Activation of a Plant NLR Complex through Heteromeric Association with an Autoimmune Risk Variant of Another NLR

Graft incompatibility between pepper and tomato can be attributed to genetic incompatibility between diverged immune systems

Graft compatibility is the capacity of two plants to form cohesive vascular connections. Tomato and pepper are incompatible graft partners; however, the underlying cause of graft rejection between these two species remains unknown.We diagnosed graft incompatibility between tomato and diverse pepper varieties based on weakened biophysical stability, decreased growth, and persistent cell death using trypan blue and TUNEL assays. Transcriptomic analysis of cell death in the junction was performed using RNA-sequencing, and molecular signatures for incompatible graft response were characterized based on meta-transcriptomic comparisons with other biotic processes.We show that tomato is broadly incompatible with diverse pepper cultivars. These incompatible graft partners activate prolonged transcriptional changes that are highly enriched for defense processes. Amongst these processes was broad NLR upregulation and hypersensitive response. Using transcriptomic datasets for a variety of biotic stress treatments, we identified a significant overlap in the genetic profile of incompatible grafting and plant parasitism. In addition, we found over 1000 genes that are uniquely upregulated in incompatible grafts.Based on NLR overactivity, DNA damage, and prolonged cell death we have determined that tomato and pepper graft incompatibility is likely caused by a form of genetic incompatibility, which triggers a hyperimmune-response.

Plant NLR immunity activation and execution: a biochemical perspective

Plants deploy cell-surface and intracellular receptors to detect pathogen attack and trigger innate immune responses. Inside host cells, families of nucleotide-binding/leucine-rich repeat (NLR) proteins serve as pathogen sensors or downstream mediators of immune defence outputs and cell death, which prevent disease. Established genetic underpinnings of NLR-mediated immunity revealed various strategies plants adopt to combat rapidly evolving microbial pathogens. The molecular mechanisms of NLR activation and signal transmission to components controlling immunity execution were less clear. Here, we review recent protein structural and biochemical insights to plant NLR sensor and signalling functions. When put together, the data show how different NLR families, whether sensors or signal transducers, converge on nucleotide-based second messengers and cellular calcium to confer immunity. Although pathogen-activated NLRs in plants engage plant-specific machineries to promote defence, comparisons with mammalian NLR immune receptor counterparts highlight some shared working principles for NLR immunity across kingdoms.

Immunobiodiversity: Conserved and specific immunity across land plants and beyond

Angiosperms represent most plants that humans cultivate, grow, and eat. However, angiosperms are only one of five major land plant lineages. As a whole lineage, plants also include algal groups. All these clades represent a tremendous genetic diversity that can be investigated to reveal the evolutionary history of any given mechanism. In this review, we describe the current model of the plant immune system, discuss its evolution based on the recent literature, and propose future directions for the field. In angiosperms, plant-microbe interactions have been intensively studied, revealing essential cell surface and intracellular immune receptors, as well as metabolic and hormonal defense pathways. Exploring diversity at the genomic and functional levels demonstrates the conservation of these pathways across land plants, some of which are beyond plants. On basis of the conserved mechanisms, lineage-specific variations have occurred, leading to diversified reservoirs of immune mechanisms. In rare cases, this diversity has been harnessed and successfully transferred to other species by integration of wild immune receptors or engineering of novel forms of receptors for improved resistance to pathogens. We propose that exploring further the diversity of immune mechanisms in the whole plant lineage will reveal completely novel sources of resistance to be deployed in crops.

NLR receptors in plant immunity: making sense of the alphabet soup

Plants coordinately use cell-surface and intracellular immune receptors to perceive pathogens and mount an immune response. Intracellular events of pathogen recognition are largely mediated by immune receptors of the nucleotide binding and leucine rich-repeat (NLR) classes. Upon pathogen perception, NLRs trigger a potent broad-spectrum immune reaction, usually accompanied by a form of programmed cell death termed the hypersensitive response. Some plant NLRs act as multifunctional singleton receptors which combine pathogen detection and immune signaling. However, NLRs can also function in higher order pairs and networks of functionally specialized interconnected receptors. In this article, we cover the basic aspects of plant NLR biology with an emphasis on NLR networks. We highlight some of the recent advances in NLR structure, function, and activation and discuss emerging topics such as modulator NLRs, pathogen suppression of NLRs, and NLR bioengineering. Multi-disciplinary approaches are required to disentangle how these NLR immune receptor pairs and networks function and evolve. Answering these questions holds the potential to deepen our understanding of the plant immune system and unlock a new era of disease resistance breeding.

Allelic compatibility in plant immune receptors facilitates engineering of new effector recognition specificities

Engineering the plant immune system offers genetic solutions to mitigate crop diseases caused by diverse agriculturally significant pathogens and pests. Modification of intracellular plant immune receptors of the nucleotide-binding leucine-rich repeat (NLR) receptor superfamily for expanded recognition of pathogen virulence proteins (effectors) is a promising approach for engineering disease resistance. However, engineering can cause NLR autoactivation, resulting in constitutive defense responses that are deleterious to the plant. This may be due to plant NLRs associating in highly complex signaling networks that coevolve together, and changes through breeding or genetic modification can generate incompatible combinations, resulting in autoimmune phenotypes. The sensor and helper NLRs of the rice (Oryza sativa) NLR pair Pik have coevolved, and mismatching between noncoevolved alleles triggers constitutive activation and cell death. This limits the extent to which protein modifications can be used to engineer pathogen recognition and enhance disease resistance mediated by these NLRs. Here, we dissected incompatibility determinants in the Pik pair in Nicotiana benthamiana and found that heavy metal-associated (HMA) domains integrated in Pik-1 not only evolved to bind pathogen effectors but also likely coevolved with other NLR domains to maintain immune homeostasis. This explains why changes in integrated domains can lead to autoactivation. We then used this knowledge to facilitate engineering of new effector recognition specificities, overcoming initial autoimmune penalties. We show that by mismatching alleles of the rice sensor and helper NLRs Pik-1 and Pik-2, we can enable the integration of synthetic domains with novel and enhanced recognition specificities. Taken together, our results reveal a strategy for engineering NLRs, which has the potential to allow an expanded set of integrations and therefore new disease resistance specificities in plants.

Pathways to engineering plant intracellular NLR immune receptors

Factors including climate change and increased global exchange are set to escalate the prevalence of plant diseases, posing an unprecedented threat to global food security and making it more challenging to meet the demands of an ever-growing population. As such, new methods of pathogen control are essential to help with the growing danger of crop losses to plant diseases. The intracellular immune system of plants utilizes nucleotide-binding leucine-rich repeat (NLR) receptors to recognize and activate defense responses to pathogen virulence proteins (effectors) delivered to the host. Engineering the recognition properties of plant NLRs toward pathogen effectors is a genetic solution to plant diseases with high specificity, and it is more sustainable than several current methods for pathogen control that frequently rely on agrochemicals. Here, we highlight the pioneering approaches toward enhancing effector recognition in plant NLRs and discuss the barriers and solutions in engineering the plant intracellular immune system.

Structural mechanism of heavy metal-associated integrated domain engineering of paired nucleotide-binding and leucine-rich repeat proteins in rice

Plant nucleotide-binding and leucine-rich repeat (NLR) proteins are immune sensors that detect pathogen effectors and initiate a strong immune response. In many cases, single NLR proteins are sufficient for both effector recognition and signaling activation. These proteins possess a conserved architecture, including a C-terminal leucine-rich repeat (LRR) domain, a central nucleotide-binding (NB) domain, and a variable N-terminal domain. Nevertheless, many paired NLRs linked in a head-to-head configuration have now been identified. The ones carrying integrated domains (IDs) can recognize pathogen effector proteins by various modes; these are known as sensor NLR (sNLR) proteins. Structural and biochemical studies have provided insights into the molecular basis of heavy metal-associated IDs (HMA IDs) from paired NLRs in rice and revealed the co-evolution between pathogens and hosts by combining naturally occurring favorable interactions across diverse interfaces. Focusing on structural and molecular models, here we highlight advances in structure-guided engineering to expand and enhance the response profile of paired NLR-HMA IDs in rice to variants of the rice blast pathogen MAX-effectors (Magnaporthe oryzae AVRs and ToxB-like). These results demonstrate that the HMA IDs-based design of rice materials with broad and enhanced resistance profiles possesses great application potential but also face considerable challenges.

NLR immune receptor-nanobody fusions confer plant disease resistance

Plant pathogens cause recurrent epidemics, threatening crop yield and global food security. Efforts to retool the plant immune system have been limited to modifying natural components and can be nullified by the emergence of new pathogen strains. Made-to-order synthetic plant immune receptors provide an opportunity to tailor resistance to pathogen genotypes present in the field. In this work, we show that plant nucleotide-binding, leucine-rich repeat immune receptors (NLRs) can be used as scaffolds for nanobody (single-domain antibody fragment) fusions that bind fluorescent proteins (FPs). These fusions trigger immune responses in the presence of the corresponding FP and confer resistance against plant viruses expressing FPs. Because nanobodies can be raised against most molecules, immune receptor-nanobody fusions have the potential to generate resistance against plant pathogens and pests delivering effectors inside host cells.

An NBS-LRR protein in the Rpp1 locus negates the dominance of Rpp1-mediated resistance against Phakopsora pachyrhizi in soybean

The soybean Rpp1 locus confers resistance to Phakopsora pachyrhizi, causal agent of rust, and resistance is usually dominant over susceptibility. However, dominance of Rpp1-mediated resistance is lost when a resistant genotype (Rpp1 or Rpp1b) is crossed with susceptible line TMG06_0011, and the mechanism of this dominant susceptibility (DS) is unknown. Sequencing the Rpp1 region reveals that the TMG06_0011 Rpp1 locus has a single nucleotide-binding site leucine-rich repeat (NBS-LRR) gene (DS-R), whereas resistant PI 594760B (Rpp1b) is similar to PI 200492 (Rpp1) and has three NBS-LRR resistance gene candidates. Evidence that DS-R is the cause of DS was reflected in virus-induced gene silencing of DS-R in Rpp1b/DS-R or Rpp1/DS-R heterozygous plants with resistance partially restored. In heterozygous Rpp1b/DS-R plants, expression of Rpp1b candidate genes was not significantly altered, indicating no effect of DS-R on transcription. Physical interaction of the DS-R protein with candidate Rpp1b resistance proteins was supported by yeast two-hybrid studies and in silico modeling. Thus, we conclude that suppression of resistance most likely does not occur at the transcript level, but instead probably at the protein level, possibly with Rpp1 function inhibited by binding to the DS-R protein. The DS-R gene was found in other soybean lines, with an estimated allele frequency of 6% in a diverse population, and also found in wild soybean (Glycine soja). The identification of a dominant susceptible NBS-LRR gene provides insight into the behavior of NBS-LRR proteins and serves as a reminder to breeders that the dominance of an R gene can be influenced by a susceptibility allele.

An atypical NLR protein modulates the NRC immune receptor network in Nicotiana benthamiana

The NRC immune receptor network has evolved in asterid plants from a pair of linked genes into a genetically dispersed and phylogenetically structured network of sensor and helper NLR (nucleotide-binding domain and leucine-rich repeat-containing) proteins. In some species, such as the model plant Nicotiana benthamiana and other Solanaceae, the NRC (NLR-REQUIRED FOR CELL DEATH) network forms up to half of the NLRome, and NRCs are scattered throughout the genome in gene clusters of varying complexities. Here, we describe NRCX, an atypical member of the NRC family that lacks canonical features of these NLR helper proteins, such as a functional N-terminal MADA motif and the capacity to trigger autoimmunity. In contrast to other NRCs, systemic gene silencing of NRCX in N. benthamiana markedly impairs plant growth resulting in a dwarf phenotype. Remarkably, dwarfism of NRCX silenced plants is partially dependent on NRCX paralogs NRC2 and NRC3, but not NRC4. Despite its negative impact on plant growth when silenced systemically, spot gene silencing of NRCX in mature N. benthamiana leaves doesn't result in visible cell death phenotypes. However, alteration of NRCX expression modulates the hypersensitive response mediated by NRC2 and NRC3 in a manner consistent with a negative role for NRCX in the NRC network. We conclude that NRCX is an atypical member of the NRC network that has evolved to contribute to the homeostasis of this genetically unlinked NLR network.

My journey studying plant immunity

Our understanding of plant immunity has taken exciting and surprising turns over the past two decades. Here, I look back on my scientific journey studying plant immunity with three publications in Cell Host &Microbe, which have provided me perspectives on future research in the area of plant-pathogen interactions.

Structural basis of NLR activation and innate immune signalling in plants

Animals and plants have NLRs (nucleotide-binding leucine-rich repeat receptors) that recognize the presence of pathogens and initiate innate immune responses. In plants, there are three types of NLRs distinguished by their N-terminal domain: the CC (coiled-coil) domain NLRs, the TIR (Toll/interleukin-1 receptor) domain NLRs and the RPW8 (resistance to powdery mildew 8)-like coiled-coil domain NLRs. CC-NLRs (CNLs) and TIR-NLRs (TNLs) generally act as sensors of effectors secreted by pathogens, while RPW8-NLRs (RNLs) signal downstream of many sensor NLRs and are called helper NLRs. Recent studies have revealed three dimensional structures of a CNL (ZAR1) including its inactive, intermediate and active oligomeric state, as well as TNLs (RPP1 and ROQ1) in their active oligomeric states. Furthermore, accumulating evidence suggests that members of the family of lipase-like EDS1 (enhanced disease susceptibility 1) proteins, which are uniquely found in seed plants, play a key role in providing a link between sensor NLRs and helper NLRs during innate immune responses. Here, we summarize the implications of the plant NLR structures that provide insights into distinct mechanisms of action by the different sensor NLRs and discuss plant NLR-mediated innate immune signalling pathways involving the EDS1 family proteins and RNLs.

The hybrid lethality of interspecific F1 hybrids of Nicotiana: a clue to understanding hybrid inviability-a major obstacle to wide hybridization and introgression breeding of plants

Reproductive isolation poses a major obstacle to wide hybridization and introgression breeding of plants. Hybrid inviability in the postzygotic isolation barrier inevitably reduces hybrid fitness, consequently causing hindrances in the establishment of novel genotypes from the hybrids among genetically divergent parents. The idea that the plant immune system is involved in the hybrid problem is applicable to the intra- and/or interspecific hybrids of many different taxa. The lethality characteristics and expression profile of genes associated with the hypersensitive response of the hybrids, along with the suppression of causative genes, support the deleterious epistatic interaction of parental NB-LRR protein genes, resulting in aberrant hyper-immunity reactions in the hybrid. Moreover, the cellular, physiological, and biochemical reactions observed in hybrid cells also corroborate this hypothesis. However, the difference in genetic backgrounds of the respective hybrids may contribute to variations in lethality phenotypes among the parental species combinations. The mixed state in parental components of the chaperone complex (HSP90-SGT1-RAR1) in the hybrid may also affect the hybrid inviability. This review article discusses the facts and hypothesis regarding hybrid inviability, alongside the findings of studies on the hybrid lethality of interspecific hybrids of the genus Nicotiana. A possible solution for averting the hybrid problem has also been scrutinized with the aim of improving the wide hybridization and introgression breeding program in plants.

Network organization of the plant immune system: from pathogen perception to robust defense induction

The plant immune system has been explored essentially through the study of qualitative resistance, a simple form of immunity, and from a reductionist point of view. The recent identification of genes conferring quantitative disease resistance revealed a large array of functions, suggesting more complex mechanisms. In addition, thanks to the advent of high-throughput analyses and system approaches, our view of the immune system has become more integrative, revealing that plant immunity should rather be seen as a distributed and highly connected molecular network including diverse functions to optimize expression of plant defenses to pathogens. Here, we review the recent progress made to understand the network complexity of regulatory pathways leading to plant immunity, from pathogen perception, through signaling pathways and finally to immune responses. We also analyze the topological organization of these networks and their emergent properties, crucial to predict novel immune functions and test them experimentally. Finally, we report how these networks might be regulated by environmental clues. Although system approaches remain extremely scarce in this area of research, a growing body of evidence indicates that the plant response to combined biotic and abiotic stresses cannot be inferred from responses to individual stresses. A view of possible research avenues in this nascent biology domain is finally proposed.

Functional diversification gave rise to allelic specialization in a rice NLR immune receptor pair

Cooperation between receptors from the nucleotide-binding, leucine-rich repeats (NLR) superfamily is important for intracellular activation of immune responses. NLRs can function in pairs that, upon pathogen recognition, trigger hypersensitive cell death and stop pathogen invasion. Natural selection drives specialization of host immune receptors towards an optimal response, whilst keeping a tight regulation of immunity in the absence of pathogens. However, the molecular basis of co-adaptation and specialization between paired NLRs remains largely unknown. Here, we describe functional specialization in alleles of the rice NLR pair Pik that confers resistance to strains of the blast fungus Magnaporthe oryzae harbouring AVR-Pik effectors. We revealed that matching pairs of allelic Pik NLRs mount effective immune responses, whereas mismatched pairs lead to autoimmune phenotypes, a hallmark of hybrid necrosis in both natural and domesticated plant populations. We further showed that allelic specialization is largely underpinned by a single amino acid polymorphism that determines preferential association between matching pairs of Pik NLRs. These results provide a framework for how functionally linked immune receptors undergo co-adaptation to provide an effective and regulated immune response against pathogens. Understanding the molecular constraints that shape paired NLR evolution has implications beyond plant immunity given that hybrid necrosis can drive reproductive isolation.

Cross-Tolerance and Autoimmunity as Missing Links in Abiotic and Biotic Stress Responses in Plants: A Perspective toward Secondary Metabolic Engineering

Plants employ a diversified array of defense activities when they encounter stress. Continuous activation of defense pathways that were induced by mutation or altered expression of disease resistance genes and mRNA surveillance mechanisms develop abnormal phenotypes. These plants show continuous defense genes' expression, reduced growth, and also manifest tissue damage by apoptosis. These macroscopic abrasions appear even in the absence of the pathogen and can be attributed to a condition known as autoimmunity. The question is whether it is possible to develop an autoimmune mutant that does not fetch yield and growth penalty and provides enhanced protection against various biotic and abiotic stresses via secondary metabolic pathways' engineering. This review is a discussion about the common stress-fighting mechanisms, how the concept of cross-tolerance instigates propitious or protective autoimmunity, and how it can be achieved by engineering secondary metabolic pathways.

How activated NLRs induce anti-microbial defenses in plants

Plants utilize cell-surface localized and intracellular leucine-rich repeat (LRR) immune receptors to detect pathogens and to activate defense responses, including transcriptional reprogramming and the initiation of a form of programmed cell death of infected cells. Cell death initiation is mainly associated with the activation of nucleotide-binding LRR receptors (NLRs). NLRs recognize the presence or cellular activity of pathogen-derived virulence proteins, so-called effectors. Effector-dependent NLR activation leads to the formation of higher order oligomeric complexes, termed resistosomes. Resistosomes can either form potential calcium-permeable cation channels at cellular membranes and initiate calcium influxes resulting in activation of immunity and cell death or function as NADases whose activity is needed for the activation of downstream immune signaling components, depending on the N-terminal domain of the NLR protein. In this mini-review, the current knowledge on the mechanisms of NLR-mediated cell death and resistance pathways during plant immunity is discussed.

One Hundred Years of Hybrid Necrosis: Hybrid Autoimmunity as a Window into the Mechanisms and Evolution of Plant-Pathogen Interactions

Hybrid necrosis in plants refers to a genetic autoimmunity syndrome in the progeny of interspecific or intraspecific crosses. Although the phenomenon was first documented in 1920, it has been unequivocally linked to autoimmunity only recently, with the discovery of the underlying genetic and biochemical mechanisms. The most common causal loci encode immune receptors, which are known to differ within and between species. One mechanism can be explained by the guard hypothesis, in which a guard protein, often a nucleotide-binding site-leucine-rich repeat protein, is activated by interaction with a plant protein that mimics standard guardees modified by pathogen effector proteins. Another surprising mechanism is the formation of inappropriately active immune receptor complexes. In this review, we summarize our current knowledge of hybrid necrosis and discuss how its study is not only informing the understanding of immune gene evolution but also revealing new aspects of plant immune signaling.

Plant "helper" immune receptors are Ca2+-permeable nonselective cation channels

Plant nucleotide-binding leucine-rich repeat receptors (NLRs) regulate immunity and cell death. In Arabidopsis, a subfamily of "helper" NLRs is required by many "sensor" NLRs. Active NRG1.1 oligomerized, was enriched in plasma membrane puncta, and conferred cytoplasmic calcium ion (Ca2+) influx in plant and human cells. NRG1.1-dependent Ca2+ influx and cell death were sensitive to Ca2+ channel blockers and were suppressed by mutations affecting oligomerization or plasma membrane enrichment. Ca2+ influx and cell death mediated by NRG1.1 and ACTIVATED DISEASE RESISTANCE 1 (ADR1), another helper NLR, required conserved negatively charged N-terminal residues. Whole-cell voltage-clamp recordings demonstrated that Arabidopsis helper NLRs form Ca2+-permeable cation channels to directly regulate cytoplasmic Ca2+ levels and consequent cell death. Thus, helper NLRs transduce cell death signals directly.

Two NLR immune receptors acquired high-affinity binding to a fungal effector through convergent evolution of their integrated domain

A subset of plant NLR immune receptors carry unconventional integrated domains in addition to their canonical domain architecture. One example is rice Pik-1 that comprises an integrated heavy metal-associated (HMA) domain. Here, we reconstructed the evolutionary history of Pik-1 and its NLR partner, Pik-2, and tested hypotheses about adaptive evolution of the HMA domain. Phylogenetic analyses revealed that the HMA domain integrated into Pik-1 before Oryzinae speciation over 15 million years ago and has been under diversifying selection. Ancestral sequence reconstruction coupled with functional studies showed that two Pik-1 allelic variants independently evolved from a weakly binding ancestral state to high-affinity binding of the blast fungus effector AVR-PikD. We conclude that for most of its evolutionary history the Pik-1 HMA domain did not sense AVR-PikD, and that different Pik-1 receptors have recently evolved through distinct biochemical paths to produce similar phenotypic outcomes. These findings highlight the dynamic nature of the evolutionary mechanisms underpinning NLR adaptation to plant pathogens.

Two NLR immune receptors acquired high-affinity binding to a fungal effector through convergent evolution of their integrated domain

A subset of plant NLR immune receptors carry unconventional integrated domains in addition to their canonical domain architecture. One example is rice Pik-1 that comprises an integrated heavy metal-associated (HMA) domain. Here, we reconstructed the evolutionary history of Pik-1 and its NLR partner, Pik-2, and tested hypotheses about adaptive evolution of the HMA domain. Phylogenetic analyses revealed that the HMA domain integrated into Pik-1 before Oryzinae speciation over 15 million years ago and has been under diversifying selection. Ancestral sequence reconstruction coupled with functional studies showed that two Pik-1 allelic variants independently evolved from a weakly binding ancestral state to high-affinity binding of the blast fungus effector AVR-PikD. We conclude that for most of its evolutionary history the Pik-1 HMA domain did not sense AVR-PikD, and that different Pik-1 receptors have recently evolved through distinct biochemical paths to produce similar phenotypic outcomes. These findings highlight the dynamic nature of the evolutionary mechanisms underpinning NLR adaptation to plant pathogens.

A Comparative Overview of the Intracellular Guardians of Plants and Animals: NLRs in Innate Immunity and Beyond

Nucleotide-binding domain leucine-rich repeat receptors (NLRs) play important roles in the innate immune systems of both plants and animals. Recent breakthroughs in NLR biochemistry and biophysics have revolutionized our understanding of how NLR proteins function in plant immunity. In this review, we summarize the latest findings in plant NLR biology and draw direct comparisons to NLRs of animals. We discuss different mechanisms by which NLRs recognize their ligands in plants and animals. The discovery of plant NLR resistosomes that assemble in a comparable way to animal inflammasomes reinforces the striking similarities between the formation of plant and animal NLR complexes. Furthermore, we discuss the mechanisms by which plant NLRs mediate immune responses and draw comparisons to similar mechanisms identified in animals. Finally, we summarize the current knowledge of the complex genetic architecture formed by NLRs in plants and animals and the roles of NLRs beyond pathogen detection.

Disentangling cause and consequence: genetic dissection of the DANGEROUS MIX2 risk locus, and activation of the DM2h NLR in autoimmunity

Nucleotide-binding domain-leucine-rich repeat-type immune receptors (NLRs) protect plants against pathogenic microbes through intracellular detection of effector proteins. However, this comes at a cost, as NLRs can also induce detrimental autoimmunity in genetic interactions with foreign alleles. This may occur when independently evolved genomes are combined in inter- or intraspecific crosses, or when foreign alleles are introduced by mutagenesis or transgenesis. Most autoimmunity-inducing NLRs are encoded within highly variable NLR gene clusters with no known immune functions, which were termed autoimmune risk loci. Whether risk NLRs differ from sensor NLRs operating in natural pathogen resistance and how risk NLRs are activated in autoimmunity is unknown. Here, we analyzed the DANGEROUS MIX2 risk locus, a major autoimmunity hotspot in Arabidopsis thaliana. By gene editing and heterologous expression, we show that a single gene, DM2h, is necessary and sufficient for autoimmune induction in three independent cases of autoimmunity in accession Landsberg erecta. We focus on autoimmunity provoked by an EDS1-yellow fluorescent protein (YFP)NLS fusion protein to characterize DM2h functionally and determine features of EDS1-YFPNLS activating the immune receptor. Our data suggest that risk NLRs function in a manner reminiscent of sensor NLRs, while autoimmunity-inducing properties of EDS1-YFPNLS in this context are unrelated to the protein's functions as an immune regulator. We propose that autoimmunity, at least in some cases, may be caused by spurious, stochastic interactions of foreign alleles with coincidentally matching risk NLRs.

The CC-NB-LRR OsRLR1 mediates rice disease resistance through interaction with OsWRKY19

Nucleotide-binding site-leucine-rich repeat (NB-LRR) resistance proteins are critical for plant resistance to pathogens; however, their mechanism of activation and signal transduction is still not well understood. We identified a mutation in an as yet uncharacterized rice coiled-coil (CC)-NB-LRR, Oryza sativa RPM1-like resistance gene 1 (OsRLR1), which leads to hypersensitive response (HR)-like lesions on the leaf blade and broad-range resistance to the fungal pathogen Pyricularia oryzae (syn. Magnaporthe oryzae) and the bacterial pathogen Xanthomonas oryzae pv. oryzae, together with strong growth reduction. Consistently, OsRLR1-overexpression lines showed enhanced resistance to both pathogens. Moreover, we found that OsRLR1 mediates the defence response through direct interaction in the nucleus with the transcription factor OsWRKY19. Down-regulation of OsWRKY19 in the rlr1 mutant compromised the HR-like phenotype and resistance response, and largely restored plant growth. OsWRKY19 binds to the promoter of OsPR10 to activate the defence response. Taken together, our data highlight the role of a new residue involved in the NB-LRR activation mechanism, allowing identification of a new NB-LRR downstream signalling pathway.

The role of chaperone complex HSP90-SGT1-RAR1 as the associated machinery for hybrid inviability between Nicotiana gossei Domin and N. tabacum L

Two cultured cell lines (GTH4 and GTH4S) of a Nicotiana interspecific F1 hybrid (N. gossei × N. tabacum) were comparatively analyzed to find genetic factors related to hybrid inviability. Both cell lines proliferated at 37 °C, but after shifting to 26 °C, GTH4 started to die similar to the F1 hybrid seedlings, whereas GTH4S survived. As cell death requires de novo expression of genes and proteins, we compared expressed protein profiles between the two cell lines, and found that NgSGT1, a cochaperone of the chaperone complex (HSP90-SGT1-RAR1), was expressed in GTH4 but not in GTH4S. Agrobacterium-mediated transient expression of NgSGT1, but not NtSGT1, induced cell death in leaves of N. tabacum, suggesting its possible role in hybrid inviability. Cell death in N. tabacum was also induced by transient expression of NgRAR1, but not NtRAR1. In contrast, transient expression of any parental combinations of three components revealed that NgRAR1 promoted cell death, whereas NtRAR1 suppressed it in N. tabacum. A specific inhibitor of HSP90, geldanamycin, inhibited the progression of hypersensitive response-like cell death in GTH4 and leaf tissue after agroinfiltration. The present study suggested that components of the chaperone complex are involved in the inviability of Nicotiana interspecific hybrid.

A New Catalog of Structural Variants in 1,301 A. thaliana Lines from Africa, Eurasia, and North America Reveals a Signature of Balancing Selection at Defense Response Genes

Genomic variation in the model plant Arabidopsis thaliana has been extensively used to understand evolutionary processes in natural populations, mainly focusing on single-nucleotide polymorphisms. Conversely, structural variation has been largely ignored in spite of its potential to dramatically affect phenotype. Here, we identify 155,440 indels and structural variants ranging in size from 1 bp to 10 kb, including presence/absence variants (PAVs), inversions, and tandem duplications in 1,301 A. thaliana natural accessions from Morocco, Madeira, Europe, Asia, and North America. We show evidence for strong purifying selection on PAVs in genes, in particular for housekeeping genes and homeobox genes, and we find that PAVs are concentrated in defense-related genes (R-genes, secondary metabolites) and F-box genes. This implies the presence of a "core" genome underlying basic cellular processes and a "flexible" genome that includes genes that may be important in spatially or temporally varying selection. Further, we find an excess of intermediate frequency PAVs in defense response genes in nearly all populations studied, consistent with a history of balancing selection on this class of genes. Finally, we find that PAVs in genes involved in the cold requirement for flowering (vernalization) and drought response are strongly associated with temperature at the sites of origin.

Hybrid Incompatibility of the Plant Immune System: An Opposite Force to Heterosis Equilibrating Hybrid Performances

Hybridization is a core element in modern rice breeding as beneficial combinations of two parental genomes often result in the expression of heterosis. On the contrary, genetic incompatibility between parents can manifest as hybrid necrosis, which leads to tissue necrosis accompanied by compromised growth and/or reduced reproductive success. Genetic and molecular studies of hybrid necrosis in numerous plant species revealed that such self-destructing symptoms in most cases are attributed to autoimmunity: plant immune responses are inadvertently activated in the absence of pathogenic invasion. Autoimmunity in hybrids predominantly occurs due to a conflict involving a member of the major plant immune receptor family, the nucleotide-binding domain and leucine-rich repeat containing protein (NLR; formerly known as NBS-LRR). NLR genes are associated with disease resistance traits, and recent population datasets reveal tremendous diversity in this class of immune receptors. Cases of hybrid necrosis involving highly polymorphic NLRs as major causes suggest that diversified R gene repertoires found in different lineages would require a compatible immune match for hybridization, which is a prerequisite to ensure increased fitness in the resulting hybrids. In this review, we overview recent genetic and molecular findings on hybrid necrosis in multiple plant species to provide an insight on how the trade-off between growth and immunity is equilibrated to affect hybrid performances. We also revisit the cases of hybrid weakness in which immune system components are found or implicated to play a causative role. Based on our understanding on the trade-off, we propose that the immune system incompatibility in plants might play an opposite force to restrict the expression of heterosis in hybrids. The antagonism is illustrated under the plant fitness equilibrium, in which the two extremes lead to either hybrid necrosis or heterosis. Practical proposition from the equilibrium model is that breeding efforts for combining enhanced disease resistance and high yield shall be achieved by balancing the two forces. Reverse breeding toward utilizing genomic data centered on immune components is proposed as a strategy to generate elite hybrids with balanced immunity and growth.

The chromatin-remodeling protein BAF60/SWP73A regulates the plant immune receptor NLRs

In both plant and animal innate immune responses, surveillance of pathogen infection is mediated by membrane-associated receptors and intracellular nucleotide-binding domain and leucine-rich-repeat receptors (NLRs). Homeostasis of NLRs is under tight multilayered regulation to avoid over-accumulation or over-activation, which often leads to autoimmune responses that have detrimental effects on growth and development. How NLRs are regulated epigenetically at the chromatin level remains unclear. Here, we report that SWP73A, an ortholog of the mammalian switch/sucrose nonfermentable (SWI/SNF) chromatin-remodeling protein BAF60, suppresses the expression of NLRs either directly by binding to the NLR promoters or indirectly by affecting the alternative splicing of some NLRs through the suppression of cell division cycle 5 (CDC5), a key regulator of RNA splicing. Upon infection, bacteria-induced small RNAs silence SWP73A to activate a group of NLRs and trigger robust immune responses. SWP73A may function as a H3K9me2 reader to enhance transcription suppression.

Genetics of autoimmunity in plants: an evolutionary genetics perspective

Autoimmunity in plants has been found in numerous hybrids as a form of hybrid necrosis and mutant panels. Uncontrolled cell death is a main cellular outcome of autoimmunity, which negatively impacts growth. Its occurrence highlights the vulnerable nature of the plant immune system. Genetic investigation of autoimmunity in hybrid plants revealed that extreme variation in the immune receptor repertoire is a major contributor, reflecting an evolutionary conundrum that plants face in nature. In this review, we discuss natural variation in the plant immune system and its contribution to fitness. The value of autoimmunity genetics lies in its ability to identify combinations of a natural immune receptor and its partner that are predisposed to triggering autoimmunity. The network of immune components for autoimmunity becomes instrumental in revealing mechanistic details of how immune receptors recognize cellular invasion and activate signaling. The list of autoimmunity-risk variants also allows us to infer evolutionary processes contributing to their maintenance in the natural population. Our approach to autoimmunity, which integrates mechanistic understanding and evolutionary genetics, has the potential to serve as a prognosis tool to optimize immunity in crops.

Structural biology of plant defence

Plants employ the innate immune system to discriminate between self and invaders through two types of immune receptors, one on the plasma membrane and the other in the intracellular space. The immune receptors on the plasma membrane are pattern recognition receptors (PRRs) that can perceive pathogen-associated molecular patterns (PAMPs) or host-derived damage-associated molecular patterns (DAMPs) leading to pattern-triggered immunity (PTI). Particular pathogens are capable of overcoming PTI by secreting specific effectors into plant cells to perturb different components of PTI signalling through various mechanisms. Most of the immune receptors from the intracellular space are the nucleotide-binding leucine-rich repeat receptors (NLRs), which specifically recognize pathogen-secreted effectors to mediate effector-triggered immunity (ETI). In this review, we will summarize recent progress in structural studies of PRRs, NLRs, and effectors, and discuss how these studies shed light on ligand recognition and activation mechanisms of the two types of immune receptors and the diversified mechanisms used by effectors to manipulate plant immune signalling.

Advancement of research on plant NLRs evolution, biochemical activity, structural association, and engineering

In this review, we have included evolution of plant intracellular immune receptors, oligomeric complex formation, enzymatic action, engineering, and mechanisms of immune inspection for appropriate defense outcomes. NLR (Nucleotide binding oligomerization domain containing leucine-rich repeat) proteins are the intracellular immune receptors that recognize pathogen-derived virulence factors to confer effector-triggered immunity (ETI). Activation of plant defense by the NLRs are often conveyed through N-terminal Toll-like/ IL-1 receptor (TIR) or non-TIR (coiled-coils or CC) domains. Homodimerization or self-association property of CC/ TIR domains of plant NLRs contribute to their auto-activity and induction of in planta ectopic cell death. High resolution crystal structures of Arabidopsis thaliana RPS4^TIR, L6^TIR, SNC1^TIR, RPP1^TIR and Muscadinia rotundifolia RPV1^TIR showed that interaction is mediated through one or two distinct interfaces i.e., αA and αE helices comprise AE interface and αD and αE helices were found to form DE interface. By contrast, conserved helical regions were determined for CC domains of plant NLRs. Evolutionary history of NLRs diversification has shown that paired forms were originated from NLR singletons. Plant TIRs executed NAD⁺ hydrolysis activity for cell death promotion. Plant NLRs were found to form large oligomeric complexes as observed in animal inflammasomes. We have also discussed different protein engineering methods includes domain shuffling, and decoy modification that increase effector recognition spectrum of plant NLRs. In summary, our review highlights structural basis of perception of the virulence factors by NLRs or NLR pairs to design novel classes of plant immune receptors.

Genomic and Molecular Perspectives of Host-pathogen Interaction and Resistance Strategies against White Rust in Oilseed Mustard

Oilseed brassicas stand as the second most valuable source of vegetable oil and the third most traded one across the globe. However, the yield can be severely affected by infections caused by phytopathogens. White rust is a major oomycete disease of oilseed brassicas resulting in up to 60% yield loss globally. So far, success in the development of oomycete resistant Brassicas through conventional breeding has been limited. Hence, there is an imperative need to blend conventional and frontier biotechnological means to breed for improved crop protection and yield.

This review provides a deep insight into the white rust disease and explains the oomycete-plant molecular events with special reference to Albugo candida describing the role of effector molecules, A. candida secretome, and disease response mechanism along with nucleotide-binding leucine-rich repeat receptor (NLR) signaling. Based on these facts, we further discussed the recent progress and future scopes of genomic approaches to transfer white rust resistance in the susceptible varieties of oilseed brassicas, while elucidating the role of resistance and susceptibility genes. Novel genomic technologies have been widely used in crop sustainability by deploying resistance in the host. Enrichment of NLR repertoire, over-expression of R genes, silencing of avirulent and disease susceptibility genes through RNA interference and CRSPR-Cas are technologies which have been successfully applied against pathogen-resistance mechanism. The article provides new insight into Albugo and Brassica genomics which could be useful for producing high yielding and WR resistant oilseed cultivars across the globe.

Cognitive and Memory Functions in Plant Immunity

From the time of Thucydides in the 5th century BC, it has been known that specific recognition of pathogens and memory formation are critical components of immune functions. In contrast to the immune system of jawed vertebrates, such as humans and mice, plants lack a circulatory system with mobile immune cells and a repertoire of clonally distributed antigen receptors with almost unlimited specificities. However, without these systems and mechanisms, plants can live and survive in the same hostile environment faced by other organisms. In fact, they achieve specific pathogen recognition and elimination, with limited self-reactivity, and generate immunological memory, sometimes with transgenerational characteristics. Thus, the plant immune system satisfies minimal conditions for constituting an immune system, namely, the recognition of signals in the milieu, integration of that information, subsequent efficient reaction based on the integrated information, and memorization of the experience. In the previous report, this set of elements was proposed as an example of minimal cognitive functions. In this essay, I will first review current understanding of plant immunity and then discuss the unique features of cognitive activities, including recognition of signals from external as well as internal environments, autoimmunity, and memory formation. In doing so, I hope to reach a deeper understanding of the significance of immunity omnipresent in the realm of living organisms.

RPS5-Mediated Disease Resistance: Fundamental Insights and Translational Applications

Focusing on the discovery and characterization of the Arabidopsis disease resistance protein RPS5 and its guardee PBS1, this review discusses work done in the Innes laboratory from the initial identification of the RPS5 gene in 1995 to the recent deployment of the PBS1 decoy system in crops. This is done through discussion of the structure, function, and signaling environment of RPS5 and PBS1, highlighting collaborations and influential ideas along the way. RPS5, a nucleotide-binding leucine-rich repeat (NLR) protein, is activated by the proteolytic cleavage of PBS1. We have shown that the cleavage site within PBS1 can be altered to contain cleavage sites for other proteases, enabling RPS5 activation by these proteases, thereby conferring resistance to different pathogens. This decoy approach has since been translated into crop species using endogenous PBS1 orthologs and holds strong potential for GMO-free development of new genetic resistance against important crop pathogens.

Variation Patterns of NLR Clusters in Arabidopsis thaliana Genomes

The nucleotide-binding domain and leucine-rich repeat (NLR) gene family is highly expanded in the plant lineage with extensive sequence and structure polymorphisms. To survey the landscape of NLR expansion, we mined the published long-read data generated by the resistance gene enrichment sequencing of 64 diverse Arabidopsis thaliana accessions. We found that the hot spots of massive multi-gene NLR cluster expansion did not typically span the whole cluster; instead, they were restricted to a handful of, or only one, dominant radiation(s). All sequences in such a radiation were distinct from other genes in the cluster but not from each other in the clade, making it difficult to assign trustworthy reference-based orthologies when multiple reference genes were present in the radiation. Consequently, NLR genes can be broadly divided into two types: radiating or high-fidelity, where high-fidelity genes are well conserved and well separated from other clades. A similar distinction could be made for NLR clusters, depending on whether cluster size was determined primarily by extensive radiation or the presence of numerous high-fidelity genes. We also identified groups of well-conserved NLR clades that were missing from the Columbia-0 reference genome. This suggests that the classification of NLRs using gene IDs from a single reference accession can rarely capture all major paralogs in a cluster accurately and representatively and that a reference-agnostic perspective is required to properly characterize these additional variations. Finally, we present a quantitative visualization method for differentiating these situations in a given clade of interest.

Cell Death in Plant Immunity

Pathogen recognition by the plant immune system leads to defense responses that are often accompanied by a form of regulated cell death known as the hypersensitive response (HR). HR shares some features with regulated necrosis observed in animals. Genetically, HR can be uncoupled from local defense responses at the site of infection and its role in immunity may be to activate systemic responses in distal parts of the organism. Recent advances in the field reveal conserved cell death-specific signaling modules that are assembled by immune receptors in response to pathogen-derived effectors. The structural elucidation of the plant resistosome-an inflammasome-like structure that may attach to the plasma membrane on activation-opens the possibility that HR cell death is mediated by the formation of pores at the plasma membrane. Necrotrophic pathogens that feed on dead tissue have evolved strategies to trigger the HR cell death pathway as a survival strategy. Ectopic activation of immunomodulators during autoimmune reactions can also promote HR cell death. In this perspective, we discuss the role and regulation of HR in these different contexts.

Molecular characterization and expression analysis of pitaya (Hylocereus polyrhizus) HpLRR genes in response to Neoscytalidium dimidiatum infection

BACKGROUND

Canker disease caused by Neoscytalidium dimidiatum is a devastating disease resulting in a major loss to the pitaya industry. However, resistance proteins in plants play crucial roles to against pathogen infection. Among resistance proteins, the leucine-rich repeat (LRR) protein is a major family that plays crucial roles in plant growth, development, and biotic and abiotic stress responses, especially in disease defense.

RESULTS

In the present study, a transcriptomics analysis identified a total of 272 LRR genes, 233 of which had coding sequences (CDSs), in the plant pitaya (Hylocereus polyrhizus) in response to fungal Neoscytalidium dimidiatum infection. These genes were divided into various subgroups based on specific domains and phylogenetic analysis. Molecular characterization, functional annotation of proteins, and an expression analysis of the LRR genes were conducted. Additionally, four LRR genes (CL445.Contig4_All, Unigene28_All, CL28.Contig2_All, and Unigene2712_All, which were selected because they had the four longest CDSs were further assessed using quantitative reverse transcription PCR (qRT-PCR) at different fungal infection stages in different pitaya species (Hylocereus polyrhizus and Hylocereus undatus), in different pitaya tissues, and after treatment with salicylic acid (SA), methyl jasmonate (MeJA), and abscisic acid (ABA) hormones. The associated protein functions and roles in signaling pathways were identified.

CONCLUSIONS

This study provides a comprehensive overview of the HpLRR family genes at transcriptional level in pitaya in response to N. dimidiatum infection, it will be helpful to understand the molecular mechanism of pitaya canker disease, and lay a strong foundation for further research.

Atypical Resistance Protein RPW8/HR Triggers Oligomerization of the NLR Immune Receptor RPP7 and Autoimmunity

In certain plant hybrids, immunity signaling is initiated when immune components interact in the absence of a pathogen trigger. In Arabidopsis thaliana, such autoimmunity and cell death are linked to variants of the NLR RPP7 and the RPW8 proteins involved in broad-spectrum resistance. We uncover the molecular basis for this autoimmunity and demonstrate that a homolog of RPW8, HR4Fei-0, can trigger the assembly of a higher-order RPP7 complex, with autoimmunity signaling as a consequence. HR4Fei-0-mediated RPP7 oligomerization occurs via the RPP7 C-terminal leucine-rich repeat (LRR) domain and ATP-binding P-loop. RPP7 forms a higher-order complex only in the presence of HR4Fei-0 and not with the standard HR4 variant, which is distinguished from HR4Fei-0 by length variation in C-terminal repeats. Additionally, HR4Fei-0 can independently form self-oligomers, which directly kill cells in an RPP7-independent manner. Our work provides evidence for a plant resistosome complex and the mechanisms by which RPW8/HR proteins trigger cell death.

A CC-NBS-LRR gene induces hybrid lethality in cotton

Hybrid lethality forms a reproductive barrier that has been found in many eukaryotes. Most cases follow the Bateson-Dobzhansky-Muller genetic incompatibility model and involve two or more loci. In this study, we demonstrate that a coiled-coil nucleotide-binding site leucine-rich repeat (CC-NBS-LRR) gene is the causal gene underlying the Le4 locus for interspecific hybrid lethality between Gossypium barbadense and G. hirsutum (cotton). Silencing this CC-NBS-LRR gene can restore F1 plants from a lethal to a normal phenotype. A total of 11 099 genes were differentially expressed between the leaves of normal and lethal F1 plants, of which genes related to autoimmune responses were highly enriched. Genes related to ATP-binding and ATPase were up-regulated before the lethal syndrome appeared; this may result in the conversion of Le4 into an active state and hence trigger immune signals in the absence of biotic/abiotic stress. We discuss our results in relation to the evolution and domestication of Sea Island cottons and the molecular mechanisms of hybrid lethality associated with autoimmune responses. Our findings provide new insights into reproductive isolation and may benefit cotton breeding.

Stronger When Together: Clustering of Plant NLR Disease resistance Genes

Gene clustering is rare in eukaryotes. However, nucleotide-binding leucine-rich repeat (NLR)-encoding disease resistance (R) genes show consistent clustering in plant genomes. These arrangements are likely to provide coregulatory benefits, as suggested by growing evidence that the gene products of both paired and larger clusters of NLRs act together in triggering immunity. Head-to-head gene pairs where one of the encoded NLRs includes an integrated decoy domain appear to behave differently than clusters evolved from closely related typical NLRs. These patterns may help to explain the broad resistance that most plants have despite their finite number of R genes. By taking into consideration the relationship between genomic arrangement and function, we can improve our understanding of and ability to predict plant immune detection.

NLR singletons, pairs, and networks: evolution, assembly, and regulation of the intracellular immunoreceptor circuitry of plants

NLRs are modular plant and animal proteins that are intracellular sensors of pathogen-associated molecules. Upon pathogen perception, NLRs trigger a potent broad-spectrum immune reaction known as the hypersensitive response. An emerging paradigm is that plant NLR immune receptors form networks with varying degrees of complexity. NLRs may have evolved from multifunctional singleton receptors, which combine pathogen detection (sensor activity) and immune signalling (helper or executor activity) into a single protein, to functionally specialized interconnected receptor pairs and networks. In this article, we highlight some of the recent advances in plant NLR biology by discussing models of NLR evolution, NLR complex formation, and how NLR (mis)regulation modulates immunity and autoimmunity. Multidisciplinary approaches are required to dissect the evolution, assembly, and regulation of the immune receptor circuitry of plants. With the new conceptual framework provided by the elucidation of the structure and activation mechanism of a plant NLR resistosome, this field is entering an exciting era of research.

Plant NLRs: From discovery to application

Plants require a complex immune system to defend themselves against a wide range of pathogens which threaten their growth and development. The nucleotide-binding leucine-rich repeat proteins (NLRs) are immune sensors that recognize effectors delivered by pathogens. The first NLR was cloned more than twenty years ago. Since this initial discovery, NLRs have been described as key components of plant immunity responsible for pathogen recognition and triggering defense responses. They have now been described in most of the well-studied mulitcellular plant species, with most having large NLR repertoires. As research has progressed so has the understanding of how NLRs interact with their recognition substrates and how they in turn activate downstream signalling. It has also become apparent that NLR regulation occurs at the transcriptional, post-transcriptional, translational, and post-translational levels. Even before the first NLR was cloned, breeders were utilising such genes to increase crop performance. Increased understanding of the mechanistic details of the plant immune system enable the generation of plants resistant against devastating pathogens. This review aims to give an updated summary of the NLR field.

A cell death assay in barley and wheat protoplasts for identification and validation of matching pathogen AVR effector and plant NLR immune receptors

BACKGROUND

Plant disease resistance to host-adapted pathogens is often mediated by host nucleotide-binding and leucine-rich repeat (NLR) receptors that detect matching pathogen avirulence effectors (AVR) inside plant cells. AVR-triggered NLR activation is typically associated with a rapid host cell death at sites of attempted infection and this response constitutes a widely used surrogate for NLR activation. However, it is challenging to assess this cell death in cereal hosts.

RESULTS

Here we quantify cell death upon NLR-mediated recognition of fungal pathogen AVRs in mesophyll leaf protoplasts of barley and wheat. We provide measurements for the recognition of the fungal AVRs AvrSr50 and AVR _a1 by their respective cereal NLRs Sr50 and Mla1 upon overexpression of the AVR and NLR pairs in mesophyll protoplast of both, wheat and barley.

CONCLUSIONS

Our data demonstrate that the here described approach can be effectively used to detect and quantify death of wheat and barley cells induced by overexpression of NLR and AVR effectors or AVR effector candidate genes from diverse fungal pathogens within 24 h.

Diverse paths to hybrid incompatibility in Arabidopsis

One of the most essential questions of biology is to understand how different species have evolved. Hybrid incompatibility, a phenomenon in which hybrids show reduced fitness in comparison with their parents, can result in reproductive isolation and speciation. Therefore, studying hybrid incompatibility provides an entry point in understanding speciation. Hybrid incompatibilities are known throughout taxa, and the underlying mechanisms have mystified scientists since the theory of evolution by means of natural selection was introduced. In plants, it is only in recent years that the high-throughput genetic and molecular tools have become available for the Arabidopsis genus, thus helping to shed light on the different genes and molecular and evolutionary mechanisms that underlie hybrid incompatibilities. In this review, we highlight the current knowledge of diverse mechanisms that are known to contribute to hybrid incompatibility.

Molecular mechanisms that limit the costs of NLR-mediated resistance in plants

Crop diseases cause significant yield losses, and the use of resistant cultivars can effectively mitigate these losses and control many plant diseases. Most plant resistance (R) genes encode immune receptors composed of nucleotide-binding and leucine-rich repeat (NLR) domains. These proteins mediate the specific recognition of pathogen avirulence effectors to induce defence responses. However, NLR-triggered immunity can be associated with a reduction in growth and yield, so-called 'fitness costs'. Recent data have shown that plants use an elaborate interplay of different mechanisms to control NLR gene transcript levels, as well as NLR protein abundance and activity, to avoid the associated cost of resistance in the absence of a pathogen. In this review, we discuss the different levels of NLR regulation (transcriptional, post-transcriptional and at the protein level). We address the apparent need for plants to maintain diverse modes of regulation. A recent model suggesting an equilibrium 'ON/OFF state' of NLR proteins, in the absence of a pathogen, provides the context for our discussion.

Autoimmunity in plants

Attenuation in the activity of the negative regulators or the hyperactivity of plant innate immune receptors often causes ectopic defense activation manifested in severe growth retardation and spontaneous lesion formations, referred to as autoimmunity. In this review, we have described the cellular and molecular basis of the development of autoimmune responses for their useful applications in plant defense. Plants are exposed to diverse disease-causing pathogens, which bring infections by taking over the control on host immune machineries. To counter the challenges of evolving pathogenic races, plants recruit specific types of intracellular immune receptors that mostly belong to the family of polymorphic nucleotide-binding oligomerization domain-containing leucine-rich repeat (NLR) proteins. Upon recognition of effector molecules, NLR triggers hyperimmune signaling, which culminates in the form of a typical programmed cell death, designated hypersensitive response. Besides, few plant NLRs also guard certain host proteins known as 'guardee' that are modified by effector proteins. However, this fine-tuned innate immune system can be lopsided upon knock-out of the alleles that correspond to the host guardees, which mimick the presence of pathogen. The absence of pathogens causes inappropriate activation of the respective NLRs and results in the constitutive activation of plant defense and exhibiting autoimmunity. In plants, autoimmune mutants are readily scorable due to their dwarf phenotype and development of characteristic macroscopic disease lesions. Here, we summarize recent reports on autoimmune response in plants, how it is triggered, and phenotypic consequences associated with this phenomenon.

A MPK3/6-WRKY33-ALD1-Pipecolic Acid Regulatory Loop Contributes to Systemic Acquired Resistance

Plants induce systemic acquired resistance (SAR) upon localized exposure to pathogens. Pipecolic acid (Pip) production via AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1) is key for SAR establishment. Here, we report a positive feedback loop important for SAR induction in Arabidopsis thaliana We showed that local activation of the MAP kinases MPK3 and MPK6 is sufficient to trigger Pip production and mount SAR. Consistent with this, mutations in MPK3 or MPK6 led to compromised Pip accumulation upon inoculation with the bacterial pathogen Pseudomonas syringae pv tomato DC3000 (Pto) AvrRpt2, which triggers strong sustained MAPK activation. By contrast, P. syringae pv maculicola and Pto, which induce transient MAPK activation, trigger Pip biosynthesis and SAR independently of MPK3/6. ALD1 expression, Pip accumulation, and SAR were compromised in mutants defective in the MPK3/6-regulated transcription factor WRKY33. Chromatin immunoprecipitation showed that WRKY33 binds to the ALD1 promoter. We found that Pip triggers activation of MPK3 and MPK6 and that MAPK activation after Pto AvrRpt2 inoculation is compromised in wrky33 and ald1 mutants. Collectively, our results reveal a positive regulatory loop consisting of MPK3/MPK6, WRKY33, ALD1, and Pip in SAR induction and suggest the existence of distinct SAR activation pathways that converge at the level of Pip biosynthesis.

Structural, Functional, and Genomic Diversity of Plant NLR Proteins: An Evolved Resource for Rational Engineering of Plant Immunity

Plants employ a diverse intracellular system of NLR (nucleotide binding-leucine-rich repeat) innate immune receptors to detect pathogens of all types. These receptors represent valuable agronomic traits that plant breeders rely on to maximize yield in the face of devastating pathogens. Despite their importance, the mechanistic underpinnings of NLR-based disease resistance remain obscure. The rapidly increasing numbers of plant genomes are revealing a diverse array of NLR-type immune receptors. In parallel, mechanistic studies are describing diverse functions for NLR immune receptors. In this review, we intend to broadly describe how the structural, functional, and genomic diversity of plant immune receptors can provide a valuable resource for rational engineering of plant immunity.

The coming of age of EvoMPMI: evolutionary molecular plant-microbe interactions across multiple timescales

Plant-microbe interactions are great model systems to study co-evolutionary dynamics across multiple timescales. However, mechanistic research on plant-microbe interactions has often been conducted with little consideration of evolutionary concepts and methods. Conversely, evolutionary research has rarely integrated the range of mechanisms and models from the molecular plant-microbe interactions field. In recent years, the incipient field of evolutionary molecular plant-microbe interactions (EvoMPMI) has emerged to bridge this gap. Here, we report on some of the recent advances in EvoMPMI. In particular, we highlight new systems to study microbe interactions with early diverging land plants, and new findings from studies of adaptive evolution in pathogens and plants. By linking mechanistic and evolutionary research, EvoMPMI promises to expand our understanding of plant-microbe interactions.

Disease Resistance Mechanisms in Plants

Plants have developed a complex defense system against diverse pests and pathogens. Once pathogens overcome mechanical barriers to infection, plant receptors initiate signaling pathways driving the expression of defense response genes. Plant immune systems rely on their ability to recognize enemy molecules, carry out signal transduction, and respond defensively through pathways involving many genes and their products. Pathogens actively attempt to evade and interfere with response pathways, selecting for a decentralized, multicomponent immune system. Recent advances in molecular techniques have greatly expanded our understanding of plant immunity, largely driven by potential application to agricultural systems. Here, we review the major plant immune system components, state of the art knowledge, and future direction of research on plant⁻pathogen interactions. In our review, we will discuss how the decentralization of plant immune systems have provided both increased evolutionary opportunity for pathogen resistance, as well as additional mechanisms for pathogen inhibition of such defense responses. We conclude that the rapid advances in bioinformatics and molecular biology are driving an explosion of information that will advance agricultural production and illustrate how complex molecular interactions evolve.