RxLR Effectors: Master Modulators, Modifiers and Manipulators

Cytoplasmic effectors with an Arg-any amino acid-Arg-Leu (RxLR) motif are encoded by hundreds of genes within the genomes of oomycete Phytophthora spp. and downy mildew pathogens. There has been a dramatic increase in our understanding of the evolution, function, and recognition of these effectors. Host proteins with a wide range of subcellular localizations and functions are targeted by RxLR effectors. Many processes are manipulated, including transcription, post-translational modifications, such as phosphorylation and ubiquitination, secretion, and intracellular trafficking. This involves an array of RxLR effector modes-of-action, including stabilization or destabilization of protein targets, altering or disrupting protein complexes, inhibition or utility of target enzyme activities, and changing the location of protein targets. Interestingly, approximately 50% of identified host proteins targeted by RxLR effectors are negative regulators of immunity. Avirulence RxLR effectors may be directly or indirectly detected by nucleotide-binding leucine-rich repeat resistance (NLR) proteins. Direct recognition by a single NLR of RxLR effector orthologues conserved across multiple Phytophthora pathogens may provide wide protection of diverse crops. Failure of RxLR effectors to interact with or appropriately manipulate target proteins in nonhost plants has been shown to restrict host range. This knowledge can potentially be exploited to alter host targets to prevent effector interaction, providing a barrier to host infection. Finally, recent evidence suggests that RxLR effectors, like cytoplasmic effectors from fungal pathogen Magnaporthe oryzae, may enter host cells via clathrin-mediated endocytosis. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Filamentous pathogen effectors enter plant cells via endocytosis

Recent findings demonstrate that cytoplasmic effectors from fungal and oomycete pathogens enter plant cells via clathrin-mediated endocytosis (CME). This raises several questions: Does effector secretion pathway facilitate host uptake? How is CME triggered in host cells? How are the effectors released from endosomal compartments to reach diverse subcellular destinations?

How to convert host plants into nonhosts

Recent research demonstrates that undermining interactions between pathogen effectors and their host target proteins can reduce infection. As more effector-target pairs are identified, their structures and interaction surfaces exposed, and there is the possibility of making multiple edits to diverse plant genomes, the desire to convert crops to nonhosts could become reality.

Uptake of oomycete RXLR effectors into host cells by clathrin-mediated endocytosis

Filamentous (oomycete and fungal) plant pathogens deliver cytoplasmic effector proteins into host cells to facilitate disease. How RXLR effectors from the potato late blight pathogen Phytophthora infestans enter host cells is unknown. One possible route involves clathrin-mediated endocytosis (CME). Transient silencing of NbCHC, encoding clathrin heavy chain, or the endosome marker gene NbAra6 encoding a Rab GTPase in the model host Nicotiana benthamiana, attenuated P. infestans infection and reduced translocation of RXLR effector fusions from transgenic pathogen strains into host cells. By contrast, silencing PP1c isoforms, susceptibility factors not required for endocytosis, reduced infection but did not attenuate RXLR effector uptake. Endosome enrichment by ultracentrifugation and sucrose gradient fractionation revealed co-localization of RXLR effector Pi04314-RFP with clathrin-coated vesicles. Immunopurification of clathrin- and NbAra6-associated vesicles during infection showed that RXLR effectors Pi04314-RFP and AvrBlb1-RFP, but not apoplastic effector PiSCR74-RFP, were co-immunoprecipitated during infection with pathogen strains secreting these effectors. Tandem mass spectrometry analyses of proteins co-immunoprecipitated with NbAra6-GFP during infection revealed enrichment of host proteins associated with endocytic vesicles alongside multiple pathogen RXLR effectors, but not apoplastic effectors, including PiSCR74, which do not enter host cells. Our data show that uptake of P. infestans RXLR effectors into plant cells occurs via CME.

A Phytophthora effector promotes homodimerization of host transcription factor StKNOX3 to enhance susceptibility

Oomycete pathogens secrete hundreds of cytoplasmic RxLR effectors to modulate host immunity by targeting diverse plant proteins. Revealing how effectors manipulate host proteins is pivotal to understanding infection processes and to developing new strategies to control plant disease. Here we show that the Phytophthora infestans RxLR effector Pi22798 interacts in the nucleus with a potato class II knotted-like homeobox (KNOX) transcription factor, StKNOX3. Silencing the ortholog NbKNOX3 in Nicotiana benthamiana reduces host colonization by P. infestans, whereas transient and stable overexpression of StKNOX3 enhances infection. StKNOX3 forms a homodimer which is dependent on its KNOX II domain. The KNOX II domain is also essential for Pi22798 interaction and for StKNOX3 to enhance P. infestans colonization, indicating that StKNOX3 homodimerization contributes to susceptibility. However, critically, the effector Pi22798 promotes StKNOX3 homodimerization, rather than heterodimerization to another KNOX transcription factor StKNOX7. These results demonstrate that the oomycete effector Pi22798 increases pathogenicity by promoting homodimerization specifically of StKNOX3 to enhance susceptibility.

Haustorium formation and a distinct biotrophic transcriptome characterize infection of Nicotiana benthamiana by the tree pathogen Phytophthora kernoviae

Phytophthora species cause some of the most serious diseases of trees and threaten forests in many parts of the world. Despite the generation of genome sequence assemblies for over 10 tree-pathogenic Phytophthora species and improved detection methods, there are many gaps in our knowledge of how these pathogens interact with their hosts. To facilitate cell biology studies of the infection cycle we examined whether the tree pathogen Phytophthora kernoviae could infect the model plant Nicotiana benthamiana. We transformed P. kernoviae to express green fluorescent protein (GFP) and demonstrated that it forms haustoria within infected N. benthamiana cells. Haustoria were also formed in infected cells of natural hosts, Rhododendron ponticum and European beech (Fagus sylvatica). We analysed the transcriptome of P. kernoviae in cultured mycelia, spores, and during infection of N. benthamiana, and detected 12,559 transcripts. Of these, 1,052 were predicted to encode secreted proteins, some of which may function as effectors to facilitate disease development. From these, we identified 87 expressed candidate RXLR (Arg-any amino acid-Leu-Arg) effectors. We transiently expressed 12 of these as GFP fusions in N. benthamiana leaves and demonstrated that nine significantly enhanced P. kernoviae disease progression and diversely localized to the cytoplasm, nucleus, nucleolus, and plasma membrane. Our results show that N. benthamiana can be used as a model host plant for studying this tree pathogen, and that the interaction likely involves suppression of host immune responses by RXLR effectors. These results establish a platform to expand the understanding of Phytophthora tree diseases.

A Conserved Oomycete CRN Effector Targets Tomato TCP14-2 to Enhance Virulence

Phytophthora spp. secrete vast arrays of effector molecules during infection to aid in host colonization. The crinkling and necrosis (CRN) protein family forms an extensive repertoire of candidate effectors that accumulate in the host nucleus to perturb processes required for immunity. Here, we show that CRN12_997 from Phytophthora capsici binds a TCP transcription factor, SlTCP14-2, to inhibit its immunity-associated activity against Phytophthora spp. Coimmunoprecipitation and bimolecular fluorescence complementation studies confirm a specific CRN12_997-SlTCP14-2 interaction in vivo. Coexpression of CRN12_997 specifically counteracts the TCP14-enhanced immunity phenotype, suggesting that CRN mediated perturbation of SlTCP14-2 function. We show that SlTCP14-2 associates with nuclear chromatin and that CRN12_997 diminishes SlTCP14-2 DNA binding. Collectively, our data support a model in which SlTCP14-2 associates with chromatin to enhance immunity. The interaction between CRN12_997 and SlTCP14-2 reduces DNA binding of the immune regulator. We propose that the modulation of SlTCP14-2 chromatin affinity, caused by CRN12-997, enhances susceptibility to P. capsici.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A Conserved Oomycete CRN Effector Targets Tomato TCP14-2 to Enhance Virulence

Phytophthora spp. secrete vast arrays of effector molecules during infection to aid in host colonization. The crinkling and necrosis (CRN) protein family forms an extensive repertoire of candidate effectors that accumulate in the host nucleus to perturb processes required for immunity. Here, we show that CRN12_997 from Phytophthora capsici binds a TCP transcription factor, SlTCP14-2, to inhibit its immunity-associated activity against Phytophthora spp. Coimmunoprecipitation and bimolecular fluorescence complementation studies confirm a specific CRN12_997-SlTCP14-2 interaction in vivo. Coexpression of CRN12_997 specifically counteracts the TCP14-enhanced immunity phenotype, suggesting that CRN mediated perturbation of SlTCP14-2 function. We show that SlTCP14-2 associates with nuclear chromatin and that CRN12_997 diminishes SlTCP14-2 DNA binding. Collectively, our data support a model in which SlTCP14-2 associates with chromatin to enhance immunity. The interaction between CRN12_997 and SlTCP14-2 reduces DNA binding of the immune regulator. We propose that the modulation of SlTCP14-2 chromatin affinity, caused by CRN12-997, enhances susceptibility to P. capsici.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Devastating intimacy: the cell biology of plant-Phytophthora interactions

An understanding of the cell biology underlying the burgeoning molecular genetic and genomic knowledge of oomycete pathogenicity is essential to gain the full context of how these pathogens cause disease on plants. An intense research focus on secreted Phytophthora effector proteins, especially those containing a conserved N-terminal RXLR motif, has meant that most cell biological studies into Phytophthora diseases have focussed on the effectors and their host target proteins. While these effector studies have provided novel insights into effector secretion and host defence mechanisms, there remain many unanswered questions about fundamental processes involved in spore biology, host penetration and haustorium formation and function.

The Ubiquitin E3 Ligase PUB17 Positively Regulates Immunity by Targeting a Negative Regulator, KH17, for Degradation

Ubiquitination is a post-translational modification that regulates many processes in plants. Several ubiquitin E3 ligases act as either positive or negative regulators of immunity by promoting the degradation of different substrates. StPUB17 is an E3 ligase that has previously been shown to positively regulate immunity to bacteria, fungi and oomycetes, including the late blight pathogen Phytophthora infestans. Silencing of StPUB17 promotes pathogen colonization and attenuates Cf4/avr4 cell death. Using yeast-2-hybrid and co-immunoprecipitation we identified the putative K-homology (KH) RNA-binding protein (RBP), StKH17, as a candidate substrate for degradation by StPUB17. StKH17 acts as a negative regulator of immunity that promotes P. infestans infection and suppresses specific immune pathways. A KH RBP domain mutant of StKH17 (StKH17GDDG) is no longer able to negatively regulate immunity, indicating that RNA binding is likely required for StKH17 function. As StPUB17 is a known target of the ubiquitin E3 ligase, StPOB1, we reveal an additional step in an E3 ligase regulatory cascade that controls plant defense.

All Roads Lead to Susceptibility: The Many Modes of Action of Fungal and Oomycete Intracellular Effectors

The ability to secrete effector proteins that can enter plant cells and manipulate host processes is a key determinant of what makes a successful plant pathogen. Here, we review intracellular effectors from filamentous (fungal and oomycete) phytopathogens and the host proteins and processes that are targeted to promote disease. We cover contrasting virulence strategies and effector modes of action. Filamentous pathogen effectors alter the fates of host proteins that they target, changing their stability, their activity, their location, and the protein partners with which they interact. Some effectors inhibit target activity, whereas others enhance or utilize it, and some target multiple host proteins. We discuss the emerging topic of effectors that target negative regulators of immunity or other plant proteins with activities that support susceptibility. We also highlight the commonly targeted host proteins that are manipulated by effectors from multiple pathogens, including those representing different kingdoms of life.

Phytophthora infestans effector SFI3 targets potato UBK to suppress early immune transcriptional responses

The potato blight agent Phytophthora infestans secretes a range of RXLR effectors to promote disease. Recent evidence indicates that some effectors suppress early pattern-triggered immunity (PTI) following perception of microbe-associated molecular patterns (MAMPs). Phytophthora infestans effector PiSFI3/Pi06087/PexRD16 has been previously shown to suppress MAMP-triggered pFRK1-Luciferase reporter gene activity. How PiSFI3 suppresses immunity is unknown. We employed yeast-two-hybrid (Y2H) assays, co-immunoprecipitation, transcriptional silencing by RNA interference and virus-induced gene silencing (VIGS), and X-ray crystallography for structure-guided mutagenesis, to investigate the function of PiSFI3 in targeting a plant U-box-kinase protein (StUBK) to suppress immunity. We discovered that PiSFI3 is active in the host nucleus and interacts in yeast and in planta with StUBK. UBK is a positive regulator of specific PTI pathways in both potato and Nicotiana benthamiana. Importantly, it contributes to early transcriptional responses that are suppressed by PiSFI3. PiSFI3 forms an unusual trans-homodimer. Mutation to disrupt dimerization prevents nucleolar localisation of PiSFI3 and attenuates both its interaction with StUBK and its ability to enhance P. infestans leaf colonisation. PiSFI3 is a 'WY-domain' RXLR effector that forms a novel trans-homodimer which is required for its ability to suppress PTI via interaction with the U-box-kinase protein StUBK.

Phytophthora infestans RXLR effectors act in concert at diverse subcellular locations to enhance host colonization

Oomycetes such as the potato blight pathogen Phytophthora infestans deliver RXLR effectors into plant cells to manipulate host processes and promote disease. Knowledge of where they localize inside host cells is important in understanding their function. Fifty-two P. infestans RXLR effectors (PiRXLRs) up-regulated during early stages of infection were expressed as fluorescent protein (FP) fusions inside cells of the model host Nicotiana benthamiana. FP-PiRXLR fusions were predominantly nucleo-cytoplasmic, nuclear, or plasma membrane-associated. Some also localized to the endoplasmic reticulum, mitochondria, peroxisomes, or microtubules, suggesting diverse sites of subcellular activity. Seven of the 25 PiRXLRs examined during infection accumulated at sites of haustorium penetration, probably due to co-localization with host target processes; Pi16663 (Avr1), for example, localized to Sec5-associated mobile bodies which showed perihaustorial accumulation. Forty-five FP-RXLR fusions enhanced pathogen leaf colonization when expressed in Nicotiana benthamiana, revealing that their presence was beneficial to infection. Co-expression of PiRXLRs that target and suppress different immune pathways resulted in an additive enhancement of colonization, indicating the potential to study effector combinations using transient expression assays. We provide a broad platform of high confidence P. infestans effector candidates from which to investigate the mechanisms, singly and in combination, by which this pathogen causes disease.

Phytophthora infestans RXLR effector SFI5 requires association with calmodulin for PTI/MTI suppressing activity

Pathogens secrete effector proteins to interfere with plant innate immunity, in which Ca2+ /calmodulin (CaM) signalling plays key roles. Thus far, few effectors have been identified that directly interact with CaM for defence suppression. Here, we report that SFI5, an RXLR effector from Phytophthora infestans, suppresses microbe-associated molecular pattern (MAMP)-triggered immunity (MTI) by interacting with host CaMs. We predicted the CaM-binding site in SFI5 using in silico analysis. The interaction between SFI5 and CaM was tested by both in vitro and in vivo assays. MTI suppression by SFI5 and truncated variants were performed in a tomato protoplast system. We found that both the predicted CaM-binding site and the full-length SFI5 protein interact with CaM in the presence of Ca2+ . MTI responses, such as FRK1 upregulation, reactive oxygen species accumulation, and mitogen-activated protein kinase activation were suppressed by truncated SFI5 proteins containing the C-terminal CaM-binding site but not by those without it. The plasma membrane localization of SFI5 and its ability to enhance infection were also perturbed by loss of the CaM-binding site. We conclude that CaM-binding is required for localization and activity of SFI5. We propose that SFI5 suppresses plant immunity by interfering with immune signalling components after activation by CaMs.

Plant pathogen effector utilizes host susceptibility factor NRL1 to degrade the immune regulator SWAP70

Plant pathogens deliver effectors into plant cells to suppress immunity. Whereas many effectors inactivate positive immune regulators, other effectors associate with negative regulators of immunity: so-called susceptibility (S) factors. Little is known about how pathogens exploit S factors to suppress immunity. Phytophthora infestans RXLR effector Pi02860 interacts with host protein NRL1, which is an S factor whose activity suppresses INF1-triggered cell death (ICD) and is required for late blight disease. We show that NRL1 interacts in yeast and in planta with a guanine nucleotide exchange factor called SWAP70. SWAP70 associates with endosomes and is a positive regulator of immunity. Virus-induced gene silencing of SWAP70 in Nicotiana benthamiana enhances P. infestans colonization and compromises ICD. In contrast, transient overexpression of SWAP70 reduces P. infestans infection and accelerates ICD. Expression of Pi02860 and NRL1, singly or in combination, results in proteasome-mediated degradation of SWAP70. Degradation of SWAP70 is prevented by silencing NRL1, or by mutation of Pi02860 to abolish its interaction with NRL1. NRL1 is a BTB-domain protein predicted to form the substrate adaptor component of a CULLIN3 ubiquitin E3 ligase. A dimerization-deficient mutant, NRL1NQ, fails to interact with SWAP70 but maintains its interaction with Pi02860. NRL1NQ acts as a dominant-negative mutant, preventing SWAP70 degradation in the presence of effector Pi02860, and reducing P. infestans infection. Critically, Pi02860 enhances the association between NRL1 and SWAP70 to promote proteasome-mediated degradation of the latter and, thus, suppress immunity. Preventing degradation of SWAP70 represents a strategy to combat late blight disease.

The Potato MAP3K StVIK Is Required for the Phytophthora infestans RXLR Effector Pi17316 to Promote Disease

Plant pathogens deliver effectors to manipulate processes in their hosts, creating a suitable environment for invasion and proliferation. Yet, little is known about the host proteins that are targeted by effectors from filamentous pathogens. Here, we show that stable transgenic expression in potato (Solanum tuberosum) and transient expression in Nicotiana benthamiana of the arginine-any amino acid-leucine-arginine effector Pi17316 enhances leaf colonization by the late blight pathogen Phytophthora infestans Expression of Pi17316 also attenuates cell death triggered by the pathogen-associated molecular pattern Infestin1 (INF1), indicating that the effector suppresses pattern-triggered immunity. However, this effector does not attenuate cell death triggered by a range of resistance proteins, showing that it specifically suppresses INF1-triggered cell death (ICD). In yeast two-hybrid assays, Pi17316 interacts directly with the potato ortholog of VASCULAR HIGHWAY1-interacting kinase (StVIK), encoding a predicted MEK kinase (MAP3K). Interaction in planta was confirmed by coimmunoprecipitation and occurs at the plant plasma membrane. Virus-induced gene silencing of VIK in N. benthamiana attenuated P. infestans colonization, whereas transient overexpression of StVIK enhanced colonization, indicating that this host protein acts as a susceptibility factor. Moreover, VIK overexpression specifically attenuated ICD, indicating that it is a negative regulator of immunity. The abilities of Pi17316 to enhance P. infestans colonization or suppress ICD were compromised significantly in NbVIK-silenced plants, demonstrating that the effector activity of Pi17316 is mediated by this MAP3K. Thus, StVIK is exploited by P. infestans as a susceptibility factor to promote late blight disease.

Exchanging missives and missiles: the roles of extracellular vesicles in plant-pathogen interactions

This article comments on: Regente M, Pinedo M, San Clemente H, Balliau T, Jamet E, de la Canal L. 2017. Plant extracellular vesicles are incorporated by a fungal pathogen and inhibit its growth. Journal of Experimental Botany 68, 5485–5495.

Delivery of cytoplasmic and apoplastic effectors from Phytophthora infestans haustoria by distinct secretion pathways

The potato blight pathogen Phytophthora infestans secretes effector proteins that are delivered inside (cytoplasmic) or can act outside (apoplastic) plant cells to neutralize host immunity. Little is known about how and where effectors are secreted during infection, yet such knowledge is essential to understand and combat crop disease. We used transient Agrobacterium tumefaciens-mediated in planta expression, transformation of P. infestans with fluorescent protein fusions and confocal microscopy to investigate delivery of effectors to plant cells during infection. The cytoplasmic effector Pi04314, expressed as a monomeric red fluorescent protein (mRFP) fusion protein with a signal peptide to secrete it from plant cells, did not passively re-enter the cells upon secretion. However, Pi04314-mRFP expressed in P. infestans was translocated from haustoria, which form intimate interactions with plant cells, to accumulate at its sites of action in the host nucleus. The well-characterized apoplastic effector EPIC1, a cysteine protease inhibitor, was also secreted from haustoria. EPIC1 secretion was inhibited by brefeldin A (BFA), demonstrating that it is delivered by conventional Golgi-mediated secretion. By contrast, Pi04314 secretion was insensitive to BFA treatment, indicating that the cytoplasmic effector follows an alternative route for delivery into plant cells. Phytophthora infestans haustoria are thus sites for delivery of both apoplastic and cytoplasmic effectors during infection, following distinct secretion pathways.

RXLR Effector AVR2 Up-Regulates a Brassinosteroid-Responsive bHLH Transcription Factor to Suppress Immunity

An emerging area in plant research focuses on antagonism between regulatory systems governing growth and immunity. Such cross talk represents a point of vulnerability for pathogens to exploit. AVR2, an RXLR effector secreted by the potato blight pathogen Phytophthora infestans, interacts with potato BSL1, a putative phosphatase implicated in growth-promoting brassinosteroid (BR) hormone signaling. Transgenic potato (Solanum tuberosum) plants expressing the effector exhibit transcriptional and phenotypic hallmarks of overactive BR signaling and show enhanced susceptibility to P. infestans Microarray analysis was used to identify a set of BR-responsive marker genes in potato, all of which are constitutively expressed to BR-induced levels in AVR2 transgenic lines. One of these genes was a bHLH transcription factor, designated StCHL1, homologous to AtCIB1 and AtHBI1, which are known to facilitate antagonism between BR and immune responses. Transient expression of either AVR2 or CHL1 enhanced leaf colonization by P. infestans and compromised immune cell death activated by perception of the elicitin Infestin1 (INF1). Knockdown of CHL1 transcript using Virus-Induced Gene Silencing (VIGS) reduced colonization of P. infestans on Nicotiana benthamiana Moreover, the ability of AVR2 to suppress INF1-triggered cell death was attenuated in NbCHL1-silenced plants, indicating that NbCHL1 was important for this effector activity. Thus, AVR2 exploits cross talk between BR signaling and innate immunity in Solanum species, representing a novel, indirect mode of innate immune suppression by a filamentous pathogen effector.

BTB-BACK Domain Protein POB1 Suppresses Immune Cell Death by Targeting Ubiquitin E3 ligase PUB17 for Degradation

Hypersensitive response programmed cell death (HR-PCD) is a critical feature in plant immunity required for pathogen restriction and prevention of disease development. The precise control of this process is paramount to cell survival and an effective immune response. The discovery of new components that function to suppress HR-PCD will be instrumental in understanding the regulation of this fundamental mechanism. Here we report the identification and characterisation of a BTB domain E3 ligase protein, POB1, that functions to suppress HR-PCD triggered by evolutionarily diverse pathogens. Nicotiana benthamiana and tobacco plants with reduced POB1 activity show accelerated HR-PCD whilst those with increased POB1 levels show attenuated HR-PCD. We demonstrate that POB1 dimerization and nuclear localization are vital for its function in HR-PCD suppression. Using protein-protein interaction assays, we identify the Plant U-Box E3 ligase PUB17, a well established positive regulator of plant innate immunity, as a target for POB1-mediated proteasomal degradation. Using confocal imaging and in planta immunoprecipitation assays we show that POB1 interacts with PUB17 in the nucleus and stimulates its degradation. Mutated versions of POB1 that show reduced interaction with PUB17 fail to suppress HR-PCD, indicating that POB1-mediated degradation of PUB17 U-box E3 ligase is an important step for negative regulation of specific immune pathways in plants. Our data reveals a new mechanism for BTB domain proteins in suppressing HR-PCD in plant innate immune responses.

The cell biology of late blight disease

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The Phytophthora haustorium is a major site of secretion during infection.

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The host endocytic cycle contributes to biogenesis of the Extra-Haustorial Membrane.

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RXLR effectors manipulate host processes at diverse subcellular locations.

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They directly manipulate the activity or location of immune proteins.

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They also promote the activity of endogenous negative regulators of immunity.

Late blight, caused by the oomycete Phytophthora infestans, is a major global disease of potato and tomato. Cell biology is teaching us much about the developmental stages associated with infection, especially the haustorium, which is a site of intimate interaction and molecular exchange between pathogen and host. Recent observations suggest a role for the plant endocytic cycle in specific recruitment of host proteins to the Extra-Haustorial Membrane, emphasising the unique nature of this membrane compartment. In addition, there has been a strong focus on the activities of RXLR effectors, which are delivered into plant cells to modulate and manipulate host processes. RXLR effectors interact directly with diverse plant proteins at a range of subcellular locations to promote disease.

Potato NPH3/RPT2-Like Protein StNRL1, Targeted by a Phytophthora infestans RXLR Effector, Is a Susceptibility Factor

Plant pathogens deliver effectors to manipulate host processes. We know little about how fungal and oomycete effectors target host proteins to promote susceptibility, yet such knowledge is vital to understand crop disease. We show that either transient expression in Nicotiana benthamiana, or stable transgenic expression in potato (Solanum tuberosum), of the Phytophthora infestans RXLR effector Pi02860 enhances leaf colonization by the pathogen. Expression of Pi02860 also attenuates cell death triggered by the P. infestans microbe-associated molecular pattern INF1, indicating that the effector suppresses pattern-triggered immunity. However, the effector does not attenuate cell death triggered by Cf4/Avr4 coexpression, showing that it does not suppress all cell death activated by cell surface receptors. Pi02860 interacts in yeast two-hybrid assays with potato NPH3/RPT2-LIKE1 (NRL1), a predicted CULLIN3-associated ubiquitin E3 ligase. Interaction of Pi02860 in planta was confirmed by coimmunoprecipitation and bimolecular fluorescence complementation assays. Virus-induced gene silencing of NRL1 in N. benthamiana resulted in reduced P. infestans colonization and accelerated INF1-mediated cell death, indicating that this host protein acts as a negative regulator of immunity. Moreover, whereas NRL1 virus-induced gene silencing had no effect on the ability of the P. infestans effector Avr3a to suppress INF1-mediated cell death, such suppression by Pi02860 was significantly attenuated, indicating that this activity of Pi02860 is mediated by NRL1. Transient overexpression of NRL1 resulted in the suppression of INF1-mediated cell death and enhanced P. infestans leaf colonization, demonstrating that NRL1 acts as a susceptibility factor to promote late blight disease.

A Phytophthora infestans RXLR effector targets plant PP1c isoforms that promote late blight disease

Plant pathogens deliver effectors to alter host processes. Knowledge of how effectors target and manipulate host proteins is critical to understand crop disease. Here, we show that in planta expression of the RXLR effector Pi04314 enhances leaf colonization by Phytophthora infestans via activity in the host nucleus and attenuates induction of jasmonic and salicylic acid-responsive genes. Pi04314 interacts with three host protein phosphatase 1 catalytic (PP1c) isoforms, causing their re-localization from the nucleolus to the nucleoplasm. Re-localization of PP1c-1 also occurs during infection and is dependent on an R/KVxF motif in the effector. Silencing the PP1c isoforms or overexpression of a phosphatase-dead PP1c-1 mutant attenuates infection, demonstrating that host PP1c activity is required for disease. Moreover, expression of PP1c-1mut abolishes enhanced leaf colonization mediated by in planta Pi04314 expression. We argue that PP1c isoforms are susceptibility factors forming holoenzymes with Pi04314 to promote late blight disease.

U-box E3 ubiquitin ligase PUB17 acts in the nucleus to promote specific immune pathways triggered by Phytophthora infestans

Ubiquitination regulates many processes in plants, including immunity. The E3 ubiquitin ligase PUB17 is a positive regulator of programmed cell death (PCD) triggered by resistance proteins CF4/9 in tomato. Its role in immunity to the potato late blight pathogen, Phytophthora infestans, was investigated here. Silencing StPUB17 in potato by RNAi and NbPUB17 in Nicotiana benthamiana by virus-induced gene silencing (VIGS) each enhanced P. infestans leaf colonization. PAMP-triggered immunity (PTI) transcriptional responses activated by flg22, and CF4/Avr4-mediated PCD were attenuated by silencing PUB17. However, silencing PUB17 did not compromise PCD triggered by P. infestans PAMP INF1, or co-expression of R3a/AVR3a, demonstrating that not all PTI- and PCD-associated responses require PUB17. PUB17 localizes to the plant nucleus and especially in the nucleolus. Transient over-expression of a dominant-negative StPUB17(V314I,V316I) mutant, which retained nucleolar localization, suppressed CF4-mediated cell death and enhanced P. infestans colonization. Exclusion of the StPUB17(V314I,V316I) mutant from the nucleus abolished its dominant-negative activity, demonstrating that StPUB17 functions in the nucleus. PUB17 is a positive regulator of immunity to late blight that acts in the nucleus to promote specific PTI and PCD pathways.

Detection of the virulent form of AVR3a from Phytophthora infestans following artificial evolution of potato resistance gene R3a

Engineering resistance genes to gain effector recognition is emerging as an important step in attaining broad, durable resistance. We engineered potato resistance gene R3a to gain recognition of the virulent AVR3a^EM effector form of Phytophthora infestans. Random mutagenesis, gene shuffling and site-directed mutagenesis of R3a were conducted to produce R3a* variants with gain of recognition towards AVR3a^EM. Programmed cell death following gain of recognition was enhanced in iterative rounds of artificial evolution and neared levels observed for recognition of AVR3a^KI by R3a. We demonstrated that R3a*-mediated recognition responses, like for R3a, are dependent on SGT1 and HSP90. In addition, this gain of response is associated with re-localisation of R3a* variants from the cytoplasm to late endosomes when co-expressed with either AVR3a^KI or AVR3a^EM a mechanism that was previously only seen for R3a upon co-infiltration with AVR3a^KI. Similarly, AVR3a^EM specifically re-localised to the same vesicles upon recognition by R3a* variants, but not with R3a. R3a and R3a* provide resistance to P. infestans isolates expressing AVR3a^KI but not those homozygous for AVR3a^EM.

Phytophthora infestans RXLR effector PexRD2 interacts with host MAPKKK ε to suppress plant immune signaling

Mitogen-activated protein kinase cascades are key players in plant immune signaling pathways, transducing the perception of invading pathogens into effective defense responses. Plant pathogenic oomycetes, such as the Irish potato famine pathogen Phytophthora infestans, deliver RXLR effector proteins to plant cells to modulate host immune signaling and promote colonization. Our understanding of the molecular mechanisms by which these effectors act in plant cells is limited. Here, we report that the P. infestans RXLR effector PexRD2 interacts with the kinase domain of MAPKKKε, a positive regulator of cell death associated with plant immunity. Expression of PexRD2 or silencing MAPKKKε in Nicotiana benthamiana enhances susceptibility to P. infestans. We show that PexRD2 perturbs signaling pathways triggered by or dependent on MAPKKKε. By contrast, homologs of PexRD2 from P. infestans had reduced or no interaction with MAPKKKε and did not promote disease susceptibility. Structure-led mutagenesis identified PexRD2 variants that do not interact with MAPKKKε and fail to support enhanced pathogen growth or perturb MAPKKKε signaling pathways. Our findings provide evidence that P. infestans RXLR effector PexRD2 has evolved to interact with a specific host MAPKKK to perturb plant immunity-related signaling.

An RxLR effector from Phytophthora infestans prevents re-localisation of two plant NAC transcription factors from the endoplasmic reticulum to the nucleus

The potato late blight pathogen Phytophthora infestans secretes an array of effector proteins thought to act in its hosts by disarming defences and promoting pathogen colonisation. However, little is known about the host targets of these effectors and how they are manipulated by the pathogen. This work describes the identification of two putative membrane-associated NAC transcription factors (TF) as the host targets of the RxLR effector PITG_03192 (Pi03192). The effector interacts with NAC Targeted by Phytophthora (NTP) 1 and NTP2 at the endoplasmic reticulum (ER) membrane, where these proteins are localised. Transcripts of NTP1 and NTP2 rapidly accumulate following treatment with culture filtrate (CF) from in vitro grown P. infestans, which acts as a mixture of Phytophthora PAMPs and elicitors, but significantly decrease during P. infestans infection, indicating that pathogen activity may prevent their up-regulation. Silencing of NTP1 or NTP2 in the model host plant Nicotiana benthamiana increases susceptibility to P. infestans, whereas silencing of Pi03192 in P. infestans reduces pathogenicity. Transient expression of Pi03192 in planta restores pathogenicity of the Pi03192-silenced line. Moreover, colonisation by the Pi03192-silenced line is significantly enhanced on N. benthamiana plants in which either NTP1 or NTP2 have been silenced. StNTP1 and StNTP2 proteins are released from the ER membrane following treatment with P. infestans CF and accumulate in the nucleus, after which they are rapidly turned over by the 26S proteasome. In contrast, treatment with the defined PAMP flg22 fails to up-regulate NTP1 and NTP2, or promote re-localisation of their protein products to the nucleus, indicating that these events follow perception of a component of CF that appears to be independent of the FLS2/flg22 pathway. Importantly, Pi03192 prevents CF-triggered re-localisation of StNTP1 and StNTP2 from the ER into the nucleus, revealing a novel effector mode-of-action to promote disease progression.

Identification and Characterisation CRN Effectors in Phytophthora capsici Shows Modularity and Functional Diversity

Phytophthora species secrete a large array of effectors during infection of their host plants. The Crinkler (CRN) gene family encodes a ubiquitous but understudied class of effectors with possible but as of yet unknown roles in infection. To appreciate CRN effector function in Phytophthora, we devised a simple Crn gene identification and annotation pipeline to improve effector prediction rates. We predicted 84 full-length CRN coding genes and assessed CRN effector domain diversity in sequenced Oomycete genomes. These analyses revealed evidence of CRN domain innovation in Phytophthora and expansion in the Peronosporales. We performed gene expression analyses to validate and define two classes of CRN effectors, each possibly contributing to infection at different stages. CRN localisation studies revealed that P. capsici CRN effector domains target the nucleus and accumulate in specific sub-nuclear compartments. Phenotypic analyses showed that few CRN domains induce necrosis when expressed in planta and that one cell death inducing effector, enhances P. capsici virulence on Nicotiana benthamiana. These results suggest that the CRN protein family form an important class of intracellular effectors that target the host nucleus during infection. These results combined with domain expansion in hemi-biotrophic and necrotrophic pathogens, suggests specific contributions to pathogen lifestyles. This work will bolster CRN identification efforts in other sequenced oomycete species and set the stage for future functional studies towards understanding CRN effector functions.

Relocalization of late blight resistance protein R3a to endosomal compartments is associated with effector recognition and required for the immune response

An important objective of plant-pathogen interactions research is to determine where resistance proteins detect pathogen effectors to mount an immune response. Many nucleotide binding-Leucine-rich repeat (NB-LRR) resistance proteins accumulate in the plant nucleus following effector recognition, where they initiate the hypersensitive response (HR). Here, we show that potato (Solanum tuberosum) resistance protein R3a relocates from the cytoplasm to endosomal compartments only when coexpressed with recognized Phytophthora infestans effector form AVR3a(KI) and not unrecognized form AVR3a(EM). Moreover, AVR3a(KI), but not AVR3a(EM), is also relocalized to endosomes in the presence of R3a. Both R3a and AVR3a(KI) colocalized in close physical proximity at endosomes in planta. Treatment with brefeldin A (BFA) or wortmannin, inhibitors of the endocytic cycle, attenuated both the relocalization of R3a to endosomes and the R3a-mediated HR. No such effect of these inhibitors was observed on HRs triggered by the gene-for-gene pairs Rx1/PVX-CP and Sto1/IpiO1. An R3a(D501V) autoactive MHD mutant, which triggered HR in the absence of AVR3a(KI), failed to localize to endosomes. Moreover, BFA and wortmannin did not alter cell death triggered by this mutant. We conclude that effector recognition and consequent HR signaling by NB-LRR resistance protein R3a require its relocalization to vesicles in the endocytic pathway.

Presence/absence, differential expression and sequence polymorphisms between PiAVR2 and PiAVR2-like in Phytophthora infestans determine virulence on R2 plants

• A detailed molecular understanding of how oomycete plant pathogens evade disease resistance is essential to inform the deployment of durable resistance (R) genes. • Map-based cloning, transient expression in planta, pathogen transformation and DNA sequence variation across diverse isolates were used to identify and characterize PiAVR2 from potato late blight pathogen Phytophthora infestans. • PiAVR2 is an RXLR-EER effector that is up-regulated during infection, accumulates at the site of haustoria formation, and is recognized inside host cells by potato protein R2. Expression of PiAVR2 in a virulent P. infestans isolate conveys a gain-of-avirulence phenotype, indicating that this is a dominant gene triggering R2-dependent disease resistance. PiAVR2 presence/absence polymorphisms and differential transcription explain virulence on R2 plants. Isolates infecting R2 plants express PiAVR2-like, which evades recognition by R2. PiAVR2 and PiAVR2-like differ in 13 amino acids, eight of which are in the C-terminal effector domain; one or more of these determines recognition by R2. Nevertheless, few polymorphisms were observed within each gene in pathogen isolates, suggesting limited selection pressure for change within PiAVR2 and PiAVR2-like. • Our results direct a search for R genes recognizing PiAVR2-like, which, deployed with R2, may exert strong selection pressure against the P. infestans population.

Imaging fluorescently tagged Phytophthora effector proteins inside infected plant tissue

Assays to determine the role of pathogen effectors within an infected plant cell are yielding valuable information about which host processes are targeted to allow successful pathogen colonization. However, this does not necessarily inform on the cellular location of these interactions, or if these effector-virulence target interactions occur only in the presence of the pathogen. Here, we describe techniques to allow the subcellular localization of pathogen effectors inside infected plant cells or tissues, based largely on infiltration of plant tissue by Agrobacterium tumefaciens and its delivery of DNA encoding fluorescent protein-tagged effectors, and subsequent confocal microscopy.

A novel Phytophthora infestans haustorium-specific membrane protein is required for infection of potato

Phytophthora infestans causes late-blight, a devastating and re-emerging disease of potato crops. During the early stages of infection, P. infestans differentiates infection-specific structures such as appressoria for host epidermal cell penetration, followed by infection vesicles, and haustoria to establish a biotrophic phase of interaction. Here we report the cloning, from a suppression subtractive hybridization library, of a P. infestans gene called Pihmp1 encoding a putative glycosylated protein with four closely spaced trans-membrane helices. Pihmp1 expression is upregulated in germinating cysts and in germinating cysts with appressoria, and significantly upregulated throughout infection of potato. Transient gene silencing of Pihmp1 led to loss of pathogenicity and indicated involvement of this gene in the penetration and early infection processes of P. infestans. P. infestans transformants expressing a Pihmp1::monomeric red fluorescent protein (mRFP) fusion demonstrated that Pihmp1 was translated in germinating sporangia, germinating cysts and appressoria, accumulated in the appressorium, and was located at the haustorial membrane during infection. Furthermore, we discovered that haustorial structures are formed over a 3 h period, maturing for up to 12 h, and that their formation is initiated only at sites on the surface of intercellular hyphae where Pihmp1::mRFP is localized. We propose that Pihmp1 is an integral membrane protein that provides physical stability to the plasma membrane of P. infestans infection structures. We have provided the first evidence that the surface of oomycete haustoria possess proteins specific to these biotrophic structures, and that formation of biotrophic structures (infection vesicles and haustoria) is essential to successful host colonization by P. infestans.

Oomycete RXLR effectors: delivery, functional redundancy and durable disease resistance

To manipulate host defences, plant pathogenic oomycetes secrete and translocate RXLR effectors into plant cells. Recent reports have indicated that RXLR effectors are translocated from the extrahaustorial matrix during the biotrophic phase of infection and that they are able to suppress PAMP-triggered immunity. Oomycete genomes contain potentially hundreds of highly diverse RXLR effector genes, providing the potential for considerable functional redundancy and the consequent ability to readily shed effectors that are recognised by plant surveillance systems without compromising pathogenic fitness. Understanding how these effectors are translocated, their precise roles in virulence, and the extent to which functional redundancy exists in oomycete RXLR effector complements, are major challenges for the coming years.

A translocation signal for delivery of oomycete effector proteins into host plant cells

Bacterial, oomycete and fungal plant pathogens establish disease by translocation of effector proteins into host cells, where they may directly manipulate host innate immunity. In bacteria, translocation is through the type III secretion system, but analogous processes for effector delivery are uncharacterized in fungi and oomycetes. Here we report functional analyses of two motifs, RXLR and EER, present in translocated oomycete effectors. We use the Phytophthora infestans RXLR-EER-containing protein Avr3a as a reporter for translocation because it triggers RXLR-EER-independent hypersensitive cell death following recognition within plant cells that contain the R3a resistance protein. We show that Avr3a, with or without RXLR-EER motifs, is secreted from P. infestans biotrophic structures called haustoria, demonstrating that these motifs are not required for targeting to haustoria or for secretion. However, following replacement of Avr3a RXLR-EER motifs with alanine residues, singly or in combination, or with residues KMIK-DDK--representing a change that conserves physicochemical properties of the protein--P. infestans fails to deliver Avr3a or an Avr3a-GUS fusion protein into plant cells, demonstrating that these motifs are required for translocation. We show that RXLR-EER-encoding genes are transcriptionally upregulated during infection. Bioinformatic analysis identifies 425 potential genes encoding secreted RXLR-EER class proteins in the P. infestans genome. Identification of this class of proteins provides unparalleled opportunities to determine how oomycetes manipulate hosts to establish infection.

Involvement of cathepsin B in the plant disease resistance hypersensitive response

A diverse range of plant proteases are implicated in pathogen perception and in subsequent signalling and execution of disease resistance. We demonstrate, using protease inhibitors and virus-induced gene silencing (VIGS), that the plant papain cysteine protease cathepsin B is required for the disease resistance hypersensitive response (HR). VIGS of cathepsin B prevented programmed cell death (PCD) and compromised disease resistance induced by two distinct non-host bacterial pathogens. It also suppressed the HR triggered by transient co-expression of potato R3a and Phytophthora infestans Avr3a genes. However, VIGS of cathepsin B did not compromise HR following recognition of Cladosporium fulvum AVR4 by tomato Cf-4, indicating that plant PCD can be independent of cathepsin B. The non-host HR to Erwinia amylovora was accompanied by a transient increase in cathepsin B transcript level and enzymatic activity and induction of the HR marker gene Hsr203. VIGS of cathepsin B significantly reduced the induction of Hsr203 following E. amylovora challenge, further demonstrating a role for this protease in PCD. Whereas cathepsin B is often relocalized from the lysosome to the cytosol during animal PCD, plant cathepsin B is secreted into the apoplast, and is activated upon secretion in the absence of pathogen challenge.