Lipid binding activities of flax rust AvrM and AvrL567 effectors

The Phytophthora sojae effector PsFYVE1 modulates immunity-related gene expression by targeting host RZ-1A protein

Oomycete pathogens secrete numerous effectors to manipulate plant immunity and promote infection. However, relatively few effector types have been well characterized. In this study, members of an FYVE domain-containing protein family that are highly expanded in oomycetes were systematically identified, and one secreted protein, PsFYVE1, was selected for further study. PsFYVE1 enhanced Phytophthora capsici infection in Nicotiana benthamiana and was necessary for Phytophthora sojae virulence. The FYVE domain of PsFYVE1 had PI3P-binding activity that depended on four conserved amino acid residues. Furthermore, PsFYVE1 targeted RNA-binding proteins RZ-1A/1B/1C in N. benthamiana and soybean (Glycine max), and silencing of NbRZ-1A/1B/1C genes attenuated plant immunity. NbRZ-1A was associated with the spliceosome complex that included three important components, glycine-rich RNA-binding protein 7 (NbGRP7), glycine-rich RNA-binding protein 8 (NbGRP8), and a specific component of the U1 small nuclear ribonucleoprotein complex (NbU1-70K). Notably, PsFYVE1 disrupted NbRZ-1A-NbGRP7 interaction. RNA-seq and subsequent experimental analysis demonstrated that PsFYVE1 and NbRZ-1A not only modulated pre-mRNA alternative splicing (AS) of the necrotic spotted lesions 1 (NbNSL1) gene, but also co-regulated transcription of hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferase (NbHCT), ethylene insensitive 2 (NbEIN2), and sucrose synthase 4 (NbSUS4) genes, which participate in plant immunity. Collectively, these findings indicate that the FYVE domain-containing protein family includes potential uncharacterized effector types and also highlight that plant pathogen effectors can regulate plant immunity-related genes at both AS and transcription levels to promote disease.

Unrelated Fungal Rust Candidate Effectors Act on Overlapping Plant Functions

Rust fungi cause epidemics that threaten the production of important plant species, such as wheat and soy. Melampsora larici-populina (Mlp) causes the poplar rust and encodes at least 1184 candidate effectors (CEs) whose functions are poorly known. In this study, we sequenced the transcriptome and used mass spectrometry to analyze the metabolome of Arabidopsis plants constitutively expressing 14 Mlp CEs and of a control line to discover alterations leading to plant susceptibility. We found 2299 deregulated genes across the experiment. Genes involved in pattern-triggered immunity, such as FRK1, PR1, RBOHD, and WRKY33, as well as AUX/IAA genes were down-regulated. We further observed that 680 metabolites were deregulated in at least one CE-expressing transgenic line, with “highly unsaturated and phenolic compounds” and “peptides” enriched among down- and up-regulated metabolites. Interestingly, transgenic lines expressing unrelated CEs had correlated patterns of gene and metabolite deregulation, while expression of CEs belonging to the same family deregulated different genes and metabolites. Thus, our results uncouple effector sequence similarity and function. This supports that effector functional investigation in the context of their virulence activity and effect on plant susceptibility requires the investigation of the individual effector and precludes generalization based on sequence similarity.

Effector Biology of Biotrophic Plant Fungal Pathogens: Current Advances and Future Prospects

The interaction of fungal pathogens with their host requires a novel invading mechanism and the presence of various virulence-associated components responsible for promoting the infection. The small secretory proteins, explicitly known as effector proteins, are one of the prime mechanisms of host manipulation utilized by the pathogen to disarm the host. Several effector proteins are known to translocate from fungus to the plant cell for host manipulation. Many fungal effectors have been identified using genomic, transcriptomic, and bioinformatics approaches. Most of the effector proteins are devoid of any conserved signatures, and their prediction based on sequence homology is very challenging, therefore by combining the sequence consensus based upon machine learning features, multiple tools have also been developed for predicting apoplastic and cytoplasmic effectors. Various post-genomics approaches like transcriptomics of virulent isolates have also been utilized for identifying active consortia of effectors. Significant progress has been made in understanding biotrophic effectors; however, most of it is underway due to their complex interaction with host and complicated recognition and signaling networks. This review discusses advances, and challenges in effector identification and highlighted various features of the potential effector proteins and approaches for understanding their genetics and strategies for regulation.

Tethering of Multi-Vesicular Bodies and the Tonoplast to the Plasma Membrane in Plants

Tethering of the plasma membrane (PM) and many organelles to the endoplasmic reticulum (ER) for communication and lipid exchange has been widely reported. However, despite growing interest in multi-vesicular bodies (MVBs) as possible sources of exosomes, tethering of MVBs to the PM has not been reported. Here we show that MVBs and the vacuolar membrane (tonoplast) could be tethered to the PM (PM-MVB/TP tethering) by artificial protein fusions or bimolecular fluorescence complementation (BiFC) complexes that contain a peripheral membrane protein that binds the PM and also a protein that binds MVBs or the tonoplast. PM-binding proteins capable of participating in PM-MVB/TP tethering included StRem1.3, BIK1, PBS1, CPK21, and the PtdIns(4)-binding proteins FAPP1 and Osh2. MVB/TP-binding proteins capable of participating in tethering included ARA6, ARA7, RHA1, RABG3f, and the PtdIns(3)P-binding proteins Vam7p and Hrs-2xFYVE. BiFC complexes or protein fusions capable of producing PM-MVB/TP tethering were visualized as large well-defined patches of fluorescence on the PM that could displace PM proteins such as AtFlotillin1 and also could displace cytoplasmic proteins such as soluble GFP. Furthermore, we identified paralogous ubiquitin E3 ligase proteins, SAUL1 (AtPUB44), and AtPUB43 that could produce PM-MVB/TP tethering. SAUL1 and AtPUB43 could produce tethering in uninfected tissue when paired with MVB-binding proteins or when their E3 ligase domain was deleted. When Nicotiana benthamiana leaf tissue was infected with Phytophthora capsici, full length SAUL1 and AtPUB43 localized in membrane patches consistent with PM-MVB/TP tethering. Our findings define new tools for studying PM-MVB/TP tethering and its possible role in plant defense.

SIGNIFICANCE STATEMENT

Although not previously observed, the tethering of multi-vesicular bodies to the plasma membrane is of interest due to the potential role of this process in producing exosomes in plants. Here we describe tools for observing and manipulating the tethering of multi-vesicular bodies and the tonoplast to the plant plasma membrane, and describe two plant proteins that may naturally regulate this process during infection.

Phytophthora parasitica Effector PpRxLR2 Suppresses Nicotiana benthamiana Immunity

Phytophthora species secrete several classes of effector proteins during interaction with their hosts. These proteins can have multiple functions including modulation of host physiology and immunity. The RxLR effectors have the ability to enter plant cells using the plant machinery. Some of these effectors have been characterized as immunity suppressors; however, very little is known about their functions in the interaction between Phytophthora parasitica and its hosts. Using a bioinformatics pipeline, we have identified 172 candidate RxLR effectors (CREs) in the isolate IAC 01_95 of P. parasitica. Of these 172 CREs, 93 were found to be also present in eight other genomes of P. parasitica, isolated from different hosts and continents. After transcriptomics and gene expression analysis, we have found five CREs to be up-regulated in in-vitro and in-planta samples. Subsequently, we selected three CREs for functional characterization in the model plant Nicotiana benthamiana. We show that PpRxLR2 is able to completely suppress INF-1-induced cell death, whereas PpRxLR3 and PpRxLR5 moderately suppressed N. benthamiana immunity in a less-extensive manner. Moreover, we confirmed the effector-triggered susceptibility activity of these proteins after transient transformation and infection of N. benthamiana plants. All three CREs enhanced virulence of P. parasitica during the interaction with N. benthamiana. These effectors, in particular PpRxLR2, can be targeted for the development of biotechnology-based control strategies of P. parasitica diseases.

The RxLR Motif of the Host Targeting Effector AVR3a of Phytophthora infestans Is Cleaved before Secretion

When plant-pathogenic oomycetes infect their hosts, they employ a large arsenal of effector proteins to establish a successful infection. Some effector proteins are secreted and are destined to be translocated and function inside host cells. The largest group of translocated proteins from oomycetes is the RxLR effectors, defined by their conserved N-terminal Arg-Xaa-Leu-Arg (RxLR) motif. However, the precise role of this motif in the host cell translocation process is unclear. Here, detailed biochemical studies of the RxLR effector AVR3a from the potato pathogen Phytophthora infestans are presented. Mass spectrometric analysis revealed that the RxLR sequence of native AVR3a is cleaved off prior to secretion by the pathogen and the N terminus of the mature effector was found likely to be acetylated. High-resolution NMR structure analysis of AVR3a indicates that the RxLR motif is well accessible to potential processing enzymes. Processing and modification of AVR3a is to some extent similar to events occurring with the export element (PEXEL) found in malaria effector proteins from Plasmodium falciparum These findings imply a role for the RxLR motif in the secretion of AVR3a by the pathogen, rather than a direct role in the host cell entry process itself.

Transformation of the cucurbit powdery mildew pathogen Podosphaera xanthii by Agrobacterium tumefaciens

The obligate biotrophic fungal pathogen Podosphaera xanthii is the main causal agent of powdery mildew in cucurbit crops all over the world. A major limitation of molecular studies of powdery mildew fungi (Erysiphales) is their genetic intractability. In this work, we describe a robust method based on the promiscuous transformation ability of Agrobacterium tumefaciens for reliable transformation of P. xanthii. The A. tumefaciens-mediated transformation (ATMT) system yielded transformants of P. xanthii with diverse transferred DNA (T-DNA) constructs. Analysis of the resultant transformants showed the random integration of T-DNA into the P. xanthii genome. The integrations were maintained in successive generations in the presence of selection pressure. Transformation was found to be transient, because in the absence of selection agent, the introduced genetic markers were lost due to excision of T-DNA from the genome. The ATMT system represents a potent tool for genetic manipulation of P. xanthii and will likely be useful for studying other biotrophic fungi. We hope that this method will contribute to the development of detailed molecular studies of the intimate interaction established between powdery mildew fungi and their host plants.

Divergent Transcriptional Responses to Physiological and Xenobiotic Stress in Giardia duodenalis

Understanding how parasites respond to stress can help to identify essential biological processes. Giardia duodenalis is a parasitic protist that infects the human gastrointestinal tract and causes 200 to 300 million cases of diarrhea annually. Metronidazole, a major antigiardial drug, is thought to cause oxidative damage within the infective trophozoite form. However, treatment efficacy is suboptimal, due partly to metronidazole-resistant infections. To elucidate conserved and stress-specific responses, we calibrated sublethal metronidazole, hydrogen peroxide, and thermal stresses to exert approximately equal pressure on trophozoite growth and compared transcriptional responses after 24 h of exposure. We identified 252 genes that were differentially transcribed in response to all three stressors, including glycolytic and DNA repair enzymes, a mitogen-activated protein (MAP) kinase, high-cysteine membrane proteins, flavin adenine dinucleotide (FAD) synthetase, and histone modification enzymes. Transcriptional responses appeared to diverge according to physiological or xenobiotic stress. Downregulation of the antioxidant system and α-giardins was observed only under metronidazole-induced stress, whereas upregulation of GARP-like transcription factors and their subordinate genes was observed in response to hydrogen peroxide and thermal stressors. Limited evidence was found in support of stress-specific response elements upstream of differentially transcribed genes; however, antisense derepression and differential regulation of RNA interference machinery suggest multiple epigenetic mechanisms of transcriptional control.

Export of malaria proteins requires co-translational processing of the PEXEL motif independent of phosphatidylinositol-3-phosphate binding

Plasmodium falciparum exports proteins into erythrocytes using the Plasmodium export element (PEXEL) motif, which is cleaved in the endoplasmic reticulum (ER) by plasmepsin V (PMV). A recent study reported that phosphatidylinositol-3-phosphate (PI(3)P) concentrated in the ER binds to PEXEL motifs and is required for export independent of PMV, and that PEXEL motifs are functionally interchangeable with RxLR motifs of oomycete effectors. Here we show that the PEXEL does not bind PI(3)P, and that this lipid is not concentrated in the ER. We find that RxLR motifs cannot mediate export in P. falciparum. Parasites expressing a mutated version of KAHRP, with the PEXEL motif repositioned near the signal sequence, prevented PMV cleavage. This mutant possessed the putative PI(3)P-binding residues but is not exported. Reinstatement of PEXEL to its original location restores processing by PMV and export. These results challenge the PI(3)P hypothesis and provide evidence that PEXEL position is conserved for co-translational processing and export.

Crystal structure of the effector AvrLm4-7 of Leptosphaeria maculans reveals insights into its translocation into plant cells and recognition by resistance proteins

The avirulence gene AvrLm4-7 of Leptosphaeria maculans, the causal agent of stem canker in Brassica napus (oilseed rape), confers a dual specificity of recognition by two resistance genes (Rlm4 and Rlm7) and is strongly involved in fungal fitness. In order to elucidate the biological function of AvrLm4-7 and understand the specificity of recognition by Rlm4 and Rlm7, the AvrLm4-7 protein was produced in Pichia pastoris and its crystal structure was determined. It revealed the presence of four disulfide bridges, but no close structural analogs could be identified. A short stretch of amino acids in the C terminus of the protein, (R/N)(Y/F)(R/S)E(F/W), was well-conserved among AvrLm4-7 homologs. Loss of recognition of AvrLm4-7 by Rlm4 is caused by the mutation of a single glycine to an arginine residue located in a loop of the protein. Loss of recognition by Rlm7 is governed by more complex mutational patterns, including gene loss or drastic modifications of the protein structure. Three point mutations altered residues in the well-conserved C-terminal motif or close to the glycine involved in Rlm4-mediated recognition, resulting in the loss of Rlm7-mediated recognition. Transient expression in Nicotiana benthamiana (tobacco) and particle bombardment experiments on leaves from oilseed rape suggested that AvrLm4-7 interacts with its cognate R proteins inside the plant cell, and can be translocated into plant cells in the absence of the pathogen. Translocation of AvrLm4-7 into oilseed rape leaves is likely to require the (R/N)(Y/F)(R/S)E(F/W) motif as well as an RAWG motif located in a nearby loop that together form a positively charged region.

Building the interaction interfaces: host responses upon infection with microorganisms

Research fields of plant symbiosis and plant immunity were relatively ignorant with each other until a little while ago. Recently, however, increasing intercommunications between those two fields have begun to provide novel aspects and knowledge for understanding relationships between plants and microorganisms. Here, we review recent reports on plant-microbe interactions, focusing on the infection processes, in order to elucidate plant cellular responses that are triggered by both symbionts and pathogens. Highlighting the core elements of host responses over biotic interactions will provide insights into general mechanisms of plant-microbe interactions.

The haustorial transcriptomes of Uromyces appendiculatus and Phakopsora pachyrhizi and their candidate effector families

Haustoria of biotrophic rust fungi are responsible for the uptake of nutrients from their hosts and for the production of secreted proteins, known as effectors, which modulate the host immune system. The identification of the transcriptome of haustoria and an understanding of the functions of expressed genes therefore hold essential keys for the elucidation of fungus-plant interactions and the development of novel fungal control strategies. Here, we purified haustoria from infected leaves and used 454 sequencing to examine the haustorial transcriptomes of Phakopsora pachyrhizi and Uromyces appendiculatus, the causal agents of soybean rust and common bean rust, respectively. These pathogens cause extensive yield losses in their respective legume crop hosts. A series of analyses were used to annotate expressed sequences, including transposable elements and viruses, to predict secreted proteins from the assembled sequences and to identify families of candidate effectors. This work provides a foundation for the comparative analysis of haustorial gene expression with further insights into physiology and effector evolution.

How do filamentous pathogens deliver effector proteins into plant cells?

Fungal and oomycete plant parasites are among the most devastating pathogens of food crops. These microbes secrete effector proteins inside plant cells to manipulate host processes and facilitate colonization. How these effectors reach the host cytoplasm remains an unclear and debated area of plant research. In this article, we examine recent conflicting findings that have generated discussion in the field. We also highlight promising approaches based on studies of both parasite and host during infection. Ultimately, this knowledge may inform future broad spectrum strategies for protecting crops from such pathogens.

Structures of the flax-rust effector AvrM reveal insights into the molecular basis of plant-cell entry and effector-triggered immunity

Fungal and oomycete pathogens cause some of the most devastating diseases in crop plants, and facilitate infection by delivering a large number of effector molecules into the plant cell. AvrM is a secreted effector protein from flax rust (Melampsora lini) that can internalize into plant cells in the absence of the pathogen, binds to phosphoinositides (PIPs), and is recognized directly by the resistance protein M in flax (Linum usitatissimum), resulting in effector-triggered immunity. We determined the crystal structures of two naturally occurring variants of AvrM, AvrM-A and avrM, and both reveal an L-shaped fold consisting of a tandem duplicated four-helix motif, which displays similarity to the WY domain core in oomycete effectors. In the crystals, both AvrM variants form a dimer with an unusual nonglobular shape. Our functional analysis of AvrM reveals that a hydrophobic surface patch conserved between both variants is required for internalization into plant cells, whereas the C-terminal coiled-coil domain mediates interaction with M. AvrM binding to PIPs is dependent on positive surface charges, and mutations that abrogate PIP binding have no significant effect on internalization, suggesting that AvrM binding to PIPs is not essential for transport of AvrM across the plant membrane. The structure of AvrM and the identification of functionally important surface regions advance our understanding of the molecular mechanisms underlying how effectors enter plant cells and how they are detected by the plant immune system.

Susceptibility to plant disease: more than a failure of host immunity

Susceptibility to infectious diseases caused by pathogens affects most plants in their natural habitat and leads to yield losses in agriculture. However, plants are not helpless because their immune system can deal with the vast majority of attackers. Nevertheless, adapted pathogens are able to circumvent or avert host immunity making plants susceptible to these uninvited guests. In addition to the failure of the plant immune system, there are other host processes that contribute to plant disease susceptibility. In this review, we discuss recent studies that show the active role played by the host in supporting disease, focusing mainly on biotrophic stages of infection. Plants attract pathogens, enable their entry and accommodation, and facilitate nutrient provision.

Solanum resistance genes against Phytophthora infestans and their corresponding avirulence genes

Resistance genes against Phytophthora infestans (Rpi genes), the most important potato pathogen, are still highly valued in the breeding of Solanum spp. for enhanced resistance. The Rpi genes hitherto explored are localized most often in clusters, which are similar between the diverse Solanum genomes. Their distribution is not independent of late maturity traits. This review provides a summary of the most recent important revelations on the genomic position and cloning of Rpi genes, and the structure, associations, mode of action and activity spectrum of Rpi and corresponding avirulence (Avr) proteins. Practical implications for research into and application of Rpi genes are deduced and combined with an outlook on approaches to address remaining issues and interesting questions. It is evident that the potential of Rpi genes has not been exploited fully.

The RXLR motif of oomycete effectors is not a sufficient element for binding to phosphatidylinositol monophosphates

The translocation of effector proteins into the host plant cells is essential for pathogens to suppress plant immune responses. The oomycete pathogen Phytophthora infestans secretes AVR3a, a crucial virulence effector protein with an N-terminal RXLR motif that is required for this translocation. It has been reported that the RXLR motif of P. sojae Avr1b, which is a close homolog of AVR3a, is required for binding to phosphatidylinositol monophosphates (PIPs). However, in our previous report, AVR3a as well as Avr1b bind to PIPs not via RXLR but via lysine residues forming a positively-charged area in the effector domain. In this report, we examined whether other RXLR effectors whose structures have been determined bind to PIPs. Both P. capsici AVR3a11 and Hyaloperonospora arabidopsidis ATR1 have an RXLR motif in their N-terminal regions but did not bind to any PIPs. These results suggest that the RXLR motif is not sufficient for PIP binding.

The Magnaporthe oryzae effector AVR1-CO39 is translocated into rice cells independently of a fungal-derived machinery

Effector proteins are key elements in plant-fungal interactions. The rice blast fungus Magnaporthe oryzae secretes numerous effectors that are suspected to be translocated inside plant cells. However, their cellular targets and the mechanisms of translocation are still unknown. Here, we have identified the open reading frame (ORF3) corresponding to the M. oryzae avirulence gene AVR1-CO39 that interacts with the rice resistance gene Pi-CO39 and encodes a small secreted protein without homology to other proteins. We demonstrate that AVR1-CO39 is specifically expressed and secreted at the plant-fungal interface during the biotrophic phase of infection. Live-cell imaging with M. oryzae transformants expressing a translational fusion between AVR1-CO39 and the monomeric red fluorescent protein (mRFP) indicated that AVR1-CO39 is translocated into the cytoplasm of infected rice cells. Transient expression of an AVR1-CO39 isoform without a signal peptide in rice protoplasts triggers a Pi-CO39-specific hypersensitive response, suggesting that recognition of AVR1-CO39 by the Pi-CO39 gene product occurs in the cytoplasm of rice cells. The native AVR1-CO39 protein enters the secretory pathway of rice protoplasts as demonstrated by the ER localization of AVR1-CO39:mRFP:HDEL translational fusions, and is correctly processed as shown by Western blotting. However, this secreted AVR1-CO39 isoform triggers a Pi-CO39-specific hypersensitive response and accumulates inside rice protoplasts as shown by Western blotting and localization of AVR1-CO39:mRFP translational fusions. This indicates that AVR1-CO39 is secreted by rice protoplasts and re-enters into the cytoplasm by unknown mechanisms, suggesting that translocation of AVR1-CO39 into rice cells occurs independently of fungal factors.

Pathogen virulence of Phytophthora infestans: from gene to functional genomics

The oomycete, Phytophthora infestans, is one of the most important plant pathogens worldwide. Much of the pathogenic success of P. infestans, the potato late blight agent, relies on its ability to generate large amounts of sporangia from mycelia, which release zoospores that encyst and form infection structures. Until recently, little was known about the molecular basis of oomycete pathogenicity by the avirulence molecules that are perceived by host defenses. To understand the molecular mechanisms interplay in the pathogen and host interactions, knowledge of the genome structure was most important, which is available now after genome sequencing. The mechanism of biotrophic interaction between potato and P. infestans could be determined by understanding the effector biology of the pathogen, which is until now poorly understood. The recent availability of oomycete genome will help in understanding of the signal transduction pathways followed by apoplastic and cytoplasmic effectors for translocation into host cell. Finally based on genomics, novel strategies could be developed for effective management of the crop losses due to the late blight disease.

Plants and pathogens: putting infection strategies and defence mechanisms on the map

All plant organs are vulnerable to colonisation and molecular manipulation by microbes. When this interaction allows proliferation of the microbe at the expense of the host, the microbe can be described as a pathogen. In our attempts to understand the full nature of the interactions that occur between a potential pathogen and its host, various aspects of the molecular mechanisms of infection and defence have begun to be characterised. There is significant variation in these mechanisms. While previous research has examined plant-pathogen interactions with whole plant/organ resolution, the specificity of infection strategies and changes in both gene expression and protein localisation of immune receptors upon infection suggest there is much to be gained from examination of plant-microbe interactions at the cellular level.

Avirulence protein 3a (AVR3a) from the potato pathogen Phytophthora infestans forms homodimers through its predicted translocation region and does not specifically bind phospholipids

The mechanism of translocation of RxLR effectors from plant pathogenic oomycetes into the cytoplasm of their host is currently the object of intense research activity and debate. Here, we report the biochemical and thermodynamic characterization of the Phytophthora infestans effector AVR3a in vitro. We show that the amino acids surrounding the RxLR leader mediate homodimerization of the protein. Dimerization was considerably attenuated by a localized mutation within the RxLR motif that was previously described to prevent translocation of the protein into host. Importantly, we confirm that the reported phospholipid-binding properties of AVR3a are mediated by its C-terminal effector domain, not its RxLR leader. However, we show that the observed phospholipid interaction is attributable to a weak association with denatured protein molecules and is therefore most likely physiologically irrelevant.

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.

Challenges and progress towards understanding the role of effectors in plant-fungal interactions

Both mutualistic and biotrophic pathogenic fungi rely on living host plants for growth and reproduction and must modify host cell structure and function for successful infection. The deployment of a diverse set of secreted virulence determinants referred to as 'effectors', many of which are directly delivered into the host cell, is postulated to be the key to host infection. This review provides a snapshot of the current progress in fungal effector biology. Recent genome sequencing of rust and powdery mildew obligate biotrophs has provided insight into the repertoires of potential effectors of these highly specialised pathogens. Identification of the first host-translocated effectors from mutualistic fungi has revealed that these fungi also manipulate host cells through effectors. The biological activities of some fungal effectors are just beginning to be revealed, while much uncertainty still surrounds the mechanisms of transport into host cells.

Patterns of plant subcellular responses to successful oomycete infections reveal differences in host cell reprogramming and endocytic trafficking

Adapted filamentous pathogens such as the oomycetes Hyaloperonospora arabidopsidis (Hpa) and Phytophthora infestans (Pi) project specialized hyphae, the haustoria, inside living host cells for the suppression of host defence and acquisition of nutrients. Accommodation of haustoria requires reorganization of the host cell and the biogenesis of a novel host cell membrane, the extrahaustorial membrane (EHM), which envelops the haustorium separating the host cell from the pathogen. Here, we applied live-cell imaging of fluorescent-tagged proteins labelling a variety of membrane compartments and investigated the subcellular changes associated with accommodating oomycete haustoria in Arabidopsis and N. benthamiana. Plasma membrane-resident proteins differentially localized to the EHM. Likewise, secretory vesicles and endosomal compartments surrounded Hpa and Pi haustoria revealing differences between these two oomycetes, and suggesting a role for vesicle trafficking pathways for the pathogen-controlled biogenesis of the EHM. The latter is supported by enhanced susceptibility of mutants in endosome-mediated trafficking regulators. These observations point at host subcellular defences and specialization of the EHM in a pathogen-specific manner. Defence-associated haustorial encasements, a double-layered membrane that grows around mature haustoria, were frequently observed in Hpa interactions. Intriguingly, all tested plant proteins accumulated at Hpa haustorial encasements suggesting the general recruitment of default vesicle trafficking pathways to defend pathogen access. Altogether, our results show common requirements of subcellular changes associated with oomycete biotrophy, and highlight differences between two oomycete pathogens in reprogramming host cell vesicle trafficking for haustoria accommodation. This provides a framework for further dissection of the pathogen-triggered reprogramming of host subcellular changes.

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.

The cellular language of myo-inositol signaling

The simple polyol, myo-inositol, is used as a building block of a cellular language that plays various roles in signal transduction. This review describes the terminology used to denote myo-inositol-containing molecules, with an emphasis on how phosphate and fatty acids are added to create second messengers used in signaling. Work in model systems has delineated the genes and enzymes required for synthesis and metabolism of many myo-inositol-containing molecules, with genetic mutants and measurement of second messengers playing key roles in developing our understanding. There is increasing evidence that molecules such as myo- inositol(1,4,5)trisphosphate and phosphatidylinositol(4,5)bisphosphate are synthesized in response to various signals plants encounter. In particular, the controversial role of myo-inositol(1,4,5)trisphosphate is addressed, accompanied by a discussion of the multiple enzymes that act to regulate this molecule. We are also beginning to understand new connections of myo-inositol signaling in plants. These recent discoveries include the novel roles of inositol phosphates in binding to plant hormone receptors and that of phosphatidylinositol(3)phosphate binding to pathogen effectors.

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.

The role of effectors of biotrophic and hemibiotrophic fungi in infection

Biotrophic and hemibiotrophic fungi are successful groups of plant pathogens that require living plant tissue to survive and complete their life cycle. Members of these groups include the rust fungi and powdery mildews and species in the Ustilago, Cladosporium and Magnaporthe genera. Collectively, they represent some of the most destructive plant parasites, causing huge economic losses and threatening global food security. During plant infection, pathogens synthesize and secrete effector proteins, some of which are translocated into the plant cytosol where they can alter the host's response to the invading pathogen. In a successful infection, pathogen effectors facilitate suppression of the plant's immune system and orchestrate the reprogramming of the infected tissue so that it becomes a source of nutrients that are required by the pathogen to support its growth and development. This review summarizes our current understanding of the function of fungal effectors in infection.

Phosphatidylinositol monophosphate-binding interface in the oomycete RXLR effector AVR3a is required for its stability in host cells to modulate plant immunity

The oomycete pathogen Phytophthora infestans causes potato late blight, one of the most economically damaging plant diseases worldwide. P. infestans produces AVR3a, an essential modular virulence effector with an N-terminal RXLR domain that is required for host-cell entry. In host cells, AVR3a stabilizes and inhibits the function of the E3 ubiquitin ligase CMPG1, a key factor in host immune responses including cell death triggered by the pathogen-derived elicitor protein INF1 elicitin. To elucidate the molecular basis of AVR3a effector function, we determined the structure of Phytophthora capsici AVR3a4, a close homolog of P. infestans AVR3a. Our structural and functional analyses reveal that the effector domain of AVR3a contains a conserved, positively charged patch and that this region, rather than the RXLR domain, is required for binding to phosphatidylinositol monophosphates (PIPs) in vitro. Mutations affecting PIP binding do not abolish AVR3a recognition by the resistance protein R3a but reduce its ability to suppress INF1-triggered cell death in planta. Similarly, stabilization of CMPG1 in planta is diminished by these mutations. The steady-state levels of non-PIP-binding mutant proteins in planta are reduced greatly, although these proteins are stable in vitro. Furthermore, overexpression of a phosphatidylinositol phosphate 5-kinase results in reduction of AVR3a levels in planta. Our results suggest that the PIP-binding ability of the AVR3a effector domain is essential for its accumulation inside host cells to suppress CMPG1-dependent immunity.

How filamentous pathogens co-opt plants: the ins and outs of fungal effectors

Research on effectors secreted by pathogens during host attack has dominated the field of molecular plant-microbe interactions over recent years. Functional analysis of type III secreted effectors injected by pathogenic bacteria into host cells has significantly advanced the field and demonstrated that many function to suppress host defense. Fungal and oomycete effectors are delivered outside the host plasma membrane, and although research has lagged behind on bacterial effectors, we are gradually learning more and more about the functions of these effectors. While some function outside the host cell to disarm defense, others exploit host cellular uptake mechanisms to suppress defense or liberate nutrients intracellularly. Comparative genomics suggests that the organization of effector genes drives effector evolution in many pathogen genomes.

How do oomycete effectors interfere with plant life?

Oomycete genomes have yielded a large number of predicted effector proteins that collectively interfere with plant life in order to create a favourable environment for pathogen infection. Oomycetes secrete effectors that can be active in the host's extracellular environment, for example inhibiting host defence enzymes, or inside host cells where they can interfere with plant processes, in particular suppression of defence. Two classes of effectors are known to be host-translocated: the RXLRs and Crinklers. Many effectors show defence-suppressive activity that is important for pathogen virulence. A striking example is AVR3a of Phytophthora infestans that targets an ubiquitin ligase, the stabilisation of which may prevent host cell death. The quest for other effector targets and mechanisms is in full swing.