Components Of Plant Immunity Research Articles

Pattern Recognition Receptors—Versatile Genetic Tools for Engineering Broad-Spectrum Disease Resistance in Crops

Infestations of crop plants with pathogens pose a major threat to global food supply. Exploiting plant defense mechanisms to produce disease-resistant crop varieties is an important strategy to control plant diseases in modern plant breeding and can greatly reduce the application of agrochemicals. The discovery of different types of immune receptors and a detailed understanding of their activation and regulation mechanisms in the last decades has paved the way for the deployment of these central plant immune components for genetic plant disease management. This review will focus on a particular class of immune sensors, termed pattern recognition receptors (PRRs), that activate a defense program termed pattern-triggered immunity (PTI) and outline their potential to provide broad-spectrum and potentially durable disease resistance in various crop species—simply by providing plants with enhanced capacities to detect invaders and to rapidly launch their natural defense program.

DNA damage as a consequence of NLR activation

DNA damage observed during plant immune responses is reported to be an intrinsic component of plant immunity. However, other immune responses may suppress DNA damage to maintain host genome integrity. Here, we show that immunity-related DNA damage can be abrogated by preventing cell death triggered by Nucleotide-binding, Leucine-rich-repeat immune Receptors (NLRs). SNI1 (suppressor of npr1-1, inducible 1), a subunit of the structural maintenance of chromosome (SMC) 5/6 complex, was reported to be a negative regulator of systemic acquired resistance (SAR) and to be necessary for controlling DNA damage. We find that cell death and DNA damage in sni1 loss-of-function mutants are prevented by mutations in the NLR signaling component EDS1. Similar to sni1, elevated DNA damage is seen in other autoimmune mutants with cell death lesions, including camta3, pub13 and vad1, but not in dnd1, an autoimmune mutant with no visible cell death. We find that as in sni1, DNA damage in camta3 is EDS1-dependent, but that it is also NLR-dependent. Using the NLR RPM1 as a model, we also show that extensive DNA damage is observed when an NLR is directly triggered by effectors. We also find that the expression of DNA damage repair (DDR) genes in mutants with cell death lesions is down regulated, suggesting that degraded DNA that accumulates during cell death is a result of cellular dismantling and is not sensed as damaged DNA that calls for repair. Our observations also indicate that SNI1 is not directly involved in SAR or DNA damage accumulation.

Cyclic Dipeptides from Bacillus vallismortis BS07 Require Key Components of Plant Immunity to Induce Disease Resistance in Arabidopsis against Pseudomonas Infection.

Cyclic dipeptides (CDPs) are one of the simplest compounds produced by living organisms. Plant-growth promoting rhizobacteria (PGPRs) also produce CDPs that can induce disease resistance. Bacillus vallismortis strain BS07 producing various CDPs has been evaluated as a potential biocontrol agent against multiple plant pathogens in chili pepper. However, plant signal pathway triggered by CDPs has not been fully elucidated yet. Here we introduce four CDPs, cyclo(Gly-L-Pro) previously identified from Aspergillus sp., and cyclo(L-Ala-L-Ile), cyclo(L-Ala-L-Leu), and cyclo(LLeu-L-Pro) identified from B. vallismortis BS07, which induce disease resistance in Arabidopsis against Pseudomonas syringae infection. The CDPs do not directly inhibit fungal and oomycete growth in vitro. These CDPs require PHYTOALEXIN DEFICIENT4, SALICYLIC ACID INDUCTION DEFICIENT2, and NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 important for salicylic acid-dependent defense to induce resistance. On the other hand, regulators involved in jasmonate-dependent event, such as ETHYLENE RECEPTOR1, JASMONATE RESPONSE1, and JASMONATE INSENSITIVE1, are necessary to the CDP-induced resistance. Furthermore, treatment of these CDPs primes Arabidopsis plants to rapidly express PATHOGENESIS-RELATED PROTEIN4 at early infection phase. Taken together, we propose that these CDPs from PGPR strains accelerate activation of jasmonate-related signaling pathway during infection.

Nucleoporin-Regulated MAP Kinase Signaling in Immunity to a Necrotrophic Fungal Pathogen.

Pathogen-responsive mitogen-activated protein kinase (MAPK or MPK) cascades relay signals from activated immune receptors across the nuclear envelope to intranuclear targets. However, in plants, little is known about the spatial control of MAPK signaling. Here, we report that the Arabidopsis (Arabidopsis thaliana) nuclear pore complex protein Nup88/MOS7 is essential for immunity to the necrotrophic fungus Botrytis cinerea The mos7-1 mutation, causing a four-amino acid deletion, compromises B. cinerea-induced activation of the key immunoregulatory MAPKs MPK3/MPK6 and reduces MPK3 protein levels posttranscriptionally. Furthermore, MOS7 contributes to retaining a sufficient MPK3 abundance in the nucleus, which is required for full immunity to B. cinerea Finally, we present a structural model of MOS7 and show that the mos7-1 mutation compromises interactions with Nup98a/b, two phenylalanine-glycine repeat nucleoporins implicated in maintaining the selective nuclear pore complex permeability barrier. Together, our analysis uncovered MOS7 and Nup98 as novel components of plant immunity toward a necrotrophic pathogen and provides mechanistic insights into how these nucleoporins coordinate nucleocytoplasmic transport to mount a robust immune response.

Recombination Rate Heterogeneity within Arabidopsis Disease Resistance Genes.

Meiotic crossover frequency varies extensively along chromosomes and is typically concentrated in hotspots. As recombination increases genetic diversity, hotspots are predicted to occur at immunity genes, where variation may be beneficial. A major component of plant immunity is recognition of pathogen Avirulence (Avr) effectors by resistance (R) genes that encode NBS-LRR domain proteins. Therefore, we sought to test whether NBS-LRR genes would overlap with meiotic crossover hotspots using experimental genetics in Arabidopsis thaliana. NBS-LRR genes tend to physically cluster in plant genomes; for example, in Arabidopsis most are located in large clusters on the south arms of chromosomes 1 and 5. We experimentally mapped 1,439 crossovers within these clusters and observed NBS-LRR gene associated hotspots, which were also detected as historical hotspots via analysis of linkage disequilibrium. However, we also observed NBS-LRR gene coldspots, which in some cases correlate with structural heterozygosity. To study recombination at the fine-scale we used high-throughput sequencing to analyze ~1,000 crossovers within the RESISTANCE TO ALBUGO CANDIDA1 (RAC1) R gene hotspot. This revealed elevated intragenic crossovers, overlapping nucleosome-occupied exons that encode the TIR, NBS and LRR domains. The highest RAC1 recombination frequency was promoter-proximal and overlapped CTT-repeat DNA sequence motifs, which have previously been associated with plant crossover hotspots. Additionally, we show a significant influence of natural genetic variation on NBS-LRR cluster recombination rates, using crosses between Arabidopsis ecotypes. In conclusion, we show that a subset of NBS-LRR genes are strong hotspots, whereas others are coldspots. This reveals a complex recombination landscape in Arabidopsis NBS-LRR genes, which we propose results from varying coevolutionary pressures exerted by host-pathogen relationships, and is influenced by structural heterozygosity.

A complex interplay of tandem- and whole-genome duplication drives expansion of the L-type lectin receptor kinase gene family in the brassicaceae.

The comparative analysis of plant gene families in a phylogenetic framework has greatly accelerated due to advances in next generation sequencing. In this study, we provide an evolutionary analysis of the L-type lectin receptor kinase and L-type lectin domain proteins (L-type LecRKs and LLPs) that are considered as components in plant immunity, in the plant family Brassicaceae and related outgroups. We combine several lines of evidence provided by sequence homology, HMM-driven protein domain annotation, phylogenetic analysis, and gene synteny for large-scale identification of L-type LecRK and LLP genes within nine core-eudicot genomes. We show that both polyploidy and local duplication events (tandem duplication and gene transposition duplication) have played a major role in L-type LecRK and LLP gene family expansion in the Brassicaceae. We also find significant differences in rates of molecular evolution based on the mode of duplication. Additionally, we show that LLPs share a common evolutionary origin with L-type LecRKs and provide a consistent gene family nomenclature. Finally, we demonstrate that the largest and most diverse L-type LecRK clades are lineage-specific. Our evolutionary analyses of these plant immune components provide a framework to support future plant resistance breeding.

Small RNAs in plant defense responses during viral and bacterial interactions: similarities and differences

Small non-coding RNAs constitute an important class of gene expression regulators that control different biological processes in most eukaryotes. In plants, several small RNA (sRNA) silencing pathways have evolved to produce a wide range of small RNAs with specialized functions. Evidence for the diverse mode of action of the small RNA pathways has been highlighted during plant–microbe interactions. Host sRNAs and small RNA silencing pathways have been recognized as essential components of plant immunity. One way plants respond and defend against pathogen infections is through the small RNA silencing immune system. To deal with plant defense responses, pathogens have evolved sophisticated mechanisms to avoid and counterattack this defense strategy. The relevance of the small RNA-mediated plant defense responses during viral infections has been well-established. Recent evidence points out its importance also during plant–bacteria interactions. Herein, this review discusses recent findings, similarities and differences about the small RNA-mediated arms race between plants and these two groups of microbes, including the small RNA silencing pathway components that contribute to plant immune responses, the pathogen-responsive endogenous sRNAs and the pathogen-delivered effector proteins.

Activation of R‐mediated innate immunity and disease susceptibility is affected by mutations in a cytosolic O‐acetylserine (thiol) lyase in Arabidopsis

O-acetylserine (thiol) lyases (OASTLs) are evolutionarily conserved proteins among many prokaryotes and eukaryotes that perform sulfur acquisition and synthesis of cysteine. A mutation in the cytosolic OASTL-A1 protein ONSET OF LEAF DEATH3 (OLD3) was previously shown to reduce the OASTL activity of the old3-1 protein in vitro and cause auto-necrosis in specific Arabidopsis accessions. Here we investigated why a mutation in this protein causes auto-necrosis in some but not other accessions. The auto-necrosis was found to depend on Recognition of Peronospora Parasitica1 (RPP1)-like disease resistance R gene(s) from an evolutionarily divergent R gene cluster that is present in Ler-0 but not the reference accession Col-0. RPP1-like gene(s) show a negative epistatic interaction with the old3-1 mutation that is not linked to reduced cysteine biosynthesis. Metabolic profiling and transcriptional analysis further indicate that an effector triggered-like immune response and metabolic disorder are associated with auto-necrosis in old3-1 mutants, probably activated by an RPP1-like gene. However, the old3-1 protein in itself results in largely neutral changes in primary plant metabolism, stress defence and immune responses. Finally, we showed that lack of a functional OASTL-A1 results in enhanced disease susceptibility against infection with virulent and non-virulent Pseudomonas syringae pv. tomato DC3000 strains. These results reveal an interaction between the cytosolic OASTL and components of plant immunity.

Mapping FLS2 function to structure: LRRs, kinase and its working bits

The plasma membrane-localised FLAGELLIN SENSING 2 (FLS2) receptor is an important component of plant immunity against potentially pathogenic bacteria, acting to recognise the conserved flg22 peptide of flagellin. FLS2 shares the common structure of transmembrane receptor kinases with a receptor-like ectodomain composed of leucine-rich repeats (LRR) and an active intracellular kinase domain. Upon ligand binding, FLS2 dimerises with the regulatory LRR-receptor kinase BRI1-associated kinase 1, which in turn triggers downstream signalling cascades. Although lacking crystal structure data, recent advances have been made in our understanding of flg22 recognition based on structural and functional analyses of FLS2. These studies have revealed critical regions/residues of FLS2 and post-translational modifications that regulate the abundance and activity of this receptor. In this review, we present the current knowledge on the structural mechanism of the FLS2-flg22 interaction and subsequent receptor-mediated signalling.

Crystallization and preliminary crystallographic analysis of Arabidopsis thaliana EDS1, a key component of plant immunity, in complex with its signalling partner SAG101.

In plants, the nucleocytoplasmic protein EDS1 (Enhanced disease susceptibility1) is an important regulator of innate immunity, coordinating host-cell defence and cell-death programs in response to pathogen attack. Arabidopsis thaliana EDS1 stabilizes and signals together with its partners PAD4 (Phytoalexin deficient4) and SAG101 (Senescence-associated gene101). Characterization of EDS1 molecular configurations in vitro and in vivo points to the formation of structurally and spatially distinct EDS1 homomeric dimers and EDS1 heteromeric complexes with either PAD4 or SAG101 as necessary components of the immune response. EDS1, PAD4 and SAG101 constitute a plant-specific protein family with a unique `EP' (EDS1-PAD4-specific) domain at their C-termini and an N-terminal domain resembling enzymes with an α/β-hydrolase fold. Here, the expression, purification and crystallization of a functional EDS1 complex formed by EDS1 and SAG101 from Arabidopsis thaliana are reported. The crystals belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 101.8, b = 115.9, c = 122.8 Å, and diffracted to 3.5 Å resolution.

PAMP (Pathogen-associated Molecular Pattern)-induced Changes in Plasma Membrane Compartmentalization Reveal Novel Components of Plant Immunity

Plasma membrane compartmentalization spatiotemporally regulates cell-autonomous immune signaling in animal cells. To elucidate immediate early protein dynamics at the plant plasma membrane in response to the bacterial pathogen-associated molecular pattern (PAMP) flagellin (flg22) we employed quantitative mass spectrometric analysis on detergent-resistant membranes (DRMs) of Arabidopsis thaliana suspension cells. This approach revealed rapid and profound changes in DRM protein composition following PAMP treatment, prominently affecting proton ATPases and receptor-like kinases, including the flagellin receptor FLS2. We employed reverse genetics to address a potential contribution of a subset of these proteins in flg22-triggered cellular responses. Mutants of three candidates (DET3, AHA1, FER) exhibited a conspicuous defect in the PAMP-triggered accumulation of reactive oxygen species. In addition, these mutants showed altered mitogen-activated protein kinase (MAPK) activation, a defect in PAMP-triggered stomatal closure as well as altered bacterial infection phenotypes, which revealed three novel players in elicitor-dependent oxidative burst control and innate immunity. Our data provide evidence for dynamic elicitor-induced changes in the membrane compartmentalization of PAMP signaling components.

Deciphering the mode of action and host recognition of bacterial type III effectors

Plant pathogenic bacteria adhere to cell walls and remain external to the cell throughout the pathogenic lifecycle, where they elicit host immunity through host plasma membrane localised receptors. To be successful pathogens, bacteria must suppress these defence responses, which they do by secreting a suite of virulence effector molecules into the host cytoplasm. However, effectors themselves can act as elicitors after perception by intracellular host immune receptors, thus, re-activating plant immunity. Bacterial effectors generally target host molecules through specific molecular activities to defeat plant defence responses. Although effectors can be used as tools to elucidate components of plant immunity, only a handful of these molecular targets are known and much remains to be learnt about effector strategies for bacterial pathogenicity. This review highlights recent advances in our understanding of the mode of action of bacterial effectors, which in the future will lead to improvements in agriculture.