Powdery mildew fungi (Ascomycota phylum) are obligate biotrophic plant pathogens that can only grow and reproduce on living host cells. They infect a wide range of plants, including many crops and the diseases they cause are common, easily recognizable and widespread. Although functional investigations in these genetically intractable organisms have been hampered by their obligate biotrophic nature, recent advances in genomics and transcriptomics have contributed tremendously to our understanding of powdery mildew biology. Comparative genomics was a powerful tool to pinpoint what distinguishes powdery mildew fungi from other filamentous plant pathogens and helped us to better understand how obligate biotrophy evolved. Comparative genome analyses among isolates in both the wheat and the barley powdery mildew lineages revealed isolate-specific mosaic genome structures of evolutionary young and old haplogroups. In addition to providing hints into the evolutionary origin of powdery mildew fungi, the observed mosaic genome structure also reflects the reproductive mode of these pathogens and explains how the large standing genetic variation is generated in powdery mildew populations. In this chapter, I discuss how the revolution in genomics has contributed and will contribute in the future to better understand the obligate biotrophic lifestyle, the virulence arsenal, the reproductive mode and the evolutionary history of powdery mildew fungi
This review focuses on plant peptides involved in defense against pathogen infection and those involved in the regulation of growth and development. Defense peptides, defensins, cyclotides and anti-microbial peptides are compared and contrasted. Signaling peptides are classified according to their major sites of activity. Finally, a network approach to creating an interactomic peptide map is described.
Plants use a range of mechanisms to respond to challenge by plant pathogens and, in turn, plant pathogens use a range of mechanisms to interfere with and evade these responses. Plant defence responses include those that are mediated by salicylic acid and jasmonic acid–ethylene but, so far, few phytopathogen effectors that interfere with these hormone-based systems have been identified. A study in PLoS Biology now shows that an effector from the biotrophic oomycete pathogen Hyaloperonospora arabidopsidis attenuates the salicylic acid response, thus enhancing biotrophy.
Many oomycete effectors contain an amino-terminal RXLR (where X is any amino acid) motif that correlates with entry into host cells. Previous characterization of the RXLR effector repertoire of H. arabidopsidis identified a subset of effectors that localize to the plant cell nucleus and interact with components of the Mediator complex — a multisubunit transcriptional regulation complex that is present in all eukaryotes. In plants, Mediator has been shown to influence key processes, including plant development and, recently, plant immunity.
Caillaud and colleagues present an in-depth characterization of one of the H. arabidopsidis RXLR nuclear effectors, HaRxL44, which interacts with the MED19a subunit of the Mediator complex. Analysis of transgenic Arabidopsis thaliana lines showed that those in which MED19a was non-functional were more susceptible than the wild type to infection with H. arabidopsidsis. By contrast, transgenic lines in which MED19a was overproduced were more resistant to infection than the wild type. This indicates that MED19a is a positive regulator of A. thaliana immunity to H. arabidopsidsisinfection. Confocal microscopy to monitor the subcellular localization of MED19a-containing and HaRxL44-containing fusion proteins showed that both localize to the plant nucleolus and nucleoplasm. However, colocalization analysis showed that when HaRxL44 was present in the nucleoplasm MED19a could not be detected. Further investigations revealed that HaRxL44 degrades MED19a in a proteasome-dependent manner.
To pinpoint the mechanism by which HaRxL44-mediated MED19a degradation affects the immune response to H. arabidopsidsis infection, the authors used quantitative reverse transcription (qRT)-PCR analysis and gene expression profiling. They found that, in the presence of HaRxL44, marker genes that were consistent with jasmonic acid–ethylene signalling were upregulated, whereas marker genes that were consistent with salicylic acid signalling — which has a key role in the response to biotrophic pathogens — were downregulated. Finally, the authors investigated the cell-specific expression patterns of marker genes that are associated with the salicylic acid response and found that H. arabidopsidsis infection only suppresses the salicylic acid response in those cells that have been parasitized by oomycete haustoria.
These data show that the H. arabidopsidsis HaRxL44 effector targets MED19a to modulate the balance between jasmonic acid–ethylene and salicylic acid signalling, such that the salicylic acid response is attenuated and biotrophic infection is favoured. It will be interesting to follow future studies that characterize the functions of the other H. arabidopsidsis RXLR nuclear effectors.
Caillaud, M.-C. et al. A downy mildew effector attenuates salicylic acid-triggered immunity inArabidopsis by interacting with the host mediator complex. PLoS Biol. 11, e1001732 (2013)
Genes encoding plant nucleotide-binding leucine-rich repeat (NB-LRR) proteins confer dominant resistance to diverse pathogens. The wild-type potato NB-LRR protein Rx confers resistance against a single strain of potato virus X (PVX), whereas LRR mutants protect against both a second PVX strain and the distantly related poplar mosaic virus (PopMV). In one of the Rx mutants there was a cost to the broad-spectrum resistance because the response to PopMV was transformed from a mild disease on plants carrying wild-type Rx to a trailing necrosis that killed the plant. To explore the use of secondary mutagenesis to eliminate this cost of broad-spectrum resistance, we performed random mutagenesis of the N-terminal domains of this broad-recognition version of Rx and isolated four mutants with a stronger response against the PopMV coat protein due to enhanced activation sensitivity. These mutations are located close to the nucleotide-binding pocket, a highly conserved structure that likely controls the “switch” between active and inactive NB-LRR conformations. Stable transgenic plants expressing one of these versions of Rx are resistant to the strains of PVX and the PopMV that previously caused trailing necrosis. We conclude from this work that artificial evolution of NB-LRR disease resistance genes in crops can be enhanced by modification of both activation and recognition phases, to both accentuate the positive and eliminate the negative aspects of disease resistance.
Live-cell imaging assisted by fluorescent markers has been fundamental to understanding the focused secretory 'warfare' that occurs between plants and biotrophic pathogens that feed on living plant cells. Pathogens succeed through the spatiotemporal deployment of a remarkably diverse range of effector proteins to control plant defences and cellular processes. Some effectors can be secreted by appressoria even before host penetration, many enter living plant cells where they target diverse subcellular compartments and others move into neighbouring cells to prepare them before invasion. This Review summarizes the latest advances in our understanding of the cell biology of biotrophic interactions between plants and their eukaryotic filamentous pathogens based on in planta analyses of effectors.
To cause plant diseases, pathogenic micro-organisms secrete effector proteins into host tissue to suppress immunity and support pathogen growth. Bacterial pathogens have evolved several distinct secretion systems to target effector proteins, but whether fungi, which cause the major diseases of most crop species, also require different secretory mechanisms is not known. Here we report that the rice blast fungus Magnaporthe oryzae possesses two distinct secretion systems to target effectors during plant infection. Cytoplasmic effectors, which are delivered into host cells, preferentially accumulate in the biotrophic interfacial complex, a novel plant membrane-rich structure associated with invasive hyphae. We show that the biotrophic interfacial complex is associated with a novel form of secretion involving exocyst components and the Sso1 t-SNARE. By contrast, effectors that are secreted from invasive hyphae into the extracellular compartment follow the conventional secretory pathway. We conclude that the blast fungus has evolved distinct secretion systems to facilitate tissue invasion.
Plant growth and response to environmental cues are largely governed by phytohormones. The plant hormones ethylene, jasmonic acid, and salicylic acid (SA) play a central role in the regulation of plant immune responses. In addition, other plant hormones, such as auxins, abscisic acid (ABA), cytokinins, gibberellins, and brassinosteroids, that have been thoroughly described to regulate plant development and growth, have recently emerged as key regulators of plant immunity. Plant hormones interact in complex networks to balance the response to developmental and environmental cues and thus limiting defense-associated fitness costs. The molecular mechanisms that govern these hormonal networks are largely unknown. Moreover, hormone signaling pathways are targeted by pathogens to disturb and evade plant defense responses. In this review, we address novel insights on the regulatory roles of the ABA, SA, and auxin in plant resistance to pathogens and we describe the complex interactions among their signal transduction pathways. The strategies developed by pathogens to evade hormone-mediated defensive responses are also described. Based on these data we discuss how hormone signaling could be manipulated to improve the resistance of crops to pathogens.
Nicolas Denancé, Andrea Sánchez-Vallet, Deborah Goffner, and Antonio Molina
Plants respond to pathogens using elaborate networks of genetic interactions. Recently, significant progress has been made in understanding RNA silencing and how viruses counter this apparently ubiquitous antiviral defense.
Plant roots are host to a multitude of filamentous microorganisms. Among these, arbuscular mycorrhizal fungi provide benefits to plants, while pathogens trigger diseases resulting in significant crop yield losses. It is therefore imperative to study processes which allow plants to discriminate detrimental and beneficial interactions in order to protect crops from diseases while retaining the ability for sustainable bio-fertilisation strategies. Accumulating evidence suggests that some symbiosis processes also affect plant–pathogen interactions. A large part of this overlap likely constitutes plant developmental processes. Moreover, microbes utilise effector proteins to interfere with plant development. Here we list relevant recent findings on how plant–microbe interactions intersect with plant development and highlight future research leads.
In plants, cell-surface receptors control immunity and development through the recognition of extracellular ligands. Leucine-rich repeat receptor-like proteins (LRR-RLPs) constitute a large multigene family of cell-surface receptors. Although this family has been intensively studied, a limited number of ligands has been identified so far, mostly because methods used for their identification and characterisation are complex and fastidious. In this study, we combined genome and transcriptome analyses to describe the LRR-RLP gene family in the model tree poplar (Populus trichocarpa). In total, 82 LRR-RLP genes have been identified in P. trichocarpa genome, among which 66 are organised in clusters of up to seven members. In these clusters, LRR-RLP genes are interspersed by orphan, poplar-specific genes encoding small proteins of unknown function (SPUFs). In particular, the nine largest clusters of LRR-RLP genes (47 LRR-RLPs) include 71 SPUF genes that account for 59% of the non-LRR-RLP gene content within these clusters. Forty-four LRR-RLP and fifty-five SPUF genes are expressed in poplar leaves, mostly at low levels, except for members of some clusters that show higher and sometimes coordinated expression levels. Notably, wounding of poplar leaves strongly induced the expression of a defense SPUF gene named Rust-Induced Secreted protein (RISP) that has been previously reported as a marker of poplar defense responses. Interestingly, we show that the RISP-associated LRR-RLP gene is highly expressed in poplar leaves and slightly induced by wounding. Both gene promoters share a highly conserved region of approx. 300 nucleotides. This led us to hypothesize that the corresponding pair of proteins could be involved in poplar immunity, possibly as a ligand/receptor couple.In conclusion, we speculate that some poplar SPUFs, such as RISP, represent candidate endogenous peptide ligands of the associated LRR-RLPs and we discuss how to investigate further this hypothesis.
Plants are members of complex communities and interact both with antagonists and beneficial organisms. An important question in plant defense-signaling research is how plants integrate signals induced by pathogens, insect herbivores and beneficial microbes into the most appropriate adaptive response. Molecular and genomic tools are now being used to uncover the complexity of the induced defense signaling networks that have evolved during the arms races between plants and the other organisms with which they intimately interact. To understand the functioning of the complex defense signaling network in nature, molecular biologists and ecologists have joined forces to place molecular mechanisms of induced plant defenses in an ecological perspective. In this Research Topic, we aim to provide an on-line, open-access snapshot of the current state of the art of the field of induced plant responses to microbes and insects, with a special focus on the translation of molecular mechanisms to ecology and vice versa. We will collect Original Research and Review papers on the topic, but also other article types, such as Methods and Opinions are welcome.
GAINESVILLE, Fla. --- Researchers from the Institute of Food and Agricultural Sciences at the University of Florida are closer to finding a possible cure for citrus canker after identifying a gene that makes citrus trees susceptible to the bacterial pathogen
RenSeq is a NB-LRR gene-targeted, Resistance gene enrichment and sequencing method that enables discovery and annotation of pathogen resistance gene family members in plant genome sequences. We successfully applied RenSeq to the sequenced potatoSolanum tuberosum clone DM, and increased the number of identified NB-LRRs from 438 to 755. The majority of these identified R gene loci reside in poorly- or previous un-annotated regions of the genome. Sequence and positional details on the twelve chromosomes have been established for 704 NB-LRRs and can be accessed through a genome browser that we provide. We compared these NB-LRR genes and the corresponding oligonucleotide baits with the highest sequence similarity and demonstrated that ~80% sequence identity is sufficient for enrichment. Analysis of the sequenced tomato S. lycopersicum extended the NB-LRR complement to 394 loci. We further describe a methodology that applies RenSeq to rapidly identify molecular markers that co-segregate with a trait of interest. In two independent segregating populations involving the wild Solanum species S. berthaultii (Rpi-ber2) and S. ruiz-ceballosii (Rpi-rzc1), we were able to apply RenSeq to successfully identify markers that co-segregate with resistance towards the late blight pathogenPhytophthora infestans. These SNP identification workflows were designed as easy-to-adapt Galaxy pipelines.
Receptor(-like) kinases with Lysin Motif (LysM) domains in their extracellular region play crucial roles during plant interactions with microorganisms; e.g. Arabidopsis thaliana CERK1 activates innate immunity upon perception of fungal chitin/chitooligosaccharides, whereas Medicago truncatula NFP and LYK3 mediate signalling upon perception of bacterial lipo-chitooligosaccharides, termed Nod factors, during the establishment of mutualism with nitrogen-fixing rhizobia. However, little is still known about the exact activation and signalling mechanisms of MtNFP and MtLYK3. We aimed at investigating putative molecular interactions of MtNFP and MtLYK3 produced in Nicotiana benthamiana. Surprisingly, heterologous co-production of these proteins resulted in an induction of defence-like responses, which included defence-related gene expression, accumulation of phenolic compounds, and cell death. Similar defence-like responses were observed upon production of AtCERK1 in N. benthamiana leaves. Production of either MtNFP or MtLYK3 alone or their co-production with other unrelated receptor(-like) kinases did not induce cell death in N. benthamiana, indicating that a functional interaction between these LysM receptor-like kinases is required for triggering this response. Importantly, structure-function studies revealed that the MtNFP intracellular region, specific features of the MtLYK3 intracellular region (including several putative phosphorylation sites), and MtLYK3 and AtCERK1 kinase activity were indispensable for cell death induction, thereby mimicking the structural requirements of nodulation or chitin-induced signalling. The observed similarity of N. benthamiana response to MtNFP and MtLYK3 co-production and AtCERK1 production suggests the existence of parallels between Nod factor-induced and chitin-induced signalling mediated by the respective LysM receptor(-like) kinases. Notably, the conserved structural requirements for MtNFP and MtLYK3 biological activity in M. truncatula (nodulation) and in N. benthamiana (cell death induction) indicates the relevance of the latter system for studies on these, and potentially other symbiotic LysM receptor-like kinases.
The plant immune system is activated by microbial patterns that are detected as nonself molecules. Such patterns are recognized by immune receptors that are cytoplasmic or localized at the plasma membrane. Cell surface receptors are represented by receptor-like kinases (RLKs) that frequently contain extracellular leucine-rich repeats and an intracellular kinase domain for activation of downstream signaling, as well as receptor-like proteins (RLPs) that lack this signaling domain. It is therefore hypothesized that RLKs are required for RLPs to activate downstream signaling. The RLPs Cf-4 and Ve1 of tomato (Solanum lycopersicum) mediate resistance to the fungal pathogens Cladosporium fulvum and Verticillium dahliae, respectively. Despite their importance, the mechanism by which these immune receptors mediate downstream signaling upon recognition of their matching ligand, Avr4 and Ave1, remained enigmatic. Here we show that the tomato ortholog of the Arabidopsis thaliana RLK Suppressor Of BIR1-1/Evershed (SOBIR1/EVR) and its close homolog S. lycopersicum (Sl)SOBIR1-like interact in planta with both Cf-4 and Ve1 and are required for the Cf-4– and Ve1-mediated hypersensitive response and immunity. Tomato SOBIR1/EVR interacts with most of the tested RLPs, but not with the RLKs FLS2, SERK1, SERK3a, BAK1, and CLV1. SOBIR1/EVR is required for stability of the Cf-4 and Ve1 receptors, supporting our observation that these RLPs are present in a complex with SOBIR1/EVR in planta. We show that SOBIR1/EVR is essential for RLP-mediated immunity and propose that the protein functions as a regulatory RLK of this type of cell-surface receptors.