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

Proteolytic processing of both RXLR and EER motifs in oomycete effectors

Arg-any amino acid-Leu-Arg (RXLR) effectors are central oomycete virulence factors that suppress plant immunity. Relatively little is known about how they are processed post-translationally before delivery into host cells. A range of molecular, cell and biochemical processes were used to investigate proteolytic processing of RXLR and Glu-Glu-Arg (EER) motifs in Phytophthora infestans effectors. Proteolytic cleavage at the RXLR motif occurred before secretion in all effectors tested, suggesting it is a general rule. Cleavage occurred between the leucine and the second arginine. There was no cleavage of a naturally occurring second RXLR motif in a structured region of Pi21388/AvrBlb1, or one introduced at a similar position in effector Pi04314, in keeping with the motif being positionally constrained, potentially to disordered regions closely following the signal peptide. Remarkably, independent proteolytic cleavage of the EER motif, often found immediately after the RXLR, was also observed, occurring immediately after the arginine. Full-length effectors expressed in host plant Nicotiana benthamiana revealed that, although secreted, they were poorly processed, suggesting that RXLR and EER cleavage does not occur in all eukaryotic cells. We conclude that, whether possessing both RXLR and EER, or either motif alone, these effectors are likely generally proteolytically processed before secretion.

Genome wide association study reveals new genes for resistance to striped stem borer in rice (Oryza sativa L.)

Rice is one of the most important food crops in the world and is important for global food security. However, damage caused by striped stem borer (SSB) seriously threatens rice production and can cause significant yield losses. The development and use of resistant rice varieties or genes is currently the most effective strategy for controlling SSB. We genotyped 201 rice samples using 2849855 high-confidence single nucleotide polymorphisms (SNPs). We conducted a genome-wide association study (GWAS) based on observed variation data of 201 rice cultivars resistant to SSB. We obtained a quantitative trait locus (QTL)-qRSSB4 that confers resistance to SSB. Through annotation and analysis of genes within the qRSSB4 locus, as well as qRT-PCR detection in resistant rice cultivars, we ultimately selected the candidate gene LOC_Os04g34140 (named OsRSSB4) for further analysis. Next, we overexpressed the candidate gene OsRSSB4 in Nipponbare through transgenic methods, resulting in OsRSSB4 overexpressing lines (OsRSSB4OE). In addition, we evaluated the insect resistance of OsRSSB4OE lines using wild type (Nipponbare) as a control. The bioassay experiment results of live plants showed that after 20 days of inoculation with SSB, the withering heart rate of OsRSSB4OE-34 and OsRSSB4OE-39 lines was only 8.3% and 0%, with resistance levels of 1 and 0, respectively; however, the withering heart rate of the wild-type reached 100%, with a resistance level of 9. The results of the in vitro stem bioassay showed that, compared with the wild-type, the average corrected mortality rate of the SSB fed on the OsRSSB4OE line reached 94.3%, and the resistance reached a high level. In summary, we preliminarily confirmed that OsRSSB4 positively regulates the defense of rice against SSB. This research findings reveal new SSB resistance gene resources, providing an important genetic basis for SSB resistance breeding in rice crops.

Temperature-dependent action of pepper mildew resistance locus O 1 in inducing pathogen immunity and thermotolerance

Plant diseases tend to be more serious under conditions of high-temperature/high-humidity (HTHH) than under moderate conditions, and hence disease resistance under HTHH is an important determinant for plant survival. However, how plants cope with diseases under HTHH remains poorly understood. In this study, we used the pathosystem consisting of pepper (Capsicum annuum) and Ralstonia solanacearum (bacterial wilt) as a model to examine the functions of the protein mildew resistance locus O 1 (CaMLO1) and U-box domain-containing protein 21 (CaPUB21) under conditions of 80% humidity and either 28 °C or 37 °C. Expression profiling, loss- and gain-of-function assays involving virus-induced gene-silencing and overexpression in pepper plants, and protein-protein interaction assays were conducted, and the results showed that CaMLO1 acted negatively in pepper immunity against R. solanacearum at 28 °C but positively at 37 °C. In contrast, CaPUB21 acted positively in immunity at 28 °C but negatively at 37 °C. Importantly, CaPUB21 interacted with CaMLO1 under all of the tested conditions, but only the interaction in response to R. solanacearum at 37 °C or to exposure to 37 °C alone led to CaMLO1 degradation, thereby turning off defence responses against R. solanacearum at 37 °C and under high-temperature stress to conserve resources. Thus, we show that CaMLO1 and CaPUB21 interact with each other and function distinctly in pepper immunity against R. solanacearum in an environment-dependent manner.

Effector-triggered susceptibility by the rice blast fungus Magnaporthe oryzae

Rice blast, the most destructive disease of cultivated rice world-wide, is caused by the filamentous fungus Magnaporthe oryzae. To cause disease in plants, M. oryzae secretes a diverse range of effector proteins to suppress plant defense responses, modulate cellular processes, and support pathogen growth. Some effectors can be secreted by appressoria even before host penetration, while others accumulate in the apoplast, or enter living plant cells where they target specific plant subcellular compartments. During plant infection, the blast fungus induces the formation of a specialized plant structure known as the biotrophic interfacial complex (BIC), which appears to be crucial for effector delivery into plant cells. Here, we review recent advances in the cell biology of M. oryzae-host interactions and show how new breakthroughs in disease control have stemmed from an increased understanding of effector proteins of M. oryzae are deployed and delivered into plant cells to enable pathogen invasion and host susceptibility.

Plant immune receptors interact with hemibiotrophic pathogens to activate plant immunity

Phytopathogens pose a devastating threat to the productivity and yield of crops by causing destructive plant diseases in natural and agricultural environments. Hemibiotrophic pathogens have a variable-length biotrophic phase before turning to necrosis and are among the most invasive plant pathogens. Plant resistance to hemibiotrophic pathogens relies mainly on the activation of innate immune responses. These responses are typically initiated after the plant plasma membrane and various plant immune receptors detect immunogenic signals associated with pathogen infection. Hemibiotrophic pathogens evade pathogen-triggered immunity by masking themselves in an arms race while also enhancing or manipulating other receptors to promote virulence. However, our understanding of plant immune defenses against hemibiotrophic pathogens is highly limited due to the intricate infection mechanisms. In this review, we summarize the strategies that different hemibiotrophic pathogens interact with host immune receptors to activate plant immunity. We also discuss the significant role of the plasma membrane in plant immune responses, as well as the current obstacles and potential future research directions in this field. This will enable a more comprehensive understanding of the pathogenicity of hemibiotrophic pathogens and how distinct plant immune receptors oppose them, delivering valuable data for the prevention and management of plant diseases.

Phosphoinositides in plant-pathogen interaction: trends and perspectives

Phosphoinositides are important regulatory membrane lipids, with a role in plant development and cellular function. Emerging evidence indicates that phosphoinositides play crucial roles in plant defence and are also utilized by pathogens for infection. In this review, we highlight the role of phosphoinositides in plant-pathogen interaction and the implication of this remarkable convergence in the battle against plant diseases.

Engineering crop Phytophthora resistance by targeting pathogen-derived PI3P for enhanced catabolism

Phytophthora pathogens lead to numerous economically damaging plant diseases worldwide, including potato late blight caused by P. infestans and soybean root rot caused by P. sojae. Our previous work showed that Phytophthora pathogens may generate abundant phosphatidylinositol 3-phosphate (PI3P) to promote infection via direct association with RxLR effectors. Here, we designed a disease control strategy for metabolizing pathogen-derived PI3P by expressing secreted Arabidopsis thaliana phosphatidylinositol-4-phosphate 5-kinase 1 (AtPIP5K1), which can phosphorylate PI3P to PI(3,4)P₂. We fused AtPIP5K1 with the soybean PR1a signal peptide (SP-PIP5K1) to enable its secretion into the plant apoplast. Transgenic soybean and potato plants expressing SP-PIP5K1 showed substantially enhanced resistance to various P. sojae and P. infestans isolates, respectively. SP-PIP5K1 significantly reduced PI3P accumulation during P. sojae and soybean interaction. Knockout or inhibition of PI3 kinases (PI3Ks) in P. sojae compromised the resistance mediated by SP-PIP5K1, indicating that SP-PIP5K1 action requires a supply of pathogen-derived PI3P. Furthermore, we revealed that SP-PIP5K1 can interfere with the action of P. sojae mediated by the RxLR effector Avr1k. This novel disease control strategy has the potential to confer durable broad-spectrum Phytophthora resistance in plants through a clear mechanism in which catabolism of PI3P interferes with RxLR effector actions.

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.

Genetic variation along an altitudinal gradient in the Phytophthora infestans effector gene Pi02860

Effector genes, together with climatic and other environmental factors, play multifaceted roles in the development of plant diseases. Understanding the role of environmental factors, particularly climate conditions affecting the evolution of effector genes, is important for predicting the long-term value of the genes in controlling agricultural diseases. Here, we collected Phytophthora infestans populations from five locations along a mountainous hill in China and sequenced the effector gene Pi02860 from >300 isolates. To minimize the influence of other ecological factors, isolates were sampled from the same potato cultivar on the same day. We also expressed the gene to visualise its cellular location, assayed its pathogenicity and evaluated its response to experimental temperatures. We found that Pi02860 exhibited moderate genetic variation at the nucleotide level which was mainly generated by point mutation. The mutations did not change the cellular location of the effector gene but significantly modified the fitness of P. infestans. Genetic variation and pathogenicity of the effector gene were positively associated with the altitude of sample sites, possibly due to increased mutation rate induced by the vertical distribution of environmental factors such as UV radiation and temperature. We further found that Pi02860 expression was regulated by experimental temperature with reduced expression as experimental temperature increased. Together, these results indicate that UV radiation and temperature are important environmental factors regulating the evolution of effector genes and provide us with considerable insight as to their future sustainable action under climate and other environmental change.

Seeing is believing: Exploiting advances in structural biology to understand and engineer plant immunity

Filamentous plant pathogens cause disease in numerous economically important crops. These pathogens secrete virulence proteins, termed effectors, that modulate host cellular processes and promote infection. Plants have evolved immunity receptors that detect effectors and activate defence pathways, resulting in resistance to the invading pathogen. This leads to an evolutionary arms race between pathogen and host that is characterised by highly diverse effector repertoires in plant pathogens. Here, we review the recent advances in understanding host-pathogen co-evolution provided by the structural determination of effectors alone, and in complex with immunity receptors. We highlight the use of recent advances in structural prediction within this field and its role for future development of designer resistance proteins.

Towards Phytopathogen Diagnostics? Coconut Bud Rot Pathogen Phytophthora palmivora Mycelial Proteome Analysis Informs Genome Annotation

Planetary agriculture stands to benefit immensely from phytopathogen diagnostics, which would enable early detection of pathogens with harmful effects on crops. For example, Phytophthora palmivora is one of the most destructive phytopathogens affecting many economically important tropical crops such as coconut. P. palmivora causes diseases in over 200 host plants, and notably, the bud rot disease in coconut and oil palm, which is often lethal because it is usually detected at advanced stages of infection. Limited availability of large-scale omics datasets for P. palmivora is an important barrier for progress toward phytopathogen diagnostics. We report here the mycelial proteome of P. palmivora using high-resolution mass spectrometry analysis. We identified 8073 proteins in the mycelium. Gene Ontology-based functional classification of detected proteins revealed 4884, 4981, and 3044 proteins, respectively, with roles in biological processes, molecular functions, and cellular components. Proteins such as P-loop, NTPase, and WD40 domains with key roles in signal transduction pathways were identified. KEGG pathway analysis annotated 2467 proteins to various signaling pathways, such as phosphatidylinositol, Ca²⁺, and mitogen-activated protein kinase, and autophagy and cell cycle. These molecular substrates might possess vital roles in filamentous growth, sporangia formation, degradation of damaged cellular content, and recycling of nutrients in P. palmivora. This large-scale proteomics data and analyses pave the way for new insights on biology, genome annotation, and vegetative growth of the important plant pathogen P. palmivora. They also can help accelerate research on future phytopathogen diagnostics and preventive interventions.

Lipid Signaling Modulates the Response to Fumonisin Contamination and Its Source, Fusarium verticillioides, in Maize

Fumonisin-contaminated maize (Zea mays L.) products are a major health concern because of their toxic effects in humans and animals. Breeding maize for increased mycotoxin resistance is one of the key sustainable strategies for mitigating the effects of fumonisin contamination. Recent studies suggest a link between fumonisin accumulation and plant lipid and oxylipin profiles. However, the data collected so far do not reveal a cause-and-effect relationship. In this study, to decipher the multifactorial nature of mycotoxin resistance and plant-pathogen interaction mechanisms, we examined the oxylipin and complex lipid profiles of two maize hybrids (H21 and H22, the latter showing significantly lower FBs content) grown in the open field in two locations over 3years. Untargeted ultra-high performance liquid chromatography coupled with quadrupole-time-of-flight (UHPLC-Q-TOF), together with chemometrics analysis, successfully distinguished between the two hybrids as having low- and high-level fumonisin contamination. Considering that H21 and H22 were exposed to the same environmental factors, the higher activation of lipid signaling systems in H22 suggests that other routes are enabled in the less susceptible hybrids to limit fumonisin B (FB) accumulation. Our results highlighted the crucial role played by oxylipin and sphingolipid signaling in modulating the complex maize response to F. verticillioides infection. Overall, our results returned a global view on the changes in lipid metabolites related to fumonisin accumulation under open field conditions, and revealed a strong activation of the lipid signaling cascade in maize in the presence of FB₁.

Targeting of anti-microbial proteins to the hyphal surface amplifies protection of crop plants against Phytophthora pathogens

Phytophthora pathogens are a persistent threat to the world's commercially important agricultural crops, including potato and soybean. Current strategies aim at reducing crop losses rely mostly on disease-resistance breeding and chemical pesticides, which can be frequently overcome by the rapid adaptive evolution of pathogens. Transgenic crops with intrinsic disease resistance offer a promising alternative and continue to be developed. Here, we explored Phytophthora-derived PI3P (phosphatidylinositol 3-phosphate) as a novel control target, using proteins that bind this lipid to direct secreted anti-microbial peptides and proteins (AMPs) to the surface of Phytophthora pathogens. In transgenic Nicotiana benthamiana, soybean, and potato plants, significantly enhanced resistance to different pathogen isolates was achieved by expression of two AMPs (GAFP1 or GAFP3 from the Chinese medicinal herb Gastrodia elata) fused with a PI3P-specific binding domain (FYVE). Using the soybean pathogen P. sojae as an example, we demonstrated that the FYVE domain could boost the activities of GAFPs in multiple independent assays, including those performed in vitro, in vivo, and in planta. Mutational analysis of P. sojae PI3K1 and PI3K2 genes of this pathogen confirmed that the enhanced activities of the targeted GAFPs were correlated with PI3P levels in the pathogen. Collectively, our study provides a new strategy that could be used to confer resistance not only to Phytophthora pathogens in many plants but also potentially to many other kinds of plant pathogens with unique targets.

The Cytospora chrysosperma Virulence Effector CcCAP1 Mainly Localizes to the Plant Nucleus To Suppress Plant Immune Responses

Canker disease is caused by the fungus Cytospora chrysosperma and damages a wide range of woody plants, causing major losses to crops and native plants. Plant pathogens secrete virulence-related effectors into host cells during infection to regulate plant immunity and promote colonization. However, the functions of C. chrysosperma effectors remain largely unknown. In this study, we used Agrobacterium tumefaciens-mediated transient expression system in Nicotiana benthamiana and confocal microscopy to investigate the immunoregulation roles and subcellular localization of CcCAP1, a virulence-related effector identified in C. chrysosperma CcCAP1 was significantly induced in the early stages of infection and contains cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins (CAP) superfamily domain with four cysteines. CcCAP1 suppressed the programmed cell death triggered by Bcl-2-associated X protein (BAX) and the elicitin infestin1 (INF1) in transient expression assays with Nicotiana benthamiana The CAP superfamily domain was sufficient for its cell death-inhibiting activity and three of the four cysteines in the CAP superfamily domain were indispensable for its activity. Pathogen challenge assays in N. benthamiana demonstrated that transient expression of CcCAP1 promoted Botrytis cinerea infection and restricted reactive oxygen species accumulation, callose deposition, and defense-related gene expression. In addition, expression of green fluorescent protein-labeled CcCAP1 in N. benthamiana showed that it localized to both the plant nucleus and the cytoplasm, but the nuclear localization was essential for its full immune inhibiting activity. These results suggest that this virulence-related effector of C. chrysosperma modulates plant immunity and functions mainly via its nuclear localization and the CAP domain.IMPORTANCE The data presented in this study provide a key resource for understanding the biology and molecular basis of necrotrophic pathogen responses to Nicotiana benthamiana resistance utilizing effector proteins, and CcCAP1 may be used in future studies to understand effector-triggered susceptibility processes in the Cytospora chrysosperma-poplar interaction system.

Comparative Transcriptome Analysis of Wheat Lines in the Field Reveals Multiple Essential Biochemical Pathways Suppressed by Obligate Pathogens

Mildew and rust are the most devastating cereal pathogens, and in wheat they can cause up to 50% yield loss every year. Wheat lines containing resistance genes are used to effectively control fungal diseases, but the molecular mechanisms underlying the interaction between wheat and its fungal pathogens are poorly understood. Here, we used RNA sequencing (RNA-Seq) to compare the transcriptomic landscape of susceptible and resistant wheat lines to identify genes and pathways that are targeted by obligate biotrophic fungal pathogens. The five lines differed in the expression of thousands of genes under infection as well as control conditions. Generally, mixed infection with powdery mildew and leaf rust resulted in downregulation of numerous genes in susceptible lines. Interestingly, transcriptomic comparison between the nearly isogenic lines Thatcher and Thatcher-Lr34 identified 753 genes that are uniquely downregulated in the susceptible line upon infection. Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis, revealed the suppression of six major biochemical pathways, namely nuclear transport, alternative splicing, DNA damage response, ubiquitin-mediated proteolysis, phosphoinositol signaling, and photosynthesis. We conclude that powdery mildew and leaf rust evade the wheat defense system by suppression of programmed cell death (PCD) and responses to cellular damage. Considering the broad range of the induced changes, we propose that the pathogen targets "master regulators" at critical steps in the respective pathways. Identification of these wheat genes targeted by the pathogen could inspire new directions for future wheat breeding.

Effector prediction and characterization in the oomycete pathogen Bremia lactucae reveal host-recognized WY domain proteins that lack the canonical RXLR motif

Pathogens that infect plants and animals use a diverse arsenal of effector proteins to suppress the host immune system and promote infection. Identification of effectors in pathogen genomes is foundational to understanding mechanisms of pathogenesis, for monitoring field pathogen populations, and for breeding disease resistance. We identified candidate effectors from the lettuce downy mildew pathogen Bremia lactucae by searching the predicted proteome for the WY domain, a structural fold found in effectors that has been implicated in immune suppression as well as effector recognition by host resistance proteins. We predicted 55 WY domain containing proteins in the genome of B. lactucae and found substantial variation in both sequence and domain architecture. These candidate effectors exhibit several characteristics of pathogen effectors, including an N-terminal signal peptide, lineage specificity, and expression during infection. Unexpectedly, only a minority of B. lactucae WY effectors contain the canonical N-terminal RXLR motif, which is a conserved feature in the majority of cytoplasmic effectors reported in Phytophthora spp. Functional analysis of 21 effectors containing WY domains revealed 11 that elicited cell death on wild accessions and domesticated lettuce lines containing resistance genes, indicative of recognition of these effectors by the host immune system. Only two of the 11 recognized effectors contained the canonical RXLR motif, suggesting that there has been an evolutionary divergence in sequence motifs between genera; this has major consequences for robust effector prediction in oomycete pathogens.

Proteinaceous effector discovery and characterization in filamentous plant pathogens

The complicated interplay of plant-pathogen interactions occurs on multiple levels as pathogens evolve to constantly evade the immune responses of their hosts. Many economically important crops fall victim to filamentous pathogens that produce small proteins called effectors to manipulate the host and aid infection/colonization. Understanding the effector repertoires of pathogens is facilitating an increased understanding of the molecular mechanisms underlying virulence as well as guiding the development of disease control strategies. The purpose of this review is to give a chronological perspective on the evolution of the methodologies used in effector discovery from physical isolation and in silico predictions, to functional characterization of the effectors of filamentous plant pathogens and identification of their host targets.

Fast and Inexpensive Phenotyping and Genotyping Methods for Evaluation of Barley Mutant Population

To further develop barley breeding and genetics, more information on gene functions based on the analysis of the mutants of each gene is needed. However, the mutant resources are not as well developed as the model plants, such as Arabidopsis and rice. Although genome editing techniques have been able to generate mutants, it is not yet an effective method as it can only be used to transform a limited number of cultivars. Here, we developed a mutant population using ‘Mannenboshi’, which produces good quality grains with high yields but is susceptible to disease, to establish a Targeting Induced Local Lesions IN Genomes (TILLING) system that can isolate mutants in a high-throughput manner. To evaluate the availability of the prepared 8043 M₃ lines, we investigated the frequency of mutant occurrence using a rapid, visually detectable waxy phenotype as an indicator. Four mutants were isolated and single nucleotide polymorphisms (SNPs) were identified in the Waxy gene as novel alleles. It was confirmed that the mutations could be easily detected using the mismatch endonuclease CELI, revealing that a sufficient number of mutants could be rapidly isolated from our TILLING population.

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.

The Phytophthora infestans AVR2 Effector Escapes R2 Recognition Through Effector Disordering

Intrinsic disorder is a common structural characteristic of proteins and a central player in the biochemical processes of species. However, the role of intrinsic disorder in the evolution of plant-pathogen interactions is rarely investigated. Here, we explored the role of intrinsic disorder in the development of the pathogenicity in the RXLR AVR2 effector of Phytophthora infestans. We found AVR2 exhibited high nucleotide diversity generated by point mutation, early-termination, altered start codon, deletion/insertion, and intragenic recombination and is predicted to be an intrinsically disordered protein. AVR2 amino acid sequences conferring a virulent phenotype had a higher disorder tendency in both the N- and C-terminal regions compared with sequences conferring an avirulent phenotype. In addition, we also found virulent AVR2 mutants gained one or two short linear interaction motifs, the critical components of disordered proteins required for protein-protein interactions. Furthermore, virulent AVR2 mutants were predicted to be unstable and have a short protein half-life. Taken together, these results support the notion that intrinsic disorder is important for the effector function of pathogens and demonstrate that SLiM-mediated protein-protein interaction in the C-terminal effector domain might contribute greatly to the evasion of resistance-protein detection in P. infestans.

Transcriptome Analysis Identifies Plasmodiophora brassicae Secondary Infection Effector Candidates

Plasmodiophora brassicae (Wor.) is an obligate intracellular plant pathogen affecting Brassicas worldwide. Identification of effector proteins is key to understanding the interaction between P. brassicae and its susceptible host plants. To date, there is very little information available on putative effector proteins secreted by P. brassicae during a secondary infection of susceptible host plants, resulting in root gall production. A bioinformatics pipeline approach to RNA-Seq data from Arabidopsis thaliana (L.) Heynh. root tissues at 17, 20, and 24 d postinoculation (dpi) identified 32 small secreted P. brassicae proteins (SSPbPs) that were highly expressed over this secondary infection time frame. Functional signal peptides were confirmed for 31 of the SSPbPs, supporting the accuracy of the pipeline designed to identify secreted proteins. Expression profiles at 0, 2, 5, 7, 14, 21, and 28 dpi verified the involvement of some of the SSPbPs in secondary infection. For seven of the SSPbPs, a functional domain was identified using Blast2GO and 3D structure analysis and domain functionality was confirmed for SSPbP22, a kinase localized to the cytoplasm and nucleus.

Two phosphatidylinositol 3-kinase components are involved in interactions between Nicotiana benthamiana and Phytophthora by regulating pathogen effectors and host cell death

Phosphatidylinositol 3-phosphate (PtdIns(3)P) has been reported to regulate different physiological processes in plants. PtdIns(3)P is synthesised by the phosphatidylinositol 3-kinase (PI3K) complex which includes common subunits of vacuolar protein sorting (VPS)15, VPS30 and VPS34. Here, we characterised the roles of the important genes NbVPS15, -30 and -34 encoding PI3K components during interactions between Nicotiana benthamiana and Phytophthora pathogens. NbVPS15 and NbVPS34 were upregulated during infection, and plants deficient in these two genes displayed higher resistance to two different Phytophthora pathogens. Silencing NbVPS15 and NbVPS34 decreased the content of PtdIns(3)P in plant cells and the stability of three RxLR (containing the characteristic amino-terminal motif of arginine-X-leucine-arginine, X is any amino acid) effectors. Furthermore, NbVPS15, -30 and -34 were essential for autolysosome formation during Phytophthora capsici infection and limiting programmed cell death (PCD) induced by effectors and elicitors. Taken together, these findings suggest that NbVPS15 and NbVPS34 play a critical role in the resistance of N. benthamiana to Phytophthora pathogens by regulating PtdIns(3)P contents and host PCD.

Structural genomics applied to the rust fungus Melampsora larici-populina reveals two candidate effector proteins adopting cystine knot and NTF2-like protein folds

Rust fungi are plant pathogens that secrete an arsenal of effector proteins interfering with plant functions and promoting parasitic infection. Effectors are often species-specific, evolve rapidly, and display low sequence similarities with known proteins. How rust fungal effectors function in host cells remains elusive, and biochemical and structural approaches have been scarcely used to tackle this question. In this study, we produced recombinant proteins of eleven candidate effectors of the leaf rust fungus Melampsora larici-populina in Escherichia coli. We successfully purified and solved the three-dimensional structure of two proteins, MLP124266 and MLP124017, using NMR spectroscopy. Although both MLP124266 and MLP124017 show no sequence similarity with known proteins, they exhibit structural similarities to knottins, which are disulfide-rich small proteins characterized by intricate disulfide bridges, and to nuclear transport factor 2-like proteins, which are molecular containers involved in a wide range of functions, respectively. Interestingly, such structural folds have not been reported so far in pathogen effectors, indicating that MLP124266 and MLP124017 may bear novel functions related to pathogenicity. Our findings show that sequence-unrelated effectors can adopt folds similar to known proteins, and encourage the use of biochemical and structural approaches to functionally characterize effector candidates.

The effect of plant stress on phosphoinositides

Phosphoinositides are very versatile molecules with a plethora of functions such as cytokinesis, chemotaxis, cell survival, and cell death. Their functions depend on the proteins with which they interact. Thus, when interacting with phospholipases, phosphatases, or kinases, they can be precursors of second messengers in different signalling pathways. They could be second messengers themselves and interact directly with other proteins to modulate their functions trough changing its localization and activity or enhancing its synthesis rate. Because they are more abundant in animal cells and their importance in diseases such as cancer has taken priority, the study of the phosphoinositides in plants has not evolved to the same extent. Nevertheless, several studies have shown the significance of these lipids in plant cells viability and environmental response. This review focuses on phosphoinositides response to abiotic and biotic stress, showing their implication in plant survival during different stages of development. SIGNIFICANCE OF THE STUDY: This review is focused on plant PIPs functions in stress, highlighting in the main differences between plant and mammal PIPs and the novel interactions that could be extrapolated to animal models to contribute in a better understanding of these pivotal molecules.

Negative regulators of plant immunity derived from cinnamyl alcohol dehydrogenases are targeted by multiple Phytophthora Avr3a-like effectors

Oomycete pathogens secrete numerous effectors to manipulate host immunity. While some effectors share a conserved structural fold, it remains unclear if any have conserved host targets. Avr3a-like family effectors, which are related to Phytophthora infestans effector PiAvr3a and are widely distributed across diverse clades of Phytophthora species, were used to study this question. By using yeast-two-hybrid, bimolecular fluorescence complementation and co-immunoprecipitation assays, we identified members of the plant cinnamyl alcohol dehydrogenase 7 (CAD7) subfamily as targets of multiple Avr3a-like effectors from Phytophthora pathogens. The CAD7 subfamily has expanded in plant genomes but lost the lignin biosynthetic activity of canonical CAD subfamilies. In turn, we identified CAD7s as negative regulators of plant immunity that are induced by Phytophthora infection. Moreover, AtCAD7 was stabilized by Avr3a-like effectors and involved in suppression of pathogen-associated molecular pattern-triggered immunity, including callose deposition, reactive oxygen species burst and WRKY33 expression. Our results reveal CAD7 subfamily proteins as negative regulators of plant immunity that are exploited by multiple Avr3a-like effectors to promote infection in different host plants.

Structural analysis of Phytophthora suppressor of RNA silencing 2 (PSR2) reveals a conserved modular fold contributing to virulence

Phytophthora are eukaryotic pathogens that cause enormous losses in agriculture and forestry. Each Phytophthora species encodes hundreds of effector proteins that collectively have essential roles in manipulating host cellular processes and facilitating disease development. Here we report the crystal structure of the effector Phytophthora suppressor of RNA silencing 2 (PSR2). PSR2 produced by the soybean pathogen Phytophthora sojae (PsPSR2) consists of seven tandem repeat units, including one W-Y motif and six L-W-Y motifs. Each L-W-Y motif forms a highly conserved fold consisting of five α-helices. Adjacent units are connected through stable, directional linkages between an internal loop at the C terminus of one unit and a hydrophobic pocket at the N terminus of the following unit. This unique concatenation results in an overall stick-like structure of PsPSR2. Genome-wide analyses reveal 293 effectors from five Phytophthora species that have the PsPSR2-like arrangement, that is, containing a W-Y motif as the "start" unit, various numbers of L-W-Y motifs as the "middle" units, and a degenerate L-W-Y as the "end" unit. Residues involved in the interunit interactions show significant conservation, suggesting that these effectors also use the conserved concatenation mechanism. Furthermore, functional analysis demonstrates differential contributions of individual units to the virulence activity of PsPSR2. These findings suggest that the L-W-Y fold is a basic structural and functional module that may serve as a "building block" to accelerate effector evolution in Phytophthora.

Early responses given distinct tactics to infection of Peronophythora litchii in susceptible and resistant litchi cultivar

Litchi downy blight, a destructive litchi disease caused by Peronophythora litchii, is controlled by intensive fungicide applying. Sources of resistance are used in conventional breeding approaches, but the mechanism is not well understood. Follow-up six years investigation, 'Guiwei' and 'Heiye' displayed stable susceptible and resistant against to P. litchii, respectively. After 72 hour inoculation, 'Heiye' showed few disease spots, while 'Guiwei' appeared brown and covered with white sporangia. Germination of sporangia and growth of mycelium in 'Guiwei' is more quickly than in 'Heiye'. Transcript levels were measured at 6, 24, and 48 hour post-inoculation. 'Oxidation-reduction process' was dramatically enhanced in 'Heiye', which could promote its resistance to pathogen infection. A small ratio (3.78%) of common DEGs indicates that resistant and susceptible cultivars take different strategies to defense against P. litchii. At early infection stage, 'Heiye' induced a larger number of genes, including seven receptor-like kinases, which quickly recognized attack of pathogen and led to a rapidly resistance by regulation of degradation of proteasome, transcription factors, and cell wall remodeling. The early DGEs were exiguous in 'Guiwei', suggesting a weak response. Once the infection was successful, the resistance was repressed by down-regulated genes involved in phenylpropanoid metabolism, ET biosynthesis and signaling conduction in 'Guiwei'. In conclusion, quickly recognition and early responses to pathogen, as well as minimal pathogen development and basal expression of resistance-related genes, were correlated with a high level of resistance in 'Heiye', while susceptible 'Guiwei' suffered massive infection due to lagging response and repressed signal transduction.

Arms race: diverse effector proteins with conserved motifs

Effector proteins play important roles in the infection by pathogenic oomycetes and fungi or the colonization by endophytic and mycorrhizal fungi. They are either translocated into the host plant cells via specific translocation mechanisms and function in the host's cytoplasm or nucleus, or they reside in the apoplast of the plant cells and act at the extracellular host-microbe interface. Many effector proteins possess conserved motifs (such as the RXLR, CRN, LysM, RGD, DELD, EAR, RYWT, Y/F/WXC or CFEM motifs) localized in their N- or C-terminal regions. Analysis of the functions of effector proteins, especially so-called "core effectors", is crucial for the understanding of pathogenicity/symbiosis mechanisms and plant defense strategies, and helps to develop breeding strategies for pathogen-resistant cultivars, and to increase crop yield and quality as well as abiotic stress resistance. This review summarizes current knowledge about these effector proteins with the conversed motifs and their involvement in pathogenic or mutualistic plant/fungal interactions.

A positive-charged patch and stabilized hydrophobic core are essential for avirulence function of AvrPib in the rice blast fungus

Fungal avirulence effectors, a key weapon utilized by pathogens to promote their infection, are recognized by immune receptors to boost host R gene-mediated resistance. Many avirulence effectors share sparse sequence homology to proteins with known functions, and their molecular and biochemical functions together with the evolutionary relationship among different members remain largely unknown. Here, the crystal structure of AvrPib, an avirulence effector from Magnaporthe oryzae, was determined and showed a high degree of similarity to the M. oryzae Avrs and ToxB (MAX) effectors. Compared with other MAX effectors, AvrPib has a distinct positive-charge patch formed by five positive-charged residues (K29, K30, R50, K52 and K70) on the surface. These five key residues were essential to avirulence function of AvrPib and affected its nuclear localization into host cells. Moreover, residues V39 and V58, which locate in the hydrophobic core of the structure, cause loss of function of AvrPib by single-point mutation in natural isolates. In comparison with the wild-type AvrPib, the V39A or V58A mutations resulted in a partial or entire loss of secondary structure elements. Taken together, our results suggest that differences in the surface charge distribution of avirulence proteins could be one of the major bases for the variation in effector-receptor specificity, and that destabilization of the hydrophobic core is one of the major mechanisms employed by AvrPib for the fungus to evade recognition by resistance factors in the host cell.

Cell entry of a host-targeting protein of oomycetes requires gp96

The animal-pathogenic oomycete Saprolegnia parasitica causes serious losses in aquaculture by infecting and killing freshwater fish. Like plant-pathogenic oomycetes, S. parasitica employs similar infection structures and secretes effector proteins that translocate into host cells to manipulate the host. Here, we show that the host-targeting protein SpHtp3 enters fish cells in a pathogen-independent manner. This uptake process is guided by a gp96-like receptor and can be inhibited by supramolecular tweezers. The C-terminus of SpHtp3 (containing the amino acid sequence YKARK), and not the N-terminal RxLR motif, is responsible for the uptake into host cells. Following translocation, SpHtp3 is released from vesicles into the cytoplasm by another host-targeting protein where it degrades nucleic acids. The effector translocation mechanism described here, is potentially also relevant for other pathogen-host interactions as gp96 is found in both animals and plants.

Crystal structure of the Melampsora lini effector AvrP reveals insights into a possible nuclear function and recognition by the flax disease resistance protein P

The effector protein AvrP is secreted by the flax rust fungal pathogen (Melampsora lini) and recognized specifically by the flax (Linum usitatissimum) P disease resistance protein, leading to effector-triggered immunity. To investigate the biological function of this effector and the mechanisms of specific recognition by the P resistance protein, we determined the crystal structure of AvrP. The structure reveals an elongated zinc-finger-like structure with a novel interleaved zinc-binding topology. The residues responsible for zinc binding are conserved in AvrP effector variants and mutations of these motifs result in a loss of P-mediated recognition. The first zinc-coordinating region of the structure displays a positively charged surface and shows some limited similarities to nucleic acid-binding and chromatin-associated proteins. We show that the majority of the AvrP protein accumulates in the plant nucleus when transiently expressed in Nicotiana benthamiana cells, suggesting a nuclear pathogenic function. Polymorphic residues in AvrP and its allelic variants map to the protein surface and could be associated with differences in recognition specificity. Several point mutations of residues on the non-conserved surface patch result in a loss of recognition by P, suggesting that these residues are required for recognition.

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.

Identification of Plasmodiophora brassicae effectors - A challenging goal

Clubroot is an economically important disease affecting Brassica plants worldwide. Plasmodiophora brassicae is the protist pathogen associated with the disease, and its soil-borne obligate parasitic nature has impeded studies related to its biology and the mechanisms involved in its infection of the plant host. The identification of effector proteins is key to understanding how the pathogen manipulates the plant's immune response and the genes involved in resistance. After more than 140 years studying clubroot and P. brassicae, very little is known about the effectors playing key roles in the infection process and subsequent disease progression. Here we analyze the information available for identified effectors and suggest several features of effector genes that can be used in the search for others. Based on the information presented in this review, we propose a comprehensive bioinformatics pipeline for effector identification and provide a list of the bioinformatics tools available for such.

How filamentous plant pathogen effectors are translocated to host cells

The interaction of microbes with "signature" plants is largely governed by secreted effector proteins, which serve to dampen plant defense responses and modulate host cell processes. Secreted effectors can function either in the apoplast or within plant cell compartments. How oomycetes and fungi translocate their effectors to plant cells is still poorly understood and controversial. While most oomycete effectors share a common 'signature' that was proposed to mediate their uptake via endocytosis, fungal effectors display no conserved motifs at the primary amino acid sequence level. Here we summarize current knowledge in the field of oomycete and fungal effector uptake and highlight emerging themes that may unite rather than set apart these unrelated filamentous pathogens.

Intrinsic disorder is a common structural characteristic of RxLR effectors in oomycete pathogens

Intrinsic disorder is common in nature and has been studied to play important biological roles in bacterial effectors. However, disorder in oomycete RxLR effectors has not been investigated previously and the roles are unknown. Our results of PONDR VL-XT disorder analysis showed that predicted oomycete RxLR effectors were significantly more disordered than other effectors and secretome. The distribution of disorder content presented preference that RxLR-dEER regions were enriched in disordered residues, suggesting potential role of disorder in effector translocation. In contrast, the disorder content was depleted in the C-terminal regions, especially for W/Y/L motifs. We also found that around 42 % of putative RxLR proteins were predicted to contain at least one α-helix-forming molecular recognition feature (α-MoRF), and most α-MoRFs were located in the C-terminal regions. Furthermore, both of the disorder mutants of PsAvh18 and PcAvh207 lost the cell death-inducing activity, indicating the potential important role of disordered structure in RxLR effector function. Overall, these results demonstrate that intrinsic disorder is a common characteristic of oomycete RxLR proteins, and we postulate that such structure feature may be important for effector translocation or function. This study extends our understanding of RxLR effectors in protein structures, and opens up new directions to explore novel mechanisms of oomycete RxLR effectors.

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.

Effectors of Filamentous Plant Pathogens: Commonalities amid Diversity

Fungi and oomycetes are filamentous microorganisms that include a diversity of highly developed pathogens of plants. These are sophisticated modulators of plant processes that secrete an arsenal of effector proteins to target multiple host cell compartments and enable parasitic infection. Genome sequencing revealed complex catalogues of effectors of filamentous pathogens, with some species harboring hundreds of effector genes. Although a large fraction of these effector genes encode secreted proteins with weak or no sequence similarity to known proteins, structural studies have revealed unexpected similarities amid the diversity. This article reviews progress in our understanding of effector structure and function in light of these new insights. We conclude that there is emerging evidence for multiple pathways of evolution of effectors of filamentous plant pathogens but that some families have probably expanded from a common ancestor by duplication and diversification. Conserved folds, such as the oomycete WY and the fungal MAX domains, are not predictive of the precise function of the effectors but serve as a chassis to support protein structural integrity while providing enough plasticity for the effectors to bind different host proteins and evolve unrelated activities inside host cells. Further effector evolution and diversification arise via short linear motifs, domain integration and duplications, and oligomerization.

Plant phosphatidylinositol-specific phospholipase C at the center of plant innate immunity

Understanding plant resistance to pathogenic microbes requires detailed information on the molecular mechanisms controlling the execution of plant innate immune responses. A growing body of evidence places phosphoinositide-specific phospholipase C (PI-PLC) enzymes immediately downstream of activated immune receptors, well upstream of the initiation of early defense responses. An increase of the cytoplasmic levels of free Ca²⁺ , lowering of the intercellular pH and the oxidative burst are a few examples of such responses and these are regulated by PI-PLCs. Consequently, PI-PLC activation represents an early primary signaling switch between elicitation and response involving the controlled hydrolysis of essential signaling phospholipids, thereby simultaneously generating lipid and non-lipid second messenger molecules required for a swift cellular defense response. Here, we elaborate on the signals generated by PI-PLCs and their respective downstream effects, while providing an inventory of different types of evidence describing the involvement of PI-PLCs in various aspects of plant immunity. We project the discussed information into a model describing the cellular events occurring after the activation of plant immune receptors. With this review we aim to provide new insights supporting future research on plant PI-PLCs and the development of plants with improved resistance.

Human β-defensin 3 contains an oncolytic motif that binds PI(4,5)P2 to mediate tumour cell permeabilisation

Cationic antimicrobial peptides (CAPs), including taxonomically diverse defensins, are innate defense molecules that display potent antimicrobial and immunomodulatory activities. Specific CAPs have also been shown to possess anticancer activities; however, their mechanisms of action are not well defined. Recently, the plant defensin NaD1 was shown to induce tumour cell lysis by directly binding to the plasma membrane phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). The NaD1–lipid interaction was structurally defined by X-ray crystallography, with the defensin forming a dimer that binds PI(4,5)P₂ via its cationic β2-β3 loops in a ‘cationic grip’ conformation. In this study, we show that human β-defensin 3 (HBD-3) contains a homologous β2-β3 loop that binds phosphoinositides. The binding of HBD-3 to PI(4,5)P₂ was shown to be critical for mediating cytolysis of tumour cells, suggesting a conserved mechanism of action for defensins across diverse species. These data not only identify an evolutionary conservation of CAP structure and function for lipid binding, but also suggest that PIP-binding CAPs could be exploited for novel multifunction therapeutics.

Genome analysis and avirulence gene cloning using a high-density RADseq linkage map of the flax rust fungus, Melampsora lini

BACKGROUND

Rust fungi are an important group of plant pathogens that cause devastating losses in agricultural, silvicultural and natural ecosystems. Plants can be protected from rust disease by resistance genes encoding receptors that trigger a highly effective defence response upon recognition of specific pathogen avirulence proteins. Identifying avirulence genes is crucial for understanding how virulence evolves in the field.

RESULTS

To facilitate avirulence gene cloning in the flax rust fungus, Melampsora lini, we constructed a high-density genetic linkage map using single nucleotide polymorphisms detected in restriction site-associated DNA sequencing (RADseq) data. The map comprises 13,412 RADseq markers in 27 linkage groups that together span 5860 cM and contain 2756 recombination bins. The marker sequences were used to anchor 68.9 % of the M. lini genome assembly onto the genetic map. The map and anchored assembly were then used to: 1) show that M. lini has a high overall meiotic recombination rate, but recombination distribution is uneven and large coldspots exist; 2) show that substantial genome rearrangements have occurred in spontaneous loss-of-avirulence mutants; and 3) identify the AvrL2 and AvrM14 avirulence genes by map-based cloning. AvrM14 is a dual-specificity avirulence gene that encodes a predicted nudix hydrolase. AvrL2 is located in the region of the M. lini genome with the lowest recombination rate and encodes a small, highly-charged proline-rich protein.

CONCLUSIONS

The M. lini high-density linkage map has greatly advanced our understanding of virulence mechanisms in this pathogen by providing novel insights into genome variability and enabling identification of two new avirulence genes.

Structural Basis of Host Autophagy-related Protein 8 (ATG8) Binding by the Irish Potato Famine Pathogen Effector Protein PexRD54

Filamentous plant pathogens deliver effector proteins to host cells to promote infection. The Phytophthora infestans RXLR-type effector PexRD54 binds potato ATG8 via its ATG8 family-interacting motif (AIM) and perturbs host-selective autophagy. However, the structural basis of this interaction remains unknown. Here, we define the crystal structure of PexRD54, which includes a modular architecture, including five tandem repeat domains, with the AIM sequence presented at the disordered C terminus. To determine the interface between PexRD54 and ATG8, we solved the crystal structure of potato ATG8CL in complex with a peptide comprising the effector's AIM sequence, and we established a model of the full-length PexRD54-ATG8CL complex using small angle x-ray scattering. Structure-informed deletion of the PexRD54 tandem domains reveals retention of ATG8CL binding in vitro and in planta This study offers new insights into structure/function relationships of oomycete RXLR effectors and how these proteins engage with host cell targets to promote disease.

Elucidating the Role of Effectors in Plant-Fungal Interactions: Progress and Challenges

Pathogenic fungi have diverse growth lifestyles that support fungal colonization on plants. Successful colonization and infection for all lifestyles depends upon the ability to modify living host plants to sequester the necessary nutrients required for growth and reproduction. Secretion of virulence determinants referred to as “effectors” is assumed to be the key governing factor that determines host infection and colonization. Effector proteins are capable of suppressing plant defense responses and alter plant physiology to accommodate fungal invaders. This review focuses on effector molecules of biotrophic and hemibiotrophic plant pathogenic fungi, and the mechanism required for the release and uptake of effector molecules by the fungi and plant cells, respectively. We also place emphasis on the discovery of effectors, difficulties associated with predicting the effector repertoire, and fungal genomic features that have helped promote effector diversity leading to fungal evolution. We discuss the role of specific effectors found in biotrophic and hemibiotrophic fungi and examine how CRISPR/Cas9 technology may provide a new avenue for accelerating our ability in the discovery of fungal effector function.

Phytophthora infestans specific phosphorylation patterns and new putative control targets

In this study we applied biomathematical searches of gene regulatory mechanisms to learn more about oomycete biology and to identify new putative targets for pesticides or biological control against Phytophthora infestans. First, oomycete phylum-specific phosphorylation motifs were found by discriminative n-gram analysis. We found 11.600 P. infestans specific n-grams, mapping 642 phosphoproteins. The most abundant group among these related to phosphatidylinositol metabolism. Due to the large number of possible targets found and our hypothesis that multi-level control is a sign of usefulness as targets for intervention, we identified overlapping targets with a second screen. This was performed to identify proteins dually regulated by small RNA and phosphorylation. We found 164 proteins to be regulated by both sRNA and phosphorylation and the dominating functions where phosphatidylinositol signalling/metabolism, endocytosis, and autophagy. Furthermore we performed a similar regulatory study and discriminative n-gram analysis of proteins with no clear orthologs in other species and proteins that are known to be unique to P. infestans such as the RxLR effectors, Crinkler (CRN) proteins and elicitins. We identified CRN proteins with specific phospho-motifs present in all life stages. PITG_12626, PITG_14042 and PITG_23175 are CRN proteins that have species-specific phosphorylation motifs and are subject to dual regulation.

Oomycete interactions with plants: infection strategies and resistance principles

The Oomycota include many economically significant microbial pathogens of crop species. Understanding the mechanisms by which oomycetes infect plants and identifying methods to provide durable resistance are major research goals. Over the last few years, many elicitors that trigger plant immunity have been identified, as well as host genes that mediate susceptibility to oomycete pathogens. The mechanisms behind these processes have subsequently been investigated and many new discoveries made, marking a period of exciting research in the oomycete pathology field. This review provides an introduction to our current knowledge of the pathogenic mechanisms used by oomycetes, including elicitors and effectors, plus an overview of the major principles of host resistance: the established R gene hypothesis and the more recently defined susceptibility (S) gene model. Future directions for development of oomycete-resistant plants are discussed, along with ways that recent discoveries in the field of oomycete-plant interactions are generating novel means of studying how pathogen and symbiont colonizations overlap.

Computational Prediction of Effector Proteins in Fungi: Opportunities and Challenges

Effector proteins are mostly secretory proteins that stimulate plant infection by manipulating the host response. Identifying fungal effector proteins and understanding their function is of great importance in efforts to curb losses to plant diseases. Recent advances in high-throughput sequencing technologies have facilitated the availability of several fungal genomes and 1000s of transcriptomes. As a result, the growing amount of genomic information has provided great opportunities to identify putative effector proteins in different fungal species. There is little consensus over the annotation and functionality of effector proteins, and mostly small secretory proteins are considered as effector proteins, a concept that tends to overestimate the number of proteins involved in a plant-pathogen interaction. With the characterization of Avr genes, criteria for computational prediction of effector proteins are becoming more efficient. There are 100s of tools available for the identification of conserved motifs, signature sequences and structural features in the proteins. Many pipelines and online servers, which combine several tools, are made available to perform genome-wide identification of effector proteins. In this review, available tools and pipelines, their strength and limitations for effective identification of fungal effector proteins are discussed. We also present an exhaustive list of classically secreted proteins along with their key conserved motifs found in 12 common plant pathogens (11 fungi and one oomycete) through an analytical pipeline.

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.

Photosynthate Regulation of the Root System Architecture Mediated by the Heterotrimeric G Protein Complex in Arabidopsis

Assimilate partitioning to the root system is a desirable developmental trait to control but little is known of the signaling pathway underlying partitioning. A null mutation in the gene encoding the Gβ subunit of the heterotrimeric G protein complex, a nexus for a variety of signaling pathways, confers altered sugar partitioning in roots. While fixed carbon rapidly reached the roots of wild type and agb1-2 mutant seedlings, agb1 roots had more of this fixed carbon in the form of glucose, fructose, and sucrose which manifested as a higher lateral root density. Upon glucose treatment, the agb1-2 mutant had abnormal gene expression in the root tip validated by transcriptome analysis. In addition, PIN2 membrane localization was altered in the agb1-2 mutant. The heterotrimeric G protein complex integrates photosynthesis-derived sugar signaling incorporating both membrane-and transcriptional-based mechanisms. The time constants for these signaling mechanisms are in the same range as photosynthate delivery to the root, raising the possibility that root cells are able to use changes in carbon fixation in real time to adjust growth behavior.