The chimeric Arabidopsis CYCLIC NUCLEOTIDE-GATED ION CHANNEL11/12 activates multiple pathogen resistance responses

From molecule to cell: the expanding frontiers of plant immunity

In recent years, the field of plant immunity has witnessed remarkable breakthroughs. During the co-evolution between plants and pathogens, plants have developed a wealth of intricate defense mechanisms to safeguard their survival. Newly identified immune receptors have added unexpected complexity to the surface and intracellular sensor networks, enriching our understanding of the ongoing plant-pathogen interplay. Deciphering the molecular mechanisms of resistosome shapes our understanding of these mysterious molecules in plant immunity. Moreover, technological innovations are expanding the horizon of the plant-pathogen battlefield into spatial and temporal scales. While the development provides new opportunities for untangling the complex realm of plant immunity, challenges remain in uncovering plant immunity across spatiotemporal dimensions from both molecular and cellular levels.

Calcium homeostasis and signaling in plant immunity

Calcium (Ca2+) signaling consists of three steps: (1) initiation of a change in cellular Ca2+ concentration in response to a stimulus, (2) recognition of the change through direct binding of Ca2+ by its sensors, (3) transduction of the signal to elicit downstream responses. Recent studies have uncovered a central role for Ca2+ signaling in both layers of immune responses initiated by plasma membrane (PM) and intracellular receptors, respectively. These advances in our understanding are attributed to several lines of research, including invention of genetically-encoded Ca2+ reporters for the recording of intracellular Ca2+ signals, identification of Ca2+ channels and their gating mechanisms, and functional analysis of Ca2+ binding proteins (Ca2+ sensors). This review analyzes the recent literature that illustrates the importance of Ca2+ homeostasis and signaling in plant innate immunity, featuring intricate Ca2+dependent positive and negative regulations.

Oomycete effector AVRblb2 targets cyclic nucleotide-gated channels through calcium sensors to suppress pattern-triggered immunity

Transient and rapid increase in cytosolic Ca2+ plays a crucial role in plant-pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). Cyclic nucleotide-gated channels (CNGCs) have been implicated in mediating this Ca2+ influx; however, their regulatory mechanisms remain poorly understood. Here, we have found that AVRblb2 requires the calmodulin (CaM) and calmodulin-like (CML) proteins as co-factors to interact with the NbCNGCs, resulting in the formation of AVRblb2-CaM/CML-NbCNGCs complex. Furthermore, CaM and CML are dissociated from NbCNGC18 during PTI response to increase Ca2+ influx; however, Avrblb2 inhibits calcium channel activation by disrupting the release of CaM and CML from NbCNGC18. Following recognition of PAMP, NbCNGC18 forms active heteromeric channels with other NbCNGCs, which may give selectivity of CNGC complex against diverse signals for fine-tuning of cytosolic Ca2+ level to mediate appropriate responses. Silencing of multiple NbCNGCs compromised the function of AVRblb2 on the pathogenicity of Phytophthora infestans, confirming that AVRblb2 contributes to pathogen virulence by targeting CNGCs. Our findings provide new insights into the regulation of CNGCs in PTI and the role of pathogen effectors in manipulating host cell physiology to promote infection.

Understanding the mechanobiology of phytoacoustics through molecular Lens: Mechanisms and future perspectives

BACKGROUND

How plants emit, perceive, and respond to sound vibrations (SVs) is a long-standing question in the field of plant sensory biology. In recent years, there have been numerous studies on how SVs affect plant morphological, physiological, and biochemical traits related to growth and adaptive responses. For instance, under drought SVs navigate plant roots towards water, activate their defence responses against stressors, and increase nectar sugar in response to pollinator SVs. Also, plants emit SVs during stresses which are informative in terms of ecological and adaptive perspective. However, the molecular mechanisms underlying the SV perception and emission in plants remain largely unknown. Therefore, deciphering the complexity of plant-SV interactions and identifying bonafide receptors and signaling players will be game changers overcoming the roadblocks in phytoacoustics.

AIM OF REVIEW

The aim of this review is to provide an overview of recent developments in phytoacoustics. We primarily focuss on SV signal perception and transduction with current challenges and future perspectives.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Timeline breakthroughs in phytoacoustics have constantly shaped our understanding and belief that plants may emit and respond to SVs like other species. However, unlike other plant mechanostimuli, little is known about SV perception and signal transduction. Here, we provide an update on phytoacoustics and its ecological importance. Next, we discuss the role of cell wall receptor-like kinases, mechanosensitive channels, intracellular organelle signaling, and other key players involved in plant-SV receptive pathways that connect them. We also highlight the role of calcium (Ca2+), reactive oxygen species (ROS), hormones, and other emerging signaling molecules in SV signal transduction. Further, we discuss the importance of molecular, biophysical, computational, and live cell imaging tools for decoding the molecular complexity of acoustic signaling in plants. Finally, we summarised the role of SV priming in plants and discuss how SVs could modulate plant defense and growth trade-offs during other stresses.

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.

The herbicide bensulfuron-methyl inhibits rice seedling development by blocking calcium ion flux in the OsCNGC12 channel

Identification of translocator protein-related genes involved in bensulfuron-methyl (BSM) uptake and transport in rice could facilitate the development of herbicide-tolerant cultivars by inactivating them. This study found that the OsCNGC12 mutants not only reduced BSM uptake but also compromised the Ca2 ⁺ efflux caused by BSM in the roots, regulating dynamic equilibrium of Ca2 ⁺ inside the cell and conferring non-target-site tolerance to BSM.

Comparative Transcriptome Analysis between Resistant and Susceptible Pakchoi Cultivars in Response to Downy Mildew

Downy mildew caused by the obligate parasite Hyaloperonospora brassicae is a devastating disease for Brassica species. Infection of Hyaloperonospora brassicae often leads to yellow spots on leaves, which significantly impacts quality and yield of pakchoi. In the present study, we conducted a comparative transcriptome between the resistant and susceptible pakchoi cultivars in response to Hyaloperonospora brassicae infection. A total of 1073 disease-resistance-related differentially expressed genes were identified using a Venn diagram. The Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that these genes were mainly involved in plant-pathogen interaction, plant hormone signal transduction, and other photosynthesis-related metabolic processes. Analysis of the phytohormone content revealed that salicylic acid increased significantly in the resistant material after inoculation with Hyaloperonospora brassicae, whereas the contents of jasmonic acid, abscisic acid, and 1-aminocyclopropane-1-carboxylic acid decreased. Exogenous salicylic acid treatment also significantly upregulated Hyaloperonospora brassicae-induced genes, which further confirmed a crucial role of salicylic acid during pakchoi defense against Hyaloperonospora brassicae. Based on these findings, we suggest that the salicylic-acid-mediated signal transduction contributes to the resistance of pakchoi to downy mildew, and PAL1, ICS1, NPR1, PR1, PR5, WRKY70, WRKY33, CML43, CNGC9, and CDPK15 were involved in this responsive process. Our findings evidently contribute to revealing the molecular mechanism of pakchoi defense against Hyaloperonospora brassicae.

Ion Channels in Electrical Signaling in Higher Plants

Electrical signals (ESs) appearing in plants under the action of various external factors play an important role in adaptation to changing environmental conditions. Generation of ES in higher plant cells is associated with activation of Ca2+, K+, and anion fluxes, as well as with changes in the activity of plasma membrane H+-ATPase. In the present review, molecular nature of the ion channels contributing to ESs transmission in higher plants is analyzed based on comparison of the data from molecular-genetic and electrophysiological studies. Based on such characteristics of ion channels as selectivity, activation mechanism, and intracellular and tissue localization, those ion channels that meet the requirements for potential participation in ES generation were selected from a wide variety of ion channels in higher plants. Analysis of the data of experimental studies performed on mutants with suppressed or enhanced expression of a certain channel gene revealed those channels whose activation contributes to ESs formation. The channels responsible for Ca2+ flux during generation of ESs include channels of the GLR family, for K+ flux - GORK, for anions - MSL. Consideration of the prospects of further studies suggests the need to combine electrophysiological and genetic approaches along with analysis of ion concentrations in intact plants within a single study.

Protein interactome of 3',5'-cAMP reveals its role in regulating the actin cytoskeleton

Identification of protein interactors is ideally suited for the functional characterization of small molecules. 3',5'-cAMP is an evolutionary ancient signaling metabolite largely uncharacterized in plants. To tap into the physiological roles of 3',5'-cAMP, we used a chemo-proteomics approach, thermal proteome profiling (TPP), for the unbiased identification of 3',5'-cAMP protein targets. TPP measures shifts in the protein thermal stability upon ligand binding. Comprehensive proteomics analysis yielded a list of 51 proteins significantly altered in their thermal stability upon incubation with 3',5'-cAMP. The list contained metabolic enzymes, ribosomal subunits, translation initiation factors, and proteins associated with the regulation of plant growth such as CELL DIVISION CYCLE 48. To functionally validate obtained results, we focused on the role of 3',5'-cAMP in regulating the actin cytoskeleton suggested by the presence of actin among the 51 identified proteins. 3',5'-cAMP supplementation affected actin organization by inducing actin-bundling. Consistent with these results, the increase in 3',5'-cAMP levels, obtained either by feeding or by chemical modulation of 3',5'-cAMP metabolism, was sufficient to partially rescue the short hypocotyl phenotype of the actin2 actin7 mutant, severely compromised in actin level. The observed rescue was specific to 3',5'-cAMP, as demonstrated using a positional isomer 2',3'-cAMP, and true for the nanomolar 3',5'-cAMP concentrations reported for plant cells. In vitro characterization of the 3',5'-cAMP-actin pairing argues against a direct interaction between actin and 3',5'-cAMP. Alternative mechanisms by which 3',5'-cAMP would affect actin dynamics, such as by interfering with calcium signaling, are discussed. In summary, our work provides a specific resource, 3',5'-cAMP interactome, as well as functional insight into 3',5'-cAMP-mediated regulation in plants.

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.

Genome-Wide Analysis and Expression of Cyclic Nucleotide-Gated Ion Channel (CNGC) Family Genes under Cold Stress in Mango (Mangifera indica)

The 'king of fruits' mango (Mangifera indica) is widely cultivated in tropical areas and has been threatened by frequent extreme cold weather. Cyclic nucleotide-gated ion channel (CNGC) genes have an important function in the calcium-mediated development and cold response of plants. However, few CNGC-related studies are reported in mango, regardless of the mango cold stress response. In this study, we identified 43 CNGC genes in mango showing tissue-specific expression patterns. Five MiCNGCs display more than 3-fold gene expression induction in the fruit peel and leaf under cold stress. Among these, MiCNGC9 and MiCNGC13 are significantly upregulated below 6 °C, suggesting their candidate functions under cold stress. Furthermore, cell membrane integrity was damaged at 2 °C in the mango leaf, as shown by the content of malondialdehyde (MDA), and eight MiCNGCs are positively correlated with MDA contents. The high correlation between MiCNGCs and MDA implies MiCNGCs might regulate cell membrane integrity by regulating MDA content. Together, these findings provide a valuable guideline for the functional characterization of CNGC genes and will benefit future studies related to cold stress and calcium transport in mango.

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.

Molecular Interactions Between Leptosphaeria maculans and Brassica Species

Canola is an important oilseed crop, providing food, feed, and fuel around the world. However, blackleg disease, caused by the ascomycete Leptosphaeria maculans, causes significant yield losses annually. With the recent advances in genomic technologies, the understanding of the Brassica napus-L. maculans interaction has rapidly increased, with numerous Avr and R genes cloned, setting this system up as a model organism for studying plant-pathogen associations. Although the B. napus-L. maculans interaction follows Flor's gene-for-gene hypothesis for qualitative resistance, it also puts some unique spins on the interaction. This review discusses the current status of the host-pathogen interaction and highlights some of the future gaps that need addressing moving forward.

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.

Con-Ca2+ -tenating plant immune responses via calcium-permeable cation channels

Calcium serves as a second messenger in a variety of developmental and physiological processes and has long been identified as important for plant immune responses. We discuss recent discoveries regarding plant immune-related calcium-permeable channels and how the two intertwined branches of the plant immune system are intricately linked to one another through calcium signalling. Cell surface immune receptors carefully tap the immense calcium gradient that exists between apoplast and cytoplasm in a short burst via tightly regulated plasma membrane (PM)-resident cation channels. Intracellular immune receptors form atypical calcium-permeable cation channels at the PM and mediate a prolonged calcium influx, overcoming the deleterious influence of pathogen effectors and enhancing plant immune responses.

A tale of many families: calcium channels in plant immunity

Plants launch a concerted immune response to dampen potential infections upon sensing microbial pathogen and insect invasions. The transient and rapid elevation of the cytosolic calcium concentration [Ca2+]cyt is among the essential early cellular responses in plant immunity. The free Ca2+ concentration in the apoplast is far higher than that in the resting cytoplasm. Thus, the precise regulation of calcium channel activities upon infection is the key for an immediate and dynamic Ca2+ influx to trigger downstream signaling. Specific Ca2+ signatures in different branches of the plant immune system vary in timing, amplitude, duration, kinetics, and sources of Ca2+. Recent breakthroughs in the studies of diverse groups of classical calcium channels highlight the instrumental role of Ca2+ homeostasis in plant immunity and cell survival. Additionally, the identification of some immune receptors as noncanonical Ca2+-permeable channels opens a new view of how immune receptors initiate cell death and signaling. This review aims to provide an overview of different Ca2+-conducting channels in plant immunity and highlight their molecular and genetic mode-of-actions in facilitating immune signaling. We also discuss the regulatory mechanisms that control the stability and activity of these channels.

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.

Flg22-induced Ca2+ increases undergo desensitization and resensitization

The flagellin epitope flg22, a pathogen-associated molecular pattern (PAMP), binds to the receptor-like kinase FLAGELLIN SENSING2 (FLS2), and triggers Ca²⁺ influx across the plasma membrane (PM). The flg22-induced increases in cytosolic Ca²⁺ concentration ([Ca²⁺ ]_i ) (FICA) play a crucial role in plant innate immunity. It's well established that the receptor FLS2 and reactive oxygen species (ROS) burst undergo sensitivity adaptation after flg22 stimulation, referred to as desensitization and resensitization, to prevent over responses to pathogens. However, whether FICA also mount adaptation mechanisms to ensure appropriate and efficient responses against pathogens remains poorly understood. Here, we analysed systematically [Ca²⁺ ]_i increases upon two successive flg22 treatments, recorded and characterized rapid desensitization but slow resensitization of FICA in Arabidopsis thaliana. Pharmacological analyses showed that the rapid desensitization might be synergistically regulated by ligand-induced FLS2 endocytosis as well as the PM depolarization. The resensitization of FICA might require de novo FLS2 protein synthesis. FICA resensitization appeared significantly slower than FLS2 protein recovery, suggesting additional regulatory mechanisms of other components, such as flg22-related Ca²⁺ permeable channels. Taken together, we have carefully defined the FICA sensitivity adaptation, which will facilitate further molecular and genetic dissection of the Ca²⁺ -mediated adaptive mechanisms in PAMP-triggered immunity.

Arabidopsis ADR1 helper NLR immune receptors localize and function at the plasma membrane in a phospholipid dependent manner

Activation of nucleotide-binding leucine-rich repeat receptors (NLRs) results in immunity and a localized cell death. NLR cell death activity requires oligomerization and in some cases plasma membrane (PM) localization. The exact mechanisms underlying PM localization of NLRs lacking predicted transmembrane domains or recognizable lipidation motifs remain elusive. We used confocal microscopy, genetically encoded molecular tools and protein-lipid overlay assays to determine whether PM localization of members of the Arabidopsis HeLo-/RPW8-like domain 'helper' NLR (RNL) family is mediated by the interaction with negatively charged phospholipids of the PM. Our results show that PM localization and stability of some RNLs and one CC-type NLR (CNL) depend on the direct interaction with PM phospholipids. Depletion of phosphatidylinositol-4-phosphate from the PM led to a mis-localization of the analysed NLRs and consequently inhibited their cell death activity. We further demonstrate homo- and hetero-association of members of the RNL family. Our results provide new insights into the molecular mechanism of NLR localization and defines an important role of phospholipids for CNL and RNL PM localization and consequently, for their function. We propose that RNLs interact with anionic PM phospholipids and that RNL-mediated cell death and immune responses happen at the PM.

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.

Activation of TIR signalling boosts pattern-triggered immunity

Plant immune responses are mainly activated by two types of receptor. Pattern recognition receptors localized on the plasma membrane perceive extracellular microbial features, and nucleotide-binding leucine-rich repeat receptors (NLRs) recognize intracellular effector proteins from pathogens1. NLRs possessing amino-terminal Toll/interleukin-1 receptor (TIR) domains activate defence responses via the NADase activity of the TIR domain2,3. Here we report that activation of TIR signalling has a key role in pattern-triggered immunity (PTI) mediated by pattern recognition receptors. TIR signalling mutants exhibit attenuated PTI responses and decreased resistance against pathogens. Consistently, PTI is compromised in plants with reduced NLR levels. Treatment with the PTI elicitor flg22 or nlp20 rapidly induces many genes encoding TIR-domain-containing proteins, which is likely to be responsible for activating TIR signalling during PTI. Overall, our study reveals that activation of TIR signalling is an important mechanism for boosting plant defence during PTI.

PTI-ETI crosstalk: an integrative view of plant immunity

Plants resist attacks by pathogens via innate immune responses, which are initiated by cell surface-localized pattern-recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat containing receptors (NLRs) leading to pattern-triggered immunity (PTI) and effector-triggered immunity (ETI), respectively. Although the two classes of immune receptors involve different activation mechanisms and appear to require different early signalling components, PTI and ETI eventually converge into many similar downstream responses, albeit with distinct amplitudes and dynamics. Increasing evidence suggests the existence of intricate interactions between PRR-mediated and NLR-mediated signalling cascades as well as common signalling components shared by both. Future investigation of the mechanisms underlying signal collaboration between PRR-initiated and NLR-initiated immunity will enable a more complete understanding of the plant immune system. This review discusses recent advances in our understanding of the relationship between the two layers of plant innate immunity.

Plant "helper" immune receptors are Ca2+-permeable nonselective cation channels

Plant nucleotide-binding leucine-rich repeat receptors (NLRs) regulate immunity and cell death. In Arabidopsis, a subfamily of "helper" NLRs is required by many "sensor" NLRs. Active NRG1.1 oligomerized, was enriched in plasma membrane puncta, and conferred cytoplasmic calcium ion (Ca2+) influx in plant and human cells. NRG1.1-dependent Ca2+ influx and cell death were sensitive to Ca2+ channel blockers and were suppressed by mutations affecting oligomerization or plasma membrane enrichment. Ca2+ influx and cell death mediated by NRG1.1 and ACTIVATED DISEASE RESISTANCE 1 (ADR1), another helper NLR, required conserved negatively charged N-terminal residues. Whole-cell voltage-clamp recordings demonstrated that Arabidopsis helper NLRs form Ca2+-permeable cation channels to directly regulate cytoplasmic Ca2+ levels and consequent cell death. Thus, helper NLRs transduce cell death signals directly.

Calcium Signaling in Plant Programmed Cell Death

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

A mis-regulated cyclic nucleotide-gated channel mediates cytosolic calcium elevation and activates immunity in Arabidopsis

Calcium (Ca²⁺ ) is a second messenger for plant cell surface and intracellular receptors mediating pattern-triggered and effector-triggered immunity (respectively, PTI and ETI). Several CYCLIC NUCLEOTIDE-GATED CHANNELS (CNGCs) were shown to control transient cytosolic Ca²⁺ influx upon PTI activation. The contributions of specific CNGC members to PTI and ETI remain unclear. ENHANCED DISEASE SUSCEPTIBLITY1 (EDS1) regulates ETI signaling. In an Arabidopsis genetic screen for suppressors of eds1, we identify a recessive gain-of-function mutation in CNGC20, denoted cngc20-4, which partially restores disease resistance in eds1. cngc20-4 enhances PTI responses and ETI hypersensitive cell death. A cngc20-4 single mutant exhibits autoimmunity, which is dependent on genetically parallel EDS1 and salicylic acid (SA) pathways. CNGC20 self-associates, forms heteromeric complexes with CNGC19, and is phosphorylated and stabilized by BOTRYTIS INDUCED KINASE1 (BIK1). The cngc20-4 L371F exchange on a predicted transmembrane channel inward surface does not disrupt these interactions but leads to increased cytosolic Ca²⁺ accumulation, consistent with mis-regulation of CNGC20 Ca²⁺ -permeable channel activity. Our data show that ectopic Ca²⁺ influx caused by a mutant form of CNGC20 in cngc20-4 affects both PTI and ETI responses. We conclude that tight control of the CNGC20 Ca²⁺ ion channel is important for regulated immunity.

Regulatory role of receptor-like cytoplasmic kinases in early immune signaling events in plants

Receptor-like cytoplasmic kinases (RLCKs) play crucial roles in regulating plant development and immunity. Conserved pathogen-associated molecular patterns (PAMPs) derived from microbes are recognized by plant pattern recognition receptors to activate PAMP-triggered immunity (PTI). Microbial effectors, whose initial function is to promote virulence, are recognized by plant intracellular nucleotide-binding domain and leucine-rich repeat receptors (NLRs) to initiate effector-triggered immunity (ETI). Both PTI and ETI trigger early immune signaling events including the production of reactive oxygen species, induction of calcium influx and activation of mitogen-activated protein kinases. Research progress has revealed the important roles of RLCKs in the regulation of early PTI signaling. Accordingly, RLCKs are often targeted by microbial effectors that are evolved to evade PTI via diverse modulations. In some cases, modulation of RLCKs by microbial effectors triggers the activation of NLRs. This review covers the mechanisms by which RLCKs engage diverse substrates to regulate early PTI signaling and the regulatory roles of RLCKs in triggering NLR activation. Accumulating evidence suggests evolutionary links and close connections between PAMP- and effector-triggered early immune signaling that are mediated by RLCKs. As key immune regulators, RLCKs can be considered targets with broad prospects for the improvement of plant resistance via genetic engineering.

Plant Defense Responses to Biotic Stress and Its Interplay With Fluctuating Dark/Light Conditions

Plants are subjected to a plethora of environmental cues that cause extreme losses to crop productivity. Due to fluctuating environmental conditions, plants encounter difficulties in attaining full genetic potential for growth and reproduction. One such environmental condition is the recurrent attack on plants by herbivores and microbial pathogens. To surmount such attacks, plants have developed a complex array of defense mechanisms. The defense mechanism can be either preformed, where toxic secondary metabolites are stored; or can be inducible, where defense is activated upon detection of an attack. Plants sense biotic stress conditions, activate the regulatory or transcriptional machinery, and eventually generate an appropriate response. Plant defense against pathogen attack is well understood, but the interplay and impact of different signals to generate defense responses against biotic stress still remain elusive. The impact of light and dark signals on biotic stress response is one such area to comprehend. Light and dark alterations not only regulate defense mechanisms impacting plant development and biochemistry but also bestow resistance against invading pathogens. The interaction between plant defense and dark/light environment activates a signaling cascade. This signaling cascade acts as a connecting link between perception of biotic stress, dark/light environment, and generation of an appropriate physiological or biochemical response. The present review highlights molecular responses arising from dark/light fluctuations vis-à-vis elicitation of defense mechanisms in plants.

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.

GCN5 modulates salicylic acid homeostasis by regulating H3K14ac levels at the 5' and 3' ends of its target genes

The modification of histones by acetyl groups has a key role in the regulation of chromatin structure and transcription. The Arabidopsis thaliana histone acetyltransferase GCN5 regulates histone modifications as part of the Spt-Ada-Gcn5 Acetyltransferase (SAGA) transcriptional coactivator complex. GCN5 was previously shown to acetylate lysine 14 of histone 3 (H3K14ac) in the promoter regions of its target genes even though GCN5 binding did not systematically correlate with gene activation. Here, we explored the mechanism through which GCN5 controls transcription. First, we fine-mapped its GCN5 binding sites genome-wide and then used several global methodologies (ATAC-seq, ChIP-seq and RNA-seq) to assess the effect of GCN5 loss-of-function on the expression and epigenetic regulation of its target genes. These analyses provided evidence that GCN5 has a dual role in the regulation of H3K14ac levels in their 5′ and 3′ ends of its target genes. While the gcn5 mutation led to a genome-wide decrease of H3K14ac in the 5′ end of the GCN5 down-regulated targets, it also led to an increase of H3K14ac in the 3′ ends of GCN5 up-regulated targets. Furthermore, genome-wide changes in H3K14ac levels in the gcn5 mutant correlated with changes in H3K9ac at both 5′ and 3′ ends, providing evidence for a molecular link between the depositions of these two histone modifications. To understand the biological relevance of these regulations, we showed that GCN5 participates in the responses to biotic stress by repressing salicylic acid (SA) accumulation and SA-mediated immunity, highlighting the role of this protein in the regulation of the crosstalk between diverse developmental and stress-responsive physiological programs. Hence, our results demonstrate that GCN5, through the modulation of H3K14ac levels on its targets, controls the balance between biotic and abiotic stress responses and is a master regulator of plant-environmental interactions.

Plant Cyclic Nucleotide-Gated Channels: New Insights on Their Functions and Regulation

Recent advances of plant cyclic nucleotide-gated channels give new insight into their molecular functions focusing on regulation, subunit assembly, and phosphorylation.

CYCLIC NUCLEOTIDE-GATED ION CHANNELs 14 and 16 Promote Tolerance to Heat and Chilling in Rice

Calcium signaling has been postulated to be critical for both heat and chilling tolerance in plants, but its molecular mechanisms are not fully understood. Here, we investigated the function of two closely related cyclic nucleotide-gated ion channel (CNGC) proteins, OsCNGC14 and OsCNGC16, in temperature-stress tolerance in rice (Oryza sativa) by examining their loss-of-function mutants generated by genome editing. Under both heat and chilling stress, both the cngc14 and cngc16 mutants displayed reduced survival rates, higher accumulation levels of hydrogen peroxide, and increased cell death. In the cngc16 mutant, the extent to which some genes were induced and repressed in response to heat stress was altered and some Heat Shock factor (HSF) and Heat Shock Protein (HSP) genes were slightly more induced compared to the wild type. Furthermore, the loss of either OsCNGC14 or OsCNGC16 reduced or abolished cytosolic calcium signals induced by either heat or chilling stress. Therefore, OsCNGC14 and OsCNGC16 are required for heat and chilling tolerance and are modulators of calcium signals in response to temperature stress. In addition, loss of their homologs AtCNGC2 and AtCNGC4 in Arabidopsis (Arabidopsis thaliana) also led to compromised tolerance of low temperature. Thus, this study indicates a critical role of CNGC genes in both chilling and heat tolerance in plants, suggesting a potential overlap in calcium signaling in response to high- and low-temperature stress.

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.

Potential Mechanisms of AtNPR1 Mediated Resistance against Huanglongbing (HLB) in Citrus

Huanglongbing (HLB), a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas), is a major threat to the citrus industry. In a previous study conducted by our laboratory, several citrus transgenic trees expressing the Arabidopsis thaliana NPR1 (AtNPR1) gene remained HLB-free when grown in a field site under high HLB disease pressure. To determine the molecular mechanisms behind AtNPR1-mediated tolerance to HLB, a transcriptome analysis was performed using AtNPR1 overexpressing transgenic trees and non-transgenic trees as control, from which we identified 57 differentially expressed genes (DEGs). Data mining revealed the enhanced transcription of genes encoding pathogen-associated molecular patterns (PAMPs), transcription factors, leucine-rich repeat receptor kinases (LRR-RKs), and putative ankyrin repeat-containing proteins. These proteins were highly upregulated in the AtNPR1 transgenic line compared to the control plant. Furthermore, analysis of protein–protein interactions indicated that AtNPR1 interacts with CsNPR3 and CsTGA5 in the nucleus. Our results suggest that AtNPR1 positively regulates the innate defense mechanisms in citrus thereby boosting resistance and effectively protecting the plant against HLB.

Salicylic Acid Suppresses Apical Hook Formation via NPR1-Mediated Repression of EIN3 and EIL1 in Arabidopsis

Salicylic acid (SA) and ethylene (ET) are important phytohormones that regulate numerous plant growth, development, and stress response processes. Previous studies have suggested functional interplay of SA and ET in defense responses, but precisely how these two hormones coregulate plant growth and development processes remains unclear. Our present work reveals antagonism between SA and ET in apical hook formation, which ensures successful soil emergence of etiolated dicotyledonous seedlings. Exogenous SA inhibited ET-induced expression of HOOKLESS1 (HLS1) in Arabidopsis (Arabidopsis thaliana) in a manner dependent on ETHYLENE INSENSITIVE3 (EIN3) and EIN3-LIKE1 (EIL1), the core transcription factors in the ET signaling pathway. SA-activated NONEXPRESSER OF PR GENES1 (NPR1) physically interacted with EIN3 and interfered with the binding of EIN3 to target gene promoters, including the HLS1 promoter. Transcriptomic analysis revealed that NPR1 and EIN3/EIL1 coordinately regulated subsets of genes that mediate plant growth and stress responses, suggesting that the interaction between NPR1 and EIN3/EIL1 is an important mechanism for integrating the SA and ET signaling pathways in multiple physiological processes. Taken together, our findings illuminate the molecular mechanism underlying SA regulation of apical hook formation as well as the antagonism between SA and ET in early seedling establishment and possibly other physiological processes.

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.

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.

The Receptor Kinases BAK1/SERK4 Regulate Ca2+ Channel-Mediated Cellular Homeostasis for Cell Death Containment

Cell death is a vital and ubiquitous process that is tightly controlled in all organisms. However, the mechanisms underlying precise cell death control remain fragmented. As an important shared module in plant growth, development, and immunity, Arabidopsis thaliana BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1 (BAK1) and somatic embryogenesis receptor kinase 4 (SERK4) redundantly and negatively regulate plant cell death. By deploying an RNAi-based genetic screen for bak1/serk4 cell death suppressors, we revealed that cyclic nucleotide-gated channel 20 (CNGC20) functions as a hyperpolarization-activated Ca²⁺-permeable channel specifically regulating bak1/serk4 cell death. BAK1 directly interacts with and phosphorylates CNGC20 at specific sites in the C-terminal cytosolic domain, which in turn regulates CNGC20 stability. CNGC19, the closest homolog of CNGC20 with a low abundance compared with CNGC20, makes a quantitative genetic contribution to bak1/serk4 cell death only in the absence of CNGC20, supporting the biochemical data showing homo- and heteromeric assembly of the CNGC20 and CNGC19 channel complexes. Transcripts of CNGC20 and CNGC19 are elevated in bak1/serk4 compared with wild-type plants, further substantiating a critical role of homeostasis of CNGC20 and CNGC19 in cell death control. Our studies not only uncover a unique regulation of ion channel stability by cell-surface-resident receptor kinase-mediated phosphorylation but also provide evidence for fine-tuning Ca²⁺ channel functions in maintaining cellular homeostasis by the formation of homo- and heterotetrameric complexes.

A host-pathogen interactome uncovers phytopathogenic strategies to manipulate plant ABA responses

The phytopathogen Pseudomonas syringae delivers into host cells type III secreted effectors (T3SEs) that promote virulence. One virulence mechanism employed by T3SEs is to target hormone signaling pathways to perturb hormone homeostasis. The phytohormone abscisic acid (ABA) influences interactions between various phytopathogens and their plant hosts, and has been shown to be a target of P. syringae T3SEs. In order to provide insight into how T3SEs manipulate ABA responses, we generated an ABA-T3SE interactome network (ATIN) between P. syringae T3SEs and Arabidopsis proteins encoded by ABA-regulated genes. ATIN consists of 476 yeast-two-hybrid interactions between 97 Arabidopsis ABA-regulated proteins and 56 T3SEs from four pathovars of P. syringae. We demonstrate that T3SE interacting proteins are significantly enriched for proteins associated with transcription. In particular, the ETHYLENE RESPONSIVE FACTOR (ERF) family of transcription factors is highly represented. We show that ERF105 and ERF8 displayed a role in defense against P. syringae, supporting our overall observation that T3SEs of ATIN converge on proteins that influence plant immunity. In addition, we demonstrate that T3SEs that interact with a large number of ABA-regulated proteins can influence ABA responses. One of these T3SEs, HopF3_Pph6 , inhibits the function of ERF8, which influences both ABA-responses and plant immunity. These results provide a potential mechanism for how HopF3_Pph6 manipulates ABA-responses to promote P. syringae virulence, and also demonstrate the utility of ATIN as a resource to study the ABA-T3SE interface.

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

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

Genetic buffering of cyclic AMP in Arabidopsis thaliana compromises the plant immune response triggered by an avirulent strain of Pseudomonas syringae pv. tomato

Cyclic AMP plays important roles in different physiological processes, including plant defence responses. However, as little information is known on plant enzymes responsible for cAMP production/degradation, studies of cAMP functions have relied, to date, on non-specific pharmacological approaches. We therefore developed a more reliable approach, producing transgenic Arabidopsis thaliana lines overexpressing the 'cAMP-sponge' (cAS), a genetic tool that specifically buffers cAMP levels. In response to an avirulent strain of Pseudomonas syringae pv. tomato (PstAvrB), cAS plants showed a higher bacterial growth and a reduced hypersensitive cell death in comparison with wild-type (WT) plants. The low cAMP availability after pathogen infection delayed cytosolic calcium elevation, as well as hydrogen peroxide increase and induction of redox systems. The proteomic analysis, performed 24 h post-infection, indicated that a core of 49 proteins was modulated in both genotypes, while 16 and 42 proteins were uniquely modulated in WT and cAS lines, respectively. The involvement of these proteins in the impairment of defence response in cAS plants is discussed in this paper. Moreover, in silico analysis revealed that the promoter regions of the genes coding for proteins uniquely accumulating in WT plants shared the CGCG motif, a target of the calcium-calmodulin-binding transcription factor AtSR1 (Arabidopsis thaliana signal responsive1). Therefore, following pathogen perception, the low free cAMP content, altering timing and levels of defence signals, and likely acting in part through the mis-regulation of AtSR1 activity, affected the speed and strength of the immune response.

Ca2+ to the rescue - Ca2+channels and signaling in plant immunity

Ca²⁺ is a universal second messenger in many signaling pathways in all eukaryotes including plants. Transient changes in [Ca²⁺]cyt are rapidly generated upon a diverse range of stimuli such as drought, heat, wounding, and biotic stresses (infection by pathogenic and symbiotic microorganisms), as well as developmental cues. It has been known for a while that [Ca²⁺]cyt transient signals play crucial roles to activate plant immunity and recently significant progresses have been made in this research field. However the identity and regulation of ion channels that are involved in defense related Ca²⁺ signals are still enigmatic. Members of two ligand gated ion channel families, glutamate receptor-like channels (GLRs) and cyclic nucleotide-gated channels (CNGCs) have been implicated in immune responses; nevertheless more precise data to understand their direct involvement in the creation of Ca²⁺ signals during immune responses is necessary. Furthermore, the study of other ion channel groups is also required to understand the whole picture of the intra- and inter-cellular Ca²⁺ signalling network. In this review we summarize Ca²⁺ signals in plant immunity from an ion channel point of view and discuss future challenges in this exciting research field.

Electrophysiological Studies Revealed CaM1-Mediated Regulation of the Arabidopsis Calcium Channel CNGC12

The Arabidopsis cyclic nucleotide-gated channel (CNGC) family consists of 20 members, which have been reported to participate in various physiological processes, such as pathogen defense, development, and thermotolerance. Although CNGC11 and CNGC12 have been identified a decade ago and their role in programmed cell death is well studied, their precise channel regulation has not been studied electrophysiologically. Here, we determined the channel activities of CNGC11 and CNGC12 utilizing the two-electrode voltage-clamp technique in the Xenopus laevis oocyte heterologous expression system. Our results suggest that CNGC12 but not CNGC11 functions as an active calcium channel. Furthermore, the cyclic nucleotide monophosphates (cNMPs) did not affect the activities of CNGC11 nor CNGC12 in Xenopus oocytes. Interestingly, while the activity of CNGC11 was not affected by co-expression with calmodulin (CaM), the activity of CNGC12 was significantly enhanced when CaM1 was co-expressed in oocytes. This study reveals that the channel activities and the mechanisms of regulation by CaM are different between CNGC11 and CNGC12.

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

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

An apple cyclic nucleotide-gated ion channel gene highly responsive to Botryosphaeria dothidea infection enhances the susceptibility of Nicotiana benthamiana to bacterial and fungal pathogens

Apple ring rot caused by the fungus Botryosphaeria dothidea is one of the devastating diseases. Up to date, the responsive mechanism of apple plant to this disease remains unclear. In the present study, an apple CNGC gene (designated as MdCNGC1) was found among highly expressed genes responding to B. dothidea infection. The expression of MdCNGC1 was different among apple cultivars with different resistance to B. dothidea. Intriguingly, MdCNGC1 expression was not induced by other two apple pathogens, Marssonina coronaria and Valsa ceratosperma. Ectopic overexpression of MdCNGC1 in Nicotiana benthamiana conferred elevated susceptibility to bacterial and fungal pathogens. Notably, overexpression of MdCNGC1 reduced salicylic acid (SA) accumulation induced by Alternaria alternata or Pseudomonas syringae. Decreased induction of pathogenesis-related (PR) genes and ROS accumulation were also observed in MdCNGC1-overexpressing plants. Up-regulated scavenging systems as indicated by enhanced expressions of CAT, APX, SOD genes and activities of antioxidative enzymes may in part contribute to reduced ROS accumulation. MdCNGC1 expression in N. benthamiana also decreased flg22 and chitosan-induced callose deposition and lowered the expression of NbPMR4, an ortholog of Arabidopsis callose synthase gene PMR4. These combined results suggested that MdCNGC1 might be a negative factor to plant resistance to bacterial and fungal pathogens.

The role of water in plant-microbe interactions

Throughout their life plants are associated with various microorganisms, including commensal, symbiotic and pathogenic microorganisms. Pathogens are genetically adapted to aggressively colonize and proliferate in host plants to cause disease. However, disease outbreaks occur only under permissive environmental conditions. The interplay between host, pathogen and environment is famously known as the 'disease triangle'. Among the environmental factors, rainfall events, which often create a period of high atmospheric humidity, have repeatedly been shown to promote disease outbreaks in plants, suggesting that the availability of water is crucial for pathogenesis. During pathogen infection, water-soaking spots are frequently observed on infected leaves as an early symptom of disease. Recent studies have shown that pathogenic bacteria dedicate specialized virulence proteins to create an aqueous habitat inside the leaf apoplast under high humidity. Water availability in the apoplastic environment, and probably other associated changes, can determine the success of potentially pathogenic microbes. These new findings reinforce the notion that the fight over water may be a major battleground between plants and pathogens. In this article, we will discuss the role of water availability in host-microbe interactions, with a focus on plant-bacterial interactions.

Plant behaviour under combined stress: tomato responses to combined salinity and pathogen stress

Crop plants are subjected to a variety of stresses during their lifecycle, including abiotic stress factors such as salinity and biotic stress factors such as pathogens. Plants have developed a multitude of defense and adaptation responses to these stress factors. In the field, different stress factors mostly occur concurrently resulting in a new state of stress, the combined stress. There is evidence that plant resistance to pathogens can be attenuated or enhanced by abiotic stress factors. With stress tolerance research being mostly focused on plant responses to individual stresses, the understanding of a plant's ability to adapt to combined stresses is limited. In the last few years, we studied powdery mildew resistance under salt stress conditions in the model crop plant tomato with the aim to understand the requirements to achieve plant resilience to a wider array of combined abiotic and biotic stress combinations. We uncovered specific responses of tomato plants to combined salinity-pathogen stress, which varied with salinity intensity and plant resistance genes. Moreover, hormones, with their complex regulation and cross-talk, were shown to play a key role in the adaptation of tomato plants to the combined stress. In this review, we attempt to understand the complexity of plant responses to abiotic and biotic stress combinations, with a focus on tomato responses (genetic control and cross-talk of signaling pathways) to combined salinity and pathogen stresses. Further, we provide recommendations on how to design novel strategies for breeding crops with a sustained performance under diverse environmental conditions.