Extracellular ATP elicits DORN1-mediated RBOHD phosphorylation to regulate stomatal aperture

FERONIA orchestrates P2K1-driven purinergic signaling in plant roots

Extracellular ATP (eATP) orchestrates vital processes in plants, akin to its role in animals. P2K1 is a crucial receptor mediating eATP effects. Immunoprecipitation tandem mass spectrometry data highlighted FERONIA's significant interaction with P2K1, driving us to explore its role in eATP signaling. Here, we investigated putative P2K1-interactor, FERONIA, which is a versatile receptor kinase pivotal in growth and stress responses. We employed a FERONIA loss-of-function mutant, fer-4, to dissect its effects on eATP signaling. Interestingly, fer-4 showed distinct calcium responses compared to wild type, while eATP-responsive genes were constitutively upregulated in fer-4. Additionally, fer-4 displayed insensitivity to eATP-regulated root growth and reduced cell wall accumulation. Together, these results uncover a role for FERONIA in regulating eATP signaling. Overall, our study deepens our understanding of eATP signaling, revealing the intricate interplay between P2K1 and FERONIA impacting the interface between growth and defense.

Identification of mitogen-activated protein kinases substrates in Arabidopsis using kinase client assay

Mitogen-activated protein kinase (MPK) cascades are essential signal transduction components that control a variety of cellular responses in all eukaryotes. MPKs convert extracellular stimuli into cellular responses by the phosphorylation of downstream substrates. Although MPK cascades are predicted to be very complex, only limited numbers of MPK substrates have been identified in plants. Here, we used the kinase client (KiC) assay to identify novel substrates of MPK3 and MPK6. Recombinant MPK3 or MPK6 were tested against a large synthetic peptide library representing in vivo phosphorylation sites, and phosphorylated peptides were identified by high-resolution tandem mass spectrometry. From this screen, we identified 23 and 21 putative client peptides of MPK3 and MPK6, respectively. To verify the phosphorylation of putative client peptides, we performed in vitro kinase assay with recombinant fusion proteins of isolated client peptides. We found that 13 and 9 recombinant proteins were phosphorylated by MPK3 and MPK6. Among them, 11 proteins were proven to be the novel substrates of two MPKs. This study suggests that the KiC assay is a useful method to identify new substrates of MPKs.

Functional analysis of reactive oxygen species-driven stress systemic signalling, interplay and acclimation

Reactive oxygen species (ROS) play a critical role in plant development and stress responses, acting as key components in rapid signalling pathways. The 'ROS wave' triggers essential acclimation processes, ultimately ensuring plant survival under diverse challenges. This review explores recent advances in understanding the composition and functionality of the ROS wave within plant cells. During their initiation and propagation, ROS waves interact with other rapid signalling pathways, hormones and various molecular compounds. Recent research sheds light on the intriguing lack of a rigid hierarchy governing these interactions, highlighting a complex interplay between diverse signals. Notably, ROS waves culminate in systemic acclimation, a crucial outcome for enhanced stress tolerance. This review emphasizes the versatility of ROS, which act as flexible players within a network of short- and long-term factors contributing to plant stress resilience. Unveiling the intricacies of these interactions between ROS and various signalling molecules holds immense potential for developing strategies to augment plant stress tolerance, contributing to improved agricultural practices and overall ecosystem well-being.

Fighting for Survival at the Stomatal Gate

Stomata serve as the battleground between plants and plant pathogens. Plants can perceive pathogens, inducing closure of the stomatal pore, while pathogens can overcome this immune response with their phytotoxins and elicitors. In this review, we summarize new discoveries in stomata-pathogen interactions. Recent studies have shown that stomatal movement continues to occur in a close-open-close-open pattern during bacterium infection, bringing a new understanding of stomatal immunity. Furthermore, the canonical pattern-triggered immunity pathway and ion channel activities seem to be common to plant-pathogen interactions outside of the well-studied Arabidopsis-Pseudomonas pathosystem. These developments can be useful to aid in the goal of crop improvement. New technologies to study intact leaves and advances in available omics data sets provide new methods for understanding the fight at the stomatal gate. Future studies should aim to further investigate the defense-growth trade-off in relation to stomatal immunity, as little is known at this time.

A novel lectin receptor kinase gene, AtG-LecRK-I.2, enhances bacterial pathogen resistance through regulation of stomatal immunity in Arabidopsis

The S-locus lectin receptor kinases (G-LecRKs) have been suggested as receptors for microbe/damage-associated molecular patterns (MAMPs/DAMPs) and to be involved in the pathogen defense responses, but the functions of most G-LecRKs in biotic stress response have not been characterized. Here, we identified a member of this family, G-LecRK-I.2, that positively regulates flg22- and Pseudomonas syringae pv. tomato (Pst) DC3000-induced stomatal closure. G-LecRK-I.2 was rapidly phosphorylated under flg22 treatment and could interact with the FLS2/BAK1 complex. Two T-DNA insertion lines, glecrk-i.2-1 and glecrk-i.2-2, had lower levels of reactive oxygen species (ROS) and nitric oxide (NO) production in guard cells, as compared with the wild-type Col-0, under Pst DC3000 infection. Also, the immunity marker genes CBP60g and PR1 were induced at lower levels under Pst DC3000 hrcC- infection in glecrk-i.2-1 and glecrk-i.2-2. The GUS reporter system also revealed that G-LecRK-I.2 was expressed only in guard cells. We also found that G-LecRK-I.2 could interact H+-ATPase AHA1 to regulate H+-ATPase activity in the guard cells. Taken together, our results show that G-LecRK-I.2 plays an important role in regulating stomatal closure under flg22 and Pst DC3000 treatments and in ROS and NO signaling specifically in guard cells.

Decoding Arabidopsis thaliana CPK/SnRK Superfamily Kinase Client Signaling Networks Using Peptide Library and Mass Spectrometry

Members of the calcium-dependent protein kinase (CDPK/CPK) and SNF-related protein kinase (SnRK) superfamilies are commonly found in plants and some protists. Our knowledge of client specificity of the members of this superfamily is fragmentary. As this family is represented by over 30 members in Arabidopsis thaliana, the identification of kinase-specific and overlapping client relationships is crucial to our understanding the nuances of this large family of kinases as directed towards signal transduction pathways. Herein, we used the kinase client (KiC) assay-a relative, quantitative, high-throughput mass spectrometry-based in vitro phosphorylation assay-to identify and characterize potential CPK/SnRK targets of Arabidopsis. Eight CPKs (1, 3, 6, 8, 17, 24, 28, and 32), four SnRKs (subclass 1 and 2), and PPCK1 and PPCK2 were screened against a synthetic peptide library that contains 2095 peptides and 2661 known phosphorylation sites. A total of 625 in vitro phosphorylation sites corresponding to 203 non-redundant proteins were identified. The most promiscuous kinase, CPK17, had 105 candidate target proteins, many of which had already been discovered. Sequence analysis of the identified phosphopeptides revealed four motifs: LxRxxS, RxxSxxR, RxxS, and LxxxxS, that were significantly enriched among CPK/SnRK clients. The results provide insight into both CPK- and SnRK-specific and overlapping signaling network architectures and recapitulate many known in vivo relationships validating this large-scale approach towards discovering kinase targets.

Pathogen perception and signaling in plant immunity

Plant diseases are a constant and serious threat to agriculture and ecological biodiversity. Plants possess a sophisticated innate immunity system capable of detecting and responding to pathogen infection to prevent disease. Our understanding of this system has grown enormously over the past century. Early genetic descriptions of plant disease resistance and pathogen virulence were embodied in the gene-for-gene hypothesis, while physiological studies identified pathogen-derived elicitors that could trigger defense responses in plant cells and tissues. Molecular studies of these phenomena have now coalesced into an integrated model of plant immunity involving cell surface and intracellular detection of specific pathogen-derived molecules and proteins culminating in the induction of various cellular responses. Extracellular and intracellular receptors engage distinct signaling processes but converge on many similar outputs with substantial evidence now for integration of these pathways into interdependent networks controlling disease outcomes. Many of the molecular details of pathogen recognition and signaling processes are now known, providing opportunities for bioengineering to enhance plant protection from disease. Here we provide an overview of the current understanding of the main principles of plant immunity, with an emphasis on the key scientific milestones leading to these insights.

CPK28 is a modulator of purinergic signaling in plant growth and defense

Extracellular ATP (eATP) is a key signaling molecule that plays a pivotal role in plant growth and defense responses. The receptor P2K1 is responsible for perceiving eATP and initiating its signaling cascade. However, the signal transduction mechanisms downstream of P2K1 activation remain incompletely understood. We conducted a comprehensive analysis of the P2K1 interactome using co-immunoprecipitation-coupled tandem mass spectrometry, leading to the identification of 121 candidate proteins interacting with P2K1. In silico analysis narrowed down the candidates to 47 proteins, including Ca2+-binding proteins, ion transport-related proteins, and receptor kinases. To investigate their involvement in eATP signaling, we employed a screening strategy based on changes in gene expression in response to eATP in mutants of the identified interactors. This screening revealed several Ca2+-dependent protein kinases (CPKs) that significantly affected the expression of eATP-responsive genes, suggesting their potential roles in eATP signaling. Notably, CPK28 and CPK6 showed physical interactions with P2K1 both in yeast and plant systems. Calcium influx and gene expression studies demonstrated that CPK28 perturbed eATP-induced Ca2+ mobilization and some early transcriptional responses. Overexpression of CPK28 resulted in an antagonistic physiological response to P2K1-mediated eATP signaling during both plant growth and defense responses to the necrotrophic pathogen Botrytis cinerea. Our findings highlight CPK28, among other CPKs, as a modulator of P2K1-mediated eATP signaling, providing valuable insights into the coordination of eATP signaling in plant growth and immunity.

ATP homeostasis and signaling in plants

ATP is the primary form of energy for plants, and a shortage of cellular ATP is generally acknowledged to pose a threat to plant growth and development, stress resistance, and crop quality. The overall metabolic processes that contribute to the ATP pool, from production, dissipation, and transport to elimination, have been studied extensively. Considerable evidence has revealed that in addition to its role in energy supply, ATP also acts as a regulatory signaling molecule to activate global metabolic responses. Identification of the eATP receptor DORN1 contributed to a better understanding of how plants cope with disruption of ATP homeostasis and of the key points at which ATP signaling pathways intersect in cells or whole organisms. The functions of SnRK1α, the master regulator of the energy management network, in restoring the equilibrium of the ATP pool have been demonstrated, and the vast and complex metabolic network mediated by SnRK1α to adapt to fluctuating environments has been characterized. This paper reviews recent advances in understanding the regulatory control of the cellular ATP pool and discusses possible interactions among key regulators of ATP-pool homeostasis and crosstalk between iATP/eATP signaling pathways. Perception of ATP deficit and modulation of cellular ATP homeostasis mediated by SnRK1α in plants are discussed at the physiological and molecular levels. Finally, we suggest future research directions for modulation of plant cellular ATP homeostasis.

Unveiling orphan receptor-like kinases in plants: novel client discovery using high-confidence library predictions in the Kinase-Client (KiC) assay

Plants are remarkable in their ability to adapt to changing environments, with receptor-like kinases (RLKs) playing a pivotal role in perceiving and transmitting environmental cues into cellular responses. Despite extensive research on RLKs from the plant kingdom, the function and activity of many kinases, i.e., their substrates or "clients", remain uncharted. To validate a novel client prediction workflow and learn more about an important RLK, this study focuses on P2K1 (DORN1), which acts as a receptor for extracellular ATP (eATP), playing a crucial role in plant stress resistance and immunity. We designed a Kinase-Client (KiC) assay library of 225 synthetic peptides, incorporating previously identified P2K phosphorylated peptides and novel predictions from a deep-learning phosphorylation site prediction model (MUsite) and a trained hidden Markov model (HMM) based tool, HMMER. Screening the library against purified P2K1 cytosolic domain (CD), we identified 46 putative substrates, including 34 novel clients, 27 of which may be novel peptides, not previously identified experimentally. Gene Ontology (GO) analysis among phosphopeptide candidates revealed proteins associated with important biological processes in metabolism, structure development, and response to stress, as well as molecular functions of kinase activity, catalytic activity, and transferase activity. We offer selection criteria for efficient further in vivo experiments to confirm these discoveries. This approach not only expands our knowledge of P2K1's substrates and functions but also highlights effective prediction algorithms for identifying additional potential substrates. Overall, the results support use of the KiC assay as a valuable tool in unraveling the complexities of plant phosphorylation and provide a foundation for predicting the phosphorylation landscape of plant species based on peptide library results.

DGK5β-derived phosphatidic acid regulates ROS production in plant immunity by stabilizing NADPH oxidase

In plant immunity, phosphatidic acid (PA) regulates reactive oxygen species (ROS) by binding to respiratory burst oxidase homolog D (RBOHD), an NADPH oxidase responsible for ROS production. Here, we analyze the influence of PA binding on RBOHD activity and the mechanism of RBOHD-bound PA generation. PA binding enhances RBOHD protein stability by inhibiting vacuolar degradation, thereby increasing chitin-induced ROS production. Mutations in diacylglycerol kinase 5 (DGK5), which phosphorylates diacylglycerol to produce PA, impair chitin-induced PA and ROS production. The DGK5 transcript DGK5β (but not DGK5α) complements reduced PA and ROS production in dgk5-1 mutants, as well as resistance to Botrytis cinerea. Phosphorylation of S506 residue in the C-terminal calmodulin-binding domain of DGK5β contributes to the activation of DGK5β to produce PA. These findings suggest that DGK5β-derived PA regulates ROS production by inhibiting RBOHD protein degradation, elucidating the role of PA-ROS interplay in immune response regulation.

Damage-Associated Molecular Patterns and Systemic Signaling

Cellular damage inflicted by wounding, pathogen infection, and herbivory releases a variety of host-derived metabolites, degraded structural components, and peptides into the extracellular space that act as alarm signals when perceived by adjacent cells. These so-called damage-associated molecular patterns (DAMPs) function through plasma membrane localized pattern recognition receptors to regulate wound and immune responses. In plants, DAMPs act as elicitors themselves, often inducing immune outputs such as calcium influx, reactive oxygen species generation, defense gene expression, and phytohormone signaling. Consequently, DAMP perception results in a priming effect that enhances resistance against subsequent pathogen infections. Alongside their established function in local tissues, recent evidence supports a critical role of DAMP signaling in generation and/or amplification of mobile signals that induce systemic immune priming. Here, we summarize the identity, signaling, and synergy of proposed and established plant DAMPs, with a focus on those with published roles in systemic signaling.

Small holes, big impact: Stomata in plant-pathogen-climate epic trifecta

The regulation of stomatal aperture opening and closure represents an evolutionary battle between plants and pathogens, characterized by adaptive strategies that influence both plant resistance and pathogen virulence. The ongoing climate change introduces further complexity, affecting pathogen invasion and host immunity. This review delves into recent advances on our understanding of the mechanisms governing immunity-related stomatal movement and patterning with an emphasis on the regulation of stomatal opening and closure dynamics by pathogen patterns and host phytocytokines. In addition, the review explores how climate changes impact plant-pathogen interactions by modulating stomatal behavior. In light of the pressing challenges associated with food security and the unpredictable nature of climate changes, future research in this field, which includes the investigation of spatiotemporal regulation and engineering of stomatal immunity, emerges as a promising avenue for enhancing crop resilience and contributing to climate control strategies.

Phosphorylation-mediated regulation of integrin-linked kinase 5 by purinoreceptor P2K2

Extracellular ATP (eATP) in plants plays a crucial role as a ligand for purinoreceptors, mediating purinergic signaling and regulating diverse biological functions, including responses to abiotic and biotic stresses. DORN1/P2K1 (LecRK I.9) was the first identified plant purinoreceptor. P2K2 (LecRK I.5) was subsequently identified as an additional plant purinoreceptor and shown to directly interact with P2K1. Recently, we reported that P2K1 interacts with Integrin-linked kinase 5 (ILK5), a Raf-like MAPKKK protein, and phosphorylates ILK5 to regulate purinergic signaling in relation to plant innate immunity. Here, we report that P2K2 also interacts with the ILK5 protein in planta. Furthermore, we demonstrate that P2K2 phosphorylates ILK5 in the presence of [γ-32P] ATP, similar to P2K1. However, unlike P2K1, P2K2 exhibits strong phosphorylation even when the Serine 192 residue of ILK5 is mutated to Alanine (ILK5S192A), suggesting the possibility of phosphorylation of other residues to fully regulate ILK5 protein function.

Computational prediction and in vitro analysis of the potential ligand binding site within the extracellular ATP receptor, P2K2

The plant extracellular ATP (eATP) receptor, P2K2, binds eATP with strong ligand affinity through its extracellular lectin domain. Ligand binding activates the intracellular kinase domain of P2K2 resulting in a variety of intracellular responses and, ultimately, increased plant immunity to invading fungal and bacterial pathogens. Here, using a computational prediction approach, we developed a tertiary structure model of the P2K2 extracellular lectin domain. In silico target docking of ATP to the P2K2-binding site predicted interaction with several residues through hydrophobic interactions and hydrogen bonding. Our confirmation of the modeling was obtained by showing that H99, R144, and S256 are key residues essential for in vitro binding of ATP by P2K2.

The Plasma Membrane Purinoreceptor P2K1/DORN1 Is Essential in Stomatal Closure Evoked by Extracellular Diadenosine Tetraphosphate (Ap4A) in Arabidopsis thaliana

Dinucleoside polyphosphates (NpnNs) are considered novel signalling molecules involved in the induction of plant defence mechanisms. However, NpnN signal recognition and transduction are still enigmatic. Therefore, the aim of our research was the identification of the NpnN receptor and signal transduction pathways evoked by these nucleotides. Earlier, we proved that purine and pyrimidine NpnNs differentially affect the phenylpropanoid pathway in Vitis vinifera suspension-cultured cells. Here, we report, for the first time, that both diadenosine tetraphosphate (Ap4A) and dicytidine tetraphosphate (Cp4C)-induced stomatal closure in Arabidopsis thaliana. Moreover, we showed that plasma membrane purinoreceptor P2K1/DORN1 (does not respond to nucleotide 1) is essential for Ap4A-induced stomata movements but not for Cp4C. Wild-type Col-0 and the dorn1-3 A. thaliana knockout mutant were used. Examination of the leaf epidermis dorn1-3 mutant provided evidence that P2K1/DORN1 is a part of the signal transduction pathway in stomatal closure evoked by extracellular Ap4A but not by Cp4C. Reactive oxygen species (ROS) are involved in signal transduction caused by Ap4A and Cp4C, leading to stomatal closure. Ap4A induced and Cp4C suppressed the transcriptional response in wild-type plants. Moreover, in dorn1-3 leaves, the effect of Ap4A on gene expression was impaired. The interaction between P2K1/DORN1 and Ap4A leads to changes in the transcription of signalling hubs in signal transduction pathways.

Soil-borne fungi alter the apoplastic purinergic signaling in plants by deregulating the homeostasis of extracellular ATP and its metabolite adenosine

Purinergic signaling activated by extracellular nucleotides and their derivative nucleosides trigger sophisticated signaling networks. The outcome of these pathways determine the capacity of the organism to survive under challenging conditions. Both extracellular ATP (eATP) and Adenosine (eAdo) act as primary messengers in mammals, essential for immunosuppressive responses. Despite the clear role of eATP as a plant damage-associated molecular pattern, the function of its nucleoside, eAdo, and of the eAdo/eATP balance in plant stress response remain to be fully elucidated. This is particularly relevant in the context of plant-microbe interaction, where the intruder manipulates the extracellular matrix. Here, we identify Ado as a main molecule secreted by the vascular fungus Fusarium oxysporum. We show that eAdo modulates the plant's susceptibility to fungal colonization by altering the eATP-mediated apoplastic pH homeostasis, an essential physiological player during the infection of this pathogen. Our work indicates that plant pathogens actively imbalance the apoplastic eAdo/eATP levels as a virulence mechanism.

Enhanced Ca2+ binding to EF-hands through phosphorylation of conserved serine residues activates MpRBOHB and chitin-triggered ROS production

NADPH oxidases/RBOHs catalyze apoplastic ROS production and act as key signaling nodes, integrating multiple signal transduction pathways regulating plant development and stress responses. Although RBOHs have been suggested to be activated by Ca2+ binding and phosphorylation by various protein kinases, a mechanism linking Ca2+ binding and phosphorylation in the activity regulation remained elusive. Chitin-triggered ROS production required cytosolic Ca2+ elevation and Ca2+ binding to MpRBOHB in a liverwort Marchantia polymorpha. Heterologous expression analysis of truncated variants revealed that a segment of the N-terminal cytosolic region highly conserved among land plant RBOHs encompassing the two EF-hand motifs is essential for the activation of MpRBOHB. Within the conserved regulatory domain, we have identified two Ser residues whose phosphorylation is critical for the activation in planta. Isothermal titration calorimetry analyses revealed that phosphorylation of the two Ser residues increased the Ca2+ binding affinity of MpRBOHB, while Ca2+ binding is indispensable for the activation, even if the two Ser residues are phosphorylated. Our findings shed light on a mechanism through which phosphorylation potentiates the Ca2+ -dependent activation of MpRBOHB, emphasizing the pivotal role of Ca2+ binding in mediating the Ca2+ and phosphorylation-driven activation of MpRBOHB, which is likely to represent a fundamental mechanism conserved among land plant RBOHs.

Salt stress releases extracellular ATP to activate purinergic signaling and inhibit plant growth

Salt stress increases extracellular ATP levels and the upregulation of P2K1 purinoreceptor transcripts, leading to the activation of purinergic signaling and consequent inhibition of plant growth.

Plant immune receptors interact with hemibiotrophic pathogens to activate plant immunity

Phytopathogens pose a devastating threat to the productivity and yield of crops by causing destructive plant diseases in natural and agricultural environments. Hemibiotrophic pathogens have a variable-length biotrophic phase before turning to necrosis and are among the most invasive plant pathogens. Plant resistance to hemibiotrophic pathogens relies mainly on the activation of innate immune responses. These responses are typically initiated after the plant plasma membrane and various plant immune receptors detect immunogenic signals associated with pathogen infection. Hemibiotrophic pathogens evade pathogen-triggered immunity by masking themselves in an arms race while also enhancing or manipulating other receptors to promote virulence. However, our understanding of plant immune defenses against hemibiotrophic pathogens is highly limited due to the intricate infection mechanisms. In this review, we summarize the strategies that different hemibiotrophic pathogens interact with host immune receptors to activate plant immunity. We also discuss the significant role of the plasma membrane in plant immune responses, as well as the current obstacles and potential future research directions in this field. This will enable a more comprehensive understanding of the pathogenicity of hemibiotrophic pathogens and how distinct plant immune receptors oppose them, delivering valuable data for the prevention and management of plant diseases.

The intrinsically disordered C-terminus of purinoceptor P2K1 fine-tunes plant responses to extracellular ATP

P2K1 is a plant-specific purinoceptor that perceives extracellular ATP (eATP), a signaling molecular implicated in various physiological processes. Interestingly, P2K1 harbors a C-terminal intrinsically disordered region (IDR). When we overexpressed a truncated P2K1 (P2K1t ) lacking the IDR, primary root growth completely ceased in response to eATP. We investigated the functional roles of the IDR in P2K1 using a combination of molecular genetics, calcium imaging, gene expression analysis, and histochemical approaches. We found that the P2K1t variant gave rise to an amplified response to eATP, through accumulation of superoxide, altered cell wall integrity, and ultimate cell death in the primary root tip. Together, these observations underscore the significant involvement of the C-terminal tail of P2K1 in root growth regulation.

Arabidopsis translation initiation factor binding protein CBE1 negatively regulates accumulation of the NADPH oxidase respiratory burst oxidase homolog D

Cell surface pattern recognition receptors sense invading pathogens by binding microbial or endogenous elicitors to activate plant immunity. These responses are under tight control to avoid excessive or untimely activation of cellular responses, which may otherwise be detrimental to host cells. How this fine-tuning is accomplished is an area of active study. We previously described a suppressor screen that identified Arabidopsis thaliana mutants with regained immune signaling in the immunodeficient genetic background bak1-5, which we named modifier of bak1-5 (mob) mutants. Here, we report that bak1-5 mob7 mutant restores elicitor-induced signaling. Using a combination of map-based cloning and whole-genome resequencing, we identified MOB7 as conserved binding of eIF4E1 (CBE1), a plant-specific protein that interacts with the highly conserved eukaryotic translation initiation factor eIF4E1. Our data demonstrate that CBE1 regulates the accumulation of respiratory burst oxidase homolog D, the NADPH oxidase responsible for elicitor-induced apoplastic reactive oxygen species production. Furthermore, several mRNA decapping and translation initiation factors colocalize with CBE1 and similarly regulate immune signaling. This study thus identifies a novel regulator of immune signaling and provides new insights into reactive oxygen species regulation, potentially through translational control, during plant stress responses.

How Extracellular Reactive Oxygen Species Reach Their Intracellular Targets in Plants

Reactive oxygen species (ROS) serve as secondary messengers that regulate various developmental and signal transduction processes, with ROS primarily generated by NADPH OXIDASEs (referred to as RESPIRATORY BURST OXIDASE HOMOLOGs [RBOHs] in plants). However, the types and locations of ROS produced by RBOHs are different from those expected to mediate intracellular signaling. RBOHs produce O2•- rather than H2O2 which is relatively long-lived and able to diffuse through membranes, and this production occurs outside the cell instead of in the cytoplasm, where signaling cascades occur. A widely accepted model explaining this discrepancy proposes that RBOH-produced extracellular O2•- is converted to H2O2 by superoxide dismutase and then imported by aquaporins to reach its cytoplasmic targets. However, this model does not explain how the specificity of ROS targeting is ensured while minimizing unnecessary damage during the bulk translocation of extracellular ROS (eROS). An increasing number of studies have provided clues about eROS action mechanisms, revealing various mechanisms for eROS perception in the apoplast, crosstalk between eROS and reactive nitrogen species, and the contribution of intracellular organelles to cytoplasmic ROS bursts. In this review, we summarize these recent advances, highlight the mechanisms underlying eROS action, and provide an overview of the routes by which eROS-induced changes reach the intracellular space.

Paradigms of receptor kinase signaling in plants

Plant receptor kinases (RKs) function as key plasma-membrane localized receptors in the perception of molecular ligands regulating development and environmental response. Through the perception of diverse ligands, RKs regulate various aspects throughout the plant life cycle from fertilization to seed set. Thirty years of research on plant RKs has generated a wealth of knowledge on how RKs perceive ligands and activate downstream signaling. In the present review, we synthesize this body of knowledge into five central paradigms of plant RK signaling: (1) RKs are encoded by expanded gene families, largely conserved throughout land plant evolution; (2) RKs perceive many different kinds of ligands through a range of ectodomain architectures; (3) RK complexes are typically activated by co-receptor recruitment; (4) post-translational modifications fulfill central roles in both the activation and attenuation of RK-mediated signaling; and, (5) RKs activate a common set of downstream signaling processes through receptor-like cytoplasmic kinases (RLCKs). For each of these paradigms, we discuss key illustrative examples and also highlight known exceptions. We conclude by presenting five critical gaps in our understanding of RK function.

Ligand recognition and signal transduction by lectin receptor-like kinases in plant immunity

Lectin receptor-like kinases (LecRKs) locate on the cell membrane and play diverse roles in perceiving environmental factors in higher plants. Studies have demonstrated that LecRKs are involved in plant development and response to abiotic and biotic stresses. In this review, we summarize the identified ligands of LecRKs in Arabidopsis, including extracellular purine (eATP), extracellular pyridine (eNAD⁺), extracellular NAD⁺ phosphate (eNADP⁺) and extracellular fatty acids (such as 3-hydroxydecanoic acid). We also discussed the posttranslational modification of these receptors in plant innate immunity and the perspectives of future research on plant LecRKs.

A histochemical reporter system to study extracellular ATP response in plants

When cells experience acute mechanical distress, they release ATP from their cellular compartment into the surrounding microenvironment. This extracellular ATP (eATP) can then act as a danger signal-signaling cellular damage. In plants, cells adjacent to damage detect rising eATP concentrations through the cell-surface receptor kinase, P2K1. Following eATP perception, P2K1 initiates a signaling cascade mobilizing plant defense. Recent transcriptome analysis revealed a profile of eATP-induced genes sharing pathogen- and wound-response hallmarks-consistent with a working model for eATP as a defense-mobilizing danger signal. To build on the transcriptional footprint and broaden our understanding of dynamic eATP signaling responses in plants, we aimed to i) generate a visual toolkit for eATP-inducible marker genes using a β-glucuronidase (GUS) reporter system and ii) evaluate the spatiotemporal response of these genes to eATP in plant tissues. Here, we demonstrate that the promoter activities of five genes, ATPR1, ATPR2, TAT3, WRKY46, and CNGC19, were highly sensitive to eATP in the primary root meristem and elongation zones with maximal responses at 2 h after treatment. These results suggest the primary root tip as a hub to study eATP-signaling activity and provide a proof-of-concept toward using these reporters to further dissect eATP and damage signaling in plants.

Phosphorylation of OsRbohB by the protein kinase OsDMI3 promotes H2O2 production to potentiate ABA responses in rice

In rice, the Ca²⁺/calmodulin-dependent protein kinase OsDMI3 is an important positive regulator of abscisic acid (ABA) signaling. In ABA signaling, H₂O₂ is required for ABA-induced activation of OsDMI3, which in turn increase H₂O₂ production. However, how OsDMI3 regulates H₂O₂ production in ABA signaling remains unknown. Here we show that OsRbohB is the main NADPH oxidase involved in ABA-induced H₂O₂ production and ABA-mediated physiological responses. OsDMI3 directly interacts with and phosphorylates OsRbohB at Ser-191, which is OsDMI3-mediated site-specific phosphorylation in ABA signaling. Further analyses revealed that OsDMI3-mediated OsRbohB Ser-191 phosphorylation positively regulates the activity of NADPH oxidase and the production of H₂O₂ in ABA signaling, thereby enhancing the sensitivity of seed germination and root growth to ABA and plant tolerance to water stress and oxidative stress. Moreover, we discovered that the OsDMI3-mediated OsRbohB phosphorylation and H₂O₂ production is dependent on the sucrose non-fermenting 1-related protein kinases SAPK8/9/10, which phosphorylate OsRbohB at Ser-140 in ABA signaling. Taken together, these results not only reveal an important regulatory mechanism that directly activates Rboh for ABA-induced H₂O₂ production but also uncover the importance of this regulatory mechanism in ABA signaling.

The Raf-like MAPKKK INTEGRIN-LINKED KINASE 5 regulates purinergic receptor-mediated innate immunity in Arabidopsis

Mitogen-activated protein (MAP) kinase signaling cascades play important roles in eukaryotic defense against various pathogens. Activation of the extracellular ATP (eATP) receptor P2K1 triggers MAP kinase 3 and 6 (MPK3/6) phosphorylation, which leads to an elevated plant defense response. However, the mechanism by which P2K1 activates the MAPK cascade is unclear. In this study, we show that in Arabidopsis thaliana, P2K1 phosphorylates the Raf-like MAP kinase kinase kinase (MAPKKK) INTEGRIN-LINKED KINASE 5 (ILK5) on serine 192 in the presence of eATP. The interaction between P2K1 and ILK5 was confirmed both in vitro and in planta and their interaction was enhanced by ATP treatment. Similar to P2K1 expression, ILK5 expression levels were highly induced by treatment with ATP, flg22, Pseudomonas syringae pv. tomato DC3000, and various abiotic stresses. ILK5 interacts with and phosphorylates the MAP kinase MKK5. Moreover, phosphorylation of MPK3/6 was significantly reduced upon ATP treatment in ilk5 mutant plants, relative to wild-type (WT). The ilk5 mutant plants showed higher susceptibility to P. syringae pathogen infection relative to WT plants. Plants expressing only the mutant ILK5S192A protein, with decreased kinase activity, did not activate the MAPK cascade upon ATP addition. These results suggest that eATP activation of P2K1 results in transphosphorylation of the Raf-like MAPKKK ILK5, which subsequently triggers the MAPK cascade, culminating in activation of MPK3/6 associated with an elevated innate immune response.

DORN1 and GORK regulate stomatal closure in Arabidopsis mediated by volatile organic compound ethyl vinyl ketone

Evk (ethyl vinyl ketone) is a signal substance for plant defense, but little is known about how evk mediates stomatal closure. Through stomatal biology experiments, we found that evk can mediate stomatal closure, and stomatal closure is weakened when DORN1 (DOES NOT RESPOND TO NUCLEOTIDES 1) and GORK (GATED OUTWARDLY-RECTIFYING K+ CHANNEL) are mutated. In addition, it was found by non-invasive micro-test technology (NMT) that the K⁺ efflux mediated by evk was significantly weakened when DORN and GORK were mutated. Yeast two-hybrid (Y2H), firefly luciferase complementation imaging (LCI), and in vitro pull-down assays demonstrated that DORN1 and GORK could interact in vitro and in vivo. It was found by molecular docking that evk could combine with MRP (Multidrug Resistance-associated Protein), thus affecting ATP transport, promoting eATP (extracellular ATP) concentration increase and realizing downstream signal transduction. Through inoculation of botrytis cinerea, it was found that evk improved the antibacterial activity of Arabidopsis thaliana. As revealed by reverse transcription quantitative PCR (RT-qPCR), the expression of defense related genes was enhanced by evk treatment. Evk is a potential green antibacterial drug.

Ultrahigh-Throughput Single-Particle Hyperspectral Imaging of Gold Nanoparticles

Gold nanoparticles (AuNPs) have become increasingly useful in recent years for their roles in nanomedicine, cellular biology, energy storage and conversion, photocatalysis, and more. At the single-particle level, AuNPs have heterogeneous physical and chemical properties which are not resolvable in ensemble measurements. In the present study, we developed an ultrahigh-throughput spectroscopy and microscopy imaging system for characterization of AuNPs at the single-particle level using phasor analysis. The developed method enables quantification of spectra and spatial information on large numbers of AuNPs with a single snapshot of an image (1024 × 1024 pixels) at high temporal resolution (26 fps) and localization precision (sub-5 nm). We characterized the localized surface plasmonic resonance (SPR) scattering spectra of gold nanospheres (AuNSs) of four different sizes (40-100 nm). Comparing to the conventional optical grating method which suffers low efficiency in characterization due to spectral interference caused by nearby nanoparticles, the phasor approach enables high-throughput analysis of single-particle SPR properties in high particle density. Up to 10-fold greater efficiency of single-particle spectro-microscopy analysis using the spectra phasor approach when compared to a conventional optical grating method was demonstrated.

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

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

Phosphate-deprivation and damage signalling by extracellular ATP

Phosphate deprivation compromises plant productivity and modulates immunity. DAMP signalling by extracellular ATP (eATP) could be compromised under phosphate deprivation by the lowered production of cytosolic ATP and the need to salvage eATP as a nutritional phosphate source. Phosphate-starved roots of Arabidopsis can still sense eATP, indicating robustness in receptor function. However, the resultant cytosolic free Ca²⁺ signature is impaired, indicating modulation of downstream components. This perspective on DAMP signalling by extracellular ATP (eATP) addresses the salvage of eATP under phosphate deprivation and its promotion of immunity, how Ca²⁺ signals are generated and how the Ca²⁺ signalling pathway could be overcome to allow beneficial fungal root colonization to fulfill phosphate demands. Safe passage for an endophytic fungus allowing root colonization could be achieved by its down-regulation of the Ca²⁺ channels that act downstream of the eATP receptors and by also preventing ROS accumulation, thus further impairing DAMP signalling.

ATANN3 Is Involved in Extracellular ATP-Regulated Auxin Distribution in Arabidopsis thaliana Seedlings

Extracellular ATP (eATP) plays multiple roles in plant growth and development, and stress responses. It has been revealed that eATP suppresses growth and alters the growth orientation of the root and hypocotyl of Arabidopsis thaliana by affecting auxin transport and localization in these organs. However, the mechanism of the eATP-stimulated auxin distribution remains elusive. Annexins are involved in multiple aspects of plant cellular metabolism, while their role in response to apoplastic signals remains unclear. Here, by using the loss-of-function mutations, we investigated the role of AtANN3 in the eATP-regulated root and hypocotyl growth. Firstly, the inhibitory effects of eATP on root and hypocotyl elongation were weakened or impaired in the AtANN3 null mutants (atann3-1 and atann3-2). Meanwhile, the distribution of DR5-GUS and DR5-GFP indicated that the eATP-induced asymmetric distribution of auxin in the root tips or hypocotyl cells occurred in wild-type control plants, while in atann3-1 mutant seedlings, it was not observed. Further, the eATP-induced asymmetric distribution of PIN2-GFP in root-tip cells or that of PIN3-GFP in hypocotyl cells was reduced in atann3-1 seedlings. Finally, the eATP-induced asymmetric distribution of cytoplasmic vesicles in root-tip cells was impaired in atann3-1 seedlings. Based on these results, we suggest that AtANN3 may be involved in eATP-regulated seedling growth by regulating the distribution of auxin and auxin transporters in vegetative organs.

DORN1 Is Involved in Drought Stress Tolerance through a Ca2+-Dependent Pathway

Water shortages caused by climate change seriously threaten the survival and production of plants and are also one of the major environmental pressures faced by plants. DORN1 was the first identified purinoceptor for the plant response to extracellular ATP. It has been established that DORN1 could play key roles in a series of biological activities in plants. However, the biological roles of DORN1 and the mechanism remain unclear under drought stress conditions in plants. Here, DORN1 was targeted for knockout by using the CRISPR/Cas 9 system. It was found that the loss function of DORN1 resulted in a significant decrease in the effective quantum yield of PSII [Y(II)], the photochemical quenching coefficient (qP), and the rate of photosynthetic electron transport through PSII (ETR), which reflected plants' photochemical efficiency. Whereas Y_(NO) values showed obvious enhancement under drought stress conditions. Further experimental results showed that the Y_(II), q_P, and ETR, which reflect plants' photochemical efficiency, increased significantly with CaCl₂ treatment. These results indicated that the drought tolerance of the mutant was decreased, and the exogenous application of calcium ions could effectively promote the drought tolerance of the dorn1 mutant. Transpiration loss controlled by stomata is closely related to drought tolerance, further, we examined the transpirational water loss in dorn1 and found that it was greater than wild-type (WT). Besides, the dorn1 mutant's stomatal aperture significantly increased compared with the WT and the stomata of dorn1 mutant plants tend to close after CaCl₂ treatment. Taken together, our results show that DORN1 plays a key role in drought stress tolerance in plants, which may depend on calcium and calcium-related signaling pathways.

Ambivalent response in pathogen defense: A double-edged sword?

Plants possess effective immune systems that defend against most microbial attackers. Recent plant immunity research has focused on the classic binary defense model involving the pivotal role of small-molecule hormones in regulating the plant defense signaling network. Although most of our current understanding comes from studies that relied on information derived from a limited number of pathosystems, newer studies concerning the incredibly diverse interactions between plants and microbes are providing additional insights into other novel mechanisms. Here, we review the roles of both classical and more recently identified components of defense signaling pathways and stress hormones in regulating the ambivalence effect during responses to diverse pathogens. Because of their different lifestyles, effective defense against biotrophic pathogens normally leads to increased susceptibility to necrotrophs, and vice versa. Given these opposing forces, the plant potentially faces a trade-off when it mounts resistance to a specific pathogen, a phenomenon referred to here as the ambivalence effect. We also highlight a novel mechanism by which translational control of the proteins involved in the ambivalence effect can be used to engineer durable and broad-spectrum disease resistance, regardless of the lifestyle of the invading pathogen.

In vivo Imaging Enables Understanding of Seamless Plant Defense Responses to Wounding and Pathogen Attack

Plants are exposed to varied biotic stresses, including sequential or simultaneous attack by insects and pathogens. To overcome these complex stresses, plants must perceive each of the stresses, then integrate and relay the information throughout the plant body and eventually activate local and systemic resistance responses. Previous molecular genetic studies identified jasmonic acid and salicylic acid as key plant hormones of wound and immune responses. These hormones, combined with their antagonistic interaction, play critical roles in the initiation and regulation of defense responses against insects and pathogens. Aside from molecular and genetic information, the latest in vivo imaging technology has revealed that plant defense responses are regulated spatially and temporally. In this review, we summarize the current knowledge of local and systemic defense responses against wounding and diseases with a focus on past and recent advances in imaging technologies. We discuss how imaging-based multiparametric analysis has improved our understanding of the spatiotemporal regulation of dynamic plant stress responses. We also emphasize the importance of compiling the knowledge generated from individual studies on plant wounding and immune responses for a more seamless understanding of plant defense responses in the natural environment.

γ-Aminobutyric acid may directly or indirectly regulate Arabidopsis ALMT9

The mechanism by which GABA regulates stomatal pore aperture and anion transport activity of ALUMINUM-ACTIVATED MALATE TRANSPORTER 9 is debated.

Regulation of PaRBOH1-mediated ROS production in Norway spruce by Ca2+ binding and phosphorylation

Plant respiratory burst oxidase homologs (RBOHs) are plasma membrane-localized NADPH oxidases that generate superoxide anion radicals, which then dismutate to H₂O₂, into the apoplast using cytoplasmic NADPH as an electron donor. PaRBOH1 is the most highly expressed RBOH gene in developing xylem as well as in a lignin-forming cell culture of Norway spruce (Picea abies L. Karst.). Since no previous information about regulation of gymnosperm RBOHs exist, our aim was to resolve how PaRBOH1 is regulated with a focus on phosphorylation. The N-terminal part of PaRBOH1 was found to contain several putative phosphorylation sites and a four-times repeated motif with similarities to the Botrytis-induced kinase 1 target site in Arabidopsis AtRBOHD. Phosphorylation was indicated for six of the sites in in vitro kinase assays using 15 amino-acid-long peptides for each of the predicted phosphotarget site in the presence of protein extracts of developing xylem. Serine and threonine residues showing positive response in the peptide assays were individually mutated to alanine (kinase-inactive) or to aspartate (phosphomimic), and the wild type PaRBOH1 and the mutated constructs transfected to human kidney embryogenic (HEK293T) cells with a low endogenous level of extracellular ROS production. ROS-producing assays with HEK cells showed that Ca²⁺ and phosphorylation synergistically activate the enzyme and identified several serine and threonine residues that are likely to be phosphorylated including a novel phosphorylation site not characterized in other plant species. These were further investigated with a phosphoproteomic study. Results of Norway spruce, the first gymnosperm species studied in relation to RBOH regulation, show that regulation of RBOH activity is conserved among seed plants.

Integration of long-range signals in plants: A model for wound-induced Ca2+, electrical, ROS, and glutamate waves

Plants show long-range cytosolic Ca²⁺ signal transduction in response to wounding. Recent advances in in vivo imaging techniques have helped visualize spatiotemporal dynamics of the systemic Ca²⁺ signals and provided new insights into underlying molecular mechanisms, in which ion channels of the GLUTAMATE RECEPTOR-LIKE (GLR) family are critical for the sensory system. These, along with MECHANOSENSITIVE CHANNEL OF SMALL CONDUCTANCE-LIKE 10 (MSL10) and Arabidopsis H⁺-ATPase (AHA1) regulate the propagation system. In addition, membrane potential, reactive oxygen species (ROS), and glutamate waves operate in parallel to long-range signal transduction. We summarize these findings and introduce a model that integrates long-range Ca²⁺, electrical, ROS, and glutamate signals in systemic wound responses.

Adenosine monophosphate deaminase modulates BIN2 activity through hydrogen peroxide-induced oligomerization

The Arabidopsis thaliana GSK3-like kinase, BRASSINOSTEROID-INSENSITIVE2 (BIN2) is a key negative regulator of brassinosteroid (BR) signaling and a hub for crosstalk with other signaling pathways. However, the mechanisms controlling BIN2 activity are not well understood. Here we performed a forward genetic screen for resistance to the plant-specific GSK3 inhibitor bikinin and discovered that a mutation in the ADENOSINE MONOPHOSPHATE DEAMINASE (AMPD)/EMBRYONIC FACTOR1 (FAC1) gene reduces the sensitivity of Arabidopsis seedlings to both bikinin and BRs. Further analyses revealed that AMPD modulates BIN2 activity by regulating its oligomerization in a hydrogen peroxide (H2O2)-dependent manner. Exogenous H2O2 induced the formation of BIN2 oligomers with a decreased kinase activity and an increased sensitivity to bikinin. By contrast, AMPD activity inhibition reduced the cytosolic reactive oxygen species (ROS) levels and the amount of BIN2 oligomers, correlating with the decreased sensitivity of Arabidopsis plants to bikinin and BRs. Furthermore, we showed that BIN2 phosphorylates AMPD to possibly alter its function. Our results uncover the existence of an H2O2 homeostasis-mediated regulation loop between AMPD and BIN2 that fine-tunes the BIN2 kinase activity to control plant growth and development.

Advances in chilling injury of postharvest fruit and vegetable: Extracellular ATP aspects

Due to the global use of cold chain, the development of postharvest technology to reduce chilling injury (CI) in postharvest fruits and vegetables during storage and transport is needed urgently. Considerable evidence shows that maintaining intracellular adenosine triphosphate (iATP) in harvested fruits and vegetables is beneficial to inhibiting CI occurrence. Extracellular ATP (eATP) is a damage-associated signal molecule and plays an important role in CI of postharvest fruits and vegetables through its receptor and subsequent signal transduction under low-temperature stress. The development of new aptasensors for the simultaneous determination of eATP level allows for better understanding of the roles of eATP in a myriad of responses mediated by low-temperature stress in relation to the chilling tolerance of postharvest fruits and vegetables. The multiple biological functions of eATP and its receptors in postharvest fruits and vegetables were attributed to interactions with reactive oxygen species (ROS) and nitric oxide (NO) in coordination with phytohormones and other signaling molecules via downstream physiological activities. The complicated interconnection among eATP in relation to its receptors, eATP/iATP homeostasis, ROS, NO, and heat shock proteins triggered by eATP recognition has been emphasized. This paper reviews recent advances in the beneficial effects of energy handling, outlines the production and homeostasis of eATP, discusses the possible mechanism of eATP and its receptors in chilling tolerance, and provides future research directions for CI in postharvest fruits and vegetables during low-temperature storage.

Ethyl Vinyl Ketone Activates K+ Efflux to Regulate Stomatal Closure by MRP4-Dependent eATP Accumulation Working Upstream of H2O2 Burst in Arabidopsis

Plants regulate stomatal mobility to limit water loss and improve pathogen resistance. Ethyl vinyl ketone (evk) is referred to as a reactive electrophilic substance (RES). In this paper, we found that evk can mediate stomatal closure and that evk-induced stomatal closure by increasing guard cell K⁺ efflux. To investigate the role of eATP, and H₂O₂ in evk-regulated K⁺ efflux, we used Arabidopsis wild-type (WT), mutant lines of mrp4, mrp5, dorn1.3 and rbohd/f. Non-invasive micro-test technology (NMT) data showed that evk-induced K⁺ efflux was diminished in mrp4, rbohd/f, and dorn1.3 mutant, which means eATP and H₂O₂ work upstream of evk-induced K⁺ efflux. According to the eATP content assay, evk stimulated eATP production mainly by MRP4. In mrp4 and mrp5 mutant groups and the ABC transporter inhibitor glibenclamide (Gli)-pretreated group, evk-regulated stomatal closure and eATP buildup were diminished, especially in the mrp4 group. According to qRT-PCR and eATP concentration results, evk regulates both relative gene expressions of MRP4/5 and eATP concentration in rbohd/f and WT group. According to the confocal data, evk-induced H₂O₂ production was lower in mrp4, mrp5 mutants, which implied that eATP works upstream of H₂O₂. Moreover, NADPH-dependent H₂O₂ burst is regulated by DORN1. A yeast two-hybrid assay, firefly luciferase complementation imaging assay, bimolecular fluorescence complementation assay, and pulldown assay showed that the interaction between DORN1 and RBOHF can be realized, which means DORN1 may control H₂O₂ burst by regulating RBOHF through interaction. This study reveals that evk-induced stomatal closure requires MRP4-dependent eATP accumulation and subsequent H₂O₂ accumulation to regulate K⁺ efflux.

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.

Extracellular ATP targets Arabidopsis RIBONUCLEASE 1 to suppress mycotoxin stress-induced cell death

Extracellular ATP is a purinergic signal with important functions in regulating plant growth and stress-adaptive responses, including programmed cell death. While signalling events proximate to receptor activation at the plasma membrane have been characterised, downstream protein targets and the mechanism of cell death activation/regulation are unknown. We designed a proteomic screen to identify ATP-responsive proteins in Arabidopsis cell cultures exposed to mycotoxin stress via fumonisin B1 (FB1) application. Arabidopsis RIBONUCLEASE 1 (RNS1) was identified by the screen, and transgenic plants overexpressing native RNS1 showed greater susceptibility to FB1, while a gene knockout rns1 mutant and antisense RNS1 transgenic plants were resistant to FB1-induced cell death. Native RNS1 complemented rns1 mutants and restored the cell death response to FB1, while a catalytically inactive version of the ribonuclease could not. The FB1 resistance of salicylic acid (SA)-depleted nahG-expressing plants was abolished by transformation with native RNS1, but not the catalytically dead version. The mechanism of FB1-induced cell death is activation of RNS1-dependent RNA cleavage, which is blocked by ATP via RNS1 suppression, or enhanced by SA through induction of RNS1 expression. Our study reveals RNS1 as a previously unknown convergence point of ATP and SA signalling in the regulation of stress-induced cell death.

Differential regulation of flower transpiration during abiotic stress in annual plants

Heat waves occurring during droughts can have a devastating impact on yield, especially if they happen during the flowering and seed set stages of the crop cycle. Global warming and climate change are driving an alarming increase in the frequency and intensity of combined drought and heat stress episodes, critically threatening global food security. Because high temperature is detrimental to reproductive processes, essential for plant yield, we measured the inner temperature, transpiration, sepal stomatal aperture, hormone concentrations and transcriptomic response of closed soybean flowers developing on plants subjected to a combination of drought and heat stress. Here, we report that, during a combination of drought and heat stress, soybean plants prioritize transpiration through flowers over transpiration through leaves by opening their flower stomata, while keeping their leaf stomata closed. This acclimation strategy, termed 'differential transpiration', lowers flower inner temperature by about 2-3°C, protecting reproductive processes at the expense of vegetative tissues. Manipulating stomatal regulation, stomatal size and/or stomatal density of flowers could serve as a viable strategy to enhance the yield of different crops and mitigate some of the current and future impacts of global warming and climate change on agriculture.

Extracellular ATP plays an important role in systemic wound response activation

Mechanical wounding occurs in plants during biotic or abiotic stresses and is associated with the activation of long-distance signaling pathways that trigger wound responses in systemic tissues. Among the different systemic signals activated by wounding are electric signals, calcium, hydraulic, and reactive oxygen species (ROS) waves. The release of glutamate (Glu) from cells at the wounded tissues was recently proposed to trigger systemic signal transduction pathways via GLU-LIKE RECEPTORs (GLRs). However, the role of another important compound released from cells during wounding (extracellular ATP [eATP]) in triggering systemic responses is not clear. Here, we show in Arabidopsis (Arabidopsis thaliana) that wounding results in the accumulation of nanomolar levels of eATP and that these levels are sufficient to trigger the systemic ROS wave. We further show that the triggering of the ROS wave by eATP during wounding requires the PURINORECEPTOR 2 KINASE (P2K) receptor. Application of eATP to unwounded leaves triggered the ROS wave, and the activation of the ROS wave by wounding or eATP application was suppressed in mutants deficient in P2Ks (e.g. p2k1-3, p2k2, and p2k1-3p2k2). In addition, expression of systemic wound response (SWR) transcripts was suppressed in mutants deficient in P2Ks during wounding. Interestingly, the effect of Glu and eATP application on ROS wave activation was not additive, suggesting that these two compounds function in the same pathway to trigger the ROS wave. Our findings reveal that in addition to sensing Glu via GLRs, eATP sensed by P2Ks plays a key role in the triggering of SWRs in plants.

Reactive oxygen species signalling in plant stress responses

Reactive oxygen species (ROS) are key signalling molecules that enable cells to rapidly respond to different stimuli. In plants, ROS play a crucial role in abiotic and biotic stress sensing, integration of different environmental signals and activation of stress-response networks, thus contributing to the establishment of defence mechanisms and plant resilience. Recent advances in the study of ROS signalling in plants include the identification of ROS receptors and key regulatory hubs that connect ROS signalling with other important stress-response signal transduction pathways and hormones, as well as new roles for ROS in organelle-to-organelle and cell-to-cell signalling. Our understanding of how ROS are regulated in cells by balancing production, scavenging and transport has also increased. In this Review, we discuss these promising developments and how they might be used to increase plant resilience to environmental stress.

Thirty years of resistance: Zig-zag through the plant immune system

Understanding the plant immune system is crucial for using genetics to protect crops from diseases. Plants resist pathogens via a two-tiered innate immune detection-and-response system. The first plant Resistance (R) gene was cloned in 1992 . Since then, many cell-surface pattern recognition receptors (PRRs) have been identified, and R genes that encode intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) have been cloned. Here, we provide a list of characterized PRRs and NLRs. In addition to immune receptors, many components of immune signaling networks were discovered over the last 30 years. We review the signaling pathways, physiological responses, and molecular regulation of both PRR- and NLR-mediated immunity. Recent studies have reinforced the importance of interactions between the two immune systems. We provide an overview of interactions between PRR- and NLR-mediated immunity, highlighting challenges and perspectives for future research.