Death don't have no mercy and neither does calcium: Arabidopsis CYCLIC NUCLEOTIDE GATED CHANNEL2 and innate immunity

Genome-wide characterization of nitric oxide-induced NBS-LRR genes from Arabidopsis thaliana and their association in monocots and dicots

BACKGROUND

Nitric oxide (NO) is pivotal in regulating the activity of NBS-LRR specific R genes, crucial components of the plant's immune system. It is noteworthy that previous research has not included a genome-wide analysis of NO-responsive NBS-LRR genes in plants.

RESULTS

The current study examined 29 NO-induced NBS-LRR genes from Arabidopsis thaliana, along with two monocots (rice and maize) and two dicots (soybean and tomato) using genome-wide analysis tools. These NBS-LRR genes were subjected to comprehensive characterization, including analysis of their physio-chemical properties, phylogenetic relationships, domain and motif identification, exon/intron structures, cis-elements, protein-protein interactions, prediction of S-Nitrosylation sites, and comparison of transcriptomic and qRT-PCR data. Results showed the diverse distribution of NBS-LRR genes across chromosomes, and variations in amino acid number, exons/introns, molecular weight, and theoretical isoelectric point, and they were found in various cellular locations like the plasma membrane, cytoplasm, and nucleus. These genes predominantly harbor the NB-ARC superfamily, LRR, LRR_8, and TIR domains, as also confirmed by motif analysis. Additionally, they feature species-specific PLN00113 superfamily and RX-CC_like domain in dicots and monocots, respectively, both responsive to defense against pathogen attacks. The NO-induced NBS-LRR genes of Arabidopsis reveal the presence of cis-elements responsive to phytohormones, light, stress, and growth, suggesting a wide range of responses mediated by NO. Protein-protein interactions, coupled with the prediction of S-Nitrosylation sites, offer valuable insights into the regulatory role of NO at the protein level within each respective species.

CONCLUSION

These above findings aimed to provide a thorough understanding of the impact of NO on NBS-LRR genes and their relationships with key plant species.

Transcriptome, hormonal, and secondary metabolite changes in leaves of DEFENSE NO DEATH 1 (DND1) silenced potato plants

DEFENSE NO DEATH 1 (DND1) is a cyclic nucleotide-gated ion channel protein. Earlier, it was shown that the silencing of DND1 in the potato (Solanum tuberosum L.) leads to resistance to late blight, powdery mildew, and gray mold diseases. At the same time, however, it can reduce plant growth and cause leaf necrosis. To obtain knowledge of the molecular events behind the pleiotropic effect of DND1 downregulation in the potato, metabolite and transcriptome analyses were performed on three DND1 silenced lines of the cultivar 'Désirée.' A massive increase in the salicylic acid content of leaves was detected. Concentrations of jasmonic acid and chlorogenic acid and their derivatives were also elevated. Expression of 1866 genes was altered in the same way in all three DND1 silenced lines, including those related to the synthesis of secondary metabolites. The activation of several alleles of leaf rust, late blight, and other disease resistance genes, as well as the induction of pathogenesis-related genes, was detected. WRKY and NAC transcription factor families were upregulated, whereas bHLHs were downregulated, indicating their central role in transcriptome changes. These results suggest that the maintenance of the constitutive defense state leads to the reduced growth of DND1 silenced potato plants.

Positive roles of the Ca2+ sensors GbCML45 and GbCML50 in improving cotton Verticillium wilt resistance

As a universal second messenger, cytosolic calcium (Ca2+) functions in multifaceted intracellular processes, including growth, development and responses to biotic/abiotic stresses in plant. The plant-specific Ca2+ sensors, calmodulin and calmodulin-like (CML) proteins, function as members of the second-messenger system to transfer Ca2+ signal into downstream responses. However, the functions of CMLs in the responses of cotton (Gossypium spp.) after Verticillium dahliae infection, which causes the serious vascular disease Verticillium wilt, remain elusive. Here, we discovered that the expression level of GbCML45 was promoted after V. dahliae infection in roots of cotton, suggesting its potential role in Verticillium wilt resistance. We found that knockdown of GbCML45 in cotton plants decreased resistance while overexpression of GbCML45 in Arabidopsis thaliana plants enhanced resistance to V. dahliae infection. Furthermore, there was physiological interaction between GbCML45 and its close homologue GbCML50 by using yeast two-hybrid and bimolecular fluorescence assays, and both proteins enhanced cotton resistance to V. dahliae infection in a Ca2+-dependent way in a knockdown study. Detailed investigations indicated that several defence-related pathways, including salicylic acid, ethylene, reactive oxygen species and nitric oxide signalling pathways, as well as accumulations of lignin and callose, are responsible for GbCML45- and GbCML50-modulated V. dahliae resistance in cotton. These results collectively indicated that GbCML45 and GbCML50 act as positive regulators to improve cotton Verticillium wilt resistance, providing potential targets for exploitation of improved Verticillium wilt-tolerant cotton cultivars by genetic engineering and molecular breeding.

Plant Immunity: At the Crossroads of Pathogen Perception and Defense Response

Plants are challenged by different microbial pathogens that affect their growth and productivity. However, to defend pathogen attack, plants use diverse immune responses, such as pattern-triggered immunity (PTI), effector-triggered immunity (ETI), RNA silencing and autophagy, which are intricate and regulated by diverse signaling cascades. Pattern-recognition receptors (PRRs) and nucleotide-binding leucine-rich repeat (NLR) receptors are the hallmarks of plant innate immunity because they can detect pathogen or related immunogenic signals and trigger series of immune signaling cascades at different cellular compartments. In plants, most commonly, PRRs are receptor-like kinases (RLKs) and receptor-like proteins (RLPs) that function as a first layer of inducible defense. In this review, we provide an update on how plants sense pathogens, microbe-associated molecular patterns (PAMPs or MAMPs), and effectors as a danger signals and activate different immune responses like PTI and ETI. Further, we discuss the role RNA silencing, autophagy, and systemic acquired resistance as a versatile host defense response against pathogens. We also discuss early biochemical signaling events such as calcium (Ca2+), reactive oxygen species (ROS), and hormones that trigger the activation of different plant immune responses. This review also highlights the impact of climate-driven environmental factors on host-pathogen interactions.

Weighted gene co-expression network analysis reveals hub genes regulating response to salt stress in peanut

Peanut (Arachis hypogaea L.) is an important oilseed crop worldwide. However, soil salinization becomes one of the main limiting factors of peanut production. Therefore, developing salt-tolerant varieties and understanding the molecular mechanisms of salt tolerance is important to protect peanut yield in saline areas. In this study, we selected four peanut varieties with contrasting response to salt challenges with T1 and T2 being tolerance and S1 and S2 being susceptible. High-throughput RNA sequencing resulted in more than 314.63 Gb of clean data from 48 samples. We identified 12,057 new genes, 7,971of which have functional annotations. KEGG pathway enrichment analysis of uniquely expressed genes in salt-tolerant peanut revealed that upregulated genes in the root are involved in the MAPK signaling pathway, fatty acid degradation, glycolysis/gluconeogenesis, and upregulated genes in the shoot were involved in plant hormone signal transduction and the MAPK signaling pathway. Na+ content, K+ content, K+/ Na+, and dry mass were measured in root and shoot tissues, and two gene co-expression networks were constructed based on weighted gene co-expression network analysis (WGCNA) in root and shoot. In this study, four key modules that are highly related to peanut salt tolerance in root and shoot were identified, plant hormone signal transduction, phenylpropanoid biosynthesis, starch and sucrose metabolism, flavonoid biosynthesis, carbon metabolism were identified as the key biological processes and metabolic pathways for improving peanut salt tolerance. The hub genes include genes encoding ion transport (such as HAK8, CNGCs, NHX, NCL1) protein, aquaporin protein, CIPK11 (CBL-interacting serine/threonine-protein kinase 11), LEA5 (late embryogenesis abundant protein), POD3 (peroxidase 3), transcription factor, and MAPKKK3. There were some new salt-tolerant genes identified in peanut, including cytochrome P450, vinorine synthase, sugar transport protein 13, NPF 4.5, IAA14, zinc finger CCCH domain-containing protein 62, beta-amylase, fatty acyl-CoA reductase 3, MLO-like protein 6, G-type lectin S-receptor-like serine/threonine-protein kinase, and kinesin-like protein KIN-7B. The identification of key modules, biological pathways, and hub genes in this study enhances our understanding of the molecular mechanisms underlying salt tolerance in peanuts. This knowledge lays a theoretical foundation for improving and innovating salt-tolerant peanut germplasm.

New insight into Ca2+ -permeable channel in plant immunity

Calcium ions (Ca2+ ) are crucial intracellular second messengers in eukaryotic cells. Upon pathogen perception, plants generate a transient and rapid increase in cytoplasmic Ca2+ levels, which is subsequently decoded by Ca2+ sensors and effectors to activate downstream immune responses. The elevation of cytosolic Ca2+ is commonly attributed to Ca2+ influx mediated by plasma membrane-localized Ca2+ -permeable channels. However, the contribution of Ca2+ release triggered by intracellular Ca2+ -permeable channels in shaping Ca2+ signaling associated with plant immunity remains poorly understood. This review discusses recent advances in understanding the mechanism underlying the shaping of Ca2+ signatures upon the activation of immune receptors, with particular emphasis on the identification of intracellular immune receptors as non-canonical Ca2+ -permeable channels. We also discuss the involvement of Ca2+ release from the endoplasmic reticulum in generating Ca2+ signaling during plant immunity.

Long small RNA76113 targets CYCLIC NUCLEOTIDE-GATED ION CHANNEL 5 to repress disease resistance in rice

Small RNAs are widely involved in plant immune responses. However, the role of long small RNAs (25 to 40 nt) in monocot plant disease resistance is largely unknown. Here, we identified a long small RNA (lsiR76113) from rice (Oryza sativa) that is downregulated by Magnaporthe oryzae infection and targets a gene encoding CYCLIC NUCLEOTIDE-GATED CHANNEL 5 (CNGC5). The cngc5 mutant lines were more susceptible to M. oryzae than the wild type, while knocking down lsiR76113 in transgenic rice plants promoted pathogen resistance. A protoplast transient expression assay showed that OsCNGC5 promotes Ca2+ influx. These results demonstrate that OsCNGC5 enhances rice resistance to rice blast by increasing the cytosolic Ca2+ concentration. Importantly, exogenous Ca2+ application enhanced rice M. oryzae resistance by affecting reactive oxygen species (ROS) production. Moreover, cngc5 mutants attenuated the PAMP-triggered immunity response, including chitin-induced and flg22-induced ROS bursts and protein phosphorylation in the mitogen-activated protein kinase cascade, indicating that OsCNGC5 is essential for PAMP-induced calcium signaling in rice. Taken together, these results suggest that lsiR76113-mediated regulation of Ca2+ influx is important for PTI responses and disease resistance in rice.

GhCNGC13 and 32 Act as Critical Links between Growth and Immunity in Cotton

Cyclic nucleotide-gated ion channels (CNGCs) remain poorly studied in crop plants, most of which are polyploid. In allotetraploid Upland cotton (Gossypium hirsutum), silencing GhCNGC13 and 32 impaired plant growth and shoot apical meristem (SAM) development, while triggering plant autoimmunity. Both growth hormones (indole-3-acetic acid and gibberellin) and stress hormones (abscisic acid, salicylic acid, and jasmonate) increased, while leaf photosynthesis decreased. The silenced plants exhibited an enhanced resistance to Botrytis cinerea; however, Verticillium wilt resistance was weakened, which was associated with LIPOXYGENASE2 (LOX2) downregulation. Transcriptomic analysis of silenced plants revealed 4835 differentially expressed genes (DEGs) with functional enrichment in immunity and photosynthesis. These DEGs included a set of transcription factors with significant over-representation in the HSF, NAC, and WRKY families. Moreover, numerous members of the GhCNGC family were identified among the DEGs, which may indicate a coordinated action. Collectively, our results suggested that GhCNGC13 and 32 functionally link to photosynthesis, plant growth, and plant immunity. We proposed that GhCNGC13 and 32 play a critical role in the "growth-defense tradeoff" widely observed in crops.

Calmodulin and calmodulin-like protein-mediated plant responses to biotic stresses

Plants have evolved a set of finely regulated mechanisms to respond to various biotic stresses. Transient changes in intracellular calcium (Ca2+ ) concentration have been well documented to act as cellular signals in coupling environmental stimuli to appropriate physiological responses with astonishing accuracy and specificity in plants. Calmodulins (CaMs) and calmodulin-like proteins (CMLs) are extensively characterized as important classes of Ca2+ sensors. The spatial-temporal coordination between Ca2+ transients, CaMs/CMLs and their target proteins is critical for plant responses to environmental stresses. Ca2+ -loaded CaMs/CMLs interact with and regulate a broad spectrum of target proteins, such as ion transporters (including channels, pumps, and antiporters), transcription factors, protein kinases, protein phosphatases, metabolic enzymes and proteins with unknown biological functions. This review focuses on mechanisms underlying how CaMs/CMLs are involved in the regulation of plant responses to diverse biotic stresses including pathogen infections and herbivore attacks. Recent discoveries of crucial functions of CaMs/CMLs and their target proteins in biotic stress resistance revealed through physiological, molecular, biochemical, and genetic analyses have been described, and intriguing insights into the CaM/CML-mediated regulatory network are proposed. Perspectives for future directions in understanding CaM/CML-mediated signalling pathways in plant responses to biotic stresses are discussed. The application of accumulated knowledge of CaM/CML-mediated signalling in biotic stress responses into crop cultivation would improve crop resistance to various biotic stresses and safeguard our food production in the future.

Arabinogalactan Alleviates Lipopolysaccharide-Induced Intestinal Epithelial Barrier Damage through Adenosine Monophosphate-Activated Protein Kinase/Silent Information Regulator 1/Nuclear Factor Kappa-B Signaling Pathways in Caco-2 Cells

Intestinal epithelial barrier (IEB) damage is an important aspect in inflammatory bowel disease (IBD). The objective of this study was to explore the protective effects and mechanisms of arabinogalactan (AG) on lipopolysaccharide (LPS)-stimulated IEB dysfunction. The results show that AG (1, 2, and 5 mg/mL) mitigated 100 μg/mL LPS-stimulated IEB dysfunction through increasing transepithelial electrical resistance (TEER), reducing fluorescein isothiocyanate (FITC)-dextran (4 kDa) flux, and up-regulating the protein and mRNA expression of tight junction (TJ) proteins (Claudin-1, Zonula occludens-1 (ZO-1) and Occludin). In addition, AG ameliorated LPS-stimulated IEB dysfunction by reducing interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and IL-1β levels, decreasing the reactive oxygen species (ROS) level, increasing superoxide dismutase (SOD) activity, increasing the glutathione (GSH) level, and decreasing the levels of malondialdehyde (MDA) and intracellular calcium ([Ca2+]i). Furthermore, 2 mg/mL AG up-regulated the expression of silent information regulator 1 (SIRT1), the phosphorylated adenosine monophosphate-activated protein kinase (AMPK), and peroxisome proliferator-activated receptor gamma coactivator (PGC)-1α and inhibited the phosphorylation of nuclear factor kappa-B (NF-κB) and the inhibitor of NF-κBα (IκBα). Therefore, AG could maintain IEB integrity by activating AMPK/SIRT1 and inhibiting the NF-κB signaling pathway. In conclusion, AG can regulate the AMPK/SIRT1/NF-κB signaling pathway to reduce inflammation and oxidative stress, thus alleviating LPS-stimulated IEB damage.

Transcriptomic Analysis of the Response of Susceptible and Resistant Bitter Melon (Momordica charantia L.) to Powdery Mildew Infection Revealing Complex Resistance via Multiple Signaling Pathways

The challenge of mitigating the decline in both yield and fruit quality due to the intrusion of powdery mildew (PM) fungus looms as a pivotal concern in the domain of bitter melon cultivation. Yet, the intricate mechanisms that underlie resistance against this pathogen remain inscrutable for the vast majority of bitter melon variants. In this inquiry, we delve deeply into the intricate spectrum of physiological variations and transcriptomic fluctuations intrinsic to the PM-resistant strain identified as '04-17-4' (R), drawing a sharp contrast with the PM-susceptible counterpart, designated as '25-15' (S), throughout the encounter with the pathogenic agent Podosphaera xanthii. In the face of the challenge presented by P. xanthii, the robust cultivar displays an extraordinary capacity to prolong the initiation of the pathogen's primary growth stage. The comprehensive exploration culminates in the discernment of 6635 and 6954 differentially expressed genes (DEGs) in R and S strains, respectively. Clarification through the lens of enrichment analyses reveals a prevalence of enriched DEGs in pathways interconnected with phenylpropanoid biosynthesis, the interaction of plants with pathogens, and the signaling of plant hormones. Significantly, in the scope of the R variant, DEGs implicated in the pathways of plant-pathogen interaction phenylpropanoid biosynthesis, encompassing components such as calcium-binding proteins, calmodulin, and phenylalanine ammonia-lyase, conspicuously exhibit an escalated tendency upon the encounter with P. xanthii infection. Simultaneously, the genes governing the synthesis and transduction of SA undergo a marked surge in activation, while their counterparts in the JA signaling pathway experience inhibition following infection. These observations underscore the pivotal role played by SA/JA signaling cascades in choreographing the mechanism of resistance against P. xanthii in the R variant. Moreover, the recognition of 40 P. xanthii-inducible genes, encompassing elements such as pathogenesis-related proteins, calmodulin, WRKY transcription factors, and Downy mildew resistant 6, assumes pronounced significance as they emerge as pivotal contenders in the domain of disease control. The zenith of this study harmonizes multiple analytical paradigms, thus capturing latent molecular participants and yielding seminal resources crucial for the advancement of PM-resistant bitter melon cultivars.

Identification of CNGCs in Glycine max and Screening of Related Resistance Genes after Fusarium solani Infection

Cyclic nucleotide-gated channels (CNGCs), non-selective cation channels localised on the plasmalemma, are involved in growth, development, and regulatory mechanisms in plants during adverse stress. To date, CNGC gene families in multiple crops have been identified and analysed. However, there have been no systematic studies on the evolution and development of CNGC gene families in legumes. Therefore, in the present study, via transcriptome analysis, we identified 143 CNGC genes in legumes, and thereafter, classified and named them according to the grouping method used for Arabidopsis thaliana. Functional verification for disease stress showed that four GmCNGCs were specifically expressed in the plasmalemma during the stress process. Further, functional enrichment analysis showed that their mode of participation and coordination included inorganic ion concentration regulation inside and outside the membrane via the transmembrane ion channel and participation in stress regulation via signal transduction. The CNGC family genes in G. max involved in disease stress were also identified and physiological stress response and omics analyses were also performed. Our preliminary results revealed the basic laws governing the involvement of CNGCs in disease resistance in G. max, providing important gene resources and a theoretical reference for the breeding of resistant soybean.

The captivating role of calcium in plant-microbe interaction

Plant immune response is fascinating due to the complete absence of a humoral system. The adaptive immune response in plants relies on the intracellular orchestration of signalling molecules or intermediates associated with transcriptional reprogramming. Plant disease response phenomena largely depend on pathogen recognition, signal perception, and intracellular signal transduction. The pathogens possess specific pathogen-associated molecular patterns (PAMP) or microbe-associated molecular patterns (MAMP), which are first identified by pattern recognition receptors (PRRs) of host plants for successful infection. After successful pathogen recognition, the defence response is initiated within plants. The first line of non-specific defence response is called PAMP-triggered immunity (PTI), followed by the specific robust signalling is called effector-triggered immunity (ETI). Calcium plays a crucial role in both PTI and ETI. The biphasic induction of reactive oxygen species (ROS) is inevitable in any plant-microbe interaction. Calcium ions play crucial roles in the initial oxidative burst and ROS induction. Different pathogens can induce calcium accumulation in the cytosol ([Ca²⁺]_Cyt), called calcium signatures. These calcium signatures further control the diverse defence-responsive proteins in the intracellular milieu. These calcium signatures then activate calcium-dependent protein kinases (CDPKs), calcium calmodulins (CaMs), calcineurin B-like proteins (CBLs), etc., to impart intricate defence signalling within the cell. Decoding this calcium ionic map is imperative to unveil any plant microbe interplay and modulate defence-responsive pathways. Hence, the present review is unique in developing concepts of calcium signature in plants and their subsequent decoding mechanism. This review also intends to articulate early sensing of calcium oscillation, signalling events, and comprehensive mechanistic roles of calcium within plants during pathogenic ingression. This will accumulate and summarize the exciting roles of calcium ions in plant immunity and provide the foundation for future research.

Phenotypes of cyclic nucleotide-gated cation channel mutants: probing the nature of native channels

Plant cyclic nucleotide gated channels (CNGCs) facilitate cytosolic Ca²⁺ influx as an early step in numerous signaling cascades. CNGC-mediated Ca²⁺ elevations are essential for plant immune defense and high temperature thermosensing. In the present study, we evaluated phenotypes of CNGC2, CNGC4, CNGC6, and CNGC12 null mutants in these two pathways. It is shown CNGC2, CNGC4, and CNGC6 physically interact in vivo, whereas CNGC12 does not. CNGC involvement in immune signaling was evaluated by monitoring mutant response to elicitor peptide Pep3. Pep3 response cascades involving CNGCs included mitogen-activated kinase activation mediated by Ca²⁺ -dependent protein kinase phosphorylation. Pep3-induced reactive oxygen species generation was impaired in cngc2, cngc4, and cngc6, but not in cngc12, suggesting that CNGC2, CNGC4, and CNGC6 (which physically interact) may be components of a multimeric CNGC channel complex for immune signaling. However, unlike cngc2 and cngc4, cngc6 is not sensitive to high Ca²⁺ and displays no pleiotropic dwarfism. All four cngc mutants showed thermotolerance compared to wild-type, although CNGC12 does not interact with the other three CNGCs. These results imply that physically interacting CNGCs may, in some cases, function in a signaling cascade as components of a heteromeric channel complex, although this may not be the case in other signaling pathways.

Multiple cyclic nucleotide-gated channels function as ABA-activated Ca2+ channels required for ABA-induced stomatal closure in Arabidopsis

Abscisic acid (ABA)-activated inward Ca2+-permeable channels in the plasma membrane (PM) of guard cells are required for the initiation and regulation of ABA-specific cytosolic Ca2+ signaling and stomatal closure in plants. But the identities of the PM Ca2+ channels are still unknown. We hypothesized that the ABA-activated Ca2+ channels consist of multiple CYCLIC NUCLEOTIDE-GATED CHANNEL (CNGC) proteins from the CNGC family, which is known as a Ca2+-permeable channel family in Arabidopsis (Arabidopsis thaliana). In this research, we observed high expression of multiple CNGC genes in Arabidopsis guard cells, namely CNGC5, CNGC6, CNGC9, and CNGC12. The T-DNA insertional loss-of-function quadruple mutant cngc5-1 cngc6-2 cngc9-1 cngc12-1 (hereafter c5/6/9/12) showed a strong ABA-insensitive phenotype of stomatal closure. Further analysis revealed that ABA-activated Ca2+ channel currents were impaired, and ABA-specific cytosolic Ca2+ oscillation patterns were disrupted in c5/6/9/12 guard cells compared with in wild-type guard cells. All ABA-related phenotypes of the c5/6/9/12 mutant were successfully rescued by the expression of a single gene out of the four CNGCs under the respective native promoter. Thus, our findings reveal a type of ABA-activated PM Ca2+ channel comprising multiple CNGCs, which is essential for ABA-specific Ca2+ signaling of guard cells and ABA-induced stomatal closure in Arabidopsis.

Expanding roles for S-nitrosylation in the regulation of plant immunity

Following pathogen recognition, plant cells produce a nitrosative burst resulting in a striking increase in nitric oxide (NO), altering the redox state of the cell, which subsequently helps orchestrate a plethora of immune responses. NO is a potent redox cue, efficiently relayed between proteins through its co-valent attachment to highly specific, powerfully reactive protein cysteine (Cys) thiols, resulting in formation of protein S-nitrosothiols (SNOs). This process, known as S-nitrosylation, can modulate the function of target proteins, enabling responsiveness to cellular redox changes. Key targets of S-nitrosylation control the production of reactive oxygen species (ROS), the transcription of immune-response genes, the triggering of the hypersensitive response (HR) and the establishment of systemic acquired resistance (SAR). Here, we bring together recent advances in the control of plant immunity by S-nitrosylation, furthering our appreciation of how changes in cellular redox status reprogramme plant immune function.

The vacuolar H+/Ca transporter CAX1 participates in submergence and anoxia stress responses

A plant's oxygen supply can vary from normal (normoxia) to total depletion (anoxia). Tolerance to anoxia is relevant to wetland species, rice (Oryza sativa) cultivation, and submergence tolerance of crops. Decoding and transmitting calcium (Ca) signals may be an important component to anoxia tolerance; however, the contribution of intracellular Ca transporters to this process is poorly understood. Four functional cation/proton exchangers (CAX1-4) in Arabidopsis (Arabidopsis thaliana) help regulate Ca homeostasis around the vacuole. Our results demonstrate that cax1 mutants are more tolerant to both anoxic conditions and submergence. Using phenotypic measurements, RNA-sequencing, and proteomic approaches, we identified cax1-mediated anoxia changes that phenocopy changes present in anoxia-tolerant crops: altered metabolic processes, diminished reactive oxygen species production post anoxia, and altered hormone signaling. Comparing wild-type and cax1 expressing genetically encoded Ca indicators demonstrated altered cytosolic Ca signals in cax1 during reoxygenation. Anoxia-induced Ca signals around the plant vacuole are involved in the control of numerous signaling events related to adaptation to low oxygen stress. This work suggests that cax1 anoxia response pathway could be engineered to circumvent the adverse effects of flooding that impair production agriculture.

Concerted actions of PRR- and NLR-mediated immunity

Plants utilise cell-surface immune receptors (functioning as pattern recognition receptors, PRRs) and intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) to detect pathogens. Perception of pathogens by these receptors activates immune signalling and resistance to infections. PRR- and NLR-mediated immunity have primarily been considered parallel processes contributing to disease resistance. Recent studies suggest that these two pathways are interdependent and converge at multiple nodes. This review summarises and provides a perspective on these convergent points.

Beauty and the pathogens: A leaf-less control presents a better image of Cymbidium orchids defense strategy

Biological control is a safe way of combating plant diseases using the living organisms. For the precise use of microbial biological control agents, the genetic information on the hypersensitive response (HR), and defense-related gene induction pathways of plants are necessary. Orchids are the most prominent stakeholders of floriculture industry, and owing to their long-awaited flowering pattern, disease control is imperative to allow healthy vegetative growth that spans more than 2 years in most of the orchids. We observed leaf-less flowering in three orchid species (Cymbidium ensifolium, C. goeringii and C. sinense). Using these materials as reference, we performed transcriptome profiling for healthy leaves from non-infected plants to identify genes specifically involved in plant-pathogen interaction pathway. For this pathway, a total of 253 differentially expressed genes (DEGs) were identified in C. ensifolium, 189 DEGs were identified in C. goeringii and 119 DEGs were found in C. sinense. These DEGs were mainly related to bacterial secretion systems, FLS2, CNGCs and EFR, regulating HR, stomatal closure and defense-related gene induction. FLS2 (LRR receptor-like serine/threonine kinase) contained the highest number of DEGs among three orchid species, followed by calmodulin. Highly upregulated gene sets were found in C. sinense as compared to other species. The great deal of DEGs, mainly the FLS2 and EFR families, related to defense and immunity responses can effectively direct the future of biological control of diseases for orchids.

Calcium channels and transporters: Roles in response to biotic and abiotic stresses

Calcium (Ca2+) serves as a ubiquitous second messenger by mediating various signaling pathways and responding to numerous environmental conditions in eukaryotes. Therefore, plant cells have developed complex mechanisms of Ca2+ communication across the membrane, receiving the message from their surroundings and transducing the information into cells and organelles. A wide range of biotic and abiotic stresses cause the increase in [Ca2+]cyt as a result of the Ca2+ influx permitted by membrane-localized Ca2+ permeable cation channels such as CYCLIC NUCLEOTIDE-GATE CHANNELs (CNGCs), and voltage-dependent HYPERPOLARIZATION-ACTIVATED CALCIUM2+ PERMEABLE CHANNELs (HACCs), as well as GLUTAMATE RECEPTOR-LIKE RECEPTORs (GLRs) and TWO-PORE CHANNELs (TPCs). Recently, resistosomes formed by some NUCLEOTIDE-BINDING LEUCINE-RICH REPEAT RECEPTORs (NLRs) are also proposed as a new type of Ca2+ permeable cation channels. On the contrary, some Ca2+ transporting membrane proteins, mainly Ca2+-ATPase and Ca2+/H+ exchangers, are involved in Ca2+ efflux for removal of the excessive [Ca2+]cyt in order to maintain the Ca2+ homeostasis in cells. The Ca2+ efflux mechanisms mediate the wide ranges of cellular activities responding to external and internal stimuli. In this review, we will summarize and discuss the recent discoveries of various membrane proteins involved in Ca2+ influx and efflux which play an essential role in fine-tuning the processing of information for plant responses to abiotic and biotic stresses.

Ca2+ signals in plant immunity

Calcium ions function as a key second messenger ion in eukaryotes. Spatially and temporally defined cytoplasmic Ca²⁺ signals are shaped through the concerted activity of ion channels, exchangers, and pumps in response to diverse stimuli; these signals are then decoded through the activity of Ca²⁺ -binding sensor proteins. In plants, Ca²⁺ signaling is central to both pattern- and effector-triggered immunity, with the generation of characteristic cytoplasmic Ca²⁺ elevations in response to potential pathogens being common to both. However, despite their importance, and a long history of scientific interest, the transport proteins that shape Ca²⁺ signals and their integration remain poorly characterized. Here, we discuss recent work that has both shed light on and deepened the mysteries of Ca²⁺ signaling in plant immunity.

Comparative transcriptome analysis of resistant and susceptible wheat in response to Rhizoctonia cerealis

BACKGROUND

Sheath blight is an important disease caused by Rhizoctonia cerealis that affects wheat yields worldwide. No wheat varieties have been identified with high resistance or immunity to sheath blight. Understanding the sheath blight resistance mechanism is essential for controlling this disease. In this study, we investigated the response of wheat to Rhizoctonia cerealis infection by analyzing the cytological changes and transcriptomes of common wheat 7182 with moderate sensitivity to sheath blight and H83 with moderate resistance.

RESULTS

The cytological observation showed that the growth of Rhizoctonia cerealis on the surface and its expansion inside the leaf sheath tissue were more rapid in the susceptible material. According to the transcriptome sequencing results, a total of 88685 genes were identified in both materials, including 20156 differentially expressed genes (DEGs) of which 12087 was upregulated genes and 8069 was downregulated genes. At 36 h post-inoculation, compared with the uninfected control, 11498 DEGs were identified in resistant materials, with 5064 downregulated genes and 6434 upregulated genes, and 13058 genes were detected in susceptible materials, with 6759 downregulated genes and 6299 upregulated genes. At 72 h post-inoculation, compared with the uninfected control, 6578 DEGs were detected in resistant materials, with 2991 downregulated genes and 3587 upregulated genes, and 7324 genes were detected in susceptible materials, with 4119 downregulated genes and 3205 upregulated genes. Functional annotation and enrichment analysis showed that the main pathways enriched for the DEGs included biosynthesis of secondary metabolites, carbon metabolism, plant hormone signal transduction, and plant-pathogen interaction. In particular, phenylpropane biosynthesis pathway is specifically activated in resistant variety H83 after infection. Many DEGs also belonged to the MYB, AP2, NAC, and WRKY transcription factor families.

CONCLUSIONS

Thus, we suggest that the normal functioning of plant signaling pathways and differences in the expression of key genes and transcription factors in some important metabolic pathways may be important for defending wheat against sheath blight. These findings may facilitate further exploration of the sheath blight resistance mechanism in wheat and the cloning of related genes.

Cyclic nucleotide-gated ion channel 6 is involved in extracellular ATP signaling and plant immunity

Extracellular ATP (eATP) is known to act as a danger signal in both plants and animals. In plants, eATP is recognized by the plasma membrane (PM)-localized receptor P2K1 (LecRK-I.9). Among the first measurable responses to eATP addition is a rapid rise in cytoplasmic free calcium levels ([Ca²⁺ ]_cyt ), which requires P2K1. However, the specific transporter/channel proteins that mediate this rise in [Ca²⁺ ]_cyt are unknown. Through a forward genetic screen, we identified an Arabidopsis ethylmethanesulfonate (EMS) mutant impaired in the [Ca²⁺ ]_cyt response to eATP. Positional cloning revealed that the mutation resided in the cngc6 gene, which encodes cyclic nucleotide-gated ion channel 6 (CNGC6). Mutation of the CNGC6 gene led to a notable decrease in the PM inward Ca²⁺ current in response to eATP. eATP-induced mitogen-activated protein kinase activation and gene expression were also significantly lower in cngc6 mutant plants. In addition, cngc6 mutant plants were also more susceptible to the bacterial pathogen Pseudomonas syringae. Taken together, our results indicate that CNGC6 plays a crucial role in mediating eATP-induced [Ca²⁺ ]_cyt signaling, as well as plant immunity.

Changes in Gene Expression in Leaves of Cacao Genotypes Resistant and Susceptible to Phytophthora palmivora Infection

Black pod rot, caused by Phytophthora palmivora, is a devastating disease of Theobroma cacao L. (cacao) leading to huge losses for farmers and limiting chocolate industry supplies. To understand resistance responses of cacao leaves to P. palmivora, Stage 2 leaves of genotypes Imperial College Selection 1 (ICS1), Colección Castro Naranjal 51 (CCN51), and Pound7 were inoculated with zoospores and monitored for symptoms up to 48 h. Pound7 consistently showed less necrosis than ICS1 and CCN51 48 h after inoculation. RNA-Seq was carried out on samples 24 h post inoculation. A total of 24,672 expressed cacao genes were identified, and 2,521 transcripts showed induction in at least one P. palmivora-treated genotype compared to controls. There were 115 genes induced in the P. palmivora-treated samples in all three genotypes. Many of the differentially expressed genes were components of KEGG pathways important in plant defense signal perception (the plant MAPK signaling pathway, plant hormone signal transduction, and plant pathogen interactions), and plant defense metabolite biosynthesis (phenylpropanoid biosynthesis, α-linolenic acid metabolism, ethylene biosynthesis, and terpenoid backbone biosynthesis). A search of putative cacao resistance genes within the cacao transcriptome identified 89 genes with prominent leucine-rich repeat (LRR) domains, 170 protein kinases encoding genes, 210 genes with prominent NB-ARC domains, 305 lectin-related genes, and 97 cysteine-rich RK genes. We further analyzed the cacao leaf transcriptome in detail focusing on gene families-encoding proteins important in signal transduction (MAP kinases and transcription factors) and direct plant defense (Germin-like, ubiquitin-associated, lectin-related, pathogenesis-related, glutathione-S-transferases, and proteases). There was a massive reprogramming of defense gene processes in susceptible cacao leaf tissue after infection, which was restricted in the resistant genotype Pound7. Most genes induced in Pound7 were induced in ICS1/CCN51. The level of induction was not always proportional to the infection level, raising the possibility that genes are responding to infection more strongly in Pound7. There were also defense-associated genes constitutively differentially expressed at higher levels in specific genotypes, possibly providing a prepositioned defense. Many of the defense genes occur in blocks where members are constitutively expressed at different levels, and some members are induced by Ppal infection. With further study, the identified candidate genes and gene blocks may be useful as markers for breeding disease-resistant cacao genotypes against P. palmivora.

Cyclic nucleotide gated channel genes (CNGCs) in Rosaceae: genome-wide annotation, evolution and the roles on Valsa canker resistance

In Rosaceae, tandem duplication caused the drastic expansion of CNGC gene family Group I. The members MdCN11 and MdCN19 negatively regulate Valsa canker resistance. Apple (Malus domestica) and pear (Pyrus bretschneideri and P. communis) are important fruit crops in Rosaceae family but are suffering from threats of Valsa canker. Cyclic nucleotide-gated ion channels (CNGCs) take crucial roles in plant immune responses. In the present study, a total of 355 CNGCs was identified from 8 Rosaceae plants. Based on phylogenetic analysis, 540 CNGCs from 18 plants (8 in Rosaceae and 10 others) could be divided into four groups. Group I was greatly expanded in Rosaceae resulted from tandem duplications. A large number of cis-acting regulatory elements (cis-elements) responsive to signals from multiple stresses and hormones were identified in the promoter regions of CNGCs in Malus spp. and Pyrus spp. Expressions of most Group I members were obviously up-regulated in Valsa canker susceptible varieties but not in the resistant ones. Furthermore, overexpression of the MdCN11 and MdCN19 in both apple fruits and 'Duli' (P. betulifolia) suspension cells compromised Valsa canker resistance. Overexpression of MdCN11 induced expression of hypersensitive response (HR)-related genes. In conclusion, tandem duplication resulted in a drastic expansion of CNGC Group I members in Rosaceae. Among these, MdCN11 and MdCN19 negatively regulate the Valsa canker resistance via inducting HR.

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.

Plant programmed cell death meets auxin signalling

Both auxin signalling and programmed cell death (PCD) are essential components of a normally functioning plant. Auxin underpins plant growth and development, as well as regulating plant defences against environmental stresses. PCD, a genetically controlled pathway for selective elimination of redundant, damaged or infected cells, is also a key element of many developmental processes and stress response mechanisms in plants. An increasing body of evidence suggests that auxin signalling and PCD regulation are often connected. While generally auxin appears to suppress cell death, it has also been shown to promote PCD events, most likely via stimulation of ethylene biosynthesis. Intriguingly, certain cells undergoing PCD have also been suggested to control the distribution of auxin in plant tissues, by either releasing a burst of auxin or creating an anatomical barrier to auxin transport and distribution. These recent findings indicate novel roles of localized PCD events in the context of plant development such as control of root architecture, or tissue regeneration following injury, and suggest exciting possibilities for incorporation of this knowledge into crop improvement strategies.

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.

The fast and the furious: rapid long-range signaling in plants

Plants possess a systemic signaling system whereby local stimuli can lead to rapid, plant-wide responses. In addition to the redistribution of chemical messengers that range from RNAs and peptides to hormones and metabolites, a communication system acting through the transmission of electrical, Ca2+, reactive oxygen species and potentially even hydraulic signals has also been discovered. This latter system can propagate signals across many cells each second and researchers are now beginning to uncover the molecular machineries behind this rapid communications network. Thus, elements such as the reactive oxygen species producing NAPDH oxidases and ion channels of the two pore channel, glutamate receptor-like and cyclic nucleotide gated families are all required for the rapid propagation of these signals. Upon arrival at their distant targets, these changes trigger responses ranging from the production of hormones, to changes in the levels of primary metabolites and shifts in patterns of gene expression. These systemic responses occur within seconds to minutes of perception of the initial, local signal, allowing for the rapid deployment of plant-wide responses. For example, an insect starting to chew on just a single leaf triggers preemptive antiherbivore defenses throughout the plant well before it has a chance to move on to the next leaf on its menu.

New Insight into Justicidin B Pathway and Production in Linum austriacum

Lignans are the main secondary metabolites synthetized by Linum species as plant defense compounds but they are also valuable for human health, in particular, for novel therapeutics. In this work, Linum austriacum in vitro cultures, cells (Cc), adventitious roots (ARc) and hairy roots (HRc) were developed for the production of justicidin B through elicitation with methyl jasmonate (MeJA) and coronatine (COR). The performances of the cultures were evaluated for their stability, total phenols content and antioxidant ability. NMR was used to identify justicidin B and isojusticidin B and HPLC to quantify the production, highlighting ARc and HRc as the highest productive tissues. MeJA and COR treatments induced the synthesis of justicidin B more than three times and the synthesis of other compounds. RNA-sequencing and a de novo assembly of L. austriacum ARc transcriptome was generated to identify the genes activated by MeJA. Furthermore, for the first time, the intracellular localization of justicidin B in ARc was investigated through microscopic analysis. Then, HRc was chosen for small-scale production in a bioreactor. Altogether, our results improve knowledge on justicidin B pathway and cellular localization in L. austriacum for future scale-up processes.

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.

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.

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.

Melatonin and Its Homologs Induce Immune Responses via Receptors trP47363-trP13076 in Nicotiana benthamiana

Melatonin (N-acetyl-5-methoxytryptamine), a naturally occurring small molecule, can protect plants against abiotic stress after exogenous treatmenting with it. It is not known if melatonin homologs, such as 5-methoxytryptamine and 5-methoxyindole, that are easy and more cost-effective to synthesize can stimulate the plant immune system in the same manner as melatonin. In the present study, we assessed the biological activity of the melatonin homologs, 5-methoxytryptamin and 5-methoxyindole. The results showed that melatonin and its homologs all induced disease resistance against Phytophthora nicotianae in Nicotiana benthamiana plants. The application of all three compounds also induced stomatal closure and the production of reactive oxygen species. Gene expression analysis indicated that the expression of genes involved in hydrogen peroxide (H₂O₂), nitric oxide (NO) production, and salicylic acid (SA) biosynthesis was significantly upregulated by all three compounds. Four homologs of the melatonin receptors were identified by blasting search with the phytomelatonin receptor in Arabidopsis. Molecular docking studies were also used to identify four putative melatonin receptors in N. benthamiana. Further experimentation revealed that silencing of the melatonin receptors trP47363 and trP13076 in N. benthamiana compromised the induction of stomatal closure, PR-1a gene expression and SA accumulation by all three compounds. Collectively, our data indicate that the induction of defense responses in N. benthamiana by melatonin, 5-methoxytryptamine, and 5-methoxyindole involves the melatonin receptors trP47363 and trP13076.

Advances and perspectives in the metabolomics of stomatal movement and the disease triangle

Crops are continuously exposed to microbial pathogens that cause tremendous yield losses worldwide. Stomatal pores formed by pairs of specialized guard cells in the leaf epidermis represent a major route of pathogen entry. Guard cells have an essential role as a first line of defense against pathogens. Metabolomics is an indispensable systems biology tool that has facilitated discovery and functional studies of metabolites that regulate stomatal movement in response to pathogens and other environmental factors. Guard cells, pathogens and environmental factors constitute the "stomatal disease triangle". The aim of this review is to highlight recent advances toward understanding the stomatal disease triangle in the context of newly discovered signaling molecules, hormone crosstalk, and consequent molecular changes that integrate pathogens and environmental sensing into stomatal immune responses. Future perspectives on emerging single-cell studies, multiomics and molecular imaging in the context of stomatal defense are discussed. Advances in this important area of plant biology will inform rational crop engineering and breeding for enhanced stomatal defense without disruption of other pathways that impact crop yield.

Lipopolysaccharide Enhances Tanshinone Biosynthesis via a Ca2+-Dependent Manner in Salvia miltiorrhiza Hairy Roots

Tanshinones, the major bioactive components in Salvia miltiorrhiza Bunge (Danshen), are synthesized via the mevalonic acid (MVA) pathway or the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway and the downstream biosynthesis pathway. In this study, the bacterial component lipopolysaccharide (LPS) was utilized as a novel elicitor to induce the wild type hairy roots of S. miltiorrhiza. HPLC analysis revealed that LPS treatment resulted in a significant accumulation of cryptotanshinone (CT) and dihydrotanshinone I (DTI). qRT-PCR analysis confirmed that biosynthesis genes such as SmAACT and SmHMGS from the MVA pathway, SmDXS and SmHDR from the MEP pathway, and SmCPS, SmKSL and SmCYP76AH1 from the downstream pathway were markedly upregulated by LPS in a time-dependent manner. Furthermore, transcription factors SmWRKY1 and SmWRKY2, which can activate the expression of SmDXR, SmDXS and SmCPS, were also increased by LPS. Since Ca²⁺ signaling is essential for the LPS-triggered immune response, Ca²⁺ channel blocker LaCl₃ and CaM antagonist W-7 were used to investigate the role of Ca²⁺ signaling in tanshinone biosynthesis. HPLC analysis demonstrated that both LaCl₃ and W-7 diminished LPS-induced tanshinone accumulation. The downstream biosynthesis genes including SmCPS and SmCYP76AH1 were especially regulated by Ca²⁺ signaling. To summarize, LPS enhances tanshinone biosynthesis through SmWRKY1- and SmWRKY2-regulated pathways relying on Ca²⁺ signaling. Ca²⁺ signal transduction plays a key role in regulating tanshinone biosynthesis in S. miltiorrhiza.

Crosstalk of hydrogen sulfide and nitric oxide requires calcium to mitigate impaired photosynthesis under cadmium stress by activating defense mechanisms in Vigna radiata

Hydrogen sulfide (H₂S) and nitric oxide (NO) have been known to affect vast number of processes in plants under abiotic stresses. Also, calcium (Ca) works as a second messenger in plants, which underpins the abiotic stress-induced damage. However, the sequence of action of these signaling molecules against cadmium (Cd)-induced cellular oxidative damage remains unidentified. Therefore, we studied the synergistic actions and/or relationship of signaling molecules and Ca-dependent activation of tolerance mechanisms in Vigna radiata seedlings under Cd stress. The present study shows that exogenous Ca supplemented to Cd-stressed V. radiata seedlings reduced Cd accumulation and improved the activity of nitrate reductase, and L/D-cysteine desulfhydrase (LCD/DCD) that resulted in improved synthesis of NO and H₂S content. Application of Ca also elevated the level of cysteine (Cys) by upregulating the activity of Cys-synthesizing enzymes serine acetyltransferase and O-acetylserine(thiol)lyase in Cd-stressed seedlings. Maintenance of Cys pool under Cd stress contributed to improved H₂S content which together with Ca and NO improved antioxidant enzymes and components of ascorbate-glutathione (AsA-GSH) cycle. All these collectively regulated the activity of NADPH oxidase and glycolate oxidase, resulting in the inhibition of Cd-induced generation of reactive oxygen species. The elevated level of Cys also assisted the Cd-stressed seedlings in maintaining GSH pool which retained normal functioning of AsA-GSH cycle and led to enhanced content of phytochelatins coupled with reduced Cd content. The positive effect of these events manifested in an enhanced rate of photosynthesis, carbohydrate accumulation, and growth attributes of the plants. On the contrary, addition of NO scavenger cPTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide], H₂S scavenger HT (Hypotaurine) and Ca-chelator EGTA (Ethylene glycol-bis(b-aminoethylether)-N,N,N',N'-tetraacetic acid) again developed a condition similar to stress and positive effect of the signaling molecules was abolished. The findings of the study postulate that Ca in association with NO and H₂S mitigates Cd-induced impairment and enhances the tolerance of the V. radiata plants against Cd stress. The results of the study also substantiate that Ca acts both upstream as well as downstream of NO signals whereas, H₂S acts downstream of Ca and NO during Cd-stress responses of the plants.

Transcriptomic analysis reveals insights into the response to Hop stunt viroid (HSVd) in sweet cherry (Prunus avium L.) fruits

Hop stunt viroid (HSVd) is a member of the genus Hostuviroid of the family Pospiviroidae and has been found in a wide range of herbaceous and woody hosts. It causes serious dapple fruit symptoms on infected sweet cherry, notably inducing cherry tree decay. In order to better understand the molecular mechanisms of HSVd infection in sweet cherry fruit, transcriptome analysis of HSVd-infected and healthy sweet cherry fruits was carried out. A total of 1,572 differentially expressed genes (DEGs) were identified, involving 961 upregulated DEGs and 611 downregulated DEGs. Functional analysis indicated that the DEGs were mainly involved in plant hormone signal transduction, plant-pathogen interactions, secondary metabolism, and the MAPK signaling pathway. In addition, C2H2 zinc finger, MYB, bHLH, AP2/ERF, C2C2-dof, NAC and WRKY transcription factors can respond to HSVd infection. In order to confirm the high-throughput sequencing results, 16 DEGs were verified by RT-qPCR analysis. The results provided insight into the pathways and genes of sweet cherry fruit in response to HSVd infection.

Stomatal immunity against fungal invasion comprises not only chitin-induced stomatal closure but also chitosan-induced guard cell death

Many pathogenic fungi exploit stomata as invasion routes, causing destructive diseases of major cereal crops. Intensive interaction is expected to occur between guard cells and fungi. In the present study, we took advantage of well-conserved molecules derived from the fungal cell wall, chitin oligosaccharide (CTOS), and chitosan oligosaccharide (CSOS) to study how guard cells respond to fungal invasion. In Arabidopsis, CTOS induced stomatal closure through a signaling mediated by its receptor CERK1, Ca2+, and a major S-type anion channel, SLAC1. CSOS, which is converted from CTOS by chitin deacetylases from invading fungi, did not induce stomatal closure, suggesting that this conversion is a fungal strategy to evade stomatal closure. At higher concentrations, CSOS but not CTOS induced guard cell death in a manner dependent on Ca2+ but not CERK1. These results suggest that stomatal immunity against fungal invasion comprises not only CTOS-induced stomatal closure but also CSOS-induced guard cell death.

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.

Calcium spikes, waves and oscillations in plant development and biotic interactions

The calcium ion (Ca²⁺) is a universal signal in all eukaryotic cells. A fundamental question is how Ca²⁺, a simple cation, encodes complex information with high specificity. Extensive research has established a two-step process (encoding and decoding) that governs the specificity of Ca²⁺ signals. While the encoding mechanism entails a complex array of channels and transporters, the decoding process features a number of Ca²⁺ sensors and effectors that convert Ca²⁺ signals into cellular effects. Along this general paradigm, some signalling components may be highly conserved, but others are divergent among different organisms. In plant cells, Ca²⁺ participates in numerous signalling processes, and here we focus on the latest discoveries on Ca²⁺-encoding mechanisms in development and biotic interactions. In particular, we use examples such as polarized cell growth of pollen tube and root hair in which tip-focused Ca²⁺ oscillations specify the signalling events for rapid cell elongation. In plant-microbe interactions, Ca²⁺ spiking and oscillations hold the key to signalling specificity: while pathogens elicit cytoplasmic spiking, symbiotic microorganisms trigger nuclear Ca²⁺ oscillations. Herbivore attacks or mechanical wounding can trigger Ca²⁺ waves traveling a long distance to transmit and convert the local signal to a systemic defence program in the whole plant. What channels and transporters work together to carve out the spatial and temporal patterns of the Ca²⁺ fluctuations? This question has remained enigmatic for decades until recent studies uncovered Ca²⁺ channels that orchestrate specific Ca²⁺ signatures in each of these processes. Future work will further expand the toolkit for Ca²⁺-encoding mechanisms and place Ca²⁺ signalling steps into larger signalling networks.

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.

CATION-CHLORIDE CO-TRANSPORTER 1 (CCC1) Mediates Plant Resistance against Pseudomonas syringae

Plasma membrane (PM) depolarization functions as an initial step in plant defense signaling pathways. However, only a few ion channels/transporters have been characterized in the context of plant immunity. Here, we show that the Arabidopsis (Arabidopsis thaliana) Na+:K+:2Cl- (NKCC) cotransporter CCC1 has a dual function in plant immunity. CCC1 functions independently of PM depolarization and negatively regulates pathogen-associated molecular pattern-triggered immunity. However, CCC1 positively regulates plant basal and effector-triggered resistance to Pseudomonas syringae pv. tomato (Pst) DC3000. In line with the compromised immunity to Pst DC3000, ccc1 mutants show reduced expression of genes encoding enzymes involved in the biosynthesis of antimicrobial peptides, camalexin, and 4-OH-ICN, as well as pathogenesis-related proteins. Moreover, genes involved in cell wall and cuticle biosynthesis are constitutively down-regulated in ccc1 mutants, and the cell walls of these mutants exhibit major changes in monosaccharide composition. The role of CCC1 ion transporter activity in the regulation of plant immunity is corroborated by experiments using the specific NKCC inhibitor bumetanide. These results reveal a function for ion transporters in immunity-related cell wall fortification and antimicrobial biosynthesis.

Calcium Signaling Commands Phagosome Maturation Process

Phagosome-lysosome (P-L) fusion is one of the central immune-effector responses of host. It is known that phagosome maturation process is associated with numerous signaling cascades and among these, important role of calcium (Ca²⁺) signaling has been realized recently. Ca²⁺ plays key roles in actin rearrangement, activation of NADPH oxidase and protein kinase C (PKC). Involvement of Ca²⁺ in these cellular processes directs phagosomal maturation process. Some of the intracellular pathogens have acquired the strategies to modulate Ca²⁺ associated pathways to block P-L fusion process. In this review we have described the mechanism of Ca²⁺ signals that influence P-L fusion by controlling ROS, actin and PKC signaling cascades. We have also discussed the strategies implemented by the intracellular pathogens to manipulate Ca²⁺ signaling to consequently subvert P-L fusion. A detail study of factors associated in manipulating Ca²⁺ signaling may provide new insights for the development of therapeutic tools for more effective treatment options against infectious diseases.

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.

Nitric Oxide, an Essential Intermediate in the Plant-Herbivore Interaction

Reactive nitrogen species (RNS), mainly nitric oxide (NO), are highly reactive molecules with a prominent role in plant response to numerous stresses including herbivores, although the information is still very limited. This perspective article compiles the current progress in determining the NO function, as either a signal molecule, a metabolic intermediate, or a toxic oxidative product, as well as the contribution of molecules associated with NO metabolic pathway in the generation of plant defenses against phytophagous arthropods, in particular to insects and acari.

Plant immune signaling: Advancing on two frontiers

Plants have evolved multiple defense strategies to cope with pathogens, among which plant immune signaling that relies on cell-surface localized and intracellular receptors takes fundamental roles. Exciting breakthroughs were made recently on the signaling mechanisms of pattern recognition receptors (PRRs) and intracellular nucleotide-binding site (NBS) and leucine-rich repeat (LRR) domain receptors (NLRs). This review summarizes the current view of PRRs activation, emphasizing the most recent discoveries about PRRs' dynamic regulation and signaling mechanisms directly leading to downstream molecular events including mitogen-activated protein kinase (MAPK) activation and calcium (Ca²⁺ ) burst. Plants also have evolved intracellular NLRs to perceive the presence of specific pathogen effectors and trigger more robust immune responses. We also discuss the current understanding of the mechanisms of NLR activation, which has been greatly advanced by recent breakthroughs including structures of the first full-length plant NLR complex, findings of NLR sensor-helper pairs and novel biochemical activity of Toll/interleukin-1 receptor (TIR) domain.

CRISPR/Cas9-Mediated Mutagenesis of MdCNGC2 in Apple Callus and VIGS-Mediated Silencing of MdCNGC2 in Fruits Improve Resistance to Botryosphaeria dothidea

Cyclic nucleotide-gated ion channels (CNGCs) have been reported to be involved in multiple plant physiological processes. Their involvement in plant immunity has been studied in several herbal plant species. It remains unclear whether CNGCs in woody plants play a similar role in plant immunity. In the present study, we identified an apple CNGC (designated as MdCNGC2), which is the homolog of Arabidopsis CNGC2. Analysis of tissue distribution revealed that MdCNGC2 was expressed in all tested tissues. Abundant transcripts of MdCNGC2 were observed in leaves and shoot bark. Low expression was observed in fruits and roots. MdCNGC2 expression was induced in apple callus and shoot bark by Botryosphaeria dothidea. The induction of MdCNGC2 was significantly higher in susceptible cultivars "Fuji," "Ralls Janet," and "Gala" compared to the resistant cultivar "Jiguan," suggesting that MdCNGC2 may be a negative regulator of resistance to B. dothidea. MdCNGC2 mutagenesis mediated by gene editing based on the CRISPR/Cas9 system led to constitutive accumulation of SA in apple callus. A culture filtrate of B. dothidea (BCF) induced the expression of several defense-related genes including MdPR1, MdPR2, MdPR4, MdPR5, MdPR8, and MdPR10a. Moreover, the induction of these genes was significantly higher in mdcngc2 mutant (MUT) callus than in wild type (WT) callus. Further analysis showed that the spread of B. dothidea was significantly lower on MUT callus than on WT callus. Knockdown of the MdCNGC2 gene reduced lesions caused by B. dothidea in apple fruits. These results collectively indicate that MdCNGC2 is a negative regulator of resistance to B. dothidea in apple callus.

Autophagy-Like Cell Death Regulates Hydrogen Peroxide and Calcium Ion Distribution in Xa3/Xa26-Mediated Resistance to Xanthomonas oryzae pv. oryzae

The broad-spectrum and durable resistance gene Xa3/Xa26 against Xanthomonas oryzae pv. oryzae (Xoo) has been widely exploited in rice production in China. But the cytological features of the Xa3/Xa26-mediated resistance reaction have been rarely reported. This study reveals the cytological characteristics of the Xa3/Xa26-mediated resistance reaction against Xoo to uncover the functions of hypersensitive response programmed cell death (HR-PCD) in rice. Autophagy-like cell death, which was characterized by double-membrane bodies appearance in xylem parenchyma cell and mesophyll cell, was inhibited by autophagy inhibitor 3-methyladenin (3-MA). The autophagy-related genes were induced to reach a high level in resistance reaction. The hydrogen peroxide (H₂O₂) maintained a low concentration on the plasma membrane. The calcium ions localized on the apoplast were transferred into the vacuole. The autophagy inhibitor (3-MA) impaired Xa3/Xa26-mediated resistance by promoting the accumulation of H₂O₂, and inhibited the transfer of extracellular calcium ions into the vacuole in the xylem parenchyma cells and mesophyll cells. Therefore, the HR-PCD belongs to autophagy-like cell death in the Xa3/Xa26-mediated resistance reaction. These results suggest that the autophagy-like cell death participates in the Xa3/Xa26-mediated resistance by negatively regulating H₂O₂ accumulation, in order to abolish oxidative stress and possibly activate calcium ion signals in xylem parenchyma cells of the rice leaf.