PTI-ETI crosstalk: an integrative view of plant immunity

Abstract Understanding the plant immune system is crucial for using genetics to protect crops from diseases. Plants resist pathogens via a two-tiered innate immune detection-and-response system. The first plant Resistance (R) gene was cloned in 1992 . Since then, many cell-surface pattern recognition receptors (PRRs) have been identified, and R genes that encode intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) have been cloned. Here, we provide a list of characterized PRRs and NLRs. In addition to immune receptors, many components of immune signaling networks were discovered over the last 30 years. We review the signaling pathways, physiological responses, and molecular regulation of both PRR- and NLR-mediated immunity. Recent studies have reinforced the importance of interactions between the two immune systems. We provide an overview of interactions between PRR- and NLR-mediated immunity, highlighting challenges and perspectives for future research.

/pdf/thirty-years-of-resistance-zig-zag-through-the-plant-immune-2s6pdgpg.pdf

Thirty years of resistance: Zig-zag through the plant immune system

Plant immune responses are mainly activated by two types of receptor. Plasma membrane-localized pattern recognition receptors (PRRs) perceive extracellular microbial features, and nucleotide-binding leucine-rich repeat receptors (NLRs) recognize intracellular effector proteins from pathogens1. NLRs possessing N-terminal Toll/interleukin-1 receptor (TIR) domains activate defence responses via the NADase activity of the TIR domain2,3. Here we report that activation of TIR signalling plays a key role in PRR-mediated pattern-triggered immunity (PTI). TIR signalling mutants exhibit attenuated PTI responses and decreased resistance against pathogens. Consistently, PTI is compromised in plants with reduced NLR levels. Treatment with PTI elicitor flg22 or nlp20 rapidly induces many genes encoding TIR domain-containing proteins, which is likely to be responsible for activating TIR signalling during PTI. Overall, our study reveals that TIR signalling activation is an important mechanism for boosting plant defence during PTI.

Activation of TIR signalling boosts pattern-triggered immunity.

Molecular mechanisms of early plant pattern-triggered immune signaling.

Plants utilise intracellular nucleotide-binding, leucine-rich repeat (NLR) immune receptors to detect pathogen effectors and activate local and systemic defence. NRG1 and ADR1 "helper" NLRs (RNLs) cooperate with enhanced disease susceptibility 1 (EDS1), senescence-associated gene 101 (SAG101) and phytoalexin-deficient 4 (PAD4) lipase-like proteins to mediate signalling from TIR domain NLR receptors (TNLs). The mechanism of RNL/EDS1 family protein cooperation is not understood. Here, we present genetic and molecular evidence for exclusive EDS1/SAG101/NRG1 and EDS1/PAD4/ADR1 co-functions in TNL immunity. Using immunoprecipitation and mass spectrometry, we show effector recognition-dependent interaction of NRG1 with EDS1 and SAG101, but not PAD4. An EDS1-SAG101 complex interacts with NRG1, and EDS1-PAD4 with ADR1, in an immune-activated state. NRG1 requires an intact nucleotide-binding P-loop motif, and EDS1 a functional EP domain and its partner SAG101, for induced association and immunity. Thus, two distinct modules (NRG1/EDS1/SAG101 and ADR1/EDS1/PAD4) mediate TNL receptor defence signalling.

/pdf/pathogen-effector-recognition-dependent-association-of-nrg1-25oangs0z7.pdf

Pathogen effector recognition-dependent association of NRG1 with EDS1 and SAG101 in TNL receptor immunity.

Plants use both cell surface and intracellular immune receptors with leucine rich-repeat (LRRs) to detect pathogens. LRR receptor kinases (LRR-RKs) and LRR receptor-like proteins (LRR-RPs) recognize extracellular microbe-derived molecules to confer pattern-triggered immunity (PTI), while nucleotide-binding LRR (NLR) proteins detect microbial effectors inside the cell to confer effector-triggered immunity (ETI). Despite PTI and ETI signaling being initiated in different compartments, both rely on the transcriptional activation of similar sets of genes, suggesting convergence in signaling upstream of nuclear events. Here we report that two sets of molecules, helper NLRs from the ADR1 (ACTIVATED DISEASE RESISTANCE 1) family as well as lipase-like proteins EDS1 (ENHANCED DISEASE SUSCEPTIBILITY 1) and PAD4 (PHYTOALEXIN DEFICIENT 4), are required not only for ETI, but also for PTI. A further similarity is seen in the evolutionary patterns of some PTI and ETI receptor genes, with both often being highly polymorphic, and with nevertheless distinct roles of LRR-RK and LRR-RP receptors in immunity. We find that the LRR-RK SOBIR1 directly links LRR-RPs with the ADR1 helper NLR as well as EDS1 and PAD4, suggesting the formation of constitutive supramolecular signalosome complexes at the inner side of the plasma membrane. We propose that the EDS1-PAD4-ADR1 node is an essential component and convergence point for immune signaling cascades activated by both surface-resident LRR-RP receptors and intracellular NLR receptors.

/pdf/arabidopsis-cell-surface-lrr-immune-receptor-signaling-49hvraht8m.pdf

Arabidopsis cell surface LRR immune receptor signaling through the EDS1-PAD4-ADR1 node

Forward genetics is a powerful tool to establish phenotype-genotype correlations in virtually all areas of plant biology and has been particularly successful in the model plant Arabidopsis This approach typically starts with a phenotype in an M2 mutant, followed by identifying a causal DNA change in F2 populations resulting from a cross between the mutant and a wildtype individual Ultimately, two additional generations are needed to pinpoint causal DNA changes upon mutant identification We postulated that genome-wide allele frequency distributions within the mutants of M2 families facilitate discrimination of causal versus non-causal mutations, essentially eliminating the need for F2 populations In a proof-of-principle experiment, we aimed to identify signalling components employed by the executor-type resistance (R) protein, Bs4C, from pepper (Capsicum pubescens) In a native setting, Bs4C is transcriptionally activated by and mediates recognition of the transcription activator-like effector AvrBs4 from the bacterial pathogen Xanthomonas Arabidopsis containing an estradiol-inducible Bs4C transgene was used in a conditionally lethal screen to identify second-site suppressor mutations Whole genome sequencing was used for M2 mutant allele-frequency distribution (MAD) mapping in three independent M2 families MAD-mapping uncovered that all three families harboured mutations in XRN4, a novel component of executor R protein pathways Our work demonstrates that causal mutations observed in forward genetic screens can be identified immediately in M2 families instead of derived F2 families Notably, the timesaving concept of MAD mapping should be applicable to most crop species and will advance the appeal of forward genetics beyond applications in fundamental research SIGNIFICANCE Forward genetics has uncovered numerous genes that govern plant immune reactions This procedure relies on mutant plants with modified immune reactions followed by identification of causal DNA changes in derived F2 progeny We developed a novel forward genetics concept where causal DNA changes are identified in the initial M2 mutants, making time consuming establishment of F2 populations obsolete To confirm the feasibility of the concept, we mutagenized transgenic Arabidopsis seeds containing the cell death executing resistance gene Bs4C from pepper Whole-genome sequencing of identified mutant families that lack a Bs4C-dependent cell death revealed the XRN4 gene as a novel component of Bs4C-dependent cell death This confirms our hypothesis that causal mutations can be identified directly within phenotypically selected mutant families

/pdf/a-shortcut-in-forward-genetics-concurrent-discovery-of-52xhoswgfn.pdf

F. El Kasmi

Papers

Arabidopsis cell surface LRR immune receptor signaling through the EDS1-PAD4-ADR1 node

A shortcut in forward genetics: concurrent discovery of mutant phenotype and causal mutation in Arabidopsis M2 families via MAD-mapping