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

A Sorghum BAK1/SERK4 Homolog Functions in Pathogen-Associated Molecular Patterns-Triggered Immunity and Cell Death in Response to Colletotrichum sublineola Infection

Sorghum bicolor is the fifth most important cereal crop and expected to gain prominence due to its versatility, low input requirements, and tolerance to hot and dry conditions. In warm and humid environments, the productivity of sorghum is severely limited by the hemibiotrophic fungal pathogen Colletotrichum sublineola, the causal agent of anthracnose. Cultivating anthracnose-resistant accessions is the most effective and environmentally benign way to safeguard yield. A previous genome-wide association study for anthracnose resistance in the Sorghum Association Panel uncovered single-nucleotide polymorphisms on chromosome 5 associated with resistance to anthracnose, including one located within the coding region of gene Sobic.005G182400. In this study, we investigated the molecular function of Sobic.005G182400 in response to C. sublineola infection. Conserved domain, phylogenetic, and structural analyses revealed that the protein encoded by Sobic.005G182400 shares significant structural similarity with the Arabidopsis BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 (BAK1)/SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE4 (SERK4). Although sequence analysis of four sorghum accessions showed no substantial variation in the coding region, accession SC1330, which carries the resistance allele, exhibited significantly higher expression of Sobic.005G182400 during early infection (≤24 h). Co-expression network analysis identified that the module associated with Sobic.005G182400 was enriched in genes involved in endocytosis, autophagy, and vesicle transport. Gene regulatory network analysis further suggested that Sobic.005G182400 regulates genes required for BAK1/SERK4-mediated cell death via protein glycosylation. Together, these findings indicate that Sobic.005G182400 encodes a protein with similarity to Arabidopsis BAK1/SERK4 that enables pathogen-associated molecular patterns-triggered immunity and regulates cell death.

The interaction of nutrient uptake with biotic and abiotic stresses in plants

Plants depend heavily on efficient nutrient uptake and utilization for optimal growth and development. However, plants are constantly subjected to a diverse array of biotic stresses, such as pathogen infections, insect pests, and herbivory, as well as abiotic stress like drought, salinity, extreme temperatures, and nutrient imbalances. These stresses significantly impact the plant's ability to take up nutrient and use it efficiency. Understanding how plants maintain nutrient uptake and use efficiency under biotic and abiotic stress conditions is crucial for improving crop resilience and sustainability. This review explores the recent advancements in elucidating the mechanisms underlying nutrient uptake and utilization efficiency in plants under such stress conditions. Our aim is to offer a comprehensive perspective that can guide the breeding of stress-tolerant and nutrition-efficient crop varieties, ultimately contributing to the advancement of sustainable agriculture.

Engineering nanozyme immunomodulator with magnetic targeting effect for cascade-enzyodynamic and ultrasound-reinforced metallo-immunotherapy in prostate carcinoma

Conventional immunotherapy exhibits low response rates due to the immunosuppressive tumor microenvironment (TME). To overcome this limitation, this study introduces ZFPG nanoparticles (ZFPG NPs) with ZnFe2O4@Pt cores and glucose oxidase (GOx) shells. The ZFPG NPs possess five-enzyme activities, good sonosensitivity, and remarkable magnetic targeting properties, which facilitate sono-metallo-immunotherapy for prostate cancer treatment in male mice. Specifically, the magnetic targeting ability effectively improves their accumulation in tumors while still showing enrichment in the liver and kidneys. The multienzyme cascade catalysis and sonosensitivity of these NPs effectively deplete glutathione and glucose, and enhance the generation and utilization of H2O2, thereby inducing multiple ROS bursts. Furthermore, these comprehensive effects up-regulate the HMOX1 to promote the Fe2+ and lipid peroxides accumulation, thereby inducing immunogenic ferroptosis. This strategy facilitates anti-tumor immunity by ameliorating the immunosuppressive TME and inhibiting lung metastatic progression. This joint warfare strategy offers a powerful solution to address conventional immunotherapy limitations.

Tolerance to multiple abiotic stresses is mediated by interacting CNGC proteins that regulate Ca2+ influx and stomatal movement in rice

Members of the cyclic nucleotide-gated channel (CNGC) proteins are reportedly involved in a variety of biotic and abiotic responses and stomatal movement. However, it is unknown if and how a single member could regulate multiple responses. Here we characterized three closely related CNGC genes in rice, OsCNGC14, OsCNGC15 and OsCNGC16, to determine whether they function in multiple abiotic stresses. The loss-of-function mutants of each of these three genes had reduced calcium ion (Ca2+) influx and slower stomatal closure in response to heat, chilling, drought and the stress hormone abscisic acid (ABA). These mutants also had reduced tolerance to heat, chilling and drought compared with the wild-type. Conversely, overexpression of OsCNGC16 led to a more rapid stomatal closure response to stresses and enhanced tolerance to heat, chilling and drought. The tight association of stomatal closure and stress tolerance strongly suggests that tolerance to multiple abiotic stresses conferred by these OsCNGC genes results at least partially from their regulation of stomatal movement. In addition, physical interactions were observed among the three OsCNGC proteins but not with a distantly related CNGC, suggesting the formation of hetero-oligomers among themselves. This study unveils the crucial role of OsCNGC14, 15 and 16 proteins in stomatal response and tolerance to multiple stresses, suggesting a mechanism of tolerance to multiple stresses that involves calcium influx and stomatal movement regulation.

Mitochondrial transfer of drug-loaded artificial mitochondria for enhanced anti-Glioma therapy through synergistic apoptosis/ferroptosis/immunogenic cell death

Mitochondrial targeting in gliomas represents a novel therapeutic strategy with significant potential to enhance drug sensitivity by effectively killing glioma cells at the mitochondrial level. In this study, we developed artificial mitochondria derived from mitochondrial membrane-based nanovesicles, enabling precise mitochondrial targeting of doxorubicin (Dox) to selectively eradicate cancer cells by amplifying multiple cell death pathways. It was found that Dox-encapsulating mitochondria-based nanovesicles (DOX-MitoNVs) exhibited an extraordinary ability to penetrate the blood-brain barrier (BBB), specifically targeting gliomas. By targeting mitochondria instead of locating at the nucleus, DOX-MitoNVs not only amplified Dox mediated apoptosis effects through the overloading of intracellular Ca2+ but also intensified ferroptosis by generating reactive oxygen species (ROS). Furthermore, DOX-MitoNVs demonstrated a significant ability to modulate the tumor immune microenvironment, thereby inducing pronounced immunogenic cell death (ICD) effects. In summary, it presents a novel therapeutic strategy utilizing DOX-MitoNVs for precise mitochondrial targeting in gliomas, enhancing drug sensitivity, inducing multiple cell death pathways, and modulating the tumor immune microenvironment to promote immunogenic cell death. STATEMENT OF SIGNIFICANCE: Mitochondrial targeting in gliomas is a promising therapeutic strategy that enhances drug sensitivity by exploiting glioma cells' mitochondrial vulnerabilities. We engineered mitochondrial membrane-based nanovesicles as artificial mitochondria for precise mitochondrial targeting of Dox. This approach facilitates selective cancer cell eradication and amplifies multiple cell death pathways alongside immunogenic chemotherapy. Notably, DOX-MitoNVs effectively cross the BBB and specifically target gliomas. By focusing on mitochondria, Dox induces apoptosis and intensifies ferroptosis through ROS generation. Additionally, DOX-MitoNVs can transform the tumor immune microenvironment, promoting ICD. Overall, DOX-MitoNVs offer a promising platform for enhanced glioma therapy.

Analysis of CNGC Family Members in Citrus clementina (Hort. ex Tan.) by a Genome-Wide Approach

The study focuses on the Cyclic nucleotide-gated ion channels (CNGCs) proteins in citrus, aiming to investigate their potential roles. A total of 33 CcCNGC proteins were identified and characterized in Citrus clementina using a genome-wide method. The study revealed that these proteins share a conserved CNGC domain structurally but exhibit significant differences in their primary sequence and motif composition. Phylogenetic analysis classified the CcCNGC proteins into 13 subgroups. The cis-elements present in all CcCNGCs promoters were identified and classified, and the number of elements was determined. The results suggested that these genes play important roles in citrus growth and development, as well as in response to biotic and abiotic stresses. Gene expression analysis further supported these findings, demonstrating that CNGC genes were responsive to various plant hormones and Phytophthora nicotianae infection, which causes citrus foot rot. Overall, the study indicated that members of the CcCNGC gene family exhibit structural and functional diversity. Further research is needed to validate the specific functions of individual family members and their roles in citrus physiology and response to stress conditions.

GhCNGC31 is critical for conferring resistance to Verticillium wilt in cotton

In the past decades, cyclic nucleotide-gated ion channels (CNGCs) have been extensively studied in diploid species Arabidopsis thaliana. However, the functional diversification of CNGCs in crop plants, mostly polyploid, remains poorly understood. In allotetraploid Upland cotton (Gossypium hirsutum), GhCNGC31 is one of the multiple orthologs of AtCNGC2, being present in the plasma membrane, capable of interacting with itself and binding to calmodulins and cyclic nucleotides. GhCNGC31 knockdown plants exhibited slight growth inhibition, and became more susceptible to Verticillium dahliae infection, which was associated with the reduced lignin and flavonoid accumulation, impaired ROS (reactive oxygen species) burst, and down-regulation of defense-related genes PR1, JAZ2, LOX2, and RBOH10. RNA-Seq analysis identified 1817 differentially expressed genes from GhCNGC31 knockdown, of which 1184 (65%) were responsive to V. dahliae infection and accounted for 57% among a total of 2065 V. dahliae-responsive genes identified in this study. These GhCNGC31-regulated genes mainly function with cell wall organization and biogenesis, cellular carbohydrate metabolic or biosynthetic process, cellular component macromolecule biosynthetic process, and rhythmic process. They are significantly enriched in the pathways of plant MAPK signaling, plant-pathogen interaction, phenylpropanoid biosynthesis, and plant hormone signal transduction. A set of transcription factors (TFs) and resistance (R) genes are among the GhCNGC31-regulated genes, which are significantly over-represented with the TCP and WRKY TFs families, as well as with the R genes of T (TIR) and TNL (TIR-NB-LRR) classes. Together, our results unraveled a critical role of GhCNGC31 for conferring resistance to Verticillium wilt in cotton.

The parallel narrative of RGF/GLV/CLEL peptide signalling

Plant peptide families share distinct characteristics, and many members are in homologous signalling pathways controlling development and responses to external signals. The root meristem growth factor (RGF) peptides/GOLVEN (GLV)/CLAVATA3-ESR-related like (CLEL) are a family of short signalling peptides that are derived from a precursor protein and undergo post-translational modifications. Their role in root meristem development is well established and recent efforts have identified subtilase processing pathways and several downstream signalling components. This discovery has enabled the convergence of previously distinct pathways and enhanced our understanding of plant developmental processes. Here, we review the structure-function relationship of RGF peptides, the post-translational modification pathways, and the downstream signalling mechanisms and highlight components of these pathways that are known in non-RGF-mediated pathways.

Transcriptome analysis to identify genes related to programmed cell death resulted from manipulating of BnaFAH ortholog by CRISPR/Cas9 in Brassica napus

Fumarylacetoacetate hydrolase (FAH) catalyzes the final step of the tyrosine degradation pathway. In this study, we isolated and characterized two homologous BnaFAH genes in Brassica napus L. variant Westar, and then used CRISPR/Cas9-mediated targeted mutagenesis to generate a series of transgene-free mutant lines either with single or double-null bnafah alleles. Among these mutant lines, the aacc (bnafah) double-null mutant line, rather than the aaCC (bnaa06fah) mutant line, exhibited programmed cell death (PCD) under short days (SD). Histochemical staining and content measurement confirmed that the accumulation of reactive oxygen species (ROS) in bnafah was significantly higher than that in bnaa06fah. To further elucidate the mechanism of PCD, we performed transcriptomic analyses of bnaa06fah and bnafah at different SD stages. A heatmap cluster of differentially expressed genes (DEGs) revealed that PCD may be related to various redox regulatory genes involved in antioxidant activity, ROS-responsive regulation and calcium signaling. Combined with the results of previous studies, our work revealed that the expression levels of BnaC04CAT2, BnaA09/C09SAL1, BnaA08/C08ACO2, BnaA07/C06ERO1, BnaA08ACA1, BnaC04BIK1, BnaA09CRK36 and BnaA03CPK4 were significantly different and that these genes might be candidate hub genes for PCD. Together, our results underscore the ability of different PCD phenotypes to alter BnaFAH orthologs through gene editing and further elucidated the molecular mechanisms of oxidative stress-induced PCD in plants.

Genome Analysis of BnCNGC Gene Family and Function Exploration of BnCNGC57 in Brassica napus L

The cyclic nucleotide-gated ion channel (CNGC), as a non-selective cation channel, plays a pivotal role in plant growth and stress response. A systematic analysis and identification of the BnCNGC gene family in Brassica napus is crucial for uncovering its biological functions and potential applications in plant science. In this study, we identified 61 BnCNGC members in the B. napus genome, which are phylogenetically similar to Arabidopsis and can be classified into Groups I-IV (with Group IV further subdivided into IV-a and IV-b). Collinearity analysis with other species provided insights into the evolution of BnCNGC. By homology modeling, we predicted the three-dimensional structure of BnCNGC proteins and analyzed cis-acting elements in their promoters, revealing diverse roles in hormone regulation, growth, and stress response. Notably, overexpression of BnCNGC57 (BnaC09g42460D) significantly increased seed size, possibly through regulating cell proliferation via the MAPK signaling pathway. Our findings contribute to a better understanding of the BnCNGC gene family and highlight the potential regulatory role of BnCNGC57 in the seed development of B. napus.

Differential phosphorylation of Ca2+-permeable channel CYCLIC NUCLEOTIDE-GATED CHANNEL20 modulates calcium-mediated freezing tolerance in Arabidopsis

Plants respond to cold stress at multiple levels, including increasing cytosolic calcium (Ca2+) influx and triggering the expression of cold-responsive genes. In this study, we show that the Ca2+-permeable channel CYCLIC NUCLEOTIDE-GATED CHANNEL20 (CNGC20) positively regulates freezing tolerance in Arabidopsis (Arabidopsis thaliana) by mediating cold-induced Ca2+ influx. Moreover, we demonstrate that the leucine-rich repeat receptor-like kinase PLANT PEPTIDE CONTAINING SULFATED TYROSINE1 RECEPTOR (PSY1R) is activated by cold, phosphorylating and enhancing the activity of CNGC20. The psy1r mutant exhibits decreased cold-evoked Ca2+ influx and freezing tolerance. Conversely, COLD-RESPONSIVE PROTEIN KINASE1 (CRPK1), a protein kinase that negatively regulates cold signaling, phosphorylates and facilitates the degradation of CNGC20 under prolonged periods of cold treatment, thereby attenuating freezing tolerance. This study thus identifies PSY1R and CRPK1 kinases that regulate CNGC20 activity and stability, respectively, thereby antagonistically modulating freezing tolerance in plants.

Genome-Wide Identification and Expression Analysis of the Cyclic Nucleotide-Gated Channel Gene Family in Zoysia japonica under Salt Stress

Salt stress severely inhibits plant growth. Understanding the mechanism of plant salt tolerance is highly important to improving plant salt tolerance. Previous studies have shown that nonselective cyclic nucleotide-gated ion channels (CNGCs) play an important role in plant salt tolerance. However, current research on CNGCs mainly focuses on CNGCs in glycophytic plants, and research on CNGCs in halophytes that exhibit special salt tolerance strategies is still scarce. This study used the halophilic plant Zoysia japonica, an excellent warm-season turfgrass, as the experimental material. Through bioinformatics analysis, 18 members of the CNGC family were identified in Zoysia japonica; they were designated ZjCNGC1 through ZjCNGC18 according to their scaffold-level chromosomal positions. ZjCNGCs are divided into four groups (I-IV), with the same groups having differentiated protein-conserved domains and gene structures. ZjCNGCs are unevenly distributed on 16 scaffold-level chromosomes. Compared with other species, the ZjCNGCs in Group III exhibit obvious gene expansion, mainly due to duplication of gene segments. The collinearity between ZjCNGCs, OsCNGCs, and SjCNGCs suggests that CNGCs are evolutionarily conserved among gramineous plants. However, the Group III ZjCNGCs are only partially collinear with OsCNGCs and SjCNGCs, implying that the expansion of Group III ZjCNGC genes may have been an independent event occurring in Zoysia japonica. Protein interaction prediction revealed that ZjCNGCs, calcium-dependent protein kinase, H+-ATPase, outwardly rectifying potassium channel protein, and polyubiquitin 3 interact with ZjCNGCs. Multiple stress response regulatory elements, including those involved in salt stress, are present on the ZjCNGC promoter. The qPCR results revealed differences in the expression patterns of ZjCNGCs in different parts of the plant. Under salt stress conditions, the expression of ZjCNGCs was significantly upregulated in roots and leaves, with ZjCNGC8 and ZjCNGC13 showing the greatest increase in expression in the roots. These results collectively suggest that ZjCNGCs play an important role in salt tolerance and that their expansion into Group III may be a special mechanism underlying the salt tolerance of Zoysia japonica.

PTI-ETI synergistic signal mechanisms in plant immunity

Plants face a relentless onslaught from a diverse array of pathogens in their natural environment, to which they have evolved a myriad of strategies that unfold across various temporal scales. Cell surface pattern recognition receptors (PRRs) detect conserved elicitors from pathogens or endogenous molecules released during pathogen invasion, initiating the first line of defence in plants, known as pattern-triggered immunity (PTI), which imparts a baseline level of disease resistance. Inside host cells, pathogen effectors are sensed by the nucleotide-binding/leucine-rich repeat (NLR) receptors, which then activate the second line of defence: effector-triggered immunity (ETI), offering a more potent and enduring defence mechanism. Moreover, PTI and ETI collaborate synergistically to bolster disease resistance and collectively trigger a cascade of downstream defence responses. This article provides a comprehensive review of plant defence responses, offering an overview of the stepwise activation of plant immunity and the interactions between PTI-ETI synergistic signal transduction.

Wound-induced small-peptide-mediated signaling cascade, regulated by OsPSKR, dictates balance between growth and defense in rice

Wounding is a general stress in plants that results from various pest and pathogenic infections in addition to environment-induced mechanical damages. Plants have sophisticated molecular mechanisms to recognize and respond to wounding, with those of monocots being distinct from dicots. Here, we show the involvement of two distinct categories of temporally separated, endogenously derived peptides, namely, plant elicitor peptides (PEPs) and phytosulfokine (PSK), mediating wound responses in rice. These peptides trigger a dynamic signal relay in which a receptor kinase involved in PSK perception named OsPSKR plays a major role. Perturbation of OsPSKR expression in rice leads to compromised development and constitutive autoimmune phenotypes. OsPSKR regulates the transitioning of defense to growth signals upon wounding. OsPSKR displays mutual antagonism with the OsPEPR1 receptor involved in PEP perception. Collectively, our work indicates the presence of a stepwise peptide-mediated signal relay that regulates the transition from defense to growth upon wounding in monocots.

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.

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.

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.

Unlocking Nature's Defense: Plant Pattern Recognition Receptors as Guardians Against Pathogenic Threats

Embedded in the plasma membrane of plant cells, receptor kinases (RKs) and receptor proteins (RPs) act as key sentinels, responsible for detecting potential pathogenic invaders. These proteins were originally characterized more than three decades ago as disease resistance (R) proteins, a concept that was formulated based on Harold Flor's gene-for-gene theory. This theory implies genetic interaction between specific plant R proteins and corresponding pathogenic effectors, eliciting effector-triggered immunity (ETI). Over the years, extensive research has unraveled their intricate roles in pathogen sensing and immune response modulation. RKs and RPs recognize molecular patterns from microbes as well as dangers from plant cells in initiating pattern-triggered immunity (PTI) and danger-triggered immunity (DTI), which have intricate connections with ETI. Moreover, these proteins are involved in maintaining immune homeostasis and preventing autoimmunity. This review showcases seminal studies in discovering RKs and RPs as R proteins and discusses the recent advances in understanding their functions in sensing pathogen signals and the plant cell integrity and in preventing autoimmunity, ultimately contributing to a robust and balanced plant defense response. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2024.

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.

A plant cell death-inducing protein from litchi interacts with Peronophythora litchii pectate lyase and enhances plant resistance

Cell wall degrading enzymes, including pectate lyases (PeLs), released by plant pathogens, break down protective barriers and/or activate host immunity. The direct interactions between PeLs and plant immune-related proteins remain unclear. We identify two PeLs, PlPeL1 and PlPeL1-like, critical for full virulence of Peronophythora litchii on litchi (Litchi chinensis). These proteins enhance plant susceptibility to oomycete pathogens in a PeL enzymatic activity-dependent manner. However, LcPIP1, a plant immune regulator secreted by litchi, binds to PlPeL1/PlPeL1-like, and attenuates PlPeL1/PlPeL1-like induced plant susceptibility to Phytophthora capsici. LcPIP1 also induces cell death and various immune responses in Nicotiana benthamiana. Conserved in plants, LcPIP1 homologs bear a conserved "VDMASG" motif and exhibit immunity-inducing activity. Furthermore, SERK3 interacts with LcPIP1 and is required for LcPIP1-induced cell death. NbPIP1 participates in immune responses triggered by the PAMP protein INF1. In summary, our study reveals the dual roles of PlPeL1/PlPeL1-like in plant-pathogen interactions: enhancing pathogen virulence through PeL enzymatic activity while also being targeted by LcPIP1, thus enhancing plant immunity.

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

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

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

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

BTL2 phospho-switch surveils plant immunity

BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) is a co-receptor involved in the recognition of pattern-associated molecular patterns (PAMPs) via plasma membrane-localized pattern recognition receptors (PRRs). Absence of BAK1/SERK4 leads to the activation of autoimmunity in plants. Yu et al. recently showed that BAK-TO-LIFE 2 (BTL2) is required for the surveillance of BAK1/SERK4 integrity to maintain immune homeostasis.

BAK-up: the receptor kinase BAK-TO-LIFE 2 enhances immunity when BAK1 is lacking

BRI1-ASSOCIATED KINASE 1 (BAK1/SERK3) and its closest homolog BAK1-LIKE 1 (BKK1/SERK4) are leucine-rich repeat receptor kinases (LRR-RKs) belonging to the SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) family. They act as co-receptors of various other LRR-RKs and participate in multiple signaling events by complexing and transphosphorylating ligand-binding receptors. Initially identified as the brassinosteroid receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) co-receptor, BAK1 also functions in plant immunity by interacting with pattern recognition receptors. Mutations in BAK1 and BKK1 cause severely stunted growth and cell death, characterized as autoimmune cell death. Several factors play a role in this type of cell death, including RKs and components of effector-triggered immunity (ETI) signaling pathways, glycosylation factors, ER quality control components, nuclear trafficking components, ion channels, and Nod-like receptors (NLRs). The Shan lab has recently discovered a novel RK BAK-TO-LIFE 2 (BTL2) that interacts with BAK1 and triggers cell death in the absence of BAK1 and BKK1. This RK compensates for the loss of BAK1-mediated pattern-triggered immunity (PTI) by activating phytocytokine-mediated immune and cell death responses.

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.

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.

A phospho-switch constrains BTL2-mediated phytocytokine signaling in plant immunity

Enabling and constraining immune activation is of fundamental importance in maintaining cellular homeostasis. Depleting BAK1 and SERK4, the co-receptors of multiple pattern recognition receptors (PRRs), abolishes pattern-triggered immunity but triggers intracellular NOD-like receptor (NLR)-mediated autoimmunity with an elusive mechanism. By deploying RNAi-based genetic screens in Arabidopsis, we identified BAK-TO-LIFE 2 (BTL2), an uncharacterized receptor kinase, sensing BAK1/SERK4 integrity. BTL2 induces autoimmunity through activating Ca2+ channel CNGC20 in a kinase-dependent manner when BAK1/SERK4 are perturbed. To compensate for BAK1 deficiency, BTL2 complexes with multiple phytocytokine receptors, leading to potent phytocytokine responses mediated by helper NLR ADR1 family immune receptors, suggesting phytocytokine signaling as a molecular link connecting PRR- and NLR-mediated immunity. Remarkably, BAK1 constrains BTL2 activation via specific phosphorylation to maintain cellular integrity. Thus, BTL2 serves as a surveillance rheostat sensing the perturbation of BAK1/SERK4 immune co-receptors in promoting NLR-mediated phytocytokine signaling to ensure plant immunity.

A novel mutant allele of AtCNGC15 reveals a dual function of nuclear calcium release in the root meristem

Calcium release to the nucleoplasm of root meristem cells was demonstrated to modulate root development. The calcium channel encoded by cyclic nucleotide-gated channel (CNGC) 15 localizes at the nuclear envelope in young Arabidopsis seedlings. In contrast, at later stages of root growth, overexpression analysis showed that AtCNGC15 can relocalize to the plasma membrane to mediate primary nitrate-induced gene expression. This raises the question as to whether nuclear localized AtCNGC15 is required for root apical meristem development in young Arabidopsis seedlings, and whether nitrate signalling occurs independently of nuclear localized AtCNGC15 at this developmental stage. In this study, we characterize a novel mutant allele of AtCNGC15 and demonstrate that the mutation of a highly conserved aspartic acid in the C-linker domain is sufficient to impair the gating of AtCNCG15. We demonstrate that AtCNGC15 mediates the nuclear calcium release that modulates root apical meristem development and nitrate-induced LBD39 expression. We also show that, in the presence of nitrate, the relocalization of AtCNGC15 at the plasma membrane occurs specifically in the columella cells. Our results further suggest that the induction of LBD37, LBD38, and LBD39 in the presence of nitrate is modulated by different inputs of cytoplasmic or nuclear calcium release.

Dual function of the CHS3-CSA1 immune receptor pair

Plant nucleotide-binding leucine-rich repeat (NLR) receptors mediate specific recognition of pathogen effectors to initiate effector-triggered immunity. Recently, studies by Schulze et al., Yang et al., and Gu et al. collectively show that the Arabidopsis (Arabidopsis thaliana) NLR pair CHS3-CSA1 acts through two distinct activation modes to recognize different pathogen effectors, thus revealing the dual function of the CHS3-CSA1 pair in plant disease resistance.

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

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

Transcriptome analysis provides insights into lignin synthesis and MAPK signaling pathway that strengthen the resistance of bitter gourd (Momordica charantia) to Fusarium wilt

Fusarium wilt is a typical soil-borne disease caused by Fusarium oxysporum f. sp. momordicae (FOM) in bitter gourd. In this study, by comparing sequencing data at multiple time points and considering the difference between resistant (R) and susceptible (S) varieties, differentially expressed genes were screened out. Short time-series expression miner analysis revealed the upregulated expression trend of genes, which were enriched in phenylpropanoid biosynthesis, plant-pathogen interaction, and mitogen-activated protein kinase signaling pathway. Further, observation of the microstructure revealed that the R variety may form tyloses earlier than the S variety to prevent mycelium diffusion from the xylem vessel. After Fusarium wilt infection, the enzymatic activities of superoxide dismutase, peroxidase, phenylalanine ammonia lyase, and catalaseas well as levels of superoxide anion and malondialdehyde were increased in the R variety higher than those in the S variety. This study provides a reference to elucidate the disease resistance mechanism of bitter gourd.

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.

Plant elicitor peptide 1 fortifies root cell walls and triggers a systemic root-to-shoot immune signaling in Arabidopsis

Plant immunity is initiated by cell surface-localized receptors upon perception of pathogen-derived microbe or pathogen-associated molecular patterns (MAMPs/PAMPs), damage/danger-associated molecular patterns (DAMPs), and phytocytokines. Different patterns activate highly overlapping immune signaling at the early stage but divergent physiological responses at the late stage. Here, we indicate that plant elicitor peptide 1 (Pep1), a well-known DAMP, induces lignin and callose depositions, two types of late immune responses for strengthening the plant cell wall. Pep1-induced lignin and callose depositions in Arabidopsis root rely on early signaling components for Pep1 perception and signaling propagation. The phytohormone jasmonic acid and ethylene differently regulate the Pep1-regulated cell wall consolidation. Pep1 application in root also triggers a systemic immune signaling in shoot, and reactive oxygen species (ROS) is essential for the signaling communication between root and shoot. Collectively, the study reveals that Pep1 strengthens cell walls in root and triggers a systemic immune signaling from root to shoot.

The Arabidopsis TIR-NBS-LRR protein CSA1 guards BAK1-BIR3 homeostasis and mediates convergence of pattern- and effector-induced immune responses

Arabidopsis BAK1/SERK3, a co-receptor of leucine-rich repeat pattern recognition receptors (PRRs), mediates pattern-triggered immunity (PTI). Genetic inactivation of BAK1 or BAK1-interacting receptor-like kinases (BIRs) causes cell death, but the direct mechanisms leading to such deregulation remains unclear. Here, we found that the TIR-NBS-LRR protein CONSTITUTIVE SHADE AVOIDANCE 1 (CSA1) physically interacts with BIR3, but not with BAK1. CSA1 mediates cell death in bak1-4 and bak1-4 bir3-2 mutants via components of effector-triggered immunity-(ETI) pathways. Effector HopB1-mediated perturbation of BAK1 also results in CSA1-dependent cell death. Likewise, microbial pattern pg23-induced cell death, but not PTI responses, requires CSA1. Thus, we show that CSA1 guards BIR3 BAK1 homeostasis and integrates pattern- and effector-mediated cell death pathways downstream of BAK1. De-repression of CSA1 in the absence of intact BAK1 and BIR3 triggers ETI cell death. This suggests that PTI and ETI pathways are activated downstream of BAK1 for efficient plant immunity.

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.

Concerted actions of PRR- and NLR-mediated immunity

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

Comparative transcriptome analysis of compatible and incompatible Brassica napus-Xanthomonas campestris interactions

Black rot caused by the vascular pathogenic bacterium Xanthomonas campestris pv. campestris (Xcc) is widespread in Brassicaceae plants and an infectious disease that causes large yield losses in oil seed rape (Brassica napus L.). Improvement of resistance through breeding is a crucial strategy to prevent black rot disease in B. napus, but presently hampered by insufficient understanding of Xcc-Brassica interactions. This study compares two EMS-mutagenized B. napus lines that show contrasting resistance levels to their susceptible progenitor. Patterns of differential gene expression between these B. napus lines were evaluated at three time points post inoculation by comparative RNA-seq analysis. In line with the observed disease phenotypes, the susceptible line ZS9mXccS-1 displayed a steady amount of differentially expressed genes (DEGs) at different time points of infection, whereas the resistant line ZS9mXccR-1 displayed a gradual increase in DEGs throughout the course of infection. Weighted gene co-expression network analysis (WGCNA) pinpointed multiple defense-related hub genes with potential central roles in immunity, including the cell surface receptor genes CRK11 and BIR1, and the associated downstream regulatory genes WRKY11 and PBL30. KEGG analysis of DEGs belonging to two distinct co-expression modules revealed enriched pathways associated with defense, including Ca²⁺-signaling, receptor-mediated immunity, and phytohormone balance. Taken together, our comparative transcriptome analysis provides new avenues to unravel the mechanisms underlying black rot resistance in B. napus.

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.

Rapid responses: Receptor-like kinases directly regulate the functions of membrane transport proteins in plants

Receptor-like kinases (RLKs) are a large group of plant-specific transmembrane proteins mainly acting as receptors or co-receptors of various extracellular signals. They usually turn extracellular signals into intracellular responses via altering gene expression profiles. However, recent studies confirmed that many RLKs can physically interact with diverse membrane-localized transport proteins and regulate their activities for speedy responses in limited tissues or cells. In this minireview, we highlight recent discoveries regarding how RLKs can work with membrane transport proteins collaboratively and thereby trigger cellular responses in a precise and rapid manner. It is anticipated that such regulation broadly presents in plants and more examples will be gradually revealed when in-depth analyses are conducted for the functions of RLKs.

Regulation of plant responses to biotic and abiotic stress by receptor-like cytoplasmic kinases

As sessile organisms, plants have to cope with environmental change and numerous biotic and abiotic stress. Upon perceiving environmental cues and stress signals using different types of receptors, plant cells initiate immediate and complicated signaling to regulate cellular processes and respond to stress. Receptor-like cytoplasmic kinases (RLCKs) transduce signals from receptors to cellular components and play roles in diverse biological processes. Recent studies have revealed the hubbing roles of RLCKs in plant responses to biotic stress. Emerging evidence indicates the important regulatory roles of RLCKs in plant responses to abiotic stress, growth, and development. As a pivot of cellular signaling, the activity and stability of RLCKs are dynamically and tightly controlled. Here, we summarize the current understanding of how RLCKs regulate plant responses to biotic and abiotic stress.

What's new in protein kinase/phosphatase signalling in the control of plant immunity?

Plant immunity is crucial to plant health but comes at an expense. For optimal plant growth, tight immune regulation is required to prevent unnecessary rechannelling of valuable resources. Pattern- and effector-triggered immunity (PTI/ETI) represent the two tiers of immunity initiated after sensing microbial patterns at the cell surface or pathogen effectors secreted into plant cells, respectively. Recent evidence of PTI-ETI cross-potentiation suggests a close interplay of signalling pathways and defense responses downstream of perception that is still poorly understood. This review will focus on controls on plant immunity through phosphorylation, a universal and key cellular regulatory mechanism. Rather than a complete overview, we highlight “what’s new in protein kinase/phosphatase signalling” in the immunity field. In addition to phosphoregulation of components in the pattern recognition receptor (PRR) complex, we will cover the actions of the major immunity-relevant intracellular protein kinases/phosphatases in the ‘signal relay’, namely calcium-regulated kinases (e.g. calcium-dependent protein kinases, CDPKs), mitogen-activated protein kinases (MAPKs), and various protein phosphatases. We discuss how these factors define a phosphocode that generates cellular decision-making ‘logic gates’, which contribute to signalling fidelity, amplitude, and duration. To underscore the importance of phosphorylation, we summarize strategies employed by pathogens to subvert plant immune phosphopathways. In view of recent game-changing discoveries of ETI-derived resistosomes organizing into calcium-permeable pores, we speculate on a possible calcium-regulated phosphocode as the mechanistic control of the PTI-ETI continuum.

A novel calmodulin-interacting Domain of Unknown Function 506 protein represses root hair elongation in Arabidopsis

Domain of Unknown Function 506 proteins are ubiquitous in plants. The phosphorus (P) stress-inducible REPRESSOR OF EXCESSIVE ROOT HAIR GROWTH1 (AtRXR1) gene encodes the first characterized DUF506. AtRXR1 inhibits root hair elongation by interacting with RabD2c GTPase. However, functions of other P-responsive DUF506 genes are still missing. Here, we selected two additional P-inducible DUF506 genes for further investigation. The expression of both genes was induced by auxin. Under P-stress, At3g07350 gene expressed ubiquitously in seedlings, whereas At1g62420 (AtRXR3) expression was strongest in roots. AtRXR3 overexpressors and knockouts had shorter and longer root hairs, respectively. A functional AtRXR3-green fluorescent protein fusion localized to root epidermal cells. Chromatin immunoprecipitation and quantitative reverse-transcriptase-polymerase chain reaction revealed that AtRXR3 was transcriptionally activated by RSL4. Bimolecular fluorescence complementation and calmodulin (CaM)-binding assays showed that AtRXR3 interacted with CaM in the presence of Ca²⁺ . Moreover, cytosolic Ca²⁺ ([Ca²⁺ ]_cyt ) oscillations in root hairs of rxr3 mutants exhibited elevated frequencies and dampened amplitudes compared to those of wild type. Thus, AtRXR3 is another DUF506 protein that attenuates P-limitation-induced root hair growth through mechanisms that involve RSL4 and interaction with CaM to modulate tip-focused [Ca²⁺ ]_cyt oscillations.

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.

Phytocytokine signalling reopens stomata in plant immunity and water loss

Stomata exert considerable effects on global carbon and water cycles by mediating gas exchange and water vapour1,2. Stomatal closure prevents water loss in response to dehydration and limits pathogen entry3,4. However, prolonged stomatal closure reduces photosynthesis and transpiration and creates aqueous apoplasts that promote colonization by pathogens. How plants dynamically regulate stomatal reopening in a changing climate is unclear. Here we show that the secreted peptides SMALL PHYTOCYTOKINES REGULATING DEFENSE AND WATER LOSS (SCREWs) and the cognate receptor kinase PLANT SCREW UNRESPONSIVE RECEPTOR (NUT) counter-regulate phytohormone abscisic acid (ABA)- and microbe-associated molecular pattern (MAMP)-induced stomatal closure. SCREWs sensed by NUT function as immunomodulatory phytocytokines and recruit SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) co-receptors to relay immune signalling. SCREWs trigger the NUT-dependent phosphorylation of ABA INSENSITIVE 1 (ABI1) and ABI2, which leads to an increase in the activity of ABI phosphatases towards OPEN STOMATA 1 (OST1)-a key kinase that mediates ABA- and MAMP-induced stomatal closure5,6-and a reduction in the activity of S-type anion channels. After induction by dehydration and pathogen infection, SCREW-NUT signalling promotes apoplastic water loss and disrupts microorganism-rich aqueous habitats to limit pathogen colonization. The SCREW-NUT system is widely distributed across land plants, which suggests that it has an important role in preventing uncontrolled stomatal closure caused by abiotic and biotic stresses to optimize plant fitness.

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.

Function of Protein Kinases in Leaf Senescence of Plants

Leaf senescence is an evolutionarily acquired process and it is critical for plant fitness. During senescence, macromolecules and nutrients are disassembled and relocated to actively growing organs. Plant leaf senescence process can be triggered by developmental cues and environmental factors, proper regulation of this process is essential to improve crop yield. Protein kinases are enzymes that modify their substrates activities by changing the conformation, stability, and localization of those proteins, to play a crucial role in the leaf senescence process. Impressive progress has been made in understanding the role of different protein kinases in leaf senescence recently. This review focuses on the recent progresses in plant leaf senescence-related kinases. We summarize the current understanding of the function of kinases on senescence signal perception and transduction, to help us better understand how the orderly senescence degeneration process is regulated by kinases, and how the kinase functions in the intricate integration of environmental signals and leaf age information.

The UBP14-CDKB1;1-CDKG2 cascade controls endoreduplication and cell growth in Arabidopsis

Endoreduplication, a process in which DNA replication occurs in the absence of mitosis, is found in all eukaryotic kingdoms, especially plants, where it is assumed to be important for cell growth and cell fate maintenance. However, a comprehensive understanding of the mechanism regulating endoreduplication is still lacking. We previously reported that UBIQUITIN-SPECIFIC PROTEASE14 (UBP14), encoded by DA3, acts upstream of CYCLIN-DEPENDENT KINASE B1;1 (CDKB1;1) to influence endoreduplication and cell growth in Arabidopsis thaliana. The da3-1 mutant possesses large cotyledons with enlarged cells due to high ploidy levels. Here, we identified a suppressor of da3-1 (SUPPRESSOR OF da3-1 6; SUD6), encoding CYCLIN-DEPENDENT KINASE G2 (CDKG2), which promotes endoreduplication and cell growth. CDKG2/SUD6 physically associates with CDKB1;1 in vivo and in vitro. CDKB1;1 directly phosphorylates SUD6 and modulates its stability. Genetic analysis indicated that SUD6 acts downstream of DA3 and CDKB1;1 to control ploidy level and cell growth. Thus, our study establishes a regulatory cascade for UBP14/DA3-CDKB1;1-CDKG2/SUD6-mediated control of endoreduplication and cell growth in Arabidopsis.

Calcium-Mobilizing Properties of Salvia miltiorrhiza-Derived Carbon Dots Confer Enhanced Environmental Adaptability in Plants

Biomass-derived carbon dots (CDs) are promising nanotools for agricultural applications and function as a reactive oxygen species (ROS) scavenger to alleviate plant oxidative stress under adverse environments. Nevertheless, plants need ROS burst to fully activate Ca²⁺-regulated defensive signaling pathway. The underlying mechanism of CDs to improve plant environmental adaptability without ROS is largely unknown. Here, Salvia miltiorrhiza-derived CDs triggered ROS-independent Ca²⁺ mobilization in plant roots. Mechanistic investigation attributed this function mainly to the hydroxyl and carboxyl groups on CDs. CDs-triggered Ca²⁺ mobilization was found to be dependent on the production of cyclic nucleotides and cyclic nucleotide-gated ion channels. Lectin receptor kinases were verified as essential for this Ca²⁺ mobilization. CDs hydroponic application promoted Ca²⁺ signaling and plant environmental adaptability under salinity and nutrient-deficient conditions. All these findings uncover that CDs have a Ca²⁺-mobilizing property and thus can be used as a simultaneous Ca²⁺ signaling amplifier and ROS scavenger for crop improvement.