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

1Plants have evolved sophisticated defense mechanisms to resist potential attacks by microbial pathogens. The first line of defense is triggered in plants by the perception of microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs) via membrane-bound pattern recognition receptors (PRRs), leading to basal immunity, known as PAMP-triggered immunity (PTI). PAMPs include conserved cell surface structures including bacterial flagellin, lipopolysaccharides, and peptidoglycan, or fungal cell wall components such as glucan or chitin. Plants may disrupt numerous non-host or host pathogen attacks via PTI; however, adapted pathogens can overcome the PTI-dependent defense response to cause disease on their host plants. PTI requires signal transduction from receptors to downstream components via mitogen-activated protein kinase (MAPK) cascade pathways. Known PAMPs activate MAP kinases in plant cells. In the second line of defense, plants have acquired a cell-based surveillance system using intracellular nucleotide-binding leucine-rich repeat (NLR) receptors to recognize specific pathogen effectors, leading to resistance (R) gene-mediated effector-triggered immunity (ETI). The two phases of plant immunity may be spatiotemporally distinct but are intimately related to the reactive oxygen species (ROS) burst. Production of ROS in plant cells is a hallmark of successful recognition of plant pathogens and activation of plant defenses. Pathogen-induced apoplastic ROS production was first demonstrated in potato tuber tissues by and indeed ROS play important roles in plant immune responses as signaling molecules. In plant cells, ROS also occur in response to many physiological stimuli.

2Reactive oxygen species are highly reactive reduced oxygen molecules, such as superoxide (⋅O2-), hydrogen peroxide (H2O2), and hydroxyl radical (⋅OH). There is comprehensive evidence that ROS can act as cellular signaling molecules, mediating various important responses of plant cells to different physiological stimuli, including pathogen attack, abiotic stress, hormone signaling, and polar growth. ROS are formed intracellularly during certain redox reactions in the cell membranes, cytoplasm, nuclei, mitochondria, chloroplasts, peroxisomes, and endoplasmic reticulum. The endomembrane and nuclear compartments are likely targets or sources for ROS signaling. Pathogen-induced ROS generation in chloroplasts is known to play a crucial role in the signaling for and/or execution of hypersensitive response (HR) cell death in plants. In addition, mitochondrial ROS associated with alteration in respiration are likely to activate defense responses, but could not be directly involved in plant cell death. The generation of extracellular ROS, including H2O2, requires the extracellular activities of cell wall peroxidases and plasma membrane NADPH oxidases in plant cells. NADPH oxidase-dependent ROS generation with electron supply provided by NADP-malic enzyme (ME) is presented in Figure 1. H2O2 is one of the most abundant and stable ROS in plants, and ROS generated inside cells can move apoplastically as H2O2 into neighboring cells. ROS signal transduction activates Ca2+ channels in the plasma membrane and may be a central step in many ROS-mediated processes regulating the physiology of plant cells. It has also been proposed that biomembrane channels (aquaporins) mediate H2O2 transport across biological membranes to control ROS signaling in plant cells. The primary ROS bursts after perception of pathogens occur in the apoplasts; however, ROS produced in different compartments inside the plant cell may function in plant defenses to pathogen invasion.

3Reactive oxygen species are an effective weapon that can be produced rapidly and utilized against pathogen infection. H2O2 and O2- are mainly produced at the site of attempted pathogen invasion in plant cells. They are secreted from the cell within 3 min of recognition of MAMPs or PAMPs. The function of H2O2 in inhibiting pathogen growth is fully understood. Plant-derived ROS act as a powerful weapon against pathogen invasion. By contrast, pathogens have evolved strategies to reduce plant ROS bursts in different ways during infection. For example, pathogens may avoid the risk of the ROS burst by using effectors. However, in the presence of effector-aware and defensible plants, pathogens are often faced with mutation pressure for virulence. The competition between pathogens and host plants, called ‘arms races’, may lead to the creation of numerous types of effectors in microbial pathogens and also R- or defense-related proteins in plants. Effectors from these evolutionarily diverse pathogens are predicted to converge on common host plant proteins, which are characterized by a high degree of interaction in host plant protein networks. Here, we review and discuss recent advances in understanding how microbial pathogen effectors have evolved toward the suppression of plant apoplastic ROS bursts during infection.

4Perception of PAMPs by plants via PRRs triggers ROS production through activation of NADPH oxidases as well as peroxidases, leading to PTI-dependent basal defenses that inhibit invading pathogens (Figure 2). The apoplastic ROS bursts generated in elicited plant cells are sufficiently cytotoxic to kill invading pathogens. ROS also act as signaling molecules, triggering plant immune and cell death responses. Thus, pathogens need to take steps to avoid exposure to toxic ROS. NADPH oxidases, also known as Respiratory Burst Oxidase Homologs (RBOHs), are responsible for production of ROS in plants during pathogen infection. NADPH oxidase (RBOHD) phosphorylation by the PRR-associated kinase BIK1 has been proposed to be essential for PAMP-triggered ROS production. In addition, the apoplastic peroxidase-dependent ROS burst plays an important role in Arabidopsis PTI mediated by the recognition of PAMPs. Antisense expression of a heterologous French bean (Phaseolus vulgaris) peroxidase (FBP1) cDNA in Arabidopsis diminishes the expression of Arabidopsis peroxidases PRX33 and PRX34, blocking the ROS burst in response to a Fusarium oxysporum cell wall elicitor, and leading to enhanced susceptibility to fungal and bacterial pathogens. Similarly, pepper extracellular peroxidase CaPO2 generates ROS bursts, activating local and systemic cell death and defense response to bacterial pathogens. Recently, the plant aquaporin AtPIP;4 has been demonstrated to trigger cytoplasmic import of apoplastic H2O2 into plant cells, activating systemic acquired resistance (SAR) and PTI pathways in response to Pseudomonas syringae pv. tomato DC3000 and two typical PAMPs (flagellin and chitin), respectively. This suggests a pivotal role for aquaporins in apocytoplastic ROS signal transduction in disease immunity pathways.

5Plants defend themselves against invading pathogens through cell wall reinforcement. Cell wall fortifications are facilitated by an apoplastic H2O2 burst, cell wall cross-linking, and callose deposition at the site of infection. Effector (Elicitor)-induced oxidative cross-linking of plant cell wall structural proteins is essential for cell maturation and toughening of cell walls in the initial stages of plant defense. Callose, a (1,3)-β-glucan, is a major component of cell wall thickening at sites of fungal penetration in plants. The Arabidopsis GTPase RabA4c physically interacts with its effector PMR4 to enhance PMR4-dependent callose biosynthesis, which ultimately results in complete penetration resistance to powdery mildew (Golovinomyces cichoracearum). Hydroxyproline-rich glycoproteins (HRGPs) are involved in cell wall strengthening by formation of intra- and intermolecular cross-links. Exogenous application of H2O2 rescues the callose deposition-deficient phenotype of peroxidase knockdown Arabidopsis lines treated with the bacterial flagellin, Flg22. This suggests that cell wall peroxidase-dependent H2O2 production is required for PAMP-triggered immune responses, such as callose deposition.

6Reactive oxygen species bursts are monitored during infection by avirulent pathogens. Pathogen avirulence (Avr) effectors interact directly or indirectly with NLR proteins, leading to a strong ROS burst and the HR cell death response, both key components of ETI (Figure 2;;;;;; ). However, whether NLR-mediated ROS themselves activate HR cell death and immune responses is not fully understood. Rice resistance to incompatible rice blast fungus (Magnaporthe oryzae) isolates is suppressed by inhibiting the accumulation of apoplastic ROS, even in the presence of the R gene. The resistant rice cultivar Hwayeonbyeo carrying Pib exhibits a compatible response to M. oryzae INA168 carrying AvrPii and AvrPib via the deletion of OsNADP-ME2. In the absence of normal production of ROS, cell death and immune responses are severely suppressed, although the Avr effector and cognate NLR proteins are not impaired. Increased ROS production during infection is essential for NLR-mediated cell death and immunity as well as disease-associated cell death. Perception of plant NLR receptors by specific pathogen effectors triggers a strong ROS burst through activation of RBOHs receiving an adequate supply of NADPH via the activity of NADP-ME (Figure 1; ).

7Host cysteine proteases targeted by the Ustilago maydis effector Pit2 are likely to be crucial determinants in apoplastic maize immune responses such as the ROS burst. The Cladosporium fulvum effector Avr2 binds and inhibits Rcr3, a extracellular tomato cysteine protease, which is required for Cf-2-dependent disease resistance. It has been proposed that the Rcr3-Avr2 complex enables the Cf-2 protein to activate a HR, including the ROS burst and cell death (Table 1). The defense-related protease Rcr3 may act as a decoy for Avr2 perception in tomato plants carrying the Cf-2 resistance gene.

8The Xanthomonas campestris pv. vesicatoria effector AvrBsT induces a H2O2 burst and HR cell death in pepper (Capsicum annuum). The AvrBsT-triggered HR cell death response is similar to the resistance (R) gene-mediated defense response in plants. AvrBsT physically binds to pepper arginine decarboxylase (CaADC1), pepper aldehyde dehydrogenase (CaALDH1), pepper heat shock protein 70a (CaHSP70a), and pepper suppressor of the G2 allele of skp1 (CaSGT1) in planta to promote the ROS burst, defense gene expression, cell death, and defense responses. AvrBsT and CaPIK1 directly bind to CaSGT1 in yeast and in planta. AvrBsT is subsequently phosphorylated by CaPIK1 and forms the active AvrBsT–SGT1–SGT1-PIK1 complex, which promotes the ROS burst, HR cell death, and defense responses in plants.

9Adapted microbial pathogens have evolved their effector proteins as virulence factors to suppress the ROS burst and PTI, causing disease on their respective host plants (Figure 2). Secreted effector proteins are delivered into host cells to protect pathogen cell walls against plant-derived hydrolytic enzymes and suppress PAMP-triggered host immunity, leading ultimately to the successful colonization of host plants. Various components of fungal cell walls such as glucans, chitin, and proteins, acting as PAMPs to trigger basal immune responses, are degraded by plant-derived hydrolytic enzymes, such as β-1,3-glucanases, chitinases, and serine and cysteine proteases.

10The C. fulvum effector Avr4, a chitin-binding lectin, binds to its own cell walls to protect chitin against hydrolysis by plant chitinases during infection of tomato, suggesting that Avr4 is a virulence factor (Table 1; ). The LysM domain-containing effector protein Ecp6 of C. fulvum mediates virulence through suppression of chitin-triggered immunity in plants. C. fulvum Ecp6 (CfEcp6) is secreted at high levels during plant infection and binds chitin, thereby blocking chitin-triggered immunity responses through sequestering chitin fragments and preventing their recognition by plant chitin receptors. By contrast to C. fulvum Ecp6, both Mg1LysM and Mg3LysM from the fungus Mycosphaerella graminicola protect fungal hyphae against plant-derived hydrolytic enzymes, such as chitinases (Table 1; ). As a virulence determinant in the rice blast fungus M. oryzae, the secreted LysM effector (Slp1) binds to chitin inside the fungal cell wall, suppressing chitin-triggered plant immune responses, including the ROS burst and plant defense gene expression. The effector Slp1 inhibits the chitin-induced ROS burst in rice suspension cells. The lectin FGB1 (Fungal Glucan Binding 1), secreted from the root endophyte Piriformospora indica, specifically interacts with β-1, 6-linked glucan, altering cell wall composition and suppressing glucan-triggered ROS production in plants. The presence of P. indica in the roots of barley inhibits laminarin-induced ROS production. Laminarin-triggered ROS production is also delayed when compared with that observed following chitin elicitation.

11Some pathogen effectors specifically bind to plant proteases and may activate downstream signaling components. Apoplast-localized plant proteases can play an important role in defense responses to microbial pathogens. The U. maydis effector Pit2 physically interacts with and inhibits apoplastic maize cysteine proteases, suppressing host immunity (Table 1; ). The secreted effector protein Pit2 is essential for maintenance of biotrophy and induction of tumors in maize. The biotrophic interaction of maize with U. maydis depends on inhibition of apoplastic cysteine proteases by the effector Pit2 with a conserved inhibitor domain. The U. maydis effector Pep1 (Protein essential during penetration-1) interferes with maize apoplastic peroxidases at the plant–pathogen interface to scavenge ROS (Table 1; ). As an inhibitor of plant peroxidases, Pep1 effectively inhibits the peroxidase-triggered ROS burst, which thereby suppresses the early immune response of maize leading to the establishment of a biotrophic interaction. Pep1 localizes to the plant apoplast, where it accumulates at sites of cell-to-cell passage of biotrophic U. maydis hyphae. The obligate biotrophic fungal pathogen of barley, Blumeria graminis f. sp. hordei, secretes an extracellular catalase B to scavenge H2O2 at sites of fungal germ tube invasion during infection (Table 1; ). A large number of catB transcripts accumulate during the mature primary germ tube and appressorium germ tube stages of fungal development on the susceptible barley plant, suggesting the upregulation of an extracellular catalase gene early during fungal invasion.

12During pathogen infection, perception of PRRs by PAMPs leads to PTI by causing a rapid ROS burst via receptor-like cytoplasmic kinases (RLCKs) such as PBL1 and BIK1 or through MAPK cascades. Pathogens overcome basal immune responses through inactivation of PAMP-induced signaling pathways that target MAPK cascade components. MAPKs are major targets for inactivation by pathogen effector proteins (Figure 3). MAPK cascades are highly conserved modules and are implicated in pathogen signaling during multiple defense responses against pathogen invasion in plants. In particular, the MAPK cascade regulates transcriptional reprogramming via the WRKY transcription factor in the early signaling events following PAMP recognition in plants. MAPK signaling is involved in the expression of genes required for apoplastic ROS production in plant defense responses. For example, WRKY transcription factors phosphorylated by MAPKs upregulate the RBOH, an NADPH oxidase, inducing pathogen-responsive ROS bursts in Nicotiana benthamiana. However, because of the complicated nature of the downstream MAPK cascade, how MAPK signaling promotes ROS generation is not fully understood.

13Pathogen effectors act on plant host target proteins, interfering with PTI-mediated defense signaling cascades, such as ROS and MAPK cascades, ultimately causing disease in host cells. For example, the P. syringae effector HopAO1 targets the Arabidopsis receptor kinase EF-TU RECEPTOR (EFR), reducing EFR phosphorylation, thereby preventing subsequent early immune responses, such as the ROS burst and MAPK activation (Table 1; ). The P. syringae effector HopF2 interacts directly with the plasma membrane-localized receptor-like kinase (RLK) BAK1 and suppresses early signaling events triggered by multiple PAMPs, including BIK1 phosphorylation, MAPK activation, and defense gene expression (Table 1;; ). In fact, BAK1 can directly phosphorylate the plasma membrane-localized RLCK BIK1. FLAGELLIN SENSING2 (FLS2)/BAK1-induced MAPK signaling enhances gene expression for plant immune responses via recognition of specific target genes by WRKY transcription factors. Multiple WRKYs bind to and activate the NADPH oxidase RBOHB promoter, followed by enhanced RBOHB expression, which subsequently leads to the RBOHB-dependent ROS burst. Type III effectors also target specific host plant proteins, such as major MAPK and WRKY modules. The P. syringae effector HopAI1, for example, inactivates MAPKs by removing the phosphate group from phosphothreonine through a unique phosphothreonine lyase activity (Table 1;, ). Another type III effector, HopF2, interacts with Arabidopsis MAP Kinase Kinase 5 (MKK5), and likely other MAPKKs (MKK1, MKK3, MKK4, MKK6, and MKK10), suppressing MAPKs and PAMP-triggered defenses (Table 1; ). The Ralstonia solanacearum acetyltransferase effector PopP2 localizes to the plant cell nucleus and acetylates lysine residues of WRKY transcription factors, blocking DNA binding (Table 1;;). The X. campestris uridylyl transferase effector AvrAC targets and inhibits BIK1 downstream of BAK1, enhancing virulence (Table 1; ). In particular, AvrAC acts upstream of MAPK cascades and ROS production to suppress PTI signaling. The PRR-associated kinase BIK1 directly phosphorylates the NADPH oxidase RBOHD, enhancing RBOHD-mediated ROS production (Figure 3;; ). The Phytophthora infestans RXLR effector PexRD2 interacts with a specific host MAPKKK, suppressing MAPKKK signaling-dependent cell death. In addition, multiple RXLR effectors from Hyaloperonospora arabidopsidis and P. infestans suppress the PAMP-elicited ROS burst. Functionally redundant effectors from different pathogen species may evolve virulence strategies to target PTI signal transduction processes, such as MAPK cascades, the ROS burst, and defense gene activation (Figure 3;;; ).

14Vesicle trafficking is an important cellular function in plants and is required for the transport of immune receptors and associated proteins, and for the extracellular secretion of immune-related molecules and antimicrobial compounds upon pathogen attack. Visible vesicle-like bodies aggregate directly beneath sites of fungal attack in the barley-B. graminis f. sp. hordei pathosystem and vesicle incidence is positively associated with levels of resistance to B. graminis f. sp. hordei penetration. Vesicles contain phytoalexins, phenolics, or ROS. The presence of ROS in the vesicles at penetration sites of B. graminis f. sp. hordei suggests that the endomembrane-associated immune response is associated with ROS signaling. One constituent of the vesicles is H2O2, a plant defense compound involved in antimicrobial, cell wall cross-linking, and signaling functions.

15Pathogen effectors target vesicle trafficking, suppressing PTI and potentially mediating immune-related ROS signaling (Figure 3; ). The Phytophthora cryptogea effector cryptogein induces an increase in abundance of the ROS-producing enzyme RBOHD, as well as ROS production at the plasma membrane of tobacco cells (Table 1; ). Plant NADPH oxidases (RBOHDs) localize to the plasma membrane and endomembranes, and vesicle trafficking may contribute to the increase in RBOH abundance at the plasma membrane. The P. syringae pv. tomato virulence effector HopM1 interacts with and degrades an immunity-associated protein AtMIN7 via the host proteasome, suppressing PTI (Table 1; ). AtMIN7 localizes to the trans-Golgi network/early endosome of plant cells and mediates immune-associated vesicle trafficking. Some type III effector proteins from X. campestris pv. vesicatoria (Xcv) target plant protein secretion pathways, suppressing PTI. The Xcv type III effector XopJ, a member of the YopJ family of SUMO peptidases and acetyltransferases, is attached to the plasma membrane of plant cells through a myristoylation motif and interferes with host-cell protein secretion, inhibiting immune-associated callose deposition at the cell wall. In addition, XopB and XopS contribute to Xcv virulence, suppressing PAMP-triggered gene expression. XopB localizes to the Golgi vesicles and cytoplasm and interferes with plant cell protein secretion. However, specific PTI-related targets of these Xcv effectors are unknown. Overall, better understanding of the role of vesicle trafficking in promoting plant immune responses related to ROS signaling requires further experimental evidence.

16Metabolic priming by secreted pathogen effectors has emerged as a common strategy for host manipulation (Figure 3). U. maydis secretes chorismate mutase (Cmu1) as a virulence factor into plant cells, suppressing plant defense responses associated with pathogen-induced salicylic acid biosynthesis. During U. maydis infection in maize, Cmu1 is translocated into plant cells, spreads to neighboring cells, and can change the metabolic status of plant cells through metabolic priming. Pathogen effectors may target plant enzymes required for ROS production in the key metabolic pathways. For instance, the chloroplastic enzyme aspartate oxidase, which is involved in NAD metabolism, is required for the NADPH oxidase RBOHD-mediated ROS burst that is triggered by the perception of several unrelated PAMPs. Notably, inducible NAD overproduction in Arabidopsis transcriptionally up-regulates aspartate oxidase during the incompatible infection with P. syringae pv. tomato (avrRpm1). Intracellular NAD acts as an integral regulator of multiple defense layers to trigger the production of ROS and defense hormones. Manipulation of abscisic acid (ABA) content in Arabidopsis modulates ROS production via the control of peroxidase activity in response to Dickeya dadantii infection. Increased ABA contents seem likely to correlate with reduced ROS production and with enhanced disease susceptibility. The M. oryzae effector AvrPii specifically interacts with rice NADP-malic enzyme2 (OsNADP-ME2) to suppress the host ROS burst (Table 1; ). Indeed, purified AvrPii proteins inhibit in vitro NADP-ME activity. NADP-ME, also known as a coenzyme, catalyzes the oxidative decarboxylation of malate and NADP+, and provides NADPH as an electron donor for plasma membrane-bound NADPH oxidase (Figure 1), which is essential for the apoplastic ROS burst at the infection site. NADP-ME is involved in the production of ROS during early plant basal defense against the hemi-biotrophic fungal pathogen Colletotrichum higginsianum. The X. oryzae pv. oryzicola effector AvrRxo1 targets and phosphorylates the central metabolite and redox carrier NAD in planta, and this catalytic activity is required for suppression of the ROS burst. The X. campestris pv. vesicatoria (Xcv) effector AvrBsT physically interacts with pepper arginine decarboxylase (CaADC1), mediating polyamine metabolism for ROS signaling, cell death, and defense responses in plants (Table 1; ). CaADC1 silencing in pepper plants greatly reduces ROS and nitric oxide (NO) bursts, as well as the cell death response during Xcv infection, suggesting that arginine decarboxylase is required for polyamine and ROS signaling in the HR cell death response.

17Microbial pathogens have evolved efficient strategies to overcome plant innate immunity for the establishment of compatible plant-pathogen interactions. Despite their evolution over a long period of time, pathogen effectors target a common host protein network, suppressing the common immune system of all plants. In particular, adapted pathogens have developed effective weaponry to compete with the host plants and defeat evolving plant immunity. In this arms race, the most frequent target of pathogen attack is a powerful plant weapon system that inflicts immense damage to invading pathogens in a short period of time.

18Effector targeting toward ROS signaling networks in plants is proposed in Figure 3. Adapted pathogens have evolved virulence effectors to inhibit both the generation and accumulation of ROS in the apoplastic space, ultimately leading to the inhibition of the intracellular signaling required for the powerful second ROS burst. Plant pathogens deliver virulence effectors into the apoplastic area and cytoplasm of plant cells. Apoplastic effectors interfere with the perception of PRRs by MAMPs or PAMPs, preventing activation of membrane-bound NADPH oxidase RBOHs. Removal of ROS from the apoplastic area reduces direct toxicity to pathogens and blocks plant cell wall reinforcement, which may be beneficial for the successful intracellular invasion and colonization of microbial pathogens. Cytoplasmic effectors target plant PRRs, the RLCK BIK1, MAPK cascades, and WRKY transcription factors, suppressing the expression of RBOHs and NADP-MEs that are essential for robust ROS generation (Figure 3; ). During pathogen infection, certain cytoplasmic effectors interfere with vesicle trafficking to suppress the transport of ROS-producing RBOH enzymes to the plasma membrane. Secreted pathogen effectors block the metabolic coenzyme NADP-ME, inhibiting the transfer of electrons to NADPH oxidases (RBOHs). However, when specific pathogen effectors interact directly or indirectly with intracellular NLR proteins, these effectors function as avirulence factors to trigger resistance (R) gene-mediated immunity, the so-called ETI. The effector-NLR complex leads to a strong apoplastic ROS burst and HR cell death responses. Apoplastic ROS are toxic to pathogens and activate MAPK cascades and NADPH oxidase (RBOH) enzymes. The first phase of ROS production by PTI following PAMP recognition may eventually have a feedback effect on the second phase of ROS production, resulting in a more powerful ETI (Figure 2;;; ).

19Pathogens have developed virulence effectors to circumvent the newly changed plant immune system during the coevolution of both pathogens and host plants. In host plants, effector-targeted host proteins inside the plant cell can be modified to restrict pathogen invasion. Virulence effectors interact with different target host proteins in the plant cell, which can ultimately result in the suppression of ROS production in the apoplastic area. Once NADP-ME fails to supply NADPH to the NADPH oxidase (RBOH) as a result of its mutation, apoplastic ROS production is inhibited and pathogens can overcome plant immunity. These results suggest that ROS contribute directly to plant immunity. PTI and ETI are closely related and a strong ROS burst is essential for ETI. The metabolic cellular processes related to ROS generation are required to sustain PTI and reinforce the immune response through ETI. The key proteins and/or enzymes involved in ROS production may be supplied by activation of MAPK signaling pathways. The biomembrane channels, aquaporins, mediate H2O2 transport across biological membranes. However, when effectors interfere with MAPK cascades and WRKY transcription factors, the second phase ROS burst and accompanying ETI are severely inhibited.

20Microbial pathogens have evolved versatile effectors to target the cellular processes associated with plant ROS production. Despite recent advances in our knowledge of pathogen effector targets in ROS signaling networks in plants (Table 1), the key host factors directly linking ROS signaling to sites of attempted pathogen invasion are not fully understood. Further elucidation of the molecular and cellular functions of pathogen effectors and host factors underlying the ROS-mediated innate immune system will provide important clues to understand better how versatile effectors have evolved to converge on ROS signaling networks in plants.

21NSJ designed the outline of the manuscript and wrote major part of the story and BKH added his previous bacterial data and expertise to upgrade the whole text and Figures.

22The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.