The leucine-rich repeat receptor kinase QSK1 regulates PRR-RBOHD complexes targeted by the bacterial effector HopF2Pto.

Plants detect pathogens using cell-surface pattern recognition receptors (PRRs) such as ELONGATION Factor-TU (EF-TU) RECEPTOR (EFR) and FLAGELLIN SENSING 2 (FLS2), which recognize bacterial EF-Tu and flagellin, respectively. These PRRs belong to the leucine-rich repeat receptor kinase (LRR-RK) family and activate the production of reactive oxygen species via the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD). The PRR-RBOHD complex is tightly regulated to prevent unwarranted or exaggerated immune responses. However, certain pathogen effectors can subvert these regulatory mechanisms, thereby suppressing plant immunity. To elucidate the intricate dynamics of the PRR-RBOHD complex, we conducted a comparative coimmunoprecipitation analysis using EFR, FLS2, and RBOHD in Arabidopsis thaliana. We identified QIAN SHOU KINASE 1 (QSK1), an LRR-RK, as a PRR-RBOHD complex-associated protein. QSK1 downregulated FLS2 and EFR abundance, functioning as a negative regulator of PRR-triggered immunity (PTI). QSK1 was targeted by the bacterial effector HopF2Pto, a mono-ADP ribosyltransferase, reducing FLS2 and EFR levels through both transcriptional and transcription-independent pathways, thereby inhibiting PTI. Furthermore, HopF2Pto transcriptionally downregulated PROSCOOP genes encoding important stress-regulated phytocytokines and their receptor MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2. Importantly, HopF2Pto requires QSK1 for its accumulation and virulence functions within plants. In summary, our results provide insights into the mechanism by which HopF2Pto employs QSK1 to desensitize plants to pathogen attack.

"Messenger RNA just entered the chat": The next layer of cross-kingdom RNA transfer.

Infectious fungi send small RNAs into plant cells to enhance their virulence by silencing defense-related genes. In this issue of Cell Host & Microbe, Wang and colleagues show that full-length messenger RNA is transported in vesicles from plants to fungi, becoming translated by fungal ribosomes and reducing fungal pathogenicity.

NET4 and RabG3 link actin to the tonoplast and facilitate cytoskeletal remodelling during stomatal immunity.

Members of the NETWORKED (NET) family are involved in actin-membrane interactions. Here we show that two members of the NET family, NET4A and NET4B, are essential for normal guard cell actin reorganization, which is a process critical for stomatal closure in plant immunity. NET4 proteins interact with F-actin and with members of the Rab7 GTPase RABG3 family through two distinct domains, allowing for simultaneous localization to actin filaments and the tonoplast. NET4 proteins interact with GTP-bound, active RABG3 members, suggesting their function being downstream effectors. We also show that RABG3b is critical for stomatal closure induced by microbial patterns. Taken together, we conclude that the actin cytoskeletal remodelling during stomatal closure involves a molecular link between actin filaments and the tonoplast, which is mediated by the NET4-RABG3b interaction. We propose that stomatal closure to microbial patterns involves the coordinated action of immune-triggered osmotic changes and actin cytoskeletal remodelling likely driving compact vacuolar morphologies.

Biophysical and proteomic analyses of Pseudomonas syringae pv. tomato DC3000 extracellular vesicles suggest adaptive functions during plant infection.

Vesiculation is a process employed by Gram-negative bacteria to release extracellular vesicles (EVs) into the environment. EVs from pathogenic bacteria play functions in host immune modulation, elimination of host defenses, and acquisition of nutrients from the host. Here, we observed EV production of the bacterial speck disease causal agent, Pseudomonas syringae pv. tomato (Pto) DC3000, as outer membrane vesicle release. Mass spectrometry identified 369 proteins enriched in Pto DC3000 EVs. The EV samples contained known immunomodulatory proteins and could induce plant immune responses mediated by bacterial flagellin. Having identified two biomarkers for EV detection, we provide evidence for Pto DC3000 releasing EVs during plant infection. Bioinformatic analysis of the EV-enriched proteins suggests a role for EVs in antibiotic defense and iron acquisition. Thus, our data provide insights into the strategies this pathogen may use to develop in a plant environment. IMPORTANCE The release of extracellular vesicles (EVs) into the environment is ubiquitous among bacteria. Vesiculation has been recognized as an important mechanism of bacterial pathogenesis and human disease but is poorly understood in phytopathogenic bacteria. Our research addresses the role of bacterial EVs in plant infection. In this work, we show that the causal agent of bacterial speck disease, Pseudomonas syringae pv. tomato, produces EVs during plant infection. Our data suggest that EVs may help the bacteria to adapt to environments, e.g., when iron could be limiting such as the plant apoplast, laying the foundation for studying the factors that phytopathogenic bacteria use to thrive in the plant environment.

S-acylation stabilizes ligand-induced receptor kinase complex formation during plant pattern-triggered immune signaling.

Plant receptor kinases are key transducers of extracellular stimuli, such as the presence of beneficial or pathogenic microbes or secreted signaling molecules. Receptor kinases are regulated by numerous post-translational modifications.1,2,3 Here, using the immune receptor kinases FLS24 and EFR,5 we show that S-acylation at a cysteine conserved in all plant receptor kinases is crucial for function. S-acylation involves the addition of long-chain fatty acids to cysteine residues within proteins, altering their biochemical properties and behavior within the membrane environment.6 We observe S-acylation of FLS2 at C-terminal kinase domain cysteine residues within minutes following the perception of its ligand, flg22, in a BAK1 co-receptor and PUB12/13 ubiquitin ligase-dependent manner. We demonstrate that S-acylation is essential for FLS2-mediated immune signaling and resistance to bacterial infection. Similarly, mutating the corresponding conserved cysteine residue in EFR suppressed elf18-triggered signaling. Analysis of unstimulated and activated FLS2-containing complexes using microscopy, detergents, and native membrane DIBMA nanodiscs indicates that S-acylation stabilizes, and promotes retention of, activated receptor kinase complexes at the plasma membrane to increase signaling efficiency.

Spotlight on plant RNA-containing extracellular vesicles.

Extracellular vesicles (EVs) carrying RNA have attracted growing attention in plant cell biology. For a long time, EV release or uptake through the rigid plant cell wall was considered to be impossible and RNA outside cells to be unstable. Identified EV biomarkers have brought new insights into functional roles of EVs to transport their RNA cargo for systemic spread in plants and into plant-invading pathogens. RNA-binding proteins supposedly take over key functions in EV-mediated RNA secretion and transport, but the mechanisms of RNA sorting and EV translocation through the plant cell wall and plasma membrane are not understood. Characterizing the molecular players and the cellular mechanisms of plant RNA-containing EVs will create new knowledge in cell-to-cell and inter-organismal communication.

Xylella fastidiosa's relationships: the bacterium, the host plants, and the plant microbiome.

Xylella fastidiosa is the causal agent of important crop diseases and is transmitted by xylem-sap-feeding insects. The bacterium colonizes xylem vessels and can persist with a commensal or pathogen lifestyle in more than 500 plant species. In the past decade, reports of X. fastidiosa across the globe have dramatically increased its known occurrence. This raises important questions: How does X. fastidiosa interact with the different host plants? How does the bacterium interact with the plant immune system? How does it influence the host's microbiome? We discuss recent strain genetic typing and plant transcriptome and microbiome analyses, which have advanced our understanding of factors that are important for X. fastidiosa plant infection.

Extracellular vesicles from phytobacteria: Properties, functions and uses.

Bacterial extracellular vesicles (EVs) are cytosol-containing membrane spheres providing a chassis for the removal and delivery of cargoes in a highly dynamic and cue-responsive manner. EVs play important roles in cell-to-cell communication, including the dialogue between recipient microbial and plant cells. Bacterial EVs are well-studied in the medical field, but their relevance for plant infection is only now being recognized. Recent studies have demonstrated the role of EVs from phytobacteria in modulating plant immunity and the outcome of disease or in symbiosis. In this review, we highlight the composition of EVs and discuss their role in the interaction with plants. Knowledge of EV composition and functions will aid their use in biotechnology and agriculture.

Host-interactor screens of Phytophthora infestans RXLR proteins reveal vesicle trafficking as a major effector-targeted process.

Pathogens modulate plant cell structure and function by secreting effectors into host tissues. Effectors typically function by associating with host molecules and modulating their activities. This study aimed to identify the host processes targeted by the RXLR class of host-translocated effectors of the potato blight pathogen Phytophthora infestans. To this end, we performed an in planta protein-protein interaction screen by transiently expressing P. infestans RXLR effectors in Nicotiana benthamiana leaves followed by coimmunoprecipitation and liquid chromatography-tandem mass spectrometry. This screen generated an effector-host protein interactome matrix of 59 P. infestans RXLR effectors x 586 N. benthamiana proteins. Classification of the host interactors into putative functional categories revealed over 35 biological processes possibly targeted by P. infestans. We further characterized the PexRD12/31 family of RXLR-WY effectors, which associate and colocalize with components of the vesicle trafficking machinery. One member of this family, PexRD31, increased the number of FYVE positive vesicles in N. benthamiana cells. FYVE positive vesicles also accumulated in leaf cells near P. infestans hyphae, indicating that the pathogen may enhance endosomal trafficking during infection. This interactome dataset will serve as a useful resource for functional studies of P. infestans effectors and of effector-targeted host processes.

Anion channel SLAH3 is a regulatory target of chitin receptor-associated kinase PBL27 in microbial stomatal closure.

In plants, antimicrobial immune responses involve the cellular release of anions and are responsible for the closure of stomatal pores. Detection of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) induces currents mediated via slow-type (S-type) anion channels by a yet not understood mechanism. Here, we show that stomatal closure to fungal chitin is conferred by the major PRRs for chitin recognition, LYK5 and CERK1, the receptor-like cytoplasmic kinase PBL27, and the SLAH3 anion channel. PBL27 has the capacity to phosphorylate SLAH3, of which S127 and S189 are required to activate SLAH3. Full activation of the channel entails CERK1, depending on PBL27. Importantly, both S127 and S189 residues of SLAH3 are required for chitin-induced stomatal closure and anti-fungal immunity at the whole leaf level. Our results demonstrate a short signal transduction module from MAMP recognition to anion channel activation, and independent of ABA-induced SLAH3 activation.

The use of quantitative imaging to investigate regulators of membrane trafficking in Arabidopsis stomatal closure.

Expansion of gene families facilitates robustness and evolvability of biological processes but impedes functional genetic dissection of signalling pathways. To address this, quantitative analysis of single cell responses can help characterize the redundancy within gene families. We developed high-throughput quantitative imaging of stomatal closure, a response of plant guard cells, and performed a reverse genetic screen in a group of Arabidopsis mutants to five stimuli. Focussing on the intersection between guard cell signalling and the endomembrane system, we identified eight clusters based on the mutant stomatal responses. Mutants generally affected in stomatal closure were mostly in genes encoding SNARE and SCAMP membrane regulators. By contrast, mutants in RAB5 GTPase genes played specific roles in stomatal closure to microbial but not drought stress. Together with timed quantitative imaging of endosomes revealing sequential patterns in FLS2 trafficking, our imaging pipeline can resolve non-redundant functions of the RAB5 GTPase gene family. Finally, we provide a valuable image-based tool to dissect guard cell responses and outline a genetic framework of stomatal closure.

Kinase activity of SOBIR1 and BAK1 is required for immune signalling.

Leucine-rich repeat-receptor-like proteins (LRR-RLPs) and LRR-receptor-like kinases (LRR-RLKs) trigger immune signalling to promote plant resistance against pathogens. LRR-RLPs lack an intracellular kinase domain, and several of these receptors have been shown to constitutively interact with the LRR-RLK Suppressor of BIR1-1/EVERSHED (SOBIR1/EVR) to form signalling-competent receptor complexes. Ligand perception by LRR-RLPs initiates recruitment of the co-receptor BRI1-Associated Kinase 1/Somatic Embryogenesis Receptor Kinase 3 (BAK1/SERK3) to the LRR-RLP/SOBIR1 complex, thereby activating LRR-RLP-mediated immunity. We employed phosphorylation analysis of in planta-produced proteins, live cell imaging, gene silencing and co-immunoprecipitation to investigate the roles of SOBIR1 and BAK1 in immune signalling. We show that Arabidopsis thaliana (At) SOBIR1, which constitutively activates immune responses when overexpressed in planta, is highly phosphorylated. Moreover, in addition to the kinase activity of SOBIR1 itself, kinase-active BAK1 is essential for AtSOBIR1-induced constitutive immunity and for the phosphorylation of AtSOBIR1. Furthermore, the defence response triggered by the tomato LRR-RLP Cf-4 on perception of Avr4 from the extracellular pathogenic fungus Cladosporium fulvum is dependent on kinase-active BAK1. We argue that, in addition to the trans-autophosphorylation of SOBIR1, it is likely that SOBIR1 and BAK1 transphosphorylate, and thereby activate the receptor complex. The signalling-competent cell surface receptor complex subsequently activates downstream cytoplasmic signalling partners to initiate RLP-mediated immunity.

Disruption of the plant-specific CFS1 gene impairs autophagosome turnover and triggers EDS1-dependent cell death.

Cell death, autophagy and endosomal sorting contribute to many physiological, developmental and immunological processes in plants. They are mechanistically interconnected and interdependent, but the molecular basis of their mutual regulation has only begun to emerge in plants. Here, we describe the identification and molecular characterization of CELL DEATH RELATED ENDOSOMAL FYVE/SYLF PROTEIN 1 (CFS1). The CFS1 protein interacts with the ENDOSOMAL SORTING COMPLEX REQUIRED FOR TRANSPORT I (ESCRT-I) component ELCH (ELC) and is localized at ESCRT-I-positive late endosomes likely through its PI3P and actin binding SH3YL1 Ysc84/Lsb4p Lsb3p plant FYVE (SYLF) domain. Mutant alleles of cfs1 exhibit auto-immune phenotypes including spontaneous lesions that show characteristics of hypersensitive response (HR). Autoimmunity in cfs1 is dependent on ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1)-mediated effector-triggered immunity (ETI) but independent from salicylic acid. Additionally, cfs1 mutants accumulate the autophagy markers ATG8 and NBR1 independently from EDS1. We hypothesize that CFS1 acts at the intersection of autophagosomes and endosomes and contributes to cellular homeostasis by mediating autophagosome turnover.

A computational approach for inferring the cell wall properties that govern guard cell dynamics.

Guard cells dynamically adjust their shape in order to regulate photosynthetic gas exchange, respiration rates and defend against pathogen entry. Cell shape changes are determined by the interplay of cell wall material properties and turgor pressure. To investigate this relationship between turgor pressure, cell wall properties and cell shape, we focused on kidney-shaped stomata and developed a biomechanical model of a guard cell pair. Treating the cell wall as a composite of the pectin-rich cell wall matrix embedded with cellulose microfibrils, we show that strong, circumferentially oriented fibres are critical for opening. We find that the opening dynamics are dictated by the mechanical stress response of the cell wall matrix, and as the turgor rises, the pectinaceous matrix stiffens. We validate these predictions with stomatal opening experiments in selected Arabidopsis cell wall mutants. Thus, using a computational framework that combines a 3D biomechanical model with parameter optimization, we demonstrate how to exploit subtle shape changes to infer cell wall material properties. Our findings reveal that proper stomatal dynamics are built on two key properties of the cell wall, namely anisotropy in the form of hoop reinforcement and strain stiffening.

An automated quantitative image analysis tool for the identification of microtubule patterns in plants.

High throughput confocal imaging poses challenges in the computational image analysis of complex subcellular structures such as the microtubule cytoskeleton. Here, we developed CellArchitect, an automated image analysis tool that quantifies changes to subcellular patterns illustrated by microtubule markers in plants. We screened microtubule-targeted herbicides and demonstrate that high throughput confocal imaging with integrated image analysis by CellArchitect can distinguish effects induced by the known herbicides indaziflam and trifluralin. The same platform was used to examine 6 other compounds with herbicidal activity, and at least 3 different effects induced by these compounds were profiled. We further show that CellArchitect can detect subcellular patterns tagged by actin and endoplasmic reticulum markers. Thus, the platform developed here can be used to automate image analysis of complex subcellular patterns for purposes such as herbicide discovery and mode of action characterisation. The capacity to use this tool to quantitatively characterize cellular responses lends itself to application across many areas of biology.

Detection and Analyses of Endocytosis of Plant Receptor Kinases.

Genetic dissection has led to a sophisticated understanding of receptor kinases in plant development and responses to abiotic and biotic stresses. Fluorescence confocal microscopy is essential to identify the (sub)cellular locations of resting and signaling receptor kinases that trigger molecular events in plant cells upon ligand perception. In this regard, the internalization of plasma membrane-localized FLAGELLIN SENSING 2 (FLS2) into endosomes induced by its ligand flg22, a peptide derived from bacterial flagellin, is a model system for studying activation status-dependent and endosomal receptor kinase trafficking routes and can be used in screens to identify pathogen effectors that target these trafficking routes for virulence promotion. In this chapter we describe approaches of visualizing fluorescently tagged FLS2, including protocols for flg22-induced endocytosis, instrument parameters, and image analysis. These approaches can be easily adapted for other receptor kinases, using the fast transient expression system in Nicotiana benthamiana for microscopic inspection.