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Allelic barley MLA immune receptors recognize sequence-unrelated avirulence effectors of the powdery mildew pathogen.

ABSTRACT: Disease-resistance genes encoding intracellular nucleotide-binding domain and leucine-rich repeat proteins (NLRs) are key components of the plant innate immune system and typically detect the presence of isolate-specific avirulence (AVR) effectors from pathogens. NLR genes define the fastest-evolving gene family of flowering plants and are often arranged in gene clusters containing multiple paralogs, contributing to copy number and allele-specific NLR variation within a host species. Barley mildew resistance locus a (Mla) has been subject to extensive functional diversification, resulting in allelic resistance specificities each recognizing a cognate, but largely unidentified, AVR_a gene of the powdery mildew fungus, Blumeria graminis f. sp. hordei (Bgh). We applied a transcriptome-wide association study among 17 Bgh isolates containing different AVR_a genes and identified AVR_a1 and AVR_a13, encoding candidate-secreted effectors recognized by Mla1 and Mla13 alleles, respectively. Transient expression of the effector genes in barley leaves or protoplasts was sufficient to trigger Mla1 or Mla13 allele-specific cell death, a hallmark of NLR receptor-mediated immunity. AVR_a1 and AVR_a13 are phylogenetically unrelated, demonstrating that certain allelic MLA receptors evolved to recognize sequence-unrelated effectors. They are ancient effectors because corresponding loci are present in wheat powdery mildew. AVR_A1 recognition by barley MLA1 is retained in transgenic Arabidopsis, indicating that AVR_A1 directly binds MLA1 or that its recognition involves an evolutionarily conserved host target of AVR_A1 Furthermore, analysis of transcriptome-wide sequence variation among the Bgh isolates provides evidence for Bgh population structure that is partially linked to geographic isolation.

SUBMITTER: Lu X

PROVIDER: S-EPMC5081590 | biostudies-literature | 2016 Oct

REPOSITORIES: biostudies-literature

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Allelic barley MLA immune receptors recognize sequence-unrelated avirulence effectors of the powdery mildew pathogen.

Lu Xunli X Kracher Barbara B Saur Isabel M L IM Bauer Saskia S Ellwood Simon R SR Wise Roger R Yaeno Takashi T Maekawa Takaki T Schulze-Lefert Paul P

Proceedings of the National Academy of Sciences of the United States of America 20161004 42

Disease-resistance genes encoding intracellular nucleotide-binding domain and leucine-rich repeat proteins (NLRs) are key components of the plant innate immune system and typically detect the presence of isolate-specific avirulence (AVR) effectors from pathogens. NLR genes define the fastest-evolving gene family of flowering plants and are often arranged in gene clusters containing multiple paralogs, contributing to copy number and allele-specific NLR variation within a host species. Barley mild ...[more]

PMID: 27702901

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Project description:Nucleotide-binding domain leucine-rich repeat-containing receptors (NLRs) in plants can detect avirulence (AVR) effectors of pathogenic microbes. The Mildew locus a (Mla) NLR gene has been shown to confer resistance against diverse fungal pathogens in cereal crops. In barley, Mla has undergone allelic diversification in the host population and confers isolate-specific immunity against the powdery mildew-causing fungal pathogen Blumeria graminis forma specialis hordei (Bgh). We previously isolated the Bgh effectors AVRA1, AVRA7, AVRA9, AVRA13, and allelic AVRA10/AVRA22, which are recognized by matching MLA1, MLA7, MLA9, MLA13, MLA10 and MLA22, respectively. Here, we extend our knowledge of the Bgh effector repertoire by isolating the AVRA6 effector, which belongs to the family of catalytically inactive RNase-Like Proteins expressed in Haustoria (RALPHs). Using structural prediction, we also identified RNase-like folds in AVRA1, AVRA7, AVRA10/AVRA22, and AVRA13, suggesting that allelic MLA recognition specificities could detect structurally related avirulence effectors. To better understand the mechanism underlying the recognition of effectors by MLAs, we deployed chimeric MLA1 and MLA6, as well as chimeric MLA10 and MLA22 receptors in plant co-expression assays, which showed that the recognition specificity for AVRA1 and AVRA6 as well as allelic AVRA10 and AVRA22 is largely determined by the receptors' C-terminal leucine-rich repeats (LRRs). The design of avirulence effector hybrids allowed us to identify four specific AVRA10 and five specific AVRA22 aa residues that are necessary to confer MLA10- and MLA22-specific recognition, respectively. This suggests that the MLA LRR mediates isolate-specific recognition of structurally related AVRA effectors. Thus, functional diversification of multi-allelic MLA receptors may be driven by a common structural effector scaffold, which could be facilitated by proliferation of the RALPH effector family in the pathogen genome.

Project description:A large-scale parallel expression analysis was conducted to elucidate Mla-specified responses to powdery mildew infection using 22K Barley1 GeneChip probe arrays. Our goal was to identify genes differentially expressed in incompatible (resistant) vs. compatible (susceptible) and Mla-specified Rar1-dependent vs. -independent interactions. A split-split-plot design with 108 experimental units (3 replications x 2 isolates x 3 genotypes x 6 time points) was used to profile near-isogenic lines containing the Mla1, Mla6, and Mla13 resistance specificities in response to inoculation with the Blumeria graminis f. sp. hordei (Bgh) isolates 5874 (AvrMla1, AvrMla6) and K1 (AvrMla1, AvrMla13). ****[PLEXdb(http://www.plexdb.org) has submitted this series at GEO on behalf of the original contributor, Rico Caldo. The equivalent experiment is BB4 at PLEXdb.] genotype: CI 16137 (Mla1) - pathogen isolates: Bgh 5874 - time: 0 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh 5874 - time: 8 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh 5874 - time: 16 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh 5874 - time: 20 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh 5874 - time: 24 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh 5874 - time: 32 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh K1 - time: 0 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh K1 - time: 8 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh K1 - time: 16 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh K1 - time: 20 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh K1 - time: 24 hai(3-replications); genotype: CI 16137 (Mla1) - pathogen isolates: Bgh K1 - time: 32 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh 5874 - time: 0 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh 5874 - time: 8 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh 5874 - time: 16 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh 5874 - time: 20 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh 5874 - time: 24 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh 5874 - time: 32 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh K1 - time: 0 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh K1 - time: 8 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh K1 - time: 16 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh K1 - time: 20 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh K1 - time: 24 hai(3-replications); genotype: CI 16151 (Mla6) - pathogen isolates: Bgh K1 - time: 32 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh 5874 - time: 0 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh 5874 - time: 8 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh 5874 - time: 16 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh 5874 - time: 20 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh 5874 - time: 24 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh 5874 - time: 32 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh K1 - time: 0 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh K1 - time: 8 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh K1 - time: 16 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh K1 - time: 20 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh K1 - time: 24 hai(3-replications); genotype: CI 16155 (Mla13) - pathogen isolates: Bgh K1 - time: 32 hai(3-replications)

Project description:BackgroundPlants encounter pathogenic and non-pathogenic microorganisms on a nearly constant basis. Small RNAs such as siRNAs and miRNAs/milRNAs influence pathogen virulence and host defense responses. We exploited the biotrophic interaction between the powdery mildew fungus, Blumeria graminis f. sp. hordei (Bgh), and its diploid host plant, barley (Hordeum vulgare) to explore fungal and plant sRNAs expressed during Bgh infection of barley leaf epidermal cells.ResultsRNA was isolated from four fast-neutron immune-signaling mutants and their progenitor over a time course representing key stages of Bgh infection, including appressorium formation, penetration of epidermal cells, and development of haustorial feeding structures. The Cereal Introduction (CI) 16151 progenitor carries the resistance allele Mla6, while Bgh isolate 5874 harbors the AVRa6 avirulence effector, resulting in an incompatible interaction. Parallel Analysis of RNA Ends (PARE) was used to verify sRNAs with likely transcript targets in both barley and Bgh. Bgh sRNAs are predicted to regulate effectors, metabolic genes, and translation-related genes. Barley sRNAs are predicted to influence the accumulation of transcripts that encode auxin response factors, NAC transcription factors, homeodomain transcription factors, and several splicing factors. We also identified phasing small interfering RNAs (phasiRNAs) in barley that overlap transcripts that encode receptor-like kinases (RLKs) and nucleotide-binding, leucine-rich domain proteins (NLRs).ConclusionsThese data suggest that Bgh sRNAs regulate gene expression in metabolism, translation-related, and pathogen effectors. PARE-validated targets of predicted Bgh milRNAs include both EKA (effectors homologous to AVRk1 and AVRa10) and CSEP (candidate secreted effector protein) families. We also identified barley phasiRNAs and miRNAs in response to Bgh infection. These include phasiRNA loci that overlap with a significant proportion of receptor-like kinases, suggesting an additional sRNA control mechanism may be active in barley leaves as opposed to predominant R-gene phasiRNA overlap in many eudicots. In addition, we identified conserved miRNAs, novel miRNA candidates, and barley genome mapped sRNAs that have PARE validated transcript targets in barley. The miRNA target transcripts are enriched in transcription factors, signaling-related proteins, and photosynthesis-related proteins. Together these results suggest both barley and Bgh control metabolism and infection-related responses via the specific accumulation and targeting of genes via sRNAs.

Project description:Protein effectors of pathogenicity are instrumental in modulating host immunity and disease resistance. The powdery mildew pathogen of grasses Blumeria graminis causes one of the most important diseases of cereal crops. B. graminis is an obligate biotrophic pathogen and as such has an absolute requirement to suppress or avoid host immunity if it is to survive and cause disease.Here we characterise a superfamily predicted to be the full complement of Candidates for Secreted Effector Proteins (CSEPs) in the fungal barley powdery mildew parasite B. graminis f.sp. hordei. The 491 genes encoding these proteins constitute over 7% of this pathogen's annotated genes and most were grouped into 72 families of up to 59 members. They were predominantly expressed in the intracellular feeding structures called haustoria, and proteins specifically associated with the haustoria were identified by large-scale mass spectrometry-based proteomics. There are two major types of effector families: one comprises shorter proteins (100-150 amino acids), with a high relative expression level in the haustoria and evidence of extensive diversifying selection between paralogs; the second type consists of longer proteins (300-400 amino acids), with lower levels of differential expression and evidence of purifying selection between paralogs. An analysis of the predicted protein structures underscores their overall similarity to known fungal effectors, but also highlights unexpected structural affinities to ribonucleases throughout the entire effector super-family. Candidate effector genes belonging to the same family are loosely clustered in the genome and are associated with repetitive DNA derived from retro-transposons.We employed the full complement of genomic, transcriptomic and proteomic analyses as well as structural prediction methods to identify and characterize the members of the CSEPs superfamily in B. graminis f.sp. hordei. Based on relative intron position and the distribution of CSEPs with a ribonuclease-like domain in the phylogenetic tree we hypothesize that the associated genes originated from an ancestral gene, encoding a secreted ribonuclease, duplicated successively by repetitive DNA-driven processes and diversified during the evolution of the grass and cereal powdery mildew lineage.

Project description:BackgroundGrass powdery mildew (Blumeria graminis, Ascomycota) is a major pathogen of cereal crops and has become a model organism for obligate biotrophic fungal pathogens of plants. The sequenced genomes of two formae speciales (ff.spp.), B.g. hordei and B.g. tritici (pathogens of barley and wheat), were found to be enriched in candidate effector genes (CEGs). Similar to other filamentous pathogens, CEGs in B. graminis are under positive selection. Additionally, effectors are more likely to have presence-absence polymorphisms than other genes among different strains.ResultsHere we identified effectors in the genomes of three additional host-specific lineages of B. graminis (B.g. poae, B.g. avenae and B.g. infecting Lolium) which diverged between 24 and 5 million years ago (Mya). We found that most CEGs in B. graminis are clustered in families and that most families are present in both reference genomes (B.g. hordei and B.g. tritici) and in the genomes of all three newly annotated lineages. We identified conserved protein domains including a novel lipid binding domain. The phylogenetic analysis showed that frequent gene duplications and losses shaped the diversity of the effector repertoires of the different lineages through their evolutionary history. We observed several lineage-specific expansions where large clades of CEGs originated in only one lineage from a single gene through repeated gene duplications. When we applied a birth-death model we found that the turnover rate (the rate at which genes are deleted and duplicated) of CEG families is much higher than for non-CEG families. The analysis of genomic context revealed that the immediate surroundings of CEGs are enriched in transposable elements (TE) which could play a role in the duplication and deletion of CEGs.ConclusionsThe CEG repertoires of related pathogens diverged dramatically in short evolutionary times because of rapid turnover and of positive selection fixing non-synonymous mutations. While signatures of positive selection on effector sequences are the expected outcome of the evolutionary "arms race" between pathogen and plant immune system, it is more difficult to infer the mechanisms and evolutionary forces that maintained an extreme turnover rate in CEG families of B. graminis for several millions of years.

Dataset Information

Allelic barley MLA immune receptors recognize sequence-unrelated avirulence effectors of the powdery mildew pathogen.

Publications

Allelic barley MLA immune receptors recognize sequence-unrelated avirulence effectors of the powdery mildew pathogen.

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