, tubular arrays of sieve element-companion cell complexes. Hence, the cell biology of sieve elements may reveal (ultra)structural and functional conditions that are of significance for survival, propagation, colonization, and effector spread of phytoplasmas. Electron microscopic images suggest that sieve elements offer facilities for mobile and stationary stages in phytoplasma movement. Stationary stages may enable phytoplasmas to interact closely with diverse sieve element compartments. The unique, reduced sieve element outfit requires permanent support by companion cells. This notion implies a future focus on the molecular biology of companion cells to understand the sieve element-phytoplasma inter-relationship. Supply of macromolecules by companion cells is channelled via specialized symplasmic connections. Ca2+-mediated gating of symplasmic corridors is decisive for the communication within and beyond the sieve element-companion cell complex and for the dissemination of phytoplasma effectors. Thus, Ca2+ homeostasis, which affects sieve element Ca2+ signatures and induces a range of modifications, is a key issue during phytoplasma infection. The exceptional physical and chemical environment in sieve elements seems an essential, though not the only factor for phytoplasma survival.

Dark septate endophytic fungi increase the activity of proton pumps, efficiency of 15N recovery from ammonium sulphate, N content, and micronutrient levels in rice plants

Plants colonised by dark septate endophytic (DSE) fungi show increased uptake of nutrients available in the environment. The objective of the present study was to evaluate the impact of DSE fungi on the activity of proton pumps, nitrogen (N) recovery from ammonium sulphate, and nutrient accumulation in rice plants. Treatments consisted of non-inoculated plants and plants inoculated with two isolates of DSE fungi, A101 and A103. To determine N recovery from the soil, ammonium sulphate enriched with 15N was added to a non-sterile substrate while parameters associated with the activity of proton pumps and with NO3- uptake were determined in a sterile environment. The A101 and A103 fungal isolates colonised the roots of rice plants, promoting 15N uptake, growth, and accumulation of nutrients as compared with the mock control. A103 induced the expression of the plasma membrane H+-ATPase (PM H+-ATPase) isoforms OsA5 and OsA8, the activity of the PM H+-ATPase and H+-pyrophosphatase. Our results suggest that the inoculation of rice plants with DSE fungi represents a strategy to improve the N recovery from ammonium sulphate and rice plant growth through the induction of OsA5 and OsA8 isoforms and stimulation of the PM H+-ATPase and H+-pyrophosphatase.

Calmodulin Is the Fundamental Regulator of NADK-Mediated NAD Signaling in Plants

Calcium (Ca2+) signaling and nicotinamide adenine dinucleotide (NAD) signaling are two basic signal regulation pathways in organisms, playing crucial roles in signal transduction, energy metabolism, stress tolerance, and various developmental processes. Notably, calmodulins (CaMs) and NAD kinases (NADKs) are important hubs for connecting these two types of signaling networks, where CaMs are the unique activators of NADKs. NADK is a key enzyme for NADP (including NADP+ and NADPH) biosynthesis by phosphorylating NAD (including NAD+ and NADH) and therefore, maintains the balance between NAD pool and NADP pool through an allosteric regulation mode. In addition, the two respective derivatives from NAD+ (substrate of NADK) and NADP+ (product of NADK), cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), have been considered to be the important messengers for intracellular Ca2+ homeostasis which could finally influence the combination between CaM and NADK, forming a feedback regulation mechanism. In this review article, we briefly summarized the major research advances related to the feedback regulation pathway, which is activated by the interaction of CaM and NADK during plant development and signaling. The theories and fact will lay a solid foundation for further studies related to CaM and NADK and their regulatory mechanisms as well as the NADK-mediated NAD signaling behavior in plant development and response to stress.

Recent Advances in Hormonal Regulation and Cross-Talk during Non-Climacteric Fruit Development and Ripening

Fleshy fruits are characterized by having a developmentally and genetically controlled, highly intricate ripening process, leading to dramatic modifications in fruit size, texture, color, flavor, and aroma. Climacteric fruits such as tomato, pear, banana, and melon show a ripening-associated increase in respiration and ethylene production and these processes are well-documented. In contrast, the hormonal mechanism of fruit development and ripening in non-climacteric fruit, such as strawberry, grape, raspberry, and citrus, is not well characterized. However, recent studies have shown that non-climacteric fruit development and ripening, involves the coordinated action of different hormones, such as abscisic acid (ABA), auxin, gibberellins, ethylene, and others. In this review, we discuss and evaluate the recent research findings concerning the hormonal regulation of non-climacteric fruit development and ripening and their cross-talk by taking grape, strawberry, and raspberry as reference fruit species.

Drought and heat stress-related proteins: an update about their functional relevance in imparting stress tolerance in agricultural crops

We describe here the recent developments about the involvement of diverse stress-related proteins in sensing, signaling, and defending the cells in plants in response to drought or/and heat stress. In the current era of global climate drift, plant growth and productivity are often limited by various environmental stresses, especially drought and heat. Adaptation to abiotic stress is a multigenic process involving maintenance of homeostasis for proper survival under adverse environment. It has been widely observed that a series of proteins respond to heat and drought conditions at both transcriptional and translational levels. The proteins are involved in various signaling events, act as key transcriptional activators and saviors of plants under extreme environments. A detailed insight about the functional aspects of diverse stress-responsive proteins may assist in unraveling various stress resilience mechanisms in plants. Furthermore, by identifying the metabolic proteins associated with drought and heat tolerance, tolerant varieties can be produced through transgenic/recombinant technologies. A large number of regulatory and functional stress-associated proteins are reported to participate in response to heat and drought stresses, such as protein kinases, phosphatases, transcription factors, and late embryogenesis abundant proteins, dehydrins, osmotins, and heat shock proteins, which may be similar or unique to stress treatments. Few studies have revealed that cellular response to combined drought and heat stresses is distinctive, compared to their individual treatments. In this review, we would mainly focus on the new developments about various stress sensors and receptors, transcription factors, chaperones, and stress-associated proteins involved in drought or/and heat stresses, and their possible role in augmenting stress tolerance in crops.

The Kinase CIPK11 Functions as a Negative Regulator in Drought Stress Response in Arabidopsis

Drought is a major limiting factor for plant growth and crop productivity. Many Calcineurin B-like interacting protein kinases (CIPKs) play crucial roles in plant adaptation to environmental stresses. It is particularly essential to find the phosphorylation targets of CIPKs and to study the underlying molecular mechanisms. In this study, we demonstrate that CIPK11 acts as a novel component to modulate drought stress in plants. The overexpression of CIPK11 (CIPK11OE) in Arabidopsis resulted in the decreased tolerance of plant to drought stress. When compared to wild type plants, CIPK11OE plants exhibited higher leaf water loss and higher content of reactive oxygen species (ROS) after drought treatment. Additionally, a yeast two hybrid screening assay by using CIPK11 as a bait captures Di19-3, a Cys2/His2-type zinc-finger transcription factor that is involved in drought stress, as a new interactor of CIPK11. Biochemical analysis revealed that CIPK11 interacted with Di19-3 in vivo and it was capable of phosphorylating Di19-3 in vitro. Genetic studies revealed that the function of CIPK11 in regulating drought stress was dependent on Di19-3. The transcripts of stress responsive genes, such as RAB18, RD29A, RD29B, and DREB2A were down-regulated in the CIPK11OE plants. Whereas overexpression of CIPK11 in di19-3 mutant background, expression levels of those marker genes were not significantly altered. Taken together, our results demonstrate that CIPK11 partly mediates the drought stress response by regulating the transcription factor Di19-3.

Interspaced Repeat Sequences Confer the Regulatory Functions of AtXTH10, Important for Root Growth in Arabidopsis

An interspaced repeat sequence (IRS) is a unique sequence similar to prokaryotic CRISPR in structure. In this study, 1343 IRSs were identified in the Arabidopsis genome. Functional annotation of the IRS-related genes showed that they were associated with various growth and development processes. More than 30% of the IRSs were located in promoter regions. Deletion of some IRSs affected promoter activity, suggesting their roles in the regulation of gene expression. Next, the function of the AtXTH10 gene was further analyzed, and the expression of this gene was regulated by IRSs in its promoter region. Transgenic and mutant plants analysis indicated that the AtXTH10 gene was associated with root development by affecting cell wall structure. Moreover, the expression profiles of some key genes involved in root development signaling pathways were also affected by AtXTH10. These results suggest that IRSs could be involved in regulating the expression of genes with important roles in plant development.

Calcium-Dependent Protein Kinase Genes in Glycyrrhiza Uralensis Appear to be Involved in Promoting the Biosynthesis of Glycyrrhizic Acid and Flavonoids under Salt Stress

As calcium signal sensors, calcium-dependent protein kinases (CPKs) play vital roles in stimulating the production of secondary metabolites to participate in plant development and response to environmental stress. However, investigations of the Glycyrrhiza uralensis CPK family genes and their multiple functions are rarely reported. In this study, a total of 23 GuCPK genes in G. uralensis were identified, and their phylogenetic relationships, evolutionary characteristics, gene structure, motif distribution, and promoter cis-acting elements were analyzed. Ten GuCPKs showed root-specific preferential expressions, and GuCPKs indicated different expression patterns under treatments of CaCl2 and NaCl. In addition, under 2.5 mM of CaCl2 and 30 mM of NaCl treatments, the diverse, induced expression of GuCPKs and significant accumulations of glycyrrhizic acid and flavonoids suggested the possible important function of GuCPKs in regulating the production of glycyrrhizic acid and flavonoids. Our results provide a genome-wide characterization of CPK family genes in G. uralensis, and serve as a foundation for understanding the potential function and regulatory mechanism of GuCPKs in promoting the biosynthesis of glycyrrhizic acid and flavonoids under salt stress.

Boron Deficiency Increases Cytosolic Ca2+ Levels Mainly via Ca2+ Influx from the Apoplast in Arabidopsis thaliana Roots

Boron (B) is a micronutrient for plant development, and its deficiency alters many physiological processes. However, the current knowledge on how plants are able to sense the B-starvation signal is still very limited. Recently, it has been reported that B deprivation induces an increase in cytosolic calcium concentration ([Ca²⁺]_cyt) in Arabidopsis thaliana roots. The aim of this work was to research in Arabidopsis whether [Ca²⁺]_cyt is restored to initial levels when B is resupplied and elucidate whether apoplastic Ca²⁺ is the major source for B-deficiency-induced rise in [Ca²⁺]_cyt. The use of chemical compounds affecting Ca²⁺ homeostasis showed that the rise in root [Ca²⁺]_cyt induced by B deficiency was predominantly owed to Ca²⁺ influx from the apoplast through plasma membrane Ca²⁺ channels in an IP₃-independent manner. Furthermore, B resupply restored the root [Ca²⁺]_cyt. Interestingly, expression levels of genes encoding Ca²⁺ transporters (ACA10, plasma membrane P_IIB-type Ca²⁺-ATPase; and CAX3, vacuolar cation/proton exchanger) were upregulated by ethylene glycol tetraacetic acid (EGTA) and abscisic acid (ABA). The results pointed out that ACA10, and especially CAX3, would play a major role in the restoration of Ca²⁺ homeostasis after 24 h of B deficiency.

What Drives Symbiotic Calcium Signalling in Legumes? Insights and Challenges of Imaging

We review the contribution of bioimaging in building a coherent understanding of Ca 2 + signalling during legume-bacteria symbiosis. Currently, two different calcium signals are believed to control key steps of the symbiosis: a Ca 2 + gradient at the tip of the legume root hair is involved in the development of an infection thread, while nuclear Ca 2 + oscillations, the hallmark signal of this symbiosis, control the formation of the root nodule, where bacteria fix nitrogen. Additionally, different Ca 2 + spiking signatures have been associated with specific infection stages. Bioimaging is intrinsically a cross-disciplinary area that requires integration of image recording, processing and analysis. We used experimental examples to critically evaluate previously-established conclusions and draw attention to challenges caused by the varying nature of the signal-to-noise ratio in live imaging. We hypothesise that nuclear Ca 2 + spiking is a wide-range signal involving the entire root hair and that the Ca 2 + signature may be related to cytoplasmic streaming.

Identification of Novel Inhibitors of Auxin-Induced Ca2+ Signaling via a Plant-Based Chemical Screen

Many signal perception mechanisms are connected to Ca2+-based second messenger signaling to modulate specific cellular responses. The well-characterized plant hormone auxin elicits a very rapid Ca2+ signal. However, the cellular targets of auxin-induced Ca2+ are largely unknown. Here, we screened a biologically annotated chemical library for inhibitors of auxin-induced Ca2+ entry in plant cell suspensions to better understand the molecular mechanism of auxin-induced Ca2+ and to explore the physiological relevance of Ca2+ in auxin signal transduction. Using this approach, we defined a set of diverse, small molecules that interfere with auxin-induced Ca2+ entry. Based on annotated biological activities of the hit molecules, we found that auxin-induced Ca2+ signaling is, among others, highly sensitive to disruption of membrane proton gradients and the mammalian Ca2+ channel inhibitor bepridil. Whereas protonophores nonselectively inhibited auxin-induced and osmotic stress-induced Ca2+ signals, bepridil specifically inhibited auxin-induced Ca2+ We found evidence that bepridil severely alters vacuolar morphology and antagonized auxin-induced vacuolar remodeling. Further exploration of this plant-tailored collection of inhibitors will lead to a better understanding of auxin-induced Ca2+ entry and its relevance for auxin responses.

Increases in Absolute Temperature Stimulate Free Calcium Concentration Elevations in the Chloroplast

Plants need to sense increases in temperature to be able to adapt their physiology and development to survive; however, the mechanisms of heat perception are currently relatively poorly understood. Here we demonstrate that in response to elevated temperature, the free calcium concentration of the stroma of chloroplasts increases. This response is specific to the chloroplast, as no corresponding increase in calcium is seen in the cytosol. The chloroplast calcium response is dose dependent above a threshold. The magnitude of this calcium response is dependent upon absolute temperature, not the rate of heating. This response is dynamic: repeated stimulation leads to rapid attenuation of the response, which can be overcome by sensitization at a higher temperature. More long-term acclimation to different temperatures resets the basal sensitivity of the system, such that plants acclimated to lower temperatures are more sensitive than those acclimated to higher temperatures. The heat-induced chloroplast calcium response was partially dependent upon the calcium-sensing receptor CAS which has been shown previously to regulate other chloroplast calcium signaling responses. Taken together, our data demonstrate the ability of chloroplasts to sense absolute high temperature and produce commensurately quantitative stromal calcium response, the magnitude of which is a function of both current temperature and stress history.

Salt acclimation in sorghum plants by exogenous proline: physiological and biochemical changes and regulation of proline metabolism

Mitigation of deleterious effects of salinity promoted by exogenous proline can be partially explained by changes in proline enzymatic metabolism and expression of specific proline-related genes. Proline accumulation is a usual response to salinity. We studied the ability of exogenous proline to mitigate the salt harmful effects in sorghum (Sorghum bicolor) leaves. Ten-day-old plants were cultivated in Hoagland's nutrient solution in either the absence or presence of salinity (NaCl at 75 mM) and sprayed with distilled water or 30 mM proline solution. Salinity deleterious effects were alleviated by exogenous proline 14 days after treatment, with a return in growth and recovery of leaf area and photosynthetic parameters. Part of the salinity response reflected an improvement in ionic homeostasis, provided by reduction in Na⁺ and Cl^- ions and increases in K⁺ and Ca²⁺ ions as well as increases of compatible solutes. In addition, the application of proline decreased membrane damage and did not increase relative water content. Proline-treated salt-stressed plants displayed increase in proline content, a response counterbalanced by punctual modulation in proline synthesis (down-regulation of Δ¹-pyrroline-5-carboxylate synthetase activity) and degradation (up-regulation of proline dehydrogenase activity) enzymes. These responses were correlated with expression of specific proline-related genes (p5cs1 and prodh). Our findings clearly show that proline treatment results in favorable changes, reducing salt-induced damage and improving salt acclimation in sorghum plants.

Genome-wide identification and functional analysis of the cyclic nucleotide-gated channel gene family in Chinese cabbage

Cyclic nucleotide-gated channels (CNGCs) are a class of nonselective cationic channels that are widely found in animals and plants. Plant CNGCs participate in numerous biological functions that vary from development to stress tolerance. Most CNGC genes have been identified in plant genomes, but no such comprehensive study has yet been conducted on Chinese cabbage. In this study, thirty BrCNGC genes were identified, divided into five groups, and used for evolutionary analysis. We assigned names of all individual CNGC members on the basis of phylogenetic relationship with A. thaliana CNGCs. All BrCNGC genes were randomly distributed on chromosomes, and the A08 chromosome did not carry any CNGC gene. The CNGC genes of Chinese cabbage and A. thaliana from the same group displayed similar conserved motifs and gene structures. Especially the closer the homology, the higher the similarity. Quantitative expression analysis showed that most of the CNGC genes were expressed under four stresses, indicating that they play a key role in the stress response of Chinese cabbage. Expression patterns of 12 BrCNGC in the roots, stems, leaves, flowers, and siliques showed that BrCNGC8 and BrCNGC16 were specifically expressed only in flowers but not in other parts. This study lays a theoretical foundation for future research on the function of the CNGC gene family in Chinese cabbage.

Transcriptome Profiling Reveals Effects of Drought Stress on Gene Expression in Diploid Potato Genotype P3-198

Potato (Solanum tuberosum L.) is one of the three most important food crops worldwide; however, it is strongly affected by drought stress. The precise molecular mechanisms of drought stress response in potato are not very well understood. The diploid potato genotype P3-198 has been verified to be highly resistant to drought stress. Here, a time-course experiment was performed to identify drought resistance response genes in P3-198 under polyethylene glycol (PEG)-induced stress using RNA-sequencing. A total of 1665 differentially expressed genes (DEGs) were specifically identified, and based on gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, the transcription factor activity, protein kinase activity, and the plant hormone signal transduction process were significantly enriched. Annotation revealed that these DEGs mainly encode transcription factors, protein kinases, and proteins related to redox regulation, carbohydrate metabolism, and osmotic adjustment. In particular, genes encoding abscisic acid (ABA)-dependent signaling molecules were significantly differentially expressed, which revealed the important roles of the ABA-dependent signaling pathway in the early response of P3-198 to drought stress. Quantitative real-time PCR experimental verification confirmed the differential expression of genes in the drought resistance signaling pathway. Our results provide valuable information for understanding potato drought-resistance mechanisms, and also enrich the gene resources available for drought-resistant potato breeding.

Inhibitor of Sarco/Endoplasmic Reticulum Calcium-ATPase Impairs Multiple Steps of Paramyxovirus Replication

Sarco/endoplasmic reticulum calcium-ATPase (SERCA) is a membrane-bound cytosolic enzyme which is known to regulate the uptake of calcium into the sarco/endoplasmic reticulum. Herein, we demonstrate for the first time that SERCA can also regulate virus replication. Treatment of Vero cells with SERCA-specific inhibitor (Thapsigargin) at a concentration that is nontoxic to the cells significantly reduced Peste des petits ruminants virus (PPRV) and Newcastle disease virus (NDV) replication. Conversely, overexpression of SERCA rescued the inhibitory effect of Thapsigargin on virus replication. PPRV and NDV infection induced SERCA expression in Vero cells, which could be blocked by Thapsigargin. Besides inducing enhanced formation of cytoplasmic foci, Thapsigargin was shown to block viral entry into the target cells as well as synthesis of viral proteins. Furthermore, NDV was shown to acquire significant resistance to Thapsigargin upon long-term passage (P) in Vero cells. As compared to the P0 and P70-Control, the fusion (F) protein of P70-Thapsigargin virus exhibited a unique mutation at amino acid residue 104 (E104K), whereas no Thapsigargin-associated mutations were observed in HN gene. To the best of our knowledge, this is the first report describing the virus-supportive role of SERCA and a rare report suggesting that viruses may acquire resistance even in the presence of an inhibitor that targets a cellular factor.

De novo assembly of Agave sisalana transcriptome in response to drought stress provides insight into the tolerance mechanisms

Agave, monocotyledonous succulent plants, is endemic to arid regions of North America, exhibiting exceptional tolerance to their xeric environments. They employ various strategies to overcome environmental constraints, such as crassulacean acid metabolism, wax depositions, and protective leaf morphology. Genomic resources of Agave species have received little attention irrespective of their cultural, economic and ecological importance, which so far prevented the understanding of the molecular bases underlying their adaptations to the arid environment. In this study, we aimed to elucidate molecular mechanism(s) using transcriptome sequencing of A. sisalana. A de novo approach was applied to assemble paired-end reads. The expression study unveiled 3,095 differentially expressed unigenes between well-irrigated and drought-stressed leaf samples. Gene ontology and KEGG analysis specified a significant number of abiotic stress responsive genes and pathways involved in processes like hormonal responses, antioxidant activity, response to stress stimuli, wax biosynthesis, and ROS metabolism. We also identified transcripts belonging to several families harboring important drought-responsive genes. Our study provides the first insight into the genomic structure of A. sisalana underlying adaptations to drought stress, thus providing diverse genetic resources for drought tolerance breeding research.

Calcium- and calmodulin-regulated microtubule-associated proteins as signal-integration hubs at the plasma membrane-cytoskeleton nexus

Plant growth and development are a genetically predetermined series of events but can change dramatically in response to environmental stimuli, involving perpetual pattern formation and reprogramming of development. The rate of growth is determined by cell division and subsequent cell expansion, which are restricted and controlled by the cell wall-plasma membrane-cytoskeleton continuum, and are coordinated by intricate networks that facilitate intra- and intercellular communication. An essential role in cellular signaling is played by calcium ions, which act as universal second messengers that transduce, integrate, and multiply incoming signals during numerous plant growth processes, in part by regulation of the microtubule cytoskeleton. In this review, we highlight recent advances in the understanding of calcium-mediated regulation of microtubule-associated proteins, their function at the microtubule cytoskeleton, and their potential role as hubs in crosstalk with other signaling pathways.

Calcium-Nutrient and Messenger

Calcium is an essential element needed for growth and development of plants under both non-stressed and stress conditions. It thereby fulfills a dual function, being not only an important factor for cell wall and membrane stability, but also serving as a second messenger in many developmental and physiological processes, including the response of plants to biotic stress. The perception of non-self hereby induces an influx of calcium ions (Ca²⁺) into the cytosol, which is decoded into downstream responses ultimately leading to defense. Maintaining intracellular Ca²⁺ homeostasis is crucial for the ability to generate this signal. This review will describe the current knowledge of the mechanisms involved in uptake and transport of calcium as well as cellular homeostasis and signal generation, describing known genes involved and discussing possible implications the plant's nutritional status with regard to calcium might have on immunity.

EGTA reduces the inflorescence stem mechanical strength of herbaceous peony by modifying secondary wall biosynthesis

The mechanical strength of inflorescence stems is an important trait in cut flowers. Calcium ions (Ca²⁺) play a pivotal role in maintaining stem strength, but little is known about the underlying molecular mechanisms. In this study, we treated herbaceous peony (Paeonia lactiflora Pall.) with ethyl glycol tetraacetic acid (EGTA), an effective Ca²⁺ chelator, and used morphology indicators, spectroscopic analysis, histochemical staining, electron microscopy, and proteomic techniques to investigate the role of Ca²⁺ in inflorescence stem mechanical strength. The EGTA treatment reduced the mechanical strength of inflorescence stems, triggered the loss of Ca²⁺ from cell walls, and reduced lignin in thickened secondary walls in xylem cells as determined by spectroscopic analysis and histochemical staining. Electron microscopy showed that the EGTA treatment also resulted in significantly fewer xylem cell layers with thickened secondary walls as well as in reducing the thickness of these secondary walls. The proteomic analysis showed 1065 differentially expressed proteins (DEPs) at the full-flowering stage (S4). By overlapping the Kyoto encyclopedia of genes and genomes (KEGG) and gene ontology (GO) analysis results, we identified 43 DEPs involved in signal transduction, transport, energy metabolism, carbohydrate metabolism, and secondary metabolite biosynthesis. Using quantitative real-time polymerase chain reaction (qRT-PCR) analysis, we showed that EGTA treatment inhibited Ca²⁺ sensors and secondary wall biosynthesis-related genes. Our findings revealed that EGTA treatment reduced the inflorescence stem mechanical strength by reducing lignin deposition in xylem cells through altering the expression of genes involved in Ca²⁺ binding and secondary wall biosynthesis.

A roadmap towards a functional paradigm for learning & memory in plants

In plants, the acquisition, processing and storage of empirical information can result in the modification of their behavior according to the nature of the stimulus, and yet this area of research remained relatively understudied until recently. As the body of evidence supporting the inclusion of plants among the higher organisms demonstrating the adaptations to accomplish these tasks keeps increasing, the resistance by traditional botanists and agricultural scientists, who were at first cautious in allowing the application of animal models onto plant physiology and development, subsides. However, the debate retains much of its heat, a good part of it originating from the controversial use of nervous system terms to describe plant processes. By focusing on the latest findings on the cellular and molecular mechanisms underlying the well established processes of Learning and Memory, recognizing what has been accomplished and what remains to be explored, and without seeking to bootstrap neuronal characteristics where none are to be found, a roadmap guiding towards a comprehensive paradigm for Learning and Memory in plants begins to emerge. Meanwhile the applications of the new field of Plant Gnosophysiology look as promising as ever.

Phytoplasmas: An Introduction

Phytoplasmas are among the most recently discovered plant pathogens. They are wall-less prokaryotes restricted to phloem tissue, associated with diseases affecting several hundred plant species. The impact of phytoplasma diseases on agriculture is impressive and, at the present day, no effective curative strategy has been developed. The availability of rapid and sensitive techniques for phytoplasma detection as well as the possibility to study their relationship with the host plants is a prerequisite for the management of phytoplasma-associated diseases.

The Phytophthora RXLR Effector Avrblb2 Modulates Plant Immunity by Interfering With Ca2+ Signaling Pathway

In plants, subcellular fluctuations in Ca²⁺ ion concentration are among the earliest responses to biotic and abiotic stresses. Calmodulin, which is a ubiquitous Ca²⁺ ion sensor in eukaryotes, plays a major role in translating these Ca²⁺ signatures to cellular responses by interacting with numerous proteins located in plasma membranes, cytoplasm, organelles and nuclei. In this report, we show that one of the Phytophthora RXLR effector, Avrblb2, interacts with calmodulin at the plasma membrane of the plant cells. Using deletion and single amino acid mutagenesis, we found that calmodulin binds to the effector domain of Avrblb2. In addition, we show that most known homologs of Avrblb2 in three different Phytophthora species interact with different isoforms of calmodulin. Type of amino acids at position 69 in Avrblb2, which determines Rbi-blb2 resistance protein-mediated defense responses, is not involved in the Avrblb2-calmodulin interaction. Using in planta functional analyses, we show that calmodulin binding to Avrblb2 is required for its recognition by Rpi-blb2 to incite hypersensitive response. These findings suggest that Avrblb2 by interacting with calmodulin interfere with plant defense associated Ca²⁺ signaling in plants.

The Glutamate Receptor-Like Protein GLR3.7 Interacts With 14-3-3ω and Participates in Salt Stress Response in Arabidopsis thaliana

Ionotropic glutamate receptors (iGluRs) are ligand-gated cation channels that mediate fast excitatory neurotransmission in the mammalian central nervous system. In the model plant Arabidopsis thaliana, a family of 20 glutamate receptor-like proteins (GLRs) shares similarities to animal iGluRs in sequence and predicted secondary structure. However, the function of GLRs in plants is little known. In the present study, a serine site (Ser-860) of AtGLR3.7 phosphorylated by a calcium-dependent protein kinase (CDPK) was identified and confirmed by an in vitro kinase assay. Using a bimolecular fluorescence complementation and quartz crystal microbalance analyses, the physical interaction between AtGLR3.7 and the 14-3-3ω protein was confirmed. The mutation of Ser-860 to alanine abolished this interaction, indicating that Ser-860 is the 14-3-3ω binding site of AtGLR3.7. Compared with wild type, seed germination of the glr3.7-2 mutant was more sensitive to salt stress. However, the primary root growth of GLR3.7-S860A overexpression lines was less sensitive to salt stress than that of the wild-type line. In addition, the increase of cytosolic calcium ion concentration by salt stress was significantly lower in the glr3.7-2 mutant line than in the wild-type line. Moreover, association of 14-3-3 proteins to microsomal fractions was less in GLR3.7-S860A overexpression lines than in GLR3.7 overexpression line under 150 mM NaCl salt stress condition. Overall, our results indicated that GLR3.7 is involved in salt stress response in A. thaliana by affecting calcium signaling.

Control of Molecular Recognition via Modulation of the Nanoenvironment

Many biological processes are driven by the interaction of a host with a guest molecule. We show such interactions can be modulated by carefully defining the local molecular environment to give a specific chemical outcome. Particularly, the selectivity of a host toward two different ions (Ca²⁺ and Al³⁺) is defined by it being in solution or the physisorbed state. In solution, the host displays greater selectivity toward Ca²⁺. When physisorbed, the selectivity profile of the host is reversed with enhanced binding of Al³⁺. This demonstrates a single host molecule can be tailored to selectively bind multiple guests by altering its nanoenvironment.

Downstream components of the calmodulin signaling pathway in the rice salt stress response revealed by transcriptome profiling and target identification

BACKGROUND

Calmodulin (CaM) is an important calcium sensor protein that transduces Ca2+ signals in plant stress signaling pathways. A previous study has revealed that transgenic rice over-expressing the calmodulin gene OsCam1-1 (LOC_Os03g20370) is more tolerant to salt stress than wild type. To elucidate the role of OsCam1-1 in the salt stress response mechanism, downstream components of the OsCam1-1-mediated response were identified and investigated by transcriptome profiling and target identification.

RESULTS

Transcriptome profiling of transgenic 'Khao Dawk Mali 105' rice over-expressing OsCam1-1 and wild type rice showed that overexpression of OsCam1-1 widely affected the expression of genes involved in several cellular processes under salt stress, including signaling, hormone-mediated regulation, transcription, lipid metabolism, carbohydrate metabolism, secondary metabolism, photosynthesis, glycolysis, tricarboxylic acid (TCA) cycle and glyoxylate cycle. Under salt stress, the photosynthesis rate in the transgenic rice was slightly lower than in wild type, while sucrose and starch contents were higher, suggesting that energy and carbon metabolism were affected by OsCam1-1 overexpression. Additionally, four known and six novel CaM-interacting proteins were identified by cDNA expression library screening with the recombinant OsCaM1. GO terms enriched in their associated proteins that matched those of the differentially expressed genes affected by OsCam1-1 overexpression revealed various downstream cellular processes that could potentially be regulated by OsCaM1 through their actions.

CONCLUSIONS

The diverse cellular processes affected by OsCam1-1 overexpression and possessed by the identified CaM1-interacting proteins corroborate the notion that CaM signal transduction pathways compose a complex network of downstream components involved in several cellular processes. These findings suggest that under salt stress, CaM activity elevates metabolic enzymes involved in central energy pathways, which promote or at least maintain the production of energy under the limitation of photosynthesis.