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

CAMTA3 repressor destabilization triggers TIR domain protein TN2-mediated autoimmunity in the Arabidopsis exo70B1 mutant

Calcium-dependent protein kinases (CPKs) can decode and translate intracellular calcium signals to induce plant immunity. Mutation of the exocyst subunit gene EXO70B1 causes autoimmunity that depends on CPK5 and the Toll/interleukin-1 receptor (TIR) domain resistance protein TIR-NBS2 (TN2), where direct interaction with TN2 stabilizes CPK5 kinase activity. However, how the CPK5-TN2 interaction initiates downstream immune responses remains unclear. Here, we show that, besides CPK5 activity, the physical interaction between CPK5 and functional TN2 triggers immune activation in exo70B1 and may represent reciprocal regulation between CPK5 and the TIR domain functions of TN2 in Arabidopsis (Arabidopsis thaliana). Moreover, we detected differential phosphorylation of the calmodulin-binding transcription activator 3 (CAMTA3) in the cpk5 background. CPK5 directly phosphorylates CAMTA3 at S964, contributing to its destabilization. The gain-of-function CAMTA3A855V variant that resists CPK5-induced degradation rescues immunity activated through CPK5 overexpression or exo70B1 mutation. Thus, CPK5-mediated immunity is executed through CAMTA3 repressor degradation via phosphorylation-induced and/or calmodulin-regulated processes. Conversely, autoimmunity in camta3 also partially requires functional CPK5. While the TIR domain activity of TN2 remains to be tested, our study uncovers a TN2-CPK5-CAMTA3 signaling module for exo70B1-mediated autoimmunity, highlighting the direct embedding of a calcium-sensing decoder element within resistance signalosomes.

Conserved transcription factors NRZ1 and NRM1 regulate NLR receptor-mediated immunity

Plant innate immunity mediated by the nucleotide-binding leucine-rich repeat (NLR) class of immune receptors plays an important role in defense against various pathogens. Although key biochemical events involving NLR activation and signaling have been recently uncovered, we know very little about the transcriptional regulation of NLRs and their downstream signaling components. Here, we show that the Toll-Interleukin 1 receptor homology domain containing NLR (TNL) gene N (Necrosis), which confers resistance to Tobacco mosaic virus, is transcriptionally induced upon immune activation. We identified two conserved transcription factors, N required C3H zinc finger 1 (NRZ1) and N required MYB-like transcription factor 1 (NRM1), that activate N in an immune responsive manner. Genetic analyses indicated that NRZ1 and NRM1 positively regulate coiled-coil domain-containing NLR- and TNL-mediated immunity and function independently of the signaling component Enhanced Disease Susceptibility 1. Furthermore, NRZ1 functions upstream of NRM1 in cell death signaling, and their gene overexpression induces ectopic cell death and expression of NLR signaling components. Our findings uncovered a conserved transcriptional regulatory network that is central to NLR-mediated cell death and immune signaling in plants.

The plant immune system: From discovery to deployment

Plant diseases cause famines, drive human migration, and present challenges to agricultural sustainability as pathogen ranges shift under climate change. Plant breeders discovered Mendelian genetic loci conferring disease resistance to specific pathogen isolates over 100 years ago. Subsequent breeding for disease resistance underpins modern agriculture and, along with the emergence and focus on model plants for genetics and genomics research, has provided rich resources for molecular biological exploration over the last 50 years. These studies led to the identification of extracellular and intracellular receptors that convert recognition of extracellular microbe-encoded molecular patterns or intracellular pathogen-delivered virulence effectors into defense activation. These receptor systems, and downstream responses, define plant immune systems that have evolved since the migration of plants to land ∼500 million years ago. Our current understanding of plant immune systems provides the platform for development of rational resistance enhancement to control the many diseases that continue to plague crop production.

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.

Arabidopsis PHYTOALEXIN DEFICIENT 4 promotes the maturation and nuclear accumulation of immune-related cysteine protease RD19

Arabidopsis PHYTOALEXIN DEFICIENT 4 (PAD4) has an essential role in pathogen resistance as a heterodimer with ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1). Here we investigated an additional PAD4 role in which it associates with and promotes the maturation of the immune-related cysteine protease RESPONSIVE TO DEHYDRATION 19 (RD19). We found that RD19 and its paralog RD19c promoted EDS1- and PAD4-mediated effector-triggered immunity to an avirulent Pseudomonas syringae strain, DC3000, expressing the effector AvrRps4 and basal immunity against the fungal pathogen Golovinomyces cichoracearum. Overexpression of RD19, but not RD19 protease-inactive catalytic mutants, in Arabidopsis transgenic lines caused EDS1- and PAD4-dependent autoimmunity and enhanced pathogen resistance. In these lines, RD19 maturation to a pro-form required its catalytic residues, suggesting that RD19 undergoes auto-processing. In transient assays, PAD4 interacted preferentially with the RD19 pro-protease and promoted its nuclear accumulation in leaf cells. Our results lead us to propose a model for PAD4-stimulated defense potentiation. PAD4 promotes maturation and nuclear accumulation of processed RD19, and RD19 then stimulates EDS1-PAD4 dimer activity to confer pathogen resistance. This study highlights potentially important additional PAD4 functions that eventually converge on canonical EDS1-PAD4 dimer signaling in plant immunity.

Nonbinary fungal signals and calcium-mediated transduction in plant immunity and symbiosis

Chitin oligomers (COs) are among the most common and active fungal elicitors of plant responses. Short-chain COs from symbiotic arbuscular mycorrhizal fungi activate accommodation responses in the host root, while long-chain COs from pathogenic fungi are acknowledged to trigger defence responses. The modulation of intracellular calcium concentration - a common second messenger in a wide variety of plant signal transduction processes - plays a central role in both signalling pathways with distinct signature features. Nevertheless, mounting evidence suggests that plant immunity and symbiosis signalling partially overlap at multiple levels. Here, we elaborate on recent findings on this topic, highlighting the nonbinary nature of chitin-based fungal signals, their perception and their interpretation through Ca2+ -mediated intracellular signals. Based on this, we propose that plant perception of symbiotic and pathogenic fungi is less clear-cut than previously described and involves a more complex scenario in which partially overlapping and blurred signalling mechanisms act upstream of the unambiguous regulation of gene expression driving accommodation or defence responses.

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.

A Ca2+ sensor BraCBL1.2 involves in BraCRa-mediated clubroot resistance in Chinese cabbage

Clubroot disease caused by Plasmodiophora brassicae (P. brassicae) severely threatens the cultivation of Cruciferous plants, especially Chinese cabbage. Recently, resistance genes in plants have been reported to encode for a Ca2+-permeable channel in the plasma membrane, which can mediate the cytosolic Ca2+ increase in plant cells upon pathogen attack. However, the downstream Ca2+ sensor and decoder are still unknown. In this study, we identified the virulent and avirulent P. brassicae isolates (Pbs) of two near isogenic lines, CR 3-2 and CS 3-2, with CR 3-2 harboring clubroot resistant gene BraCRa. The transcriptomic analysis was then conducted with CR 3-2 after inoculating with virulent isolate PbE and avirulent isolate Pb4. From the differentially expressed genes of transcriptomic data, we identified a Ca2+-sensor encoding gene, BraCBL1.2, that was highly induced in CR 3-2 during infection by Pb4 but not by PbE. Moreover, GUS histochemical staining and subcellular localization analysis revealed that BraCBL1.2 was specifically expressed in the root hair cells of Arabidopsis and encoded a putative Ca2+ sensor localized in the plasma membrane. We also developed an assay to investigate the BraCRa-mediated hypersensitive response (HR) in tobacco leaves. The results suggest that BraCBL1.2 is involved in the BraCRa-mediated plant ETI immune response against P. brassicae. In addition, we verified that overexpression of BraCBL1.2 enhanced clubroot resistance in Arabidopsis. Collectively, our data identified the involvement of a Ca2+ sensor in BraCRa-mediated clubroot resistance in Chinese cabbage, providing a theoretical basis for further research on the resistance of Chinese cabbage to P. brassicae.

Plant NLR immunity activation and execution: a biochemical perspective

Plants deploy cell-surface and intracellular receptors to detect pathogen attack and trigger innate immune responses. Inside host cells, families of nucleotide-binding/leucine-rich repeat (NLR) proteins serve as pathogen sensors or downstream mediators of immune defence outputs and cell death, which prevent disease. Established genetic underpinnings of NLR-mediated immunity revealed various strategies plants adopt to combat rapidly evolving microbial pathogens. The molecular mechanisms of NLR activation and signal transmission to components controlling immunity execution were less clear. Here, we review recent protein structural and biochemical insights to plant NLR sensor and signalling functions. When put together, the data show how different NLR families, whether sensors or signal transducers, converge on nucleotide-based second messengers and cellular calcium to confer immunity. Although pathogen-activated NLRs in plants engage plant-specific machineries to promote defence, comparisons with mammalian NLR immune receptor counterparts highlight some shared working principles for NLR immunity across kingdoms.

Transcriptomic data exploring the effect of agave fructans on the induction of the defense system in avocado fruit

The effect of 20% high degree polymerized agave fructans (HDPAF) on the induction of the defense system in avocado fruits was investigated by transcriptomic analysis at 1, 24 and 72 h after treatment, and the effect of HDPAF on respiration rate and ethylene production was also analyzed. Transcriptomic profiling revealed 5425 differentially expressed genes (DEGs), 55 of which were involved in the pathways related to plant defense response to pathogens. Key genes were associated with phenylpropanoid biosynthesis, mitogen-activated protein signaling, plant hormone signaling, calcium ion signal decoding, and pathogenesis-related proteins. Dysregulated genes involved in ethylene biosynthesis were also identified, and the reduction in ethylene production by HDPAF was corroborated by gas chromatography, where three days of delayed peak production was observed compared to that in water-treated fruits. These results help to understand the mechanism of induction of the avocado defense system by applying HDPAF and support the application of HDPAF as an efficient postharvest treatment to extend the shelf life of the fruit.

Metabolic and Genetic Association of Vitamin D with Calcium Signaling and Insulin Resistance

Various evidences have unveiled the significance of Vitamin D in diverse processes which include its action in prevention of immune dysfunction, cancer and cardiometabolic disorders. Studies have confirmed the function of VD in controlling the expression of approximately nine hundred genes including gene expression of insulin. VD insufficiency may be linked with the pathogenesis of diseases that are associated with insulin resistance (IR) including diabetes as well as obesity. Thus, VD lowers IR-related disorders such as inflammation and oxidative stress. This review provides an insight regarding the molecular mechanism manifesting, how insufficiency of VD may be connected with the IR and diabetes. It also discusses the effect of VD in maintaining the Ca2+ levels in beta cells of the pancreas and in the tissues that are responsive to insulin.

NLR signaling in plants: from resistosomes to second messengers

Nucleotide binding and leucine-rich repeat-containing receptors (NLRs) have a critical role in plant immunity through direct or indirect recognition of pathogen effectors. Recent studies have demonstrated that such recognition induces formation of large protein complexes called resistosomes to mediate NLR immune signaling. Some NLR resistosomes activate Ca2+ influx by acting as Ca2+-permeable channels, whereas others function as active NADases to catalyze the production of nucleotide-derived second messengers. In this review we summarize these studies on pathogen effector-induced assembly of NLR resistosomes and resistosome-mediated production of the second messengers of Ca2+ and nucleotide derivatives. We also discuss downstream events and regulation of resistosome signaling.

Is localized acquired resistance the mechanism for effector-triggered disease resistance in plants?

Plant nucleotide-binding leucine-rich repeat receptors (NLRs) are intracellular immune receptors that are activated by their direct or indirect interactions with virulence effectors. NLR activation triggers a strong immune response and consequent disease resistance. However, the NLR-driven immune response can be targeted by virulence effectors. It is thus unclear how immune activation can occur concomitantly with virulence effector suppression of immunity. Recent observations suggest that the activation of effector-triggered immunity does not sustain defence gene expression in tissues in contact with the hemi-biotrophic pathogen Pseudomonas syringae pv. tomato. Instead, strong defence was observed on the border of the infection area. This response is reminiscent of localized acquired resistance (LAR). LAR is a strong defence response occurring in a ~2 mm area around cells in contact with the pathogen and probably serves to prevent the spread of pathogens. Here we propose that effector-triggered immunity is essentially a quarantining mechanism to prevent systemic pathogen spread and disease, and that the induction of LAR is a key component of this mechanism.

TIR-domain enzymatic activities at the heart of plant immunity

Toll/interleukin-1/resistance (TIR) domain proteins contribute to innate immunity in all cellular kingdoms. TIR modules are activated by self-association and in plants, mammals and bacteria, some TIRs have enzymatic functions that are crucial for disease resistance and/or cell death. Many plant TIR-only proteins and pathogen effector-activated TIR-domain NLR receptors are NAD+ hydrolysing enzymes. Biochemical, structural and functional studies established that for both plant TIR-protein types, and certain bacterial TIRs, NADase activity generates bioactive signalling intermediates which promote resistance. A set of plant TIR-catalysed nucleotide isomers was discovered which bind to and activate EDS1 complexes, promoting their interactions with co-functioning helper NLRs. Analysis of TIR enzymes across kingdoms fills an important gap in understanding how pathogen disturbance induces TIR-regulated immune responses.

Role of Gamma-Aminobutyric Acid in Plant Defense Response

Gamma-aminobutyric acid (GABA) is a four-carbon non-protein amino acid that acts as a defense substance and a signaling molecule in various physiological processes, and which helps plants respond to biotic and abiotic stresses. This review focuses on the role of GABA's synthetic and metabolic pathways in regulating primary plant metabolism, redistributing carbon and nitrogen resources, reducing the accumulation of reactive oxygen species, and improving plants' tolerance of oxidative stress. This review also highlights the way in which GABA maintains intracellular pH homeostasis by acting as a buffer and activating H⁺-ATPase. In addition, calcium signals participate in the accumulation process of GABA under stress. Moreover, GABA also transmits calcium signals through receptors to trigger downstream signaling cascades. In conclusion, understanding the role of GABA in this defense response provides a theoretical basis for applying GABA in agriculture and forestry and feasible coping strategies for plants in complex and changeable environments.

WeiTsing, a pericycle-expressed ion channel, safeguards the stele to confer clubroot resistance

Plant roots encounter numerous pathogenic microbes that often cause devastating diseases. One such pathogen, Plasmodiophora brassicae (Pb), causes clubroot disease and severe yield losses on cruciferous crops worldwide. Here, we report the isolation and characterization of WeiTsing (WTS), a broad-spectrum clubroot resistance gene from Arabidopsis. WTS is transcriptionally activated in the pericycle upon Pb infection to prevent pathogen colonization in the stele. Brassica napus carrying the WTS transgene displayed strong resistance to Pb. WTS encodes a small protein localized in the endoplasmic reticulum (ER), and its expression in plants induces immune responses. The cryoelectron microscopy (cryo-EM) structure of WTS revealed a previously unknown pentameric architecture with a central pore. Electrophysiology analyses demonstrated that WTS is a calcium-permeable cation-selective channel. Structure-guided mutagenesis indicated that channel activity is strictly required for triggering defenses. The findings uncover an ion channel analogous to resistosomes that triggers immune signaling in the pericycle.

TIR-catalyzed nucleotide signaling molecules in plant defense

Toll and interleukin-1 receptor (TIR) domain is a conserved immune module in prokaryotes and eukaryotes. Signaling regulated by TIR-only proteins or TIR domain-containing intracellular immune receptors is critical for plant immunity. Recent studies demonstrated that TIR domains function as enzymes encoding a variety of activities, which manifest different mechanisms for regulation of plant immunity. These enzymatic activities catalyze metabolism of NAD+, ATP and other nucleic acids, generating structurally diversified nucleotide metabolites. Signaling roles have been revealed for some TIR enzymatic products that can act as second messengers to induce plant immunity. Herein, we summarize our current knowledge about catalytic production of these nucleotide metabolites and their roles in plant immune signaling. We also highlight outstanding questions that are likely to be the focus of future investigations about TIR-produced signaling molecules.

Pea Aphid (Acyrthosiphon pisum) Host Races Reduce Heat-Induced Forisome Dispersion in Vicia faba and Trifolium pratense

Although phloem-feeding insects such as aphids can cause significant damage to plants, relatively little is known about early plant defenses against these insects. As a first line of defense, legumes can stop the phloem mass flow through a conformational change in phloem proteins known as forisomes in response to Ca²⁺ influx. However, specialized phloem-feeding insects might be able to suppress the conformational change of forisomes and thereby prevent sieve element occlusion. To investigate this possibility, we triggered forisome dispersion through application of a local heat stimulus to the leaf tips of pea (Pisum sativum), clover (Trifolium pratense) and broad bean (Vicia faba) plants infested with different pea aphid (Acyrthosiphon pisum) host races and monitored forisome responses. Pea aphids were able to suppress forisome dispersion, but this depended on the infesting aphid host race, the plant species, and the age of the plant. Differences in the ability of aphids to suppress forisome dispersion may be explained by differences in the composition and quantity of the aphid saliva injected into the plant. Various mechanisms of how pea aphids might suppress forisome dispersion are discussed.

Seed Nano-Priming with Calcium Oxide Maintains the Redox State by Boosting the Antioxidant Defense System in Water-Stressed Carom (Trachyspermum ammi L.) Plants to Confer Drought Tolerance

This paper explores the potential of nano seed priming with calcium oxide nanoparticles in maintaining the redox status in carom (Trachyspermum ammi L.) plants by modulating non-enzymatic antioxidants and enzymatic antioxidants. Calcium oxide nanoparticles were prepared in four testing regimes comprising 25, 50, 75, and 100 ppm along with the control treatment of 0 ppm (distilled water). Priming was performed by soaking the carom seeds in the aerated water, and plants were grown under split plots corresponding to drought and water. Seed priming with 75 ppm CaONPs reduced hydrogen peroxide, malondialdehyde contents and electrolyte leakage by 23.3%, 35.9% and 31.6%, respectively, in the water-stressed carom plants. The glutathione s-transferase, superoxide dismutase and peroxidase functions improved under water stress by 42.3%, 24.1% and 44.8%, respectively, in the carom plants raised through 100 ppm primed seeds with CaO_NPs. Priming induced better Ca²⁺ signaling, which affected the enzymes of the ascorbate glutathione cycle, enabling them to maintain redox status in the carom plants exposed to drought stress. The morpho-agronomic traits of carom plants in terms of number of umbels, hundred seeds weights, shoot and root length and biomass improved significantly upon seed priming treatments. Seed priming with CaO_NPs is a viable strategy to combat reactive oxygen species-mediated damages in the carom plants.

Exogenous calcium regulates the growth and development of Pinus massoniana detecting by physiological, proteomic, and calcium-related genes expression analysis

Pinus massoniana is an important industrial crop tree species commonly used for timber and wood pulp for papermaking, rosin, and turpentine. This study investigated the effects of exogenous calcium (Ca) on P. massoniana seedling growth, development, and various biological processes and revealed the underlying molecular mechanisms. The results showed that Ca deficiency led to severe inhibition of seedling growth and development, whereas adequate exogenous Ca markedly improved growth and development. Many physiological processes were regulated by exogenous Ca. The underlying mechanisms involved diverse Ca-influenced biological processes and metabolic pathways. Calcium deficiency inhibited or impaired these pathways and processes, whereas sufficient exogenous Ca improved and benefited these cellular events by regulating several related enzymes and proteins. High levels of exogenous Ca facilitated photosynthesis and material metabolism. Adequate exogenous Ca supply relieved oxidative stress that occurred at low Ca levels. Enhanced cell wall formation, consolidation, and cell division also played a role in exogenous Ca-improved P. massoniana seedling growth and development. Calcium ion homeostasis and Ca signal transduction-related gene expression were also activated at high exogenous Ca levels. Our study facilitates the elucidation of the potential regulatory role of Ca in P. massoniana physiology and biology and is of guiding significance in Pinaceae plant forestry.

Dying in self-defence: a comparative overview of immunogenic cell death signalling in animals and plants

Host organisms utilise a range of genetically encoded cell death programmes in response to pathogen challenge. Host cell death can restrict pathogen proliferation by depleting their replicative niche and at the same time dying cells can alert neighbouring cells to prepare environmental conditions favouring future pathogen attacks. As expected, many pathogenic microbes have strategies to subvert host cell death to promote their virulence. The structural and lifestyle differences between animals and plants have been anticipated to shape very different host defence mechanisms. However, an emerging body of evidence indicates that several components of the host-pathogen interaction machinery are shared between the two major branches of eukaryotic life. Many proteins involved in cell death execution or cell death-associated immunity in plants and animals exert direct effects on endomembrane and loss of membrane integrity has been proposed to explain the potential immunogenicity of dying cells. In this review we aim to provide a comparative view on how cell death processes are linked to anti-microbial defence mechanisms in plants and animals and how pathogens interfere with these cell death programmes. In comparison to the several well-defined cell death programmes in animals, immunogenic cell death in plant defence is broadly defined as the hypersensitive response. Our comparative overview may help discerning whether specific types of immunogenic cell death exist in plants, and correspondingly, it may provide new hints for previously undiscovered cell death mechanism in animals.

Cytoplasmic regulation of chloroplast ROS accumulation during effector-triggered immunity

Accumulating evidence suggests that chloroplasts are an important battleground during various microbe-host interactions. Plants have evolved layered strategies to reprogram chloroplasts to promote de novo biosynthesis of defense-related phytohormones and the accumulation of reactive oxygen species (ROS). In this minireview, we will discuss how the host controls chloroplast ROS accumulation during effector-triggered immunity (ETI) at the level of selective mRNA decay, translational regulation, and autophagy-dependent formation of Rubisco-containing bodies (RCBs). We hypothesize that regulation at the level of cytoplasmic mRNA decay impairs the repair cycle of photosystem II (PSII) and thus facilitates ROS generation at PSII. Meanwhile, removing Rubisco from chloroplasts potentially reduces both O₂ and NADPH consumption. As a consequence, an over-reduced stroma would further exacerbate PSII excitation pressure and enhance ROS production at photosystem I.

Structure, biochemical function, and signaling mechanism of plant NLRs

To counter pathogen invasion, plants have evolved a large number of immune receptors, including membrane-resident pattern recognition receptors (PRRs) and intracellular nucleotide-binding and leucine-rich repeat receptors (NLRs). Our knowledge about PRR and NLR signaling mechanisms has expanded significantly over the past few years. Plant NLRs form multi-protein complexes called resistosomes in response to pathogen effectors, and the signaling mediated by NLR resistosomes converges on Ca²⁺-permeable channels. Ca²⁺-permeable channels important for PRR signaling have also been identified. These findings highlight a crucial role of Ca²⁺ in triggering plant immune signaling. In this review, we first discuss the structural and biochemical mechanisms of non-canonical NLR Ca²⁺ channels and then summarize our knowledge about immune-related Ca²⁺-permeable channels and their roles in PRR and NLR signaling. We also discuss the potential role of Ca²⁺ in the intricate interaction between PRR and NLR signaling.

Calcium signaling in plant immunity: a spatiotemporally controlled symphony

Calcium ions (Ca²⁺) are prominent intracellular messengers in all eukaryotic cells. Recent studies have emphasized the crucial roles of Ca²⁺ in plant immunity. Here, we review the latest progress on the spatiotemporal control of Ca²⁺ function in plant immunity. We discuss discoveries of how Ca²⁺ influx is triggered upon the activation of immune receptors, how Ca²⁺-permeable channels are activated, how Ca²⁺ signals are decoded inside plant cells, and how these signals are switched off. Despite recent advances, many open questions remain and we highlight the existing toolkit and the new technologies to address the outstanding questions of Ca²⁺ signaling in plant immunity.

The Nucleotide Revolution: Immunity at the Intersection of Toll/Interleukin-1 Receptor Domains, Nucleotides, and Ca2

The discovery of the enzymatic activity of the toll/interleukin-1 receptor (TIR) domain protein SARM1 five years ago preceded a flood of discoveries regarding the nucleotide substrates and products of TIR domains in plants, animals, bacteria, and archaea. These discoveries into the activity of TIR domains coincide with major advances in understanding the structure and mechanisms of NOD-like receptors and the mutual dependence of pattern recognition receptor- and effector-triggered immunity (PTI and ETI, respectively) in plants. It is quickly becoming clear that TIR domains and TIR-produced nucleotides are ancestral signaling molecules that modulate immunity and that their activity is closely associated with Ca²⁺ signaling. TIR domain research now bridges the separate disciplines of molecular plant- and animal-microbe interactions, neurology, and prokaryotic immunity. A cohesive framework for understanding the role of enzymatic TIR domains in diverse organisms will help unite the research of these disparate fields. Here, we review known products of TIR domains in plants, animals, bacteria, and archaea and use context gained from animal and prokaryotic TIR domain systems to present a model for TIR domains, nucleotides, and Ca²⁺ at the intersection of PTI and ETI in plant immunity. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

The Arabidopsis thaliana-Fusarium oxysporum strain 5176 pathosystem: an overview

Fusarium oxysporum is a soil-borne fungal pathogen of several major food crops. Research on understanding the molecular details of fungal infection and the plant's defense mechanisms against this pathogen has long focused mainly on the tomato-infecting F. oxysporum strains and their specific host plant. However, in recent years, the Arabidopsis thaliana-Fusarium oxysporum strain 5176 (Fo5176) pathosystem has additionally been established to study this plant-pathogen interaction with all the molecular biology, genetic, and genomic tools available for the A. thaliana model system. Work on this system has since produced several new insights, especially with regards to the role of phytohormones involved in the plant's defense response, and the receptor proteins and peptide ligands involved in pathogen detection. Furthermore, work with the pathogenic strain Fo5176 and the related endophytic strain Fo47 has demonstrated the suitability of this system for comparative studies of the plant's specific responses to general microbe- or pathogen-associated molecular patterns. In this review, we highlight the advantages of this specific pathosystem, summarize the advances made in studying the molecular details of this plant-fungus interaction, and point out open questions that remain to be answered.

The significance of calcium-sensing receptor in sustaining photosynthesis and ameliorating stress responses in plants

Calcium ions (Ca²⁺) regulate plant growth and development during exposure to multiple biotic and abiotic stresses as the second signaling messenger in cells. The extracellular calcium-sensing receptor (CAS) is a specific protein spatially located on the thylakoid membrane. It regulates the intracellular Ca²⁺ responses by sensing changes in extracellular Ca²⁺ concentration, thereby affecting a series of downstream signal transduction processes and making plants more resilient to respond to stresses. Here, we summarized the discovery process, structure, and location of CAS in plants and the effects of Ca²⁺ and CAS on stomatal functionality, photosynthesis, and various environmental adaptations. Under changing environmental conditions and global climate, our study enhances the mechanistic understanding of calcium-sensing receptors in sustaining photosynthesis and mediating abiotic stress responses in plants. A better understanding of the fundamental mechanisms of Ca²⁺ and CAS in regulating stress responses in plants may provide novel mitigation strategies for improving crop yield in a world facing more extreme climate-changed linked weather events with multiple stresses during cultivation.

Seed priming with calcium chloride enhances stress tolerance in rice seedlings

Calcium is a crucial second messenger in plant cells and contributes to plant resistance against biotic and abiotic stress. Plant defense priming with natural or synthetic compounds leads to quicker and stronger resistance responses. However, whether pretreatment of plant seeds with calcium could improve their resistance to stress remains poorly understood. In this study, we showed that rice seedlings grown from calcium chloride (CaCl₂)-pretreated seeds displayed enhanced resistance to the rice blast fungus Magnaporthe oryzae and the rice bacterial pathogen Xanthomonas oryzae pv. Oryzae (Xoo). Seed priming with CaCl₂ also led to enhanced rice tolerance to salt and cold. Furthermore, the reactive oxygen species (ROS) burst increased significantly upon immunity activation in the leaves of rice seedlings grown from CaCl₂-pretreated seeds. Additionally, we analyzed the rice calmodulin-binding protein 60 (OsCBP60) family and found that there were 19 OsCBP60s in rice cultivar Zhonghua 11 (ZH11). The transcripts of several OsCBP60s were chitin- and M. oryzae-inducible, suggesting that they may contribute to rice resistance. Taken together, these data indicate that seed priming with CaCl₂ can effectively enhance rice tolerance to multiple stresses, perhaps by boosting the burst of ROS, and OsCBP60 family members may also play an essential role in this process.

Connecting the dots between cell surface- and intracellular-triggered immune pathways in plants

Plants can detect microbial molecules via surface-localized pattern-recognition receptors (PRRs) and intracellular immune receptors from the nucleotide-binding, leucine-rich repeat receptor (NLR) family. The corresponding pattern-triggered (PTI) and effector-triggered (ETI) immunity were long considered separate pathways, although they converge on largely similar cellular responses, such as calcium influx and overlapping gene reprogramming. A number of studies recently uncovered genetic and molecular interconnections between PTI and ETI, highlighting the complexity of the plant immune network. Notably, PRR- and NLR-mediated immune responses require and potentiate each other to reach an optimal immune output. How PTI and ETI connect to confer robust immunity in different plant species, including crops will be an exciting future research area.

NLR we there yet? Nucleocytoplasmic coordination of NLR-mediated immunity

Plant intracellular nucleotide-binding leucine-rich repeat immune receptors (NLRs) perceive the activity of pathogen-secreted effector molecules that, when undetected, promote colonisation of hosts. Signalling from activated NLRs converges with and potentiates downstream responses from activated pattern recognition receptors (PRRs) that sense microbial signatures at the cell surface. Efficient signalling of both receptor branches relies on the host cell nucleus as an integration point for transcriptional reprogramming, and on the macromolecular transport processes that mediate the communication between cytoplasm and nucleoplasm. Studies on nuclear pore complexes (NPCs), the nucleoporin proteins (NUPs) that compose NPCs, and nuclear transport machinery constituents that control nucleocytoplasmic transport, have revealed that they play important roles in regulating plant immune responses. Here, we discuss the contributions of nucleoporins and nuclear transport receptor (NTR)-mediated signal transduction in plant immunity with an emphasis on NLR immune signalling across the nuclear compartment boundary and within the nucleus. We also highlight and discuss cytoplasmic and nuclear functions of NLRs and their signalling partners and further consider the potential implications of NLR activation and resistosome formation in both cellular compartments for mediating plant pathogen resistance and programmed host cell death.

Back From the Dead: The Atypical Kinase Activity of a Pseudokinase Regulator of Cation Fluxes During Inducible Immunity

Pseudokinases are thought to lack phosphotransfer activity due to altered canonical catalytic residues within their kinase domain. However, a subset of pseudokinases maintain activity through atypical phosphotransfer mechanisms. The Arabidopsis ILK1 is a pseudokinase from the Raf-like MAP3K family and is the only known plant pseudokinase with confirmed protein kinase activity. ILK1 activity promotes disease resistance and molecular pattern-induced root growth inhibition through its stabilization of the HAK5 potassium transporter with the calmodulin-like protein CML9. ILK1 also has a kinase-independent function in salt stress suggesting that it interacts with additional proteins. We determined that members of the ILK subfamily are the sole pseudokinases within the Raf-like MAP3K family and identified 179 novel putative ILK1 protein interactors. We also identified 70 novel peptide targets for ILK1, the majority of which were phosphorylated in the presence of Mn²⁺ instead of Mg²⁺ in line with modifications in ILK1's DFG cofactor binding domain. Overall, the ILK1-targeted or interacting proteins included diverse protein types including transporters (HAK5, STP1), protein kinases (MEKK1, MEKK3), and a cytokinin receptor (AHK2). The expression of 31 genes encoding putative ILK1-interacting or phosphorylated proteins, including AHK2, were altered in the root and shoot in response to molecular patterns suggesting a role for these genes in immunity. We describe a potential role for ILK1 interactors in the context of cation-dependent immune signaling, highlighting the importance of K⁺ in MAMP responses. This work further supports the notion that ILK1 is an atypical kinase with an unusual cofactor dependence that may interact with multiple proteins in the cell.

The secret life of insect-associated microbes and how they shape insect-plant interactions

Insects are associated with a plethora of different microbes of which we are only starting to understand their role in shaping insect–plant interactions. Besides directly benefitting from symbiotic microbial metabolism, insects obtain and transmit microbes within their environment, making them ideal vectors and potential beneficiaries of plant diseases and microbes that alter plant defenses. To prevent damage, plants elicit stress-specific defenses to ward off insects and their microbiota. However, both insects and microbes harbor a wealth of adaptations that allow them to circumvent effective plant defense activation. In the past decades, it has become apparent that the enormous diversity and metabolic potential of insect-associated microbes may play a far more important role in shaping insect–plant interactions than previously anticipated. The latter may have implications for the development of sustainable pest control strategies. Therefore, this review sheds light on the current knowledge on multitrophic insect–microbe–plant interactions in a rapidly expanding field of research.