Bacterial effector HopF2 suppresses arabidopsis innate immunity at the plasma membrane

A high-efficiency transient expression system mediated by Agrobacterium tumefaciens in Spinacia oleracea leaves

BACKGROUND

Optimization of a highly efficient transient expression system is critical for the study of gene function, particularly in those plants in which stable transformation methods are not widely available. Agrobacterium tumefaciens‑mediated transient transformation is a simple and low-cost method that has been developed and applied to a wide variety of plant species. However, the transient expression in spinach (Spinacia oleracea L.) is still not reported.

RESULTS

We developed a transient expression system in spinach leaves of the Sp75 and Sp73 varieties. Several factors influencing the transformation efficiency were optimized such as Agrobacterium strain, spinach seedling stage, leaf position, and the expression time after injection. Agrobacterium strain GV3101 (pSoup-p19) was more efficient than AGL1 in expressing recombinant protein in spinach leaves. In general, Sp75 leaves were more suitable than Sp73 leaves, regardless of grow stage. At four-leaf stage, higher intensity and efficiency of transient expression were observed in group 1 (G1) of Sp75 at 53 h after injection (HAI) and in G1 of Sp73 at 64 HAI. At six-leaf stage of Sp75, group 3 (G3) at 72 HAI were the most effective condition for transient expression. Using the optimized expression system, we detected the subcellular localization of a transcriptional co-activator SoMBF1c and a NADPH oxidase SoRbohF. We also detected the interaction of the protein kinase SoCRK10 and the NADPH oxidase SoRbohB.

CONCLUSION

This study established a method of highly efficient transient expression mediated by Agrobacterium in spinach leaves. The transient expression system will facilitate the analysis of gene function and lay a solid foundation for molecular design breeding of spinach.

Association mechanism between Arabidopsis immune coreceptor BAK1 and Pseudomonas syringae effector HopF2

Brassinosteroid activated kinase 1 (BAK1) is a cell-surface coreceptor which plays multiple roles in innate immunity of plants. HopF2 is an effector secreted by the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 into Arabidopsis and suppresses host immune system through interaction with BAK1 as well as its downstream kinase MKK5. The association mechanism of HopF2 to BAK1 remains unclear, which prohibits our understanding and subsequent interfering of their interaction for pathogen management. Herein, we found the kinase domain of BAK1 (BAK1-KD) is sufficient for HopF2 association. With a combination of hydrogen/deuterium exchange mass spectrometry and mutational assays, we found a region of BAK1-KD N-lobe and a region of HopF2 head subdomain are critical for intermolecular interaction, which is also supported by unbiased protein-protein docking with ClusPro and kinase activity assay. Collectively, this research presents the interaction mechanism between Arabidopsis BAK1 and P. syringae HopF2, which could pave the way for bactericide development that blocking the functioning of HopF2 toward BAK1.

Suppression of NLR-mediated plant immune detection by bacterial pathogens

The plant immune system is constituted of two functionally interdependent branches that provide the plant with an effective defense against microbial pathogens. They can be considered separate since one detects extracellular pathogen-associated molecular patterns by means of receptors on the plant surface, while the other detects pathogen-secreted virulence effectors via intracellular receptors. Plant defense depending on both branches can be effectively suppressed by host-adapted microbial pathogens. In this review we focus on bacterially driven suppression of the latter, known as effector-triggered immunity (ETI) and dependent on diverse NOD-like receptors (NLRs). We examine how some effectors secreted by pathogenic bacteria carrying type III secretion systems can be subject to specific NLR-mediated detection, which can be evaded by the action of additional co-secreted effectors (suppressors), implying that virulence depends on the coordinated action of the whole repertoire of effectors of any given bacterium and their complex epistatic interactions within the plant. We consider how ETI activation can be avoided by using suppressors to directly alter compromised co-secreted effectors, modify plant defense-associated proteins, or occasionally both. We also comment on the potential assembly within the plant cell of multi-protein complexes comprising both bacterial effectors and defense protein targets.

Multiple host targets of Pseudomonas effector protein HopM1 form a protein complex regulating apoplastic immunity and water homeostasis

Bacterial type III effector proteins injected into the host cell play a critical role in mediating bacterial interactions with plant and animal hosts. Notably, some bacterial effectors are reported to target sequence-unrelated host proteins with unknown functional relationships. The Pseudomonas syringae effector HopM1 is such an example; it interacts with and/or degrades several HopM1-interacting (MIN) Arabidopsis proteins, including HopM1-interacting protein 2 (MIN2/RAD23), HopM1-interacting protein 7 (MIN7/BIG5), HopM1-interacting protein 10 (MIN10/14-3-3ĸ), and HopM1-interacting protein 13 (MIN13/BIG2). In this study, we purified the MIN7 complex formed in planta and found that it contains MIN7, MIN10, MIN13, as well as a tetratricopeptide repeat protein named HLB1. Mutational analysis showed that, like MIN7, HLB1 is required for pathogen-associated molecular pattern (PAMP)-, effector-, and benzothiadiazole (BTH)-triggered immunity. HLB1 is recruited to the trans-Golgi network (TGN)/early endosome (EE) in a MIN7-dependent manner. Both min7 and hlb1 mutant leaves contained elevated water content in the leaf apoplast and artificial water infiltration into the leaf apoplast was sufficient to phenocopy immune-suppressing phenotype of HopM1. These results suggest that multiple HopM1-targeted MIN proteins form a protein complex with a dual role in modulating water level and immunity in the apoplast, which provides an explanation for the dual phenotypes of HopM1 during bacterial pathogenesis.

Prevention of Stomatal Entry as a Strategy for Plant Disease Control against Foliar Pathogenic Pseudomonas Species

The genus Pseudomonas includes some of the most problematic and studied foliar bacterial pathogens. Generally, in a successful disease cycle there is an initial epiphytic lifestyle on the leaf surface and a subsequent aggressive endophytic stage inside the leaf apoplast. Leaf-associated bacterial pathogens enter intercellular spaces and internal leaf tissues by natural surface opening sites, such as stomata. The stomatal crossing is complex and dynamic, and functional genomic studies have revealed several virulence factors required for plant entry. Currently, treatments with copper-containing compounds, where authorized and admitted, and antibiotics are commonly used against bacterial plant pathogens. However, strains resistant to these chemicals occur in the fields. Therefore, the demand for alternative control strategies has been increasing. This review summarizes efficient strategies to prevent bacterial entry. Virulence factors required for entering the leaf in plant-pathogenic Pseudomonas species are also discussed.

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.

Metaeffector interactions modulate the type III effector-triggered immunity load of Pseudomonas syringae

The bacterial plant pathogen Pseudomonas syringae requires type III secreted effectors (T3SEs) for pathogenesis. However, a major facet of plant immunity entails the recognition of a subset of P. syringae’s T3SEs by intracellular host receptors in a process called Effector-Triggered Immunity (ETI). Prior work has shown that ETI-eliciting T3SEs are pervasive in the P. syringae species complex raising the question of how P. syringae mitigates its ETI load to become a successful pathogen. While pathogens can evade ETI by T3SE mutation, recombination, or loss, there is increasing evidence that effector-effector (a.k.a., metaeffector) interactions can suppress ETI. To study the ETI-suppression potential of P. syringae T3SE repertoires, we compared the ETI-elicitation profiles of two genetically divergent strains: P. syringae pv. tomato DC3000 (PtoDC3000) and P. syringae pv. maculicola ES4326 (PmaES4326), which are both virulent on Arabidopsis thaliana but harbour largely distinct effector repertoires. Of the 529 T3SE alleles screened on A. thaliana Col-0 from the P. syringae T3SE compendium (PsyTEC), 69 alleles from 21 T3SE families elicited ETI in at least one of the two strain backgrounds, while 50 elicited ETI in both backgrounds, resulting in 19 differential ETI responses including two novel ETI-eliciting families: AvrPto1 and HopT1. Although most of these differences were quantitative, three ETI responses were completely absent in one of the pathogenic backgrounds. We performed ETI suppression screens to test if metaeffector interactions contributed to these ETI differences, and found that HopQ1a suppressed AvrPto1m-mediated ETI, while HopG1c and HopF1g suppressed HopT1b-mediated ETI. Overall, these results show that P. syringae strains leverage metaeffector interactions and ETI suppression to overcome the ETI load associated with their native T3SE repertoires.

Noncanonical mono(ADP-ribosyl)ation of zinc finger SZF proteins counteracts ubiquitination for protein homeostasis in plant immunity

Protein ADP-ribosylation is a reversible post-translational modification that transfers ADP-ribose from NAD+ onto acceptor proteins. Poly(ADP-ribosyl)ation (PARylation), catalyzed by poly(ADP-ribose) polymerases (PARPs) and poly(ADP-ribose) glycohydrolases (PARGs), which remove the modification, regulates diverse cellular processes. However, the chemistry and physiological functions of mono(ADP-ribosyl)ation (MARylation) remain elusive. Here, we report that Arabidopsis zinc finger proteins SZF1 and SZF2, key regulators of immune gene expression, are MARylated by the noncanonical ADP-ribosyltransferase SRO2. Immune elicitation promotes MARylation of SZF1/SZF2 via dissociation from PARG1, which has an unconventional activity in hydrolyzing both poly(ADP-ribose) and mono(ADP-ribose) from acceptor proteins. MARylation antagonizes polyubiquitination of SZF1 mediated by the SH3 domain-containing proteins SH3P1/SH3P2, thereby stabilizing SZF1 proteins. Our study uncovers a noncanonical ADP-ribosyltransferase mediating MARylation of immune regulators and underpins the molecular mechanism of maintaining protein homeostasis by the counter-regulation of ADP-ribosylation and polyubiquitination to ensure proper immune responses.

Development of potent promoters that drive the efficient expression of genes in apple protoplasts

Protoplast transient expression is a powerful strategy for gene functional characterization, especially in biochemical mechanism studies. We herein developed a highly efficient transient expression system for apple protoplasts. The abilities of the Arabidopsis thaliana and Malus domestica ubiquitin-10 (AtUBQ10 and MdUBQ10) promoters to drive the expression of multiple genes were compared with that of the CaMV 35S promoter, and the results revealed that the AtUBQ10 and MdUBQ10 promoters were more efficient in apple protoplasts. With this system, we demonstrated that active AtMKK7ac could activate MAPK6/3/4 signaling cascades, which further regulated MdWRKY33 phosphorylation and stability in apple. Furthermore, the ligand-induced interaction between the immune receptor AtFLS2 and the coreceptor AtBAK1 was reconstituted in apple protoplasts. We also found that the stability of the bacterial effector AvrRpt2 was regulated by feedback involving auxin and the immune regulator RIN4. The system established herein will serve as a useful tool for the molecular and biochemical analyses of apple genes.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

Interior design: how plant pathogens optimize their living conditions

Pathogens use effectors to suppress host defence mechanisms, promote the derivation of nutrients, and facilitate infection within the host plant. Much is now known about effectors that target biotic pathways, particularly those that interfere with plant innate immunity. By contrast, an understanding of how effectors manipulate nonimmunity pathways is only beginning to emerge. Here, we focus on exciting new insights into effectors that target abiotic stress adaptation pathways, tampering with key functions within the plant to promote colonization. We critically assess the role of various signalling agents in linking different pathways upon perturbation by pathogen effectors. Additionally, this review provides a summary of currently known bacterial, fungal, and oomycete pathogen effectors that induce biotic and abiotic stress responses in the plant, as a first step towards establishing a comprehensive picture for linking effector targets to pathogenic lifestyles.

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

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

An Arabidopsis protoplast isolation method reduces cytosolic acidification and activation of the chloroplast stress sensor SENSITIVE TO FREEZING 2

Chloroplasts adapt to freezing and other abiotic stresses in part by modifying their membranes. One key-remodeling enzyme is SENSITIVE TO FREEZING2 (SFR2). SFR2 is unusual because it does not respond to initial cold stress or cold acclimation, instead it responds during freezing conditions in Arabidopsis. This response has been shown to be sensitive to cytosolic acidification. The unique lipid products of SFR2 have also been detected in response to non-freezing stresses, but what causes SFR2 to respond in these stresses is unknown. Here, we investigate protoplast isolation as a representative of wounding stress. We show that SFR2 oligogalactolipid products accumulate during protoplast isolation. Notably, we show that protoplast cytosol is acidified during isolation. Modification of the buffers reduces oligogalactolipid accumulation, while prolonged incubation in the isolated state increases it. We conclude that SFR2 activation during protoplast isolation correlates with cytosolic acidification, implying that all SFR2 activation may be dependent on cytosolic acidification. We also conclude that protoplasts can be more gently isolated, reducing their stress.

The mitogen activated protein kinase (MAPK) gene family functions as a cohort during the Glycine max defense response to Heterodera glycines

Mitogen activated protein kinases (MAPKs) play important signal transduction roles. However, little is known regarding how they influence the gene expression of other family members and the relationship to a biological process, including the Glycine max defense response to Heterodera glycines. Transcriptomics have identified MAPK gene expression occurring within root cells undergoing a defense response to a pathogenic event initiated by H. glycines in the allotetraploid Glycine max. Functional analyses are presented for its 32 MAPKs revealing 9 have a defense role, including homologs of Arabidopsis thaliana MAPK (MPK) MPK2, MPK3, MPK4, MPK5, MPK6, MPK13, MPK16 and MPK20. Defense signaling occurring through pathogen activated molecular pattern (PAMP) triggered immunity (PTI) and effector triggered immunity (ETI) have been determined in relation to these MAPKs. Five different types of gene expression relate to MAPK expression, influencing PTI and ETI gene expression and proven defense genes including an ABC-G transporter, 20S membrane fusion particle components, glycoside biosynthesis, carbon metabolism, hemicellulose modification, transcription and secretion. The experiments show MAPKs broadly influence defense MAPK gene expression, including the co-regulation of parologous MAPKs and reveal its relationship to proven defense genes. The experiments reveal each defense MAPK induces the expression of a G. max homolog of a PATHOGENESIS RELATED1 (PR1), itself shown to function in defense in the studied pathosystem.

Comparative genomics of Pseudomonas syringae reveals convergent gene gain and loss associated with specialization onto cherry (Prunus avium)

Genome-wide analyses of the effector- and toxin-encoding genes were used to examine the phylogenetics and evolution of pathogenicity amongst diverse strains of Pseudomonas syringae causing bacterial canker of cherry (Prunus avium), including pathovars P. syringae pv morsprunorum (Psm) races 1 and 2, P. syringae pv syringae (Pss) and P. syringae pv avii. Phylogenetic analyses revealed Psm races and P. syringae pv avii clades were distinct and were each monophyletic, whereas cherry-pathogenic strains of Pss were interspersed amongst strains from other host species. A maximum likelihood approach was used to predict effectors associated with pathogenicity on cherry. Pss possesses a smaller repertoire of type III effectors but has more toxin biosynthesis clusters than Psm and P. syringae pv avii. Evolution of cherry pathogenicity was correlated with gain of genes such as hopAR1 and hopBB1 through putative phage transfer and horizontal transfer respectively. By contrast, loss of the avrPto/hopAB redundant effector group was observed in cherry-pathogenic clades. Ectopic expression of hopAB and hopC1 triggered the hypersensitive reaction in cherry leaves, confirming computational predictions. Cherry canker provides a fascinating example of convergent evolution of pathogenicity that is explained by the mix of effector and toxin repertoires acting on a common host.

Pseudomonas syringae: what it takes to be a pathogen

Pseudomonas syringae is one of the best-studied plant pathogens and serves as a model for understanding host-microorganism interactions, bacterial virulence mechanisms and host adaptation of pathogens as well as microbial evolution, ecology and epidemiology. Comparative genomic studies have identified key genomic features that contribute to P. syringae virulence. P. syringae has evolved two main virulence strategies: suppression of host immunity and creation of an aqueous apoplast to form its niche in the phyllosphere. In addition, external environmental conditions such as humidity profoundly influence infection. P. syringae may serve as an excellent model to understand virulence and also of how pathogenic microorganisms integrate environmental conditions and plant microbiota to become ecologically robust and diverse pathogens of the plant kingdom.

Roles of Nicotinamide Adenine Dinucleotide (NAD+) in Biological Systems

NAD+ has emerged as a crucial element in both bioenergetic and signaling pathways since it acts as a key regulator of cellular and organism homeostasis. NAD+ is a coenzyme in redox reactions, a donor of adenosine diphosphate-ribose (ADPr) moieties in ADP-ribosylation reactions, a substrate for sirtuins, a group of histone deacetylase enzymes that use NAD+ to remove acetyl groups from proteins; NAD+ is also a precursor of cyclic ADP-ribose, a second messenger in Ca++ release and signaling, and of diadenosine tetraphosphate (Ap4A) and oligoadenylates (oligo2′-5′A), two immune response activating compounds. In the biological systems considered in this review, NAD+ is mostly consumed in ADP-ribose (ADPr) transfer reactions. In this review the roles of these chemical products are discussed in biological systems, such as in animals, plants, fungi and bacteria. In the review, two types of ADP-ribosylating enzymes are introduced as well as the pathways to restore the NAD+ pools in these systems.

Behind the lines-actions of bacterial type III effector proteins in plant cells

Pathogenicity of most Gram-negative plant-pathogenic bacteria depends on the type III secretion (T3S) system, which translocates bacterial effector proteins into plant cells. Type III effectors modulate plant cellular pathways to the benefit of the pathogen and promote bacterial multiplication. One major virulence function of type III effectors is the suppression of plant innate immunity, which is triggered upon recognition of pathogen-derived molecular patterns by plant receptor proteins. Type III effectors also interfere with additional plant cellular processes including proteasome-dependent protein degradation, phytohormone signaling, the formation of the cytoskeleton, vesicle transport and gene expression. This review summarizes our current knowledge on the molecular functions of type III effector proteins with known plant target molecules. Furthermore, plant defense strategies for the detection of effector protein activities or effector-triggered alterations in plant targets are discussed.

The Bacterial Effector AvrPto Targets the Regulatory Coreceptor SOBIR1 and Suppresses Defense Signaling Mediated by the Receptor-Like Protein Cf-4

Receptor-like proteins (RLPs) and receptor-like kinases (RLKs) are cell-surface receptors that are essential for detecting invading pathogens and subsequent activation of plant defense responses. RLPs lack a cytoplasmic kinase domain to trigger downstream signaling leading to host resistance. The RLK SOBIR1 constitutively interacts with the tomato RLP Cf-4, thereby providing Cf-4 with a kinase domain. SOBIR1 is required for Cf-4-mediated resistance to strains of the fungal tomato pathogen Cladosporium fulvum that secrete the effector Avr4. Upon perception of this effector by the Cf-4/SOBIR1 complex, the central regulatory RLK SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3a (SERK3a) is recruited to the complex and defense signaling is triggered. SOBIR1 is also required for RLP-mediated resistance to bacterial, fungal ,and oomycete pathogens, and we hypothesized that SOBIR1 is targeted by effectors of such pathogens to suppress host defense responses. In this study, we show that Pseudomonas syringae pv. tomato DC3000 effector AvrPto interacts with Arabidopsis SOBIR1 and its orthologs of tomato and Nicotiana benthamiana, independent of SOBIR1 kinase activity. Interestingly, AvrPto suppresses Arabidopsis SOBIR1-induced cell death in N. benthamiana. Furthermore, AvrPto compromises Avr4-triggered cell death in Cf-4-transgenic N. benthamiana, without affecting Cf-4/SOBIR1/SERK3a complex formation. Our study shows that the RLP coreceptor SOBIR1 is targeted by a bacterial effector, which results in compromised defense responses.

The HopF family of Pseudomonas syringae type III secreted effectors

Pseudomonas syringae is a bacterial phytopathogen that utilizes the type III secretion system to inject effector proteins into plant host cells. Pseudomonas syringae can infect a wide range of plant hosts, including agronomically important crops such as tomatoes and beans. The ability of P. syringae to infect such numerous hosts is caused, in part, by the diversity of effectors employed by this phytopathogen. Over 60 different effector families exist in P. syringae; one such family is HopF, which contains over 100 distinct alleles. Despite this diversity, research has focused on only two members of this family: HopF1 from P. syringae pathovar phaseolicola 1449B and HopF2 from P. syringae pathovar tomato DC3000. In this study, we review the research on HopF family members, including their host targets and molecular mechanisms of immunity suppression, and their enzymatic function. We also provide a phylogenetic analysis of this expanding effector family which provides a basis for a proposed nomenclature to guide future research. The extensive genetic diversity that exists within the HopF family presents a great opportunity to study how functional diversification on an effector family contributes to host specialization.

Convergent Evolution of Pathogen Effectors toward Reactive Oxygen Species Signaling Networks in Plants

Microbial pathogens have evolved protein effectors to promote virulence and cause disease in host plants. Pathogen effectors delivered into plant cells suppress plant immune responses and modulate host metabolism to support the infection processes of pathogens. Reactive oxygen species (ROS) act as cellular signaling molecules to trigger plant immune responses, such as pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity. In this review, we discuss recent insights into the molecular functions of pathogen effectors that target multiple steps in the ROS signaling pathway in plants. The perception of PAMPs by pattern recognition receptors leads to the rapid and strong production of ROS through activation of NADPH oxidase Respiratory Burst Oxidase Homologs (RBOHs) as well as peroxidases. Specific pathogen effectors directly or indirectly interact with plant nucleotide-binding leucine-rich repeat receptors to induce ROS production and the hypersensitive response in plant cells. By contrast, virulent pathogens possess effectors capable of suppressing plant ROS bursts in different ways during infection. PAMP-triggered ROS bursts are suppressed by pathogen effectors that target mitogen-activated protein kinase cascades. Moreover, pathogen effectors target vesicle trafficking or metabolic priming, leading to the suppression of ROS production. Secreted pathogen effectors block the metabolic coenzyme NADP-malic enzyme, inhibiting the transfer of electrons to the NADPH oxidases (RBOHs) responsible for ROS generation. Collectively, pathogen effectors may have evolved to converge on a common host protein network to suppress the common plant immune system, including the ROS burst and cell death response in plants.

SERKing Coreceptors for Receptors

Plants have evolved a large number of cell surface-resident receptor-like kinases (RLKs) and receptor-like proteins (RLPs), many of which are implicated in sensing extrinsic and intrinsic signals, and govern diverse cellular responses. The signaling pathways mediated by RLKs and RLPs converge at a small group of RLKs, somatic embryogenesis receptor kinases (SERKs), via ligand-induced heterodimerization and transphosphorylation. As shared coreceptors in diverse signaling receptorsomes, SERKs exhibit functional plasticity yet maintain a high degree of signaling specificity. Here, we review recent advances in newly identified SERK functions in plant cell differentiation, growth, and immunity; discuss the regulation and activation mechanisms of SERK-associated receptorsomes; and provide insights into how SERKs maintain signaling specificity as convergent hubs in various signaling pathways.

Using Arabidopsis Protoplasts to Study Cellular Responses to Environmental Stress

Arabidopsis mesophyll protoplasts can be readily isolated and transfected in order to transiently express proteins of interest. As freshly isolated mesophyll protoplasts maintain essentially the same physiological characteristics of whole leaves, this cell-based transient expression system can be used to molecularly dissect the responses to various stress conditions. The response of stress-responsive promoters to specific stimuli can be accessed via reporter gene assays. Additionally, reporter systems can be easily engineered to address other levels of regulation, such as transcript and/or protein stability. Here we present a detailed protocol for using the Arabidopsis mesophyll protoplast system to study responses to environmental stress, including preparation of reporter and effector constructs, large scale DNA purification, protoplast isolation, transfection, treatment, and quantification of luciferase-based reporter gene activities.

Signaling mechanisms in pattern-triggered immunity (PTI)

In nature, plants constantly have to face pathogen attacks. However, plant disease rarely occurs due to efficient immune systems possessed by the host plants. Pathogens are perceived by two different recognition systems that initiate the so-called pattern-triggered immunity (PTI) and effector-triggered immunity (ETI), both of which are accompanied by a set of induced defenses that usually repel pathogen attacks. Here we discuss the complex network of signaling pathways occurring during PTI, focusing on the involvement of mitogen-activated protein kinases.

The role of cell wall-based defences in the early restriction of non-pathogenic hrp mutant bacteria in Arabidopsis

We have investigated the cause of the restricted multiplication of hrp mutant bacteria in leaves of Arabidopsis. Our focus was on early interactions leading to differentiation between virulent wild-type and non-pathogenic hrpA mutant strains of Pseudomonas syringae pv. tomato. An initial drop in recoverable bacteria detected 0-4 h after inoculation with either strain was dependent on a functional FLS2 receptor and H2O2 accumulation in challenged leaves. Wild-type bacteria subsequently multiplied rapidly whereas the hrpA mutant was restricted within 6 h. Despite the early restriction, the hrpA mutant was still viable several days after inoculation. Analysis of intercellular washing fluids (IWFs), showed that high levels of nutrients were readily available to bacteria in the apoplast and that no diffusible inhibitors were produced in response to bacterial challenge. Histochemical and immunocytochemical methods were used to detect changes in polysaccharides (callose, two forms of cellulose, and pectin), arabinogalactan proteins (AGPs), H2O2 and peroxidase. Quantitative analysis showed very similar changes in localisation of AGPs, cellulose epitopes and callose 2 and 4 h after inoculation with either strain. However from 6 to 12 h after inoculation papillae expanded only next to the hrp mutant. In contrast to the similar patterns of secretory activity recorded from mesophyll cells, accumulation of H2O2 and peroxidase was significantly greater around the hrpA mutant within the first 4h after inoculation. A striking differential accumulation of H2O2 was also found in chloroplasts in cells next to the mutant. Ascorbate levels were lower in the IWFs recovered from sites inoculated with the hrp mutant than with wild-type bacteria. The critical response, observed at the right time and place to explain the observed differential behaviour of wild-type and hrpA mutant bacteria was the accumulation of H2O2, probably generated through Type III peroxidase activity and in chloroplasts. It is proposed that H2O2 and apoplastic peroxidase cross-link secreted glycoproteins and polysaccharides to agglutinate the hrp mutant. Generation of H2O2 has been identified as a likely target for effector proteins injected into plant cells by the wild-type bacteria.

Syntaxin 31 functions in Glycine max resistance to the plant parasitic nematode Heterodera glycines

A Glycine max syntaxin 31 homolog (Gm-SYP38) was identified as being expressed in nematode-induced feeding structures known as syncytia undergoing an incompatible interaction with the plant parasitic nematode Heterodera glycines. The observed Gm-SYP38 expression was consistent with prior gene expression analyses that identified the alpha soluble NSF attachment protein (Gm-α-SNAP) resistance gene because homologs of these genes physically interact and function together in other genetic systems. Syntaxin 31 is a protein that resides on the cis face of the Golgi apparatus and binds α-SNAP-like proteins, but has no known role in resistance. Experiments presented here show Gm-α-SNAP overexpression induces Gm-SYP38 transcription. Overexpression of Gm-SYP38 rescues G. max [Williams 82/PI 518671], genetically rhg1 (-/-), by suppressing H. glycines parasitism. In contrast, Gm-SYP38 RNAi in the rhg1 (+/+) genotype G. max [Peking/PI 548402] increases susceptibility. Gm-α-SNAP and Gm-SYP38 overexpression induce the transcriptional activity of the cytoplasmic receptor-like kinase BOTRYTIS INDUCED KINASE 1 (Gm-BIK1-6) which is a family of defense proteins known to anchor to membranes through a 5' MGXXXS/T(R) N-myristoylation sequence. Gm-BIK1-6 had been identified previously by RNA-seq experiments as expressed in syncytia undergoing an incompatible reaction. Gm-BIK1-6 overexpression rescues the resistant phenotype. In contrast, Gm-BIK1-6 RNAi increases parasitism. The analysis demonstrates a role for syntaxin 31-like genes in resistance that until now was not known.

The Pseudomonas syringae effector HopF2 suppresses Arabidopsis immunity by targeting BAK1

Pseudomonas syringae delivers a plethora of effector proteins into host cells to sabotage immune responses and modulate physiology to favor infection. The P. syringae pv. tomato DC3000 effector HopF2 suppresses Arabidopsis innate immunity triggered by multiple microbe-associated molecular patterns (MAMP) at the plasma membrane. We show here that HopF2 possesses distinct mechanisms for suppression of two branches of MAMP-activated MAP kinase (MAPK) cascades. In addition to blocking MKK5 (MAPK kinase 5) activation in the MEKK1 (MAPK kinase kinase 1)/MEKKs-MKK4/5-MPK3/6 cascade, HopF2 targets additional component(s) upstream of MEKK1 in the MEKK1-MKK1/2-MPK4 cascade and the plasma membrane-localized receptor-like cytoplasmic kinase BIK1 and its homologs. We further show that HopF2 directly targets BAK1, a plasma membrane-localized receptor-like kinase that is involved in multiple MAMP signaling. The interaction between BAK1 and HopF2 and between two other P. syringae effectors, AvrPto and AvrPtoB, was confirmed in vivo and in vitro. Consistent with BAK1 as a physiological target of AvrPto, AvrPtoB and HopF2, the strong growth defects or lethality associated with ectopic expression of these effectors in wild-type Arabidopsis transgenic plants were largely alleviated in bak1 mutant plants. Thus, our results provide genetic evidence to show that BAK1 is a physiological target of AvrPto, AvrPtoB and HopF2. Identification of BAK1 as an additional target of HopF2 virulence not only explains HopF2 suppression of multiple MAMP signaling at the plasma membrane, but also supports the notion that pathogen virulence effectors act through multiple targets in host cells.

Dual regulation of gene expression mediated by extended MAPK activation and salicylic acid contributes to robust innate immunity in Arabidopsis thaliana

Network robustness is a crucial property of the plant immune signaling network because pathogens are under a strong selection pressure to perturb plant network components to dampen plant immune responses. Nevertheless, modulation of network robustness is an area of network biology that has rarely been explored. While two modes of plant immunity, Effector-Triggered Immunity (ETI) and Pattern-Triggered Immunity (PTI), extensively share signaling machinery, the network output is much more robust against perturbations during ETI than PTI, suggesting modulation of network robustness. Here, we report a molecular mechanism underlying the modulation of the network robustness in Arabidopsis thaliana. The salicylic acid (SA) signaling sector regulates a major portion of the plant immune response and is important in immunity against biotrophic and hemibiotrophic pathogens. In Arabidopsis, SA signaling was required for the proper regulation of the vast majority of SA-responsive genes during PTI. However, during ETI, regulation of most SA-responsive genes, including the canonical SA marker gene PR1, could be controlled by SA-independent mechanisms as well as by SA. The activation of the two immune-related MAPKs, MPK3 and MPK6, persisted for several hours during ETI but less than one hour during PTI. Sustained MAPK activation was sufficient to confer SA-independent regulation of most SA-responsive genes. Furthermore, the MPK3 and SA signaling sectors were compensatory to each other for inhibition of bacterial growth as well as for PR1 expression during ETI. These results indicate that the duration of the MAPK activation is a critical determinant for modulation of robustness of the immune signaling network. Our findings with the plant immune signaling network imply that the robustness level of a biological network can be modulated by the activities of network components.

Robustness of a network is defined by how consistently it performs upon removal of some of its components. It is a common strategy for plant pathogens to attack components of the plant immune signaling network in an attempt to dampen plant immunity. Therefore, it is crucial for the plant immune signaling network to have a high level of robustness. We previously reported that the robustness level of the plant immune signaling network is higher during Effector-Triggered Immunity (ETI) than Pattern-Triggered Immunity (PTI). Here we discovered a molecular switch that determines two robustness levels during ETI and PTI. Salicylic acid (SA) is a major plant immune signal molecule that regulates many immune-related genes. SA-independent alternative mechanisms also regulated the majority of SA-responsive genes during ETI but not PTI. One of the SA-independent mechanisms was mediated by prolonged activation of MAP kinases (MAPKs). MAPK activation was prolonged during ETI but transient during PTI. Thus, the duration of MAPK activation switches the robustness level of the plant immune signaling network. Our findings imply that the robustness level of a biological network can be modulated by activities of its components.

Pseudomonas HopU1 modulates plant immune receptor levels by blocking the interaction of their mRNAs with GRP7

Pathogens target important components of host immunity to cause disease. The Pseudomonas syringae type III-secreted effector HopU1 is a mono-ADP-ribosyltransferase required for full virulence on Arabidopsis thaliana. HopU1 targets several RNA-binding proteins including GRP7, whose role in immunity is still unclear. Here, we show that GRP7 associates with translational components, as well as with the pattern recognition receptors FLS2 and EFR. Moreover, GRP7 binds specifically FLS2 and EFR transcripts in vivo through its RNA recognition motif. HopU1 does not affect the protein-protein associations between GRP7, FLS2 and translational components. Instead, HopU1 blocks the interaction between GRP7 and FLS2 and EFR transcripts in vivo. This inhibition correlates with reduced FLS2 protein levels upon Pseudomonas infection in a HopU1-dependent manner. Our results reveal a novel virulence strategy used by a microbial effector to interfere with host immunity.

Wide screening of phage-displayed libraries identifies immune targets in planta

Microbe-Associated Molecular Patterns and virulence effectors are recognized by plants as a first step to mount a defence response against potential pathogens. This recognition involves a large family of extracellular membrane receptors and other immune proteins located in different sub-cellular compartments. We have used phage-display technology to express and select for Arabidopsis proteins able to bind bacterial pathogens. To rapidly identify microbe-bound phage, we developed a monitoring method based on microarrays. This combined strategy allowed for a genome-wide screening of plant proteins involved in pathogen perception. Two phage libraries for high-throughput selection were constructed from cDNA of plants infected with Pseudomonas aeruginosa PA14, or from combined samples of the virulent isolate DC3000 of Pseudomonas syringae pv. tomato and its avirulent variant avrRpt2. These three pathosystems represent different degrees in the specificity of plant-microbe interactions. Libraries cover up to 2×10⁷ different plant transcripts that can be displayed as functional proteins on the surface of T7 bacteriophage. A number of these were selected in a bio-panning assay for binding to Pseudomonas cells. Among the selected clones we isolated the ethylene response factor ATERF-1, which was able to bind the three bacterial strains in competition assays. ATERF-1 was rapidly exported from the nucleus upon infiltration of either alive or heat-killed Pseudomonas. Moreover, aterf-1 mutants exhibited enhanced susceptibility to infection. These findings suggest that ATERF-1 contains a microbe-recognition domain with a role in plant defence. To identify other putative pathogen-binding proteins on a genome-wide scale, the copy number of selected-vs.-total clones was compared by hybridizing phage cDNAs with Arabidopsis microarrays. Microarray analysis revealed a set of 472 candidates with significant fold change. Within this set defence-related genes, including well-known targets of bacterial effectors, are over-represented. Other genes non-previously related to defence can be associated through this study with general or strain-specific recognition of Pseudomonas.

Pseudomonas syringae pv. tomato DC3000: a model pathogen for probing disease susceptibility and hormone signaling in plants

Since the early 1980s, various strains of the gram-negative bacterial pathogen Pseudomonas syringae have been used as models for understanding plant-bacterial interactions. In 1991, a P. syringae pathovar tomato (Pst) strain, DC3000, was reported to infect not only its natural host tomato but also Arabidopsis in the laboratory, a finding that spurred intensive efforts in the subsequent two decades to characterize the molecular mechanisms by which this strain causes disease in plants. Genomic analysis shows that Pst DC3000 carries a large repertoire of potential virulence factors, including proteinaceous effectors that are secreted through the type III secretion system and a polyketide phytotoxin called coronatine, which structurally mimics the plant hormone jasmonate (JA). Study of Pst DC3000 pathogenesis has not only provided several conceptual advances in understanding how a bacterial pathogen employs type III effectors to suppress plant immune responses and promote disease susceptibility but has also facilitated the discovery of the immune function of stomata and key components of JA signaling in plants. The concepts derived from the study of Pst DC3000 pathogenesis may prove useful in understanding pathogenesis mechanisms of other plant pathogens.

Catch me if you can: bacterial effectors and plant targets

To suppress plant defense responses and favor the establishment of disease, phytopathogenic bacteria have gained the ability to deliver effector molecules inside host cells through the type III secretion system. Inside plant cells, bacterial effector proteins may be addressed to different subcellular compartments where they are able to manipulate a variety of host cellular components and molecular functions. Here we review how the recent identification and functional characterization of plant components targeted by bacterial effectors, as well as the discovery of new pathogen recognition capabilities evolved in turn by plant cells, have significantly contributed to further our knowledge about the intricate molecular interactions that are established between plants and their invading bacteria.

The RxLR effector Avh241 from Phytophthora sojae requires plasma membrane localization to induce plant cell death

• The Phytophthora sojae genome encodes hundreds of RxLR effectors predicted to manipulate various plant defense responses, but the molecular mechanisms involved are largely unknown. Here we have characterized in detail the P. sojae RxLR effector Avh241. • To determine the function and localization of Avh241, we transiently expressed it on different plants. Silencing of Avh241 in P. sojae, we determined its virulence during infection. Through the assay of promoting infection by Phytophthora capsici to Nicotiana benthamiana, we further confirmed this virulence role. • Avh241 induced cell death in several different plants and localized to the plant plasma membrane. An N-terminal motif within Avh241 was important for membrane localization and cell death-inducing activity. Two mitogen-activated protein kinases, NbMEK2 and NbWIPK, were required for the cell death triggered by Avh241 in N. benthamiana. Avh241 was important for the pathogen's full virulence on soybean. Avh241 could also promote infection by P. capsici and the membrane localization motif was not required to promote infection. • This work suggests that Avh241 interacts with the plant immune system via at least two different mechanisms, one recognized by plants dependent on subcellular localization and one promoting infection independent on membrane localization.

Bacterial toxin effector-membrane targeting: outside in, then back again

Pathogenic bacteria utilize multiple approaches to establish infection and mediate their toxicity to eukaryotic cells. Dedicated protein machines deposit toxic effectors directly inside the host, whereas secreted toxins must enter cells independently of other bacterial components. Regardless of how they reach the cytosol, these bacterial proteins must accurately identify their intracellular target before they can manipulate the host cell to benefit their associated bacteria. Within eukaryotic cells, post-translational modifications and individual targeting motifs spatially regulate endogenous host proteins. This review focuses on the strategies employed by bacterial effectors to associate with a frequently targeted location within eukaryotic cells, the plasma membrane.

Subcellular localization of the Hpa RxLR effector repertoire identifies a tonoplast-associated protein HaRxL17 that confers enhanced plant susceptibility

Filamentous phytopathogens form sophisticated intracellular feeding structures called haustoria in plant cells. Pathogen effectors are likely to play a role in the establishment and maintenance of haustoria in addition to their better-characterized role in suppressing plant defence. However, the specific mechanisms by which these effectors promote virulence remain unclear. To address this question, we examined changes in subcellular architecture using live-cell imaging during the compatible interaction between the oomycete Hyaloperonospora arabidopsidis (Hpa) and its host Arabidopsis. We monitored host-cell restructuring of subcellular compartments within plant mesophyll cells during haustoria ontogenesis. Live-cell imaging highlighted rearrangements in plant cell membranes upon infection, in particular to the tonoplast, which was located close to the extra-haustorial membrane surrounding the haustorium. We also investigated the subcellular localization patterns of Hpa RxLR effector candidates (HaRxLs) in planta. We identified two major classes of HaRxL effector based on localization: nuclear-localized effectors and membrane-localized effectors. Further, we identified a single effector, HaRxL17, that associated with the tonoplast in uninfected cells and with membranes around haustoria, probably the extra-haustorial membrane, in infected cells. Functional analysis of selected effector candidates in planta revealed that HaRxL17 enhances plant susceptibility. The roles of subcellular changes and effector localization, with specific reference to the potential role of HaRxL17 in plant cell membrane trafficking, are discussed with respect to Hpa virulence.

Phytobacterial type III effectors HopX1, HopAB1 and HopF2 enhance sense-post-transcriptional gene silencing independently of plant R gene-effector recognition

Plant- and animal-pathogenic bacteria deploy a variable arsenal of type III effector proteins (T3EP) to manipulate host defense. Specific biochemical functions and molecular or subcellular targets have been demonstrated or proposed for a growing number of T3EP but remain unknown for the majority of them. Here, we show that transient expression of genes coding certain bacterial T3EP (HopAB1, HopX1, and HopF2), which did not elicit hypersensitive response (HR) in transgenic green fluorescent protein (GFP) Nicotiana benthamiana 16C line, enhanced the sense post-transcriptional gene silencing (S-PTGS) triggered by agrodelivery of a GFP-expressing cassette and the silencing enhancement could be blocked by two well-known viral silencing suppressors. Further analysis using genetic truncations and site-directed mutations showed that the receptor recognition domains of HopAB1 and HopX1 are not involved in enhancing silencing. Our studies provide new evidence that phytobacterial pathogen T3EP manipulate the plant small interfering RNA pathways by enhancing silencing efficiency in the absence of effector-triggered immunity signaling and suggest that phytopathogenic bacterial effectors affect host RNA silencing in yet other ways than previously described.