Transcriptional programming and functional interactions within the Phytophthora sojae RXLR effector repertoire

The Phytophthora sojae Avr1d gene encodes an RxLR-dEER effector with presence and absence polymorphisms among pathogen strains

Soybean root and stem rot is caused by the oomycete pathogen Phytophthora sojae. The interaction between P. sojae and soybean fits the "gene-for-gene" hypothesis. Although more than 10 P. sojae avirulence (Avr) effectors have been genetically identified, nearly half of genetically defined avr genes have been cloned. In a previous bioinformatic and global transcriptional analysis, we identified a P. sojae RxLR effector, Avr1d, which was 125 amino acids in length. Mapping data demonstrated that Avr1d presence or absence in the genome was co-segregated with the Avr1d avirulence phenotype in F2 populations. Transient expression of the Avr1d gene using co-bombardment in soybean isogenic lines revealed that this gene triggered a hypersensitive response (HR) in the presence of Rps1d. Sequencing of Avr1d genes in different P. sojae strains revealed two Avr1d alleles. Although polymorphic, the two Avr1d alleles could trigger Rps1d-mediated HR. P. sojae strains carrying either of the alleles were avirulent on Rps1d soybean lines. Avr1d was upregulated during the germinating cyst and early infection stages. Furthermore, transient expression of Avr1d in Nicotiana benthamiana suppressed BAX-induced cell death and enhanced P. capsici infection. Avr1d also suppressed effector-triggered immunity induction by associating with Avr1b and Rps1b, suggestive of a role in suppressing plant immunity.

Gene duplication and fragment recombination drive functional diversification of a superfamily of cytoplasmic effectors in Phytophthora sojae

Phytophthora and other oomycetes secrete a large number of putative host cytoplasmic effectors with conserved FLAK motifs following signal peptides, termed crinkling and necrosis inducing proteins (CRN), or Crinkler. Here, we first investigated the evolutionary patterns and mechanisms of CRN effectors in Phytophthora sojae and compared them to two other Phytophthora species. The genes encoding CRN effectors could be divided into 45 orthologous gene groups (OGG), and most OGGs unequally distributed in the three species, in which each underwent large number of gene gains or losses, indicating that the CRN genes expanded after species evolution in Phytophthora and evolved through pathoadaptation. The 134 expanded genes in P. sojae encoded family proteins including 82 functional genes and expressed at higher levels while the other 68 genes encoding orphan proteins were less expressed and contained 50 pseudogenes. Furthermore, we demonstrated that most expanded genes underwent gene duplication or/and fragment recombination. Three different mechanisms that drove gene duplication or recombination were identified. Finally, the expanded CRN effectors exhibited varying pathogenic functions, including induction of programmed cell death (PCD) and suppression of PCD through PAMP-triggered immunity or/and effector-triggered immunity. Overall, these results suggest that gene duplication and fragment recombination may be two mechanisms that drive the expansion and neofunctionalization of the CRN family in P. sojae, which aids in understanding the roles of CRN effectors within each oomycete pathogen.

The Phytophthora parasitica RXLR effector penetration-specific effector 1 favours Arabidopsis thaliana infection by interfering with auxin physiology

Pathogenic oomycetes have evolved RXLR effectors to thwart plant defense mechanisms and invade host tissues. We analysed the function of one of these effectors (Penetration-Specific Effector 1 (PSE1)) whose transcript is transiently accumulated during penetration of host roots by the oomycete Phytophthora parasitica. Expression of PSE1 protein in tobacco (Nicotiana tabacum and Nicotiana benthamiana) leaves and in Arabidopsis thaliana plants was used to assess the role of this effector in plant physiology and in interactions with pathogens. A pharmacological approach and marker lines were used to charcterize the A. thaliana phenotypes. Expression of PSE1 in A. thaliana led to developmental perturbations associated with low concentrations of auxin at the root apex. This modification of auxin content was associated with an altered distribution of the PIN4 and PIN7 auxin efflux carriers. The PSE1 protein facilitated plant infection: it suppressed plant cell death activated by Pseudomonas syringae avirulence gene AvrPto and Phytophthora cryptogea elicitin cryptogein in tobacco and exacerbated disease symptoms upon inoculation of transgenic A. thaliana plantlets with P. parasitica in an auxin-dependant manner. We propose that P. parasitica secretes the PSE1 protein during the penetration process to favour the infection by locally modulating the auxin content. These results support the hypothesis that effectors from plant pathogens may act on a limited set of targets, including hormones.

Two RxLR avirulence genes in Phytophthora sojae determine soybean Rps1k-mediated disease resistance

Resistance to Phytophthora sojae (Rps) genes have been widely used in soybean against root and stem rot diseases caused by this oomycete. Among 15 known soybean Rps genes, Rps1k has been the most widely used in the past four decades. Here, we show that the products of two distinct but closely linked RxLR effector genes are detected by Rps1k-containing plants, resulting in disease resistance. One of the genes is Avr1b-1, that confers avirulence in the presence of Rps1b. Three lines of evidence, including overexpression and gene silencing of Avr1b-1 in stable P. sojae transformants, as well as transient expression of this gene in soybean, indicated that Avr1b could trigger an Rps1k-mediated defense response. Some isolates of P. sojae that do not express Avr1b are nevertheless unable to infect Rps1k plants. In those isolates, we identified a second RxLR effector gene (designated Avr1k), located 5 kb away from Avr1b-1. Silencing or overexpression of Avr1k in P. sojae stable transformants resulted in the loss or gain, respectively, of the avirulence phenotype in the presence of Rps1k. Only isolates of P. sojae with mutant alleles of both Avr1b-1 and Avr1k could evade perception by the soybean plants carrying Rps1k.

From pathogen genomes to host plant processes: the power of plant parasitic oomycetes

Recent pathogenomic research on plant parasitic oomycete effector function and plant host responses has resulted in major conceptual advances in plant pathology, which has been possible thanks to the availability of genome sequences.

PsVPS1, a dynamin-related protein, is involved in cyst germination and soybean infection of Phytophthora sojae

Plant pathogens secrete effector proteins to suppress plant immunity. However, the mechanism by which oomycete pathogens deliver effector proteins during plant infection remains unknown. In this report, we characterized a Phytophthora sojae vps1 gene. This gene encodes a homolog of the Saccharomyces cerevisiae vacuolar protein sorting gene vps1 that mediates budding of clathrin-coated vesicles from the late Golgi, which are diverted from the general secretory pathway to the vacuole. PsVPS1-silenced mutants were generated using polyethylene glycol-mediated protoplast stable transformation and were viable but had reduced extracellular protein activity. The PsVPS1-silenced mutants showed impaired hyphal growth, and the shapes of the vacuoles were highly fragmented. Silencing of PsVPS1 affected cyst germination as well as the polarized growth of germinated cysts. Silenced mutants showed impaired invasion of susceptible soybean plants regardless of wounding. These results suggest that PsVPS1 is involved in vacuole morphology and cyst development. Moreover, it is essential for the virulence of P. sojae and extracellular protein secretion.

Genome-wide prediction and functional validation of promoter motifs regulating gene expression in spore and infection stages of Phytophthora infestans

Most eukaryotic pathogens have complex life cycles in which gene expression networks orchestrate the formation of cells specialized for dissemination or host colonization. In the oomycete Phytophthora infestans, the potato late blight pathogen, major shifts in mRNA profiles during developmental transitions were identified using microarrays. We used those data with search algorithms to discover about 100 motifs that are over-represented in promoters of genes up-regulated in hyphae, sporangia, sporangia undergoing zoosporogenesis, swimming zoospores, or germinated cysts forming appressoria (infection structures). Most of the putative stage-specific transcription factor binding sites (TFBSs) thus identified had features typical of TFBSs such as position or orientation bias, palindromy, and conservation in related species. Each of six motifs tested in P. infestans transformants using the GUS reporter gene conferred the expected stage-specific expression pattern, and several were shown to bind nuclear proteins in gel-shift assays. Motifs linked to the appressoria-forming stage, including a functionally validated TFBS, were over-represented in promoters of genes encoding effectors and other pathogenesis-related proteins. To understand how promoter and genome architecture influence expression, we also mapped transcription patterns to the P. infestans genome assembly. Adjacent genes were not typically induced in the same stage, including genes transcribed in opposite directions from small intergenic regions, but co-regulated gene pairs occurred more than expected by random chance. These data help illuminate the processes regulating development and pathogenesis, and will enable future attempts to purify the cognate transcription factors.

The genus Phytophthora includes over one hundred species of plant pathogens that have devastating effects worldwide in agriculture and natural environments. Its most notorious member is P. infestans, which causes the late blight diseases of potato and tomato. Their success as pathogens is dependent on the formation of specialized cells for plant-to-plant transmission and host infection, but little is known about how this is regulated. Recognizing that changes in gene expression drive the formation of these cell types, we used a computational approach to predict the sequences of about one hundred transcription factor binding sites associated with expression in either of five life stages, including several types of spores and infection structures. We then used a functional testing strategy to prove their biological activity by showing that the DNA motifs enabled the stage-specific expression of a transgene. Our work lays the groundwork for dissecting the molecular mechanisms that regulate life-stage transitions and pathogenesis in Phytophthora. A similar approach should be useful for other plant and animal pathogens.

Phytopathogen effectors subverting host immunity: different foes, similar battleground

Phytopathogenic bacteria, fungi, and oomycetes invade and colonize their host plants through distinct routes. These pathogens secrete diverse groups of effector proteins that aid infection and establishment of different parasitic lifestyles. Despite this diversity, a comparison of different plant-pathogen systems has revealed remarkable similarities in the host immune pathways targeted by effectors from distinct pathogen groups. Immune signaling pathways mediated by pattern recognition receptors, phytohormone homeostasis or signaling, defenses associated with host secretory pathways and pathogen penetrations, and plant cell death represent some of the key processes controlling disease resistance against diverse pathogens. These immune pathways are targeted by effectors that carry a wide range of biochemical functions and are secreted by completely different pathogen groups, suggesting that these pathways are a common battleground encountered by many plant pathogens.

Structural basis for interactions of the Phytophthora sojae RxLR effector Avh5 with phosphatidylinositol 3-phosphate and for host cell entry

Oomycetes such as Phytophthora sojae employ effector proteins that enter plant cells to facilitate infection. Entry of some effector proteins is mediated by RxLR motifs in the effectors and phosphoinositides (PIP) resident in the host plasma membrane such as phosphatidylinositol 3-phosphate (PtdIns(3)P). Recent reports differ regarding the regions on RxLR effectors involved in PIP recognition. We have structurally and functionally characterized the P. sojae effector, avirulence homolog-5 (Avh5). Using nuclear magnetic resonance (NMR) spectroscopy, we demonstrate that Avh5 is helical in nature, with a long N-terminal disordered region. NMR titrations of Avh5 with the PtdIns(3)P head group, inositol 1,3-bisphosphate, directly identified the ligand-binding residues. A C-terminal lysine-rich helical region (helix 2) was the principal lipid-binding site, with the N-terminal RxLR (RFLR) motif playing a more minor role. Mutations in the RFLR motif affected PtdIns(3)P binding, while mutations in the basic helix almost abolished it. Mutations in the RFLR motif or in the basic region both significantly reduced protein entry into plant and human cells. Both regions independently mediated cell entry via a PtdIns(3)P-dependent mechanism. Based on these findings, we propose a model where Avh5 interacts with PtdIns(3)P through its C terminus, and by binding of the RFLR motif, which promotes host cell entry.

Homologous RXLR effectors from Hyaloperonospora arabidopsidis and Phytophthora sojae suppress immunity in distantly related plants

Diverse pathogens secrete effector proteins into plant cells to manipulate host cellular processes. Oomycete pathogens contain large complements of predicted effector genes defined by an RXLR host cell entry motif. The genome of Hyaloperonospora arabidopsidis (Hpa, downy mildew of Arabidopsis) contains at least 134 candidate RXLR effector genes. Only a small subset of these genes is conserved in related oomycetes from the Phytophthora genus. Here, we describe a comparative functional characterization of the Hpa RXLR effector gene HaRxL96 and a homologous gene, PsAvh163, from the Glycine max (soybean) pathogen Phytophthora sojae. HaRxL96 and PsAvh163 are induced during the early stages of infection and carry a functional RXLR motif that is sufficient for protein uptake into plant cells. Both effectors can suppress immune responses in soybean. HaRxL96 suppresses immunity in Nicotiana benthamiana, whereas PsAvh163 induces an HR-like cell death response in Nicotiana that is dependent on RAR1 and Hsp90.1. Transgenic Arabidopsis plants expressing HaRxL96 or PsAvh163 exhibit elevated susceptibility to virulent and avirulent Hpa, as well as decreased callose deposition in response to non-pathogenic Pseudomonas syringae. Both effectors interfere with defense marker gene induction, but do not affect salicylic acid biosynthesis. Together, these experiments demonstrate that evolutionarily conserved effectors from different oomycete species can suppress immunity in plant species that are divergent from the source pathogen's host.

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.

Oomycetes, effectors, and all that jazz

Plant pathogenic oomycetes secrete a diverse repertoire of effector proteins that modulate host innate immunity and enable parasitic infection. Understanding how effectors evolve, translocate and traffic inside host cells, and perturb host processes are major themes in the study of oomycete-plant interactions. The last year has seen important progress in the study of oomycete effectors with, notably, the elucidation of the 3D structures of five RXLR effectors, and novel insights into how cytoplasmic effectors subvert host cells. In this review, we discuss these and other recent advances and highlight the most important open questions in oomycete effector biology.

The NLP toxin family in Phytophthora sojae includes rapidly evolving groups that lack necrosis-inducing activity

Necrosis- and ethylene-inducing-like proteins (NLP) are widely distributed in eukaryotic and prokaryotic plant pathogens and are considered to be important virulence factors. We identified, in total, 70 potential Phytophthora sojae NLP genes but 37 were designated as pseudogenes. Sequence alignment of the remaining 33 NLP delineated six groups. Three of these groups include proteins with an intact heptapeptide (Gly-His-Arg-His-Asp-Trp-Glu) motif, which is important for necrosis-inducing activity, whereas the motif is not conserved in the other groups. In total, 19 representative NLP genes were assessed for necrosis-inducing activity by heterologous expression in Nicotiana benthamiana. Surprisingly, only eight genes triggered cell death. The expression of the NLP genes in P. sojae was examined, distinguishing 20 expressed and 13 nonexpressed NLP genes. Real-time reverse-transcriptase polymerase chain reaction results indicate that most NLP are highly expressed during cyst germination and infection stages. Amino acid substitution ratios (Ka/Ks) of 33 NLP sequences from four different P. sojae strains resulted in identification of positive selection sites in a distinct NLP group. Overall, our study indicates that expansion and pseudogenization of the P. sojae NLP family results from an ongoing birth-and-death process, and that varying patterns of expression, necrosis-inducing activity, and positive selection suggest that NLP have diversified in function.

Arabidopsis nonhost resistance gene PSS1 confers immunity against an oomycete and a fungal pathogen but not a bacterial pathogen that cause diseases in soybean

BACKGROUND

Nonhost resistance (NHR) provides immunity to all members of a plant species against all isolates of a microorganism that is pathogenic to other plant species. Three Arabidopsis thaliana PEN (penetration deficient) genes, PEN1, 2 and 3 have been shown to provide NHR against the barley pathogen Blumeria graminis f. sp. hordei at the prehaustorial level. Arabidopsis pen1-1 mutant lacking the PEN1 gene is penetrated by the hemibiotrophic oomycete pathogen Phytophthora sojae, the causal organism of the root and stem rot disease in soybean. We investigated if there is any novel nonhost resistance mechanism in Arabidopsis against the soybean pathogen, P. sojae.

RESULTS

The P.sojaesusceptible (pss) 1 mutant was identified by screening a mutant population created in the Arabidopsis pen1-1 mutant that lacks penetration resistance against the non adapted barley biotrophic fungal pathogen, Blumeria graminis f. sp. hordei. Segregation data suggested that PEN1 is not epistatic to PSS1. Responses of pss1 and pen1-1 to P. sojae invasion were distinct and suggest that PSS1 may act at both pre- and post-haustorial levels, while PEN1 acts at the pre-haustorial level against this soybean pathogen. Therefore, PSS1 encodes a new form of nonhost resistance. The pss1 mutant is also infected by the necrotrophic fungal pathogen, Fusarium virguliforme, which causes sudden death syndrome in soybean. Thus, a common NHR mechanism is operative in Arabidopsis against both hemibiotrophic oomycetes and necrotrophic fungal pathogens that are pathogenic to soybean. However, PSS1 does not play any role in immunity against the bacterial pathogen, Pseudomonas syringae pv. glycinea, that causes bacterial blight in soybean. We mapped PSS1 to a region very close to the southern telomere of chromosome 3 that carries no known disease resistance genes.

CONCLUSIONS

The study revealed that Arabidopsis PSS1 is a novel nonhost resistance gene that confers a new form of nonhost resistance against both a hemibiotrophic oomycete pathogen, P. sojae and a necrotrophic fungal pathogen, F. virguliforme that cause diseases in soybean. However, this gene does not play any role in the immunity of Arabidopsis to the bacterial pathogen, P. syringae pv. glycinea, which causes bacterial blight in soybean. Identification and further characterization of the PSS1 gene would provide further insights into a new form of nonhost resistance in Arabidopsis, which could be utilized in improving resistance of soybean to two serious pathogens.

TaMCA4, a novel wheat metacaspase gene functions in programmed cell death induced by the fungal pathogen Puccinia striiformis f. sp. tritici

Programmed cell death (PCD) is a physiological process to remove redundant or harmful cells, for the development of multicellular organisms, or for restricting the spread of pathogens (hypersensitive response). Metacaspases are cysteine-dependent proteases which play an essential role in PCD. Triticum aestivum metacaspase 4 (TaMCA4) is a type II metacaspase gene cloned from 'Suwon11' wheat, with typical structural features such as peptidase C14 caspase domain and a long linker sequence between the two subunits. Transient expression of TaMCA4 in tobacco leaves failed to induce PCD directly but enhanced cell death triggered by a mouse Bax gene or a candidate effector gene from Puccinia striiformis f. sp. tritici. Enhancement of PCD was also observed in wheat leaves co-bombarded with TaMCA4. When challenged with the avirulent race of P. striiformis f. sp. tritici, the expression level of TaMCA4 in wheat leaves was sharply upregulated, whereas the transcript level was not significantly induced by the virulent race. Moreover, knocking down TaMCA4 expression by virus-induced gene silencing enhanced the susceptibility of Suwon11 to the avirulent race of P. striiformis f. sp. tritici and reduced the necrotic area at infection sites.

Cell death of Nicotiana benthamiana is induced by secreted protein NIS1 of Colletotrichum orbiculare and is suppressed by a homologue of CgDN3

Colletotrichum orbiculare, the causal agent of cucumber anthracnose, infects Nicotiana benthamiana. Functional screening of C. orbiculare cDNAs in a virus vector-based plant expression system identified a novel secreted protein gene, NIS1, whose product induces cell death in N. benthamiana. Putative homologues of NIS1 are present in selected members of fungi belonging to class Sordariomycetes, Dothideomycetes, or Orbiliomycetes. Green fluorescent protein-based expression studies suggested that NIS1 is preferentially expressed in biotrophic invasive hyphae. NIS1 lacking signal peptide did not induce NIS1-triggered cell death (NCD), suggesting apoplastic recognition of NIS1. NCD was prevented by virus-induced gene silencing of SGT1 and HSP90, indicating the dependency of NCD on SGT1 and HSP90. Deletion of NIS1 had little effect on the virulence of C. orbiculare against N. benthamiana, suggesting possible suppression of NCD by C. orbiculare at the postinvasive stage. The CgDN3 gene of C. gloeosporioides was previously identified as a secreted protein gene involved in suppression of hypersensitive-like response in Stylosanthes guianensis. Notably, we found that NCD was suppressed by the expression of a CgDN3 homologue of C. orbiculare. Our findings indicate that C. orbiculare expresses NIS1 at the postinvasive stage and suggest that NCD could be repressed via other effectors, including the CgDN3 homologue.

Sequential delivery of host-induced virulence effectors by appressoria and intracellular hyphae of the phytopathogen Colletotrichum higginsianum

Phytopathogens secrete effector proteins to manipulate their hosts for effective colonization. Hemibiotrophic fungi must maintain host viability during initial biotrophic growth and elicit host death for subsequent necrotrophic growth. To identify effectors mediating these opposing processes, we deeply sequenced the transcriptome of Colletotrichum higginsianum infecting Arabidopsis. Most effector genes are host-induced and expressed in consecutive waves associated with pathogenic transitions, indicating distinct effector suites are deployed at each stage. Using fluorescent protein tagging and transmission electron microscopy-immunogold labelling, we found effectors localised to stage-specific compartments at the host-pathogen interface. In particular, we show effectors are focally secreted from appressorial penetration pores before host invasion, revealing new levels of functional complexity for this fungal organ. Furthermore, we demonstrate that antagonistic effectors either induce or suppress plant cell death. Based on these results we conclude that hemibiotrophy in Colletotrichum is orchestrated through the coordinated expression of antagonistic effectors supporting either cell viability or cell death.

Oomycete and fungal effector entry, a microbial Trojan horse

Oomycete and fungal symbionts have significant impacts on most commercially important crop and forest species, and on natural ecosystems, both negatively as pathogens and positively as mutualists. Symbiosis may be facilitated through the secretion of effector proteins, some of which modulate a variety of host defense mechanisms. A subset of these secreted proteins are able to translocate into host cells. In the oomycete pathogens, two conserved N-terminal motifs, RXLR and dEER, mediate translocation of effector proteins into host cells independent of any pathogen-encoded machinery. An expanded 'RXLR-like' motif [R/K/H]X[L/M/I/F/Y/W]X has been used to identify functional translocation motifs in host-cell-entering fungal effector proteins from pathogens and a mutualist. The RXLR-like translocation motifs were required for the fungal effectors to enter host cells in the absence of any pathogen-encoded machinery. Oomycete and fungal effectors with RXLR and RXLR-like motifs can bind phospholipids, specifically phosphatidylinositol-3-phosphate (PtdIns-3-P). Effector-PtdIns-3-P binding appears to mediate cell entry via lipid raft-mediated endocytosis, and could be blocked by sequestering cell surface PtdIns-3-P or by utilizing inositides that competitively inhibit effector binding to PtdIns-3-P. These findings suggest that effector blocking technologies could be developed and utilized in a variety of important crop species against a broad spectrum of plant pathogens.

Mechanisms and evolution of virulence in oomycetes

Many destructive diseases of plants and animals are caused by oomycetes, a group of eukaryotic pathogens important to agricultural, ornamental, and natural ecosystems. Understanding the mechanisms underlying oomycete virulence and the genomic processes by which those mechanisms rapidly evolve is essential to developing effective long-term control measures for oomycete diseases. Several common mechanisms underlying oomycete virulence, including protein toxins and cell-entering effectors, have emerged from comparing oomycetes with different genome characteristics, parasitic lifestyles, and host ranges. Oomycete genomes display a strongly bipartite organization in which conserved housekeeping genes are concentrated in syntenic gene-rich blocks, whereas virulence genes are dispersed into highly dynamic, repeat-rich regions. There is also evidence that key virulence genes have been acquired by horizontal transfer from other eukaryotic and prokaryotic species.

Rust secreted protein Ps87 is conserved in diverse fungal pathogens and contains a RXLR-like motif sufficient for translocation into plant cells

BACKGROUND

Effector proteins of biotrophic plant pathogenic fungi and oomycetes are delivered into host cells and play important roles in both disease development and disease resistance response. How obligate fungal pathogen effectors enter host cells is poorly understood. The Ps87 gene of Puccinia striiformis encodes a protein that is conserved in diverse fungal pathogens. Ps87 homologs from a clade containing rust fungi are predicted to be secreted. The aim of this study is to test whether Ps87 may act as an effector during Puccinia striiformis infection.

METHODOLOGY/PRINCIPAL FINDINGS

Yeast signal sequence trap assay showed that the rust protein Ps87 could be secreted from yeast cells, but a homolog from Magnaporthe oryzae that was not predicted to be secreted, could not. Cell re-entry and protein uptake assays showed that a region of Ps87 containing a conserved RXLR-like motif [K/R]RLTG was confirmed to be capable of delivering oomycete effector Avr1b into soybean leaf cells and carrying GFP into soybean root cells. Mutations in the Ps87 motif (KRLTG) abolished the protein translocation ability.

CONCLUSIONS/SIGNIFICANCE

The results suggest that Ps87 and its secreted homologs could utilize similar protein translocation machinery as those of oomycete and other fungal pathogens. Ps87 did not show direct suppression activity on plant defense responses. These results suggest Ps87 may represent an "emerging effector" that has recently acquired the ability to enter plant cells but has not yet acquired the ability to alter host physiology.

Tyler B, Wang Y. Phytophthora sojae avirulence effector Avr3b is a secreted NADH and ADP-ribose pyrophosphorylase that modulates plant immunity

Plants have evolved pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) to protect themselves from infection by diverse pathogens. Avirulence (Avr) effectors that trigger plant ETI as a result of recognition by plant resistance (R) gene products have been identified in many plant pathogenic oomycetes and fungi. However, the virulence functions of oomycete and fungal Avr effectors remain largely unknown. Here, we combined bioinformatics and genetics to identify Avr3b, a new Avr gene from Phytophthora sojae, an oomycete pathogen that causes soybean root rot. Avr3b encodes a secreted protein with the RXLR host-targeting motif and C-terminal W and Nudix hydrolase motifs. Some isolates of P. sojae evade perception by the soybean R gene Rps3b through sequence mutation in Avr3b and lowered transcript accumulation. Transient expression of Avr3b in Nicotiana benthamiana increased susceptibility to P. capsici and P. parasitica, with significantly reduced accumulation of reactive oxygen species (ROS) around invasion sites. Biochemical assays confirmed that Avr3b is an ADP-ribose/NADH pyrophosphorylase, as predicted from the Nudix motif. Deletion of the Nudix motif of Avr3b abolished enzyme activity. Mutation of key residues in Nudix motif significantly impaired Avr3b virulence function but not the avirulence activity. Some Nudix hydrolases act as negative regulators of plant immunity, and thus Avr3b might be delivered into host cells as a Nudix hydrolase to impair host immunity. Avr3b homologues are present in several sequenced Phytophthora genomes, suggesting that Phytophthora pathogens might share similar strategies to suppress plant immunity.

Entry of oomycete and fungal effectors into plant and animal host cells

Fungal and oomycete pathogens cause many destructive diseases of plants and important diseases of humans and other animals. Fungal and oomycete plant pathogens secrete numerous effector proteins that can enter inside host cells to condition susceptibility. Until recently it has been unknown if these effectors enter via pathogen-encoded translocons or via pathogen-independent mechanisms. Here we review recent evidence that many fungal and oomycete effectors enter via receptor-mediated endocytosis, and can do so in the absence of the pathogen. Surprisingly, a large number of these effectors utilize cell surface phosphatidyinositol-3-phosphate (PI-3-P) as a receptor, a molecule previously known only inside cells. Binding of effectors to PI-3-P appears to be mediated by the cell entry motif RXLR in oomycetes, and by diverse RXLR-like variants in fungi. PI-3-P appears to be present on the surface of animal cells also, suggesting that it may mediate entry of effectors of fungal and oomycete animal pathogens, for example, RXLR effectors found in the oomycete fish pathogen, Saprolegnia parasitica. Reagents that can block PI-3-P-mediated entry have been identified, suggesting new therapeutic strategies.