Species-wide genetic incompatibility analysis identifies immune genes as hot spots of deleterious epistasis

Epidermal electronic-tattoo for plant immune response monitoring

Real-time monitoring of plant immune responses is crucial for understanding plant immunity and mitigating economic losses from pathogen and pest attacks. However, current methods relying on molecular-level assessment are destructive and time-consuming. Here, we report an ultrathin, substrate-free, and highly conductive electronic tattoo (e-tattoo) designed for plants, enabling immune response monitoring through non-invasive electrical impedance spectroscopy (EIS). The e-tattoo's biocompatibility, high conductivity, and sub-100 nm thickness allow it to conform to leaf tissue morphology and provide robust impedance data. We demonstrate continuous EIS analysis of live transgenic Arabidopsis thaliana plants for over 24 h, capturing the onset of NLR-mediated acute immune responses within three hours post-induction, prior to visible symptoms. RNA-seq and tissue ion leakage tests validate that EIS data accurately represent the physiological and molecular changes associated with immune activation. This non-invasive tissue-assessment technology has the potential to enhance our comprehension of immune activation mechanisms in plants and paves the way for real-time monitoring for plant health management.

Genomic insights into evolution of parthenogenesis and triploidy in the flowerpot snake

The flowerpot snake (Indotyphlops braminus) is the only known parthenogenetic and triploid species within Serpentes. However, the genetic basis underlying this phenomenon remains unresolved. We investigated the genomic complexities of this rare all-female triploid reptile. On the basis of the newly assembled genome, we revealed 40 chromosomes grouped into three subgenomes (A, B, and C). Comparative genomic analysis with related diploid species revealed a chromosome fusion event in ancestral genomes. This event shaped the unique genetic landscape of the flowerpot snake. We examined gene expression specificity in ovarian tissues and identified pathways essential for DNA replication and repair. Our findings suggest a potential mechanism of homologous chromosome pairing during meiosis in allopolyploid parthenogenesis. This research provides insights into the evolutionary adaptations and genetic mechanisms underlying parthenogenesis in reptilian species, challenging traditional views on reproductive strategies and genomic evolution in asexual organisms.

Pan-analysis of intra- and inter-species diversity reveals a group of highly variable immune receptor genes in rice

Plant immune receptors and their natural variations play a central role in combating disease-causing pathogens. These immune receptors include intracellular nucleotide-binding leucine-rich repeat (LRR) receptors (NLRs) and cell-surface pattern recognition receptors (PRRs) that can be further classified as receptor-like proteins (RLPs) and receptor-like kinases (RLKs). Although the NLRome has been characterized, the repertoire and extent of diversity of PRRome remain undetermined in rice. In this study, we examined the diversity of immune receptor genes using high-quality genomes of 309 rice accessions from 8 species within the genus Oryza. A total of 376 310 immune receptor genes were identified, including 149 592 NLR-coding genes and 226 718 PRR coding genes. Shannon entropy analysis revealed a set of immune receptors that display significant intra-species and inter-species diversity in rice. In general, RLPs are more variable than RLKs, while NLRs and LRR-RLPs are more variable than LRR-RLKs. Additionally, NLR and PRR genes exhibit contrasting shoot/root expression patterns, with NLRs generally skewed towards root expression. Furthermore, we found that the size of the LRR-RLK gene families correlates with local annual precipitation, suggesting a stronger selection pressure on LRR-RLK genes in rice accessions grown under wet conditions than dry conditions. In sum, this pan-genomic analysis not only reveals the extensive diversity of the immune receptor repertoires in rice but also provides potential target genes for improving disease resistance in rice.

Genomic structural variation in an alpha/beta hydrolase triggers hybrid necrosis in wheat

Hybrid necrosis, a century-old mystery in wheat, is caused by complementary genes Ne1 and Ne2. Ne2, encoding a nucleotide-binding leucine-rich repeat (NLR) immune receptor, has been cloned, yet Ne1 remains elusive. Here, we report that Ne1, which encodes an alpha/beta hydrolase (ABH) protein generated by structural variation, triggers hybrid necrosis with Ne2 by activating autoimmune responses. We further verify that not only allelic variation but also copy number variation (CNV) of Ne1 are pivotal for hybrid necrosis diversity in wheat. Ne1 likely originates from wild emmer wheat, potentially through duplication and ectopic recombination events. Unlike Ne2, which is frequently selected for rust resistance in wheat breeding, the lower prevalence of Ne1 in modern wheat cultivars is attributed to its association with hybrid necrosis. Altogether, these findings illuminate the co-evolution of the NLR/ABH gene pair in plant development and innate immunity, offering potential benefits for wheat breeding.

It's Complicated: Why Are There So Few Commercially Successful Crop Varieties Engineered for Disease Resistance?

It is more than 40 years since the era of transgenic plants began and more than 30 years after the cloning of the first plant disease resistance genes. Despite extensive progress in our mechanistic understanding and despite considerable sustained efforts in the commercial, nonprofit, academic and governmental sectors, the prospect of commercially viable plant varieties carrying disease resistance traits endowed by biotechnological approaches remains elusive. The cost of complying with the regulations governing the release of transgenic plants is often cited as the main reason for this lack of success. While this is undeniably a substantial hurdle, other transgenic traits have been successfully commercialised. We argue that a significant portion of the challenges of producing crop varieties engineered for disease resistance is intrinsic to the trait itself. In this review, we briefly discuss the main approaches used to engineer plant disease resistance. We further discuss possible reasons why they have not been successful in a commercial context and, finally, we try to derive some lessons to apply to future efforts.

Majority of the Highly Variable NLRs in Maize Share Genomic Location and Contain Additional Target-Binding Domains

Nucleotide-binding, leucine-rich repeat (LRR) proteins (NLRs) are a major class of immune receptors in plants. NLRs include both conserved and rapidly evolving members; however, their evolutionary trajectory in crops remains understudied. Availability of crop pan-genomes enables analysis of the recent events in the evolution of this highly complex gene family within domesticated species. Here, we investigated the NLR complement of 26 nested association mapping (NAM) founder lines of maize. We found that maize has just four main subfamilies containing rapidly evolving highly variable NLR (hvNLR) receptors. Curiously, three of these phylogenetically distinct hvNLR lineages are located in adjacent clusters on chromosome 10. Members of the same hvNLR clade show variable expression and methylation across lines and tissues, which is consistent with their rapid evolution. By combining sequence diversity analysis and AlphaFold2 computational structure prediction, we predicted ligand-binding sites in the hvNLRs. We also observed novel insertion domains in the LRR regions of two hvNLR subfamilies that likely contribute to target recognition. To make this analysis accessible, we created NLRCladeFinder, a Google Colaboratory notebook, that accepts any newly identified NLR sequence, places it in the evolutionary context of the maize pan-NLRome, and provides an updated clade alignment, phylogenetic tree, and sequence diversity information for the gene of interest. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Contact Cooling-Induced ELOVL4 Enhances Skin Wound Healing by Promoting the Inflammation-to-Proliferation Phase Transition

Empirical evidence indicates that the rate of wound healing varies through different seasons, where it is higher in spring and fall but lower in summer and winter, suggesting adequate temperatures may promote wound healing via an unknown mechanism. Here we show that adequate temperature facilitates wound healing by inducing the expression of Elongation of Very Long Chain Fatty Acid Elongase 4 (ELOVL4) that curtails the inflammation phase. Using skin injury and skin organoids models, bulk- and single-cell RNA-sequencing and spatial transcriptomics analysis, and in vivo functional perturbations, we first demonstrate that adjusting skin surface temperature to 20°C by contact cooling markedly increases the rate of wound healing via upregulating ELOVL4 in the injured epidermis. We then reveal docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) as the key products of ELOVL4 that independently control wound healing by dampening the expression of pro-inflammatory cytokines such as tumor necrosis factor α (TNFα). This chain of physiological events enhances wound healing via its timely exit of the inflammatory phase and entry into the proliferation phase of tissue repair. Our findings highlight the skin's adaptability to different temperatures and link the evolutionarily conserved mechanism of long-chain fatty acid synthesis to wound repair while demonstrating the potential application of contact cooling therapy in wound healing.

Generation of Inheritable A-to-G Transitions Using Adenine Base Editing and NG-PAM Cas9 in Arabidopsis thaliana

Towards precise genome editing, base editors have been developed by fusing catalytically compromised Cas9 with deaminase components, mediating C-to-T (cytosine base editors) or A-to-G (adenine base editors) transition. We developed a set of vectors consisting of a 5'-NG-3' PAM-recognizing variant of SpCas9 with adenosine deaminases TadA7.10 or TadA8e. Using a phenotype-based screen in Arabidopsis thaliana targeting multiple PDS3 intron splice sites, we achieved up to 81% somatic A-to-G editing in primary transformants at a splice acceptor site with NGG PAM, while 35% was achieved for the same target adenine with NGA PAM. Among tested vectors, pECNUS4 (Addgene #184887), carrying TadA8e, showed the highest adenine base editor (ABE) efficiency. With pECNUS4, we recreated a naturally occurring allele of DANGEROUS MIX3 (DM3) in two generations, transgene-free, for NGC PAM. We also simultaneously base-edited four redundant DM1/SSI4 homologs, encoding nucleotide-binding leucine-rich repeat (NLR) proteins, using a single gRNA with NGA PAM targeting the conserved yet functionally crucial P-loop motif of NLR proteins. We found fixation of A-to-G in three NLR genes for all three possible adenine sites within base-editing window 3-9, as the edited genes segregate in T2. Multigene targeting succeeded in rescuing the previously reported autoimmune phenotype in two generations. Mediating desired ABE on seven NLR genes simultaneously was successful as well; above 77% editing was achieved in six of the seven possible targets in a T1 plant, with the remaining having a moderately high (32%) editing. ABE application to specifically inactivate functional motifs is anticipated to expedite the discovery of novel roles for proteins. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Patterns of presence-absence variation of NLRs across populations of Solanum chilense are clade-dependent and mainly shaped by past demographic history

Understanding the evolution of pathogen resistance genes (nucleotide-binding site-leucine-rich repeats, NLRs) within a species requires a comprehensive examination of factors that affect gene loss and gain. We present a new reference genome of Solanum chilense, which leads to an increased number and more accurate annotation of NLRs. Using a target capture approach, we quantify the presence-absence variation (PAV) of NLR loci across 20 populations from different habitats. We build a rigorous pipeline to validate the identification of PAV of NLRs and then show that PAV is larger within populations than between populations, suggesting that maintenance of NLR diversity is linked to population dynamics. The amount of PAV appears not to be correlated with the NLR presence in gene clusters in the genome, but rather with the past demographic history of the species, with loss of NLRs in diverging (smaller) populations at the distribution edges. Finally, using a redundancy analysis, we find limited evidence of PAV being linked to environmental gradients. Our results suggest that random processes (genetic drift and demography) and weak positive selection for local adaptation shape the evolution of NLRs at the single nucleotide polymorphism and PAV levels in an outcrossing plant with high nucleotide diversity.

Decoding resilience: ecology, regulation, and evolution of biosynthetic gene clusters

Secondary metabolism is crucial for plant survival and can generate chemistry with nutritional, therapeutic, and industrial value. Biosynthetic genes of selected secondary metabolites cluster within localised chromosomal regions. The arrangement of these biosynthetic gene clusters (BGCs) challenges the long-held model of random gene order in eukaryotes, raising questions about their regulation, ecological significance, and evolution. In this review, we address these questions by exploring the contribution of BGCs to ecologically relevant plant-biotic interactions, while also evaluating the molecular-(epi)genetic mechanisms controlling their coordinated stress- and tissue-specific expression. Based on evidence that BGCs have distinct chromatin signatures and are enriched with transposable elements (TEs), we integrate emerging hypotheses into an updated evolutionary model emphasising how stress-induced epigenetic processes have shaped BGC formation.

The pan-NLRome analysis based on 23 genomes reveals the diversity of NLRs in Brassica napus

Brassica napus, a globally significant oilseed crop, exhibits a wide distribution across diverse climatic zones. B. napus is being increasingly susceptible to distinct diseases, such as blackleg, clubroot and sclerotinia stem rot, leading to substantial reductions in yield. Nucleotide-binding site leucine-rich repeat genes (NLRs), the most pivotal family of resistance genes, can be effectively harnessed by identifying and uncovering their diversity to acquire premium disease-resistant gene resources. Here, we collected the genomes of 23 accessions and established the first comprehensive pan-NLRome in B. napus by leveraging multiple genomic resources. We observe significant variation in the number of NLR genes across different B. napus accessions, ranging from 189 to 474. Notably, TNL (TIR-NBS-LRR) genes constitute approximately half of the total count, indicating their predominant presence in B. napus. The number of NLRs in the C subgenome is significantly higher than that in the A subgenome, and chromosome C09 exhibits the highest density of NLR genes with featuring multiple NLR clusters. Domain analysis reveals that the integrated domains significantly enhance the diversity of NLRs, with B3 DNA binding, VQ, and zinc fingers being the most prevalent integrated domains. Pan-genomic analysis reveals that the core type of NLR genes, which is present in most accessions, constitutes approximately 58% of the total NLRs. Furthermore, we conduct a comparative analysis of the diversity of NLR genes across distinct ecotypes, leading to the identification of ecotype-specific NLRs and their integrated domains. In conclusion, our study effectively addresses the limitations of a single reference genome and provides valuable insights into the diversity of NLR genes in B. napus, thereby contributing to disease resistance breeding.

A newborn F-box gene blocks gene flow by selectively degrading phosphoglucomutase in species hybrids

The establishment of reproductive barriers such as postzygotic hybrid incompatibility (HI) remains the key to speciation. Gene duplication followed by differential functionalization has long been proposed as a major model underlying HI, but little supporting evidence exists. Here, we demonstrate that a newborn F-box gene, Cni-neib-1, of the nematode Caenorhabditis nigoni specifically inactivates an essential phosphoglucomutase encoded by Cbr-shls-1 in its sister species Caenorhabditis briggsae and their hybrids. Zygotic expression of Cni-neib-1 specifically depletes Cbr-SHLS-1, but not Cni-SHLS-1, in approximately 40 min starting from gastrulation, causing embryonic death. Cni-neib-1 is one of thirty-three paralogues emerging from a recent surge in F-box gene duplication events within C. nigoni, all of which are evolving under positive selection. Cni-neib-1 undergoes turnover even among C. nigoni populations. Differential expansion of F-box genes between the two species could reflect their distinctive innate immune responses. Collectively, we demonstrate how recent duplication of genes involved in protein degradation can cause incidental destruction of targets in hybrids that leads to HI, providing an invaluable insight into mechanisms of speciation.

The Ecology of Hybrid Incompatibilities

Ecologically mediated selection against hybrids, caused by hybrid phenotypes fitting poorly into available niches, is typically viewed as distinct from selection caused by epistatic Dobzhansky-Muller hybrid incompatibilities. Here, we show how selection against transgressive phenotypes in hybrids manifests as incompatibility. After outlining our logic, we summarize current approaches for studying ecology-based selection on hybrids. We then quantitatively review QTL-mapping studies and find traits differing between parent taxa are typically polygenic. Next, we describe how verbal models of selection on hybrids translate to phenotypic and genetic fitness landscapes, highlighting emerging approaches for detecting polygenic incompatibilities. Finally, in a synthesis of published data, we report that trait transgression-and thus possibly extrinsic hybrid incompatibility in hybrids-escalates with the phenotypic divergence between parents. We discuss conceptual implications and conclude that studying the ecological basis of hybrid incompatibility will facilitate new discoveries about mechanisms of speciation.

Phenotypic characterization and gene mapping of hybrid necrosis in Triticum durum-Haynaldia villosa amphiploids

KEY MESSAGE

Phenotypical, physiological and genetic characterization was carried out on the hybrid necrosis gene from Haynaldia villosa, and the related gene Ne-V was mapped to chromosome arm 2VL. Introducing genetic variation from wild relatives into common wheat through wide crosses is a vital strategy for enriching genetic diversity and promoting wheat breeding. However, hybrid necrosis, a genetic autoimmunity syndrome, often occurs in the offspring of interspecific or intraspecific crosses, restricting both the selection of hybrid parents and the pyramiding of beneficial genes. To utilize the germplasms of Haynaldia villosa (2n = 2x = 14, VV), we conducted wide hybridization between durum wheat (2n = 4x = 28, AABB) and multiple H. villosa accessions to synthesize the amphiploids (2n = 6x = 42, AABBVV). This study revealed that 61.5% of amphiploids derived from the above crosses exhibited hybrid necrosis, with some amphiploids even dying before reaching maturity. However, the initiation time and severity of necrosis varied dramatically among the progenies, suggesting that there were multiple genetic loci or multiple alleles in the same genetic locus conferring to hybrid necrosis in H. villosa accessions. Genetic analysis was performed on the F2 and derived F2:3 populations, which were constructed between amphiploid STH59-1 with normal leaves and amphiploid STH59-2 with necrotic leaves. A semidominant hybrid necrosis-related gene, Ne-V, was mapped to an 11.8-cM genetic interval on the long arm of chromosome 2V, representing a novel genetic locus identified in Triticum-related species. In addition, the hybrid necrosis was correlated with enhanced H2O2 accumulation and cell death, and it was influenced by the temperature and light. Our findings provide a foundation for cloning the Ne-V gene and exploring its molecular mechanism.

Genetic and molecular mechanisms of reproductive isolation in the utilization of heterosis for breeding hybrid rice

Heterosis, also known as hybrid vigor, is commonly observed in rice crosses. The hybridization of rice species or subspecies exhibits robust hybrid vigor, however, the direct harnessing of this vigor is hindered by reproductive isolation. Here, we review recent advances in the understanding of the molecular mechanisms governing reproductive isolation in inter-subspecific and inter-specific hybrids. This review encompasses the genetic model of reproductive isolation within and among Oryza sativa species, emphasizing the essential role of mitochondria in this process. Additionally, we delve into the molecular intricacies governing the interaction between mitochondria and autophagosomes, elucidating their significant contribution to reproductive isolation. Furthermore, our exploration extends to comprehending the evolutionary dynamics of reproductive isolation and speciation in rice. Building on these advances, we offer a forward-looking perspective on how to overcome the challenges of reproductive isolation and facilitate the utilization of heterosis in future hybrid rice breeding endeavors.

Revisiting growth-defence trade-offs and breeding strategies in crops

Plants have evolved a multi-layered immune system to fight off pathogens. However, immune activation is costly and is often associated with growth and development penalty. In crops, yield is the main breeding target and is usually affected by high disease resistance. Therefore, proper balance between growth and defence is critical for achieving efficient crop improvement. This review highlights recent advances in attempts designed to alleviate the trade-offs between growth and disease resistance in crops mediated by resistance (R) genes, susceptibility (S) genes and pleiotropic genes. We also provide an update on strategies for optimizing the growth-defence trade-offs to breed future crops with desirable disease resistance and high yield.

High allelic diversity in Arabidopsis NLRs is associated with distinct genomic features

Plants rely on Nucleotide-binding, Leucine-rich repeat Receptors (NLRs) for pathogen recognition. Highly variable NLRs (hvNLRs) show remarkable intraspecies diversity, while their low-variability paralogs (non-hvNLRs) are conserved between ecotypes. At a population level, hvNLRs provide new pathogen-recognition specificities, but the association between allelic diversity and genomic and epigenomic features has not been established. Our investigation of NLRs in Arabidopsis Col-0 has revealed that hvNLRs show higher expression, less gene body cytosine methylation, and closer proximity to transposable elements than non-hvNLRs. hvNLRs show elevated synonymous and nonsynonymous nucleotide diversity and are in chromatin states associated with an increased probability of mutation. Diversifying selection maintains variability at a subset of codons of hvNLRs, while purifying selection maintains conservation at non-hvNLRs. How these features are established and maintained, and whether they contribute to the observed diversity of hvNLRs is key to understanding the evolution of plant innate immune receptors.

Detailing Early Shoot Growth Arrest in Kro-0 x BG-5 Hybrids of Arabidopsis thaliana

Shoot growth directly impacts plant productivity. Plants adjust their shoot growth in response to varying environments to maximize resource capture and stress resilience. While several factors controlling shoot growth are known, the complexity of the regulation and the input of the environment are not fully understood. We have investigated shoot growth repression induced by low ambient temperatures in hybrids of Arabidopsis thaliana Kro-0 and BG-5 accessions. To continue our previous studies, we confirmed that the Kro-0 allele of DYNAMIN-RELATED PROTEIN 3B causes stunted shoot growth in the BG-5 background. We also found that shoot growth repression was most pronounced near the apex at a lower temperature and that the cells in the hybrid stem failed to elongate correctly. Furthermore, we observed that shoot growth repression in hybrids depended on light availability. Global gene expression analysis indicated the involvement of hormones, especially strigolactone, associated with the dwarf phenotype. Altogether, this study enhances our knowledge on the genetic, physiological and environmental factors associated with shoot growth regulation.

Deleterious phenotypes in wild Arabidopsis arenosa populations are common and linked to runs of homozygosity

In this study, we aimed to systematically assess the frequency at which potentially deleterious phenotypes appear in natural populations of the outcrossing model plant Arabidopsis arenosa, and to establish their underlying genetics. For this purpose, we collected seeds from wild A. arenosa populations and screened over 2,500 plants for unusual phenotypes in the greenhouse. We repeatedly found plants with obvious phenotypic defects, such as small stature and necrotic or chlorotic leaves, among first-generation progeny of wild A. arenosa plants. Such abnormal plants were present in about 10% of maternal sibships, with multiple plants with similar phenotypes in each of these sibships, pointing to a genetic basis of the observed defects. A combination of transcriptome profiling, linkage mapping and genome-wide runs of homozygosity patterns using a newly assembled reference genome indicated a range of underlying genetic architectures associated with phenotypic abnormalities. This included evidence for homozygosity of certain genomic regions, consistent with alleles that are identical by descent being responsible for these defects. Our observations suggest that deleterious alleles with different genetic architectures are segregating at appreciable frequencies in wild A. arenosa populations.

Reproductive isolation in a three-way contact zone

Contact zones between divergent forms within a species provide insight into the role of gene flow in adaptation and speciation. Previous work has focused on contact zones involving only two divergent forms, but in nature, many more than two populations may overlap simultaneously and experience gene flow. Patterns of introgression in wild populations are, therefore, likely much more complicated than is often assumed. We begin to address this gap in current knowledge by investigating patterns of divergence and introgression across a complex natural contact zone. We use phenotypic and genomic data to confirm the existence of a three-way contact zone among divergent freshwater resident, saltwater resident and saltwater migratory three-spined stickleback (Gasterosteus aculeatus) on the island of North Uist, Scottish Western Isles. We find evidence for hybridization, mostly between saltwater resident and saltwater migratory forms. Despite hybridization, genomic analyses reveal pairwise islands of divergence between all forms that are maintained across the contact zone. Genomic cline analyses also provide evidence for selection and/or hybrid incompatibilities in divergent regions. Divergent genomic regions occur across multiple chromosomes and involve many known adaptive loci and several chromosomal inversions. We also identify distinct immune gene expression profiles between forms, but no evidence for transgressive expression in hybrids. Our results suggest that reproductive isolation is maintained in this three-way contact zone, despite some hybridization, and that reduced recombination in chromosomal inversions may play an important role in maintaining this isolation.

Dysfunction of duplicated pair rice histone acetyltransferases causes segregation distortion and an interspecific reproductive barrier

Postzygotic reproductive isolation, which results in the irreversible divergence of species, is commonly accompanied by hybrid sterility, necrosis/weakness, or lethality in the F1 or other offspring generations. Here we show that the loss of function of HWS1 and HWS2, a couple of duplicated paralogs, together confer complete interspecific incompatibility between Asian and African rice. Both of these non-Mendelian determinants encode the putative Esa1-associated factor 6 (EAF6) protein, which functions as a characteristic subunit of the histone H4 acetyltransferase complex regulating transcriptional activation via genome-wide histone modification. The proliferating tapetum and inappropriate polar nuclei arrangement cause defective pollen and seeds in F2 hybrid offspring due to the recombinant HWS1/2-mediated misregulation of vitamin (biotin and thiamine) metabolism and lipid synthesis. Evolutionary analysis of HWS1/2 suggests that this gene pair has undergone incomplete lineage sorting (ILS) and multiple gene duplication events during speciation. Our findings have not only uncovered a pair of speciation genes that control hybrid breakdown but also illustrate a passive mechanism that could be scaled up and used in the guidance and optimization of hybrid breeding applications for distant hybridization.

Paternal imprinting in Marchantia polymorpha

We are becoming aware of a growing number of organisms that do not express genetic information equally from both parents as a result of an epigenetic phenomenon called genomic imprinting. Recently, it was shown that the entire paternal genome is repressed during the diploid phase of the life cycle of the liverwort Marchantia polymorpha. The deposition of the repressive epigenetic mark H3K27me3 on the male pronucleus is responsible for the imprinted state, which is reset by the end of meiosis. Here, we put these recent reports in perspective of other forms of imprinting and discuss the potential mechanisms of imprinting in bryophytes and the causes of its evolution.

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.

The genetic and physiological basis of Arabidopsis thaliana tolerance to Pseudomonas viridiflava

The opportunistic pathogen Pseudomonas viridiflava colonizes > 50 agricultural crop species and is the most common Pseudomonas in the phyllosphere of European Arabidopsis thaliana populations. Belonging to the P. syringae complex, it is genetically and phenotypically distinct from well-characterized P. syringae sensu stricto. Despite its prevalence, we lack knowledge of how A. thaliana responds to its native isolates at the molecular level. Here, we characterize the host response in an A. thaliana - P. viridiflava pathosystem. We measured host and pathogen growth in axenic infections and used immune mutants, transcriptomics, and metabolomics to determine defense pathways influencing susceptibility to P. viridiflava infection. Infection with P. viridiflava increased jasmonic acid (JA) levels and the expression of ethylene defense pathway marker genes. The immune response in a susceptible host accession was delayed compared with a tolerant one. Mechanical injury rescued susceptibility, consistent with an involvement of JA. The JA/ethylene pathway is important for suppression of P. viridiflava, yet suppression capacity varies between accessions. Our results shed light on how A. thaliana can suppress the ever-present P. viridiflava, but further studies are needed to understand how P. viridiflava evades this suppression to spread broadly across A. thaliana populations.

The role of conflict in shaping plant biodiversity

Although intrinsic postzygotic reproductive barriers can play a fundamental role in speciation, their underlying evolutionary causes are widely debated. One hypothesis is that incompatibilities result from genomic conflicts. Here, I synthesize the evidence that conflict generates incompatibilities in plants, thus playing a creative role in plant biodiversity. While much evidence supports a role for conflict in several classes of incompatibility, integrating knowledge of incompatibility alleles with natural history can provide further essential tests. Moreover, comparative work can shed light on the relative importance of conflict in causing incompatibilities, including the extent to which their evolution is repeatable. Together, these approaches can provide independent lines of evidence that conflict causes incompatibilities, cementing its role in plant speciation.

Global expression patterns of R-genes in tomato and potato

Introduction

As key-players of plant immunity, the proteins encoded by resistance genes (R-genes) recognize pathogens and initiate pathogen-specific defense responses. The expression of some R-genes carry fitness costs and therefore inducible immune responses are likely advantageous. To what degree inducible resistance driven by R-genes is triggered by pathogen infection is currently an open question.

Methods

In this study we analyzed the expression of 940 R-genes of tomato and potato across 315 transcriptome libraries to investigate how interspecific interactions with microbes influence R-gene expression in plants.

Results

We found that most R-genes are expressed at a low level. A small subset of R-genes had moderate to high levels of expression and were expressed across many independent libraries, irrespective of infection status. These R-genes include members of the class of genes called NRCs (NLR required for cell death). Approximately 10% of all R-genes were differentially expressed during infection and this included both up- and down-regulation. One factor associated with the large differences in R-gene expression was host tissue, reflecting a considerable degree of tissue-specific transcriptional regulation of this class of genes.

Discussion

These results call into question the widespread view that R-gene expression is induced upon pathogen attack. Instead, a small core set of R-genes is constitutively expressed, imparting upon the plant a ready-to-detect and defend status.

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.

Genomics of plant speciation

Studies of plants have been instrumental for revealing how new species originate. For several decades, botanical research has complemented and, in some cases, challenged concepts on speciation developed via the study of other organisms while also revealing additional ways in which species can form. Now, the ability to sequence genomes at an unprecedented pace and scale has allowed biologists to settle decades-long debates and tackle other emerging challenges in speciation research. Here, we review these recent genome-enabled developments in plant speciation. We discuss complications related to identification of reproductive isolation (RI) loci using analyses of the landscape of genomic divergence and highlight the important role that structural variants have in speciation, as increasingly revealed by new sequencing technologies. Further, we review how genomics has advanced what we know of some routes to new species formation, like hybridization or whole-genome duplication, while casting doubt on others, like population bottlenecks and genetic drift. While genomics can fast-track identification of genes and mutations that confer RI, we emphasize that follow-up molecular and field experiments remain critical. Nonetheless, genomics has clarified the outsized role of ancient variants rather than new mutations, particularly early during speciation. We conclude by highlighting promising avenues of future study. These include expanding what we know so far about the role of epigenetic and structural changes during speciation, broadening the scope and taxonomic breadth of plant speciation genomics studies, and synthesizing information from extensive genomic data that have already been generated by the plant speciation community.

Pervasive under-dominance in gene expression underlying emergent growth trajectories in Arabidopsis thaliana hybrids

BACKGROUND

Complex traits, such as growth and fitness, are typically controlled by a very large number of variants, which can interact in both additive and non-additive fashion. In an attempt to gauge the relative importance of both types of genetic interactions, we turn to hybrids, which provide a facile means for creating many novel allele combinations.

RESULTS

We focus on the interaction between alleles of the same locus, i.e., dominance, and perform a transcriptomic study involving 141 random crosses between different accessions of the plant model species Arabidopsis thaliana. Additivity is rare, consistently observed for only about 300 genes enriched for roles in stress response and cell death. Regulatory rare-allele burden affects the expression level of these genes but does not correlate with F1 rosette size. Non-additive, dominant gene expression in F1 hybrids is much more common, with the vast majority of genes (over 90%) being expressed below the parental average. Unlike in the additive genes, regulatory rare-allele burden in the dominant gene set is strongly correlated with F1 rosette size, even though it only mildly covaries with the expression level of these genes.

CONCLUSIONS

Our study underscores under-dominance as the predominant gene action associated with emergence of rosette growth trajectories in the A. thaliana hybrid model. Our work lays the foundation for understanding molecular mechanisms and evolutionary forces that lead to dominance complementation of rare regulatory alleles.

A natural gene drive system confers reproductive isolation in rice

Hybrid sterility restricts the utilization of superior heterosis of indica-japonica inter-subspecific hybrids. In this study, we report the identification of RHS12, a major locus controlling male gamete sterility in indica-japonica hybrid rice. We show that RHS12 consists of two genes (iORF3/DUYAO and iORF4/JIEYAO) that confer preferential transmission of the RHS12-i type male gamete into the progeny, thereby forming a natural gene drive. DUYAO encodes a mitochondrion-targeted protein that interacts with OsCOX11 to trigger cytotoxicity and cell death, whereas JIEYAO encodes a protein that reroutes DUYAO to the autophagosome for degradation via direct physical interaction, thereby detoxifying DUYAO. Evolutionary trajectory analysis reveals that this system likely formed de novo in the AA genome Oryza clade and contributed to reproductive isolation (RI) between different lineages of rice. Our combined results provide mechanistic insights into the genetic basis of RI as well as insights for strategic designs of hybrid rice breeding.

A recessive LRR-RLK gene causes hybrid breakdown in cotton

KEY MESSAGE

An LRR-RLK gene causing interspecific hybrid breakdown between Gossypium. anomalum and G. hirsutum was identified by deploying a map-based cloning strategy. The self-destructing symptoms of hybrid incompatibility in most cases are attributed to autoimmunity. The cloning of genes responsible for hybrid incompatibility in cotton is helpful to clarify the mechanisms underlying hybrid incompatibility and can break the barriers in distant hybridization. In this study, a temperature-dependent lethality was identified in CSSL11-9 (chromosome segment substitution line) with Gossypium anomalum chromosome segment on chromosome A11. Transcriptome analysis showed the differentially expressed genes related to autoimmune responses were highly enriched, suggesting that expression of CSSL11-9 plant lethal gene activated autoimmunity in the absence of any pathogen or external stimulus, inducing programmed cell death (PCD) and causing a lethal phenotype. The lethal phenotype was controlled by a pair of recessive genes and then fine mapped between JAAS3191-JAAS3050 interval, which covered 63.87 kb in G. hirsutum genome and 98.66 kb in G. anomalum. We demonstrated that an LRR-RLK gene designated as hybrid breakdown 1 (GoanoHBD1) was the causal gene underlying this locus for interspecific hybrid incompatibility between G. anomalum and G. hirsutum. Silencing this LRR-RLK gene could restore CSSL11-9 plants from a lethal to a normal phenotype. Our findings provide new insights into reproductive isolation and may benefit cotton breeding.

Maximization through optimization? On the relationship between hybrid performance and parental genetic distance

Heterosis is the improved performance of hybrids compared with their parental components and is widely exploited in agriculture. According to quantitative genetic theory, genetic distance between parents at heterotic quantitative trait loci is required for heterosis, but how heterosis varies with genetic distance has remained elusive, despite intensive research on the topic. Experimental studies have often found a positive association between heterosis and genetic distance that, however, varied in strength. Most importantly, it has remained unclear whether heterosis increases continuously with genetic distance or whether there is an optimum genetic distance after which heterosis declines again. Here, we revisit the relationship between heterosis and genetic distance and provide perspectives on how to maximize heterosis and hybrid performance in breeding, as well as the consequences for the design of heterotic groups and the utilization of more exotic material and genetic resources.

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.

Understanding and overcoming hybrid lethality in seed and seedling stages as barriers to hybridization and gene flow

Hybrid lethality is a type of reproductive isolation barrier observed in two developmental stages, hybrid embryos (hybrid seeds) and hybrid seedlings. Hybrid lethality has been reported in many plant species and limits distant hybridization breeding including interspecific and intergeneric hybridization, which increases genetic diversity and contributes to produce new germplasm for agricultural purposes. Recent studies have provided molecular and genetic evidence suggesting that underlying causes of hybrid lethality involve epistatic interaction of one or more loci, as hypothesized by the Bateson-Dobzhansky-Muller model, and effective ploidy or endosperm balance number. In this review, we focus on the similarities and differences between hybrid seed lethality and hybrid seedling lethality, as well as methods of recovering seed/seedling activity to circumvent hybrid lethality. Current knowledge summarized in our article will provides new insights into the mechanisms of hybrid lethality and effective methods for circumventing hybrid lethality.

The role of gene presence-absence variations on genetic incompatibility in Asian rice

Genetic incompatibilities are widespread between species. However, it remains unclear whether they all originated after population divergence as suggested by the Bateson-Dobzhansky-Muller model, and if not, what is their prevalence and distribution within populations. The gene presence-absence variations (PAVs) provide an opportunity for investigating gene-gene incompatibility. Here, we searched for the repulsion of coexistence between gene PAVs to identify the negative interaction of gene functions separately in two Oryza sativa subspecies. Many PAVs are involved in subspecies-specific negative epistasis and segregate at low-to-intermediate frequencies in focal subspecies but at low or high frequencies in the other subspecies. Incompatible PAVs are enriched in two functional groups, defense response and protein phosphorylation, which are associated with plant immunity and consistent with autoimmunity being a known mechanism of hybrid incompatibility in plants. Genes in the two enriched functional groups are older and seldom directly interact with each other. Instead, they interact with other younger gene PAVs with diverse functions. Our results illustrate the landscape of genetic incompatibility at gene PAVs in rice, where many incompatible pairs have already segregated as polymorphisms within subspecies, and many are novel negative interactions between older defense-related genes and younger genes with diverse functions.

Antibodies for a bespoke plant immunity

Plants do not have antibodies. However, in a recent Science article, Kourelis and Marchal et al. have demonstrated that plant immune receptors can be retrofitted with animal antibodies to provide plants potentially with hundreds and thousands of options to perceive attacking microbes. This is the dawn of bespoke plant immunity.

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.

Tight genetic linkage of genes causing hybrid necrosis and pollinator isolation between young species

The mechanisms of reproductive isolation that cause phenotypic diversification and eventually speciation are a major topic of evolutionary research. Hybrid necrosis is a post-zygotic isolation mechanism in which cell death develops in the absence of pathogens. It is often due to the incompatibility between proteins from two parents. Here we describe a unique case of hybrid necrosis due to an incompatibility between loci on chromosomes 2 and 7 between two pollinator-isolated Petunia species. Typical immune responses as well as endoplasmic reticulum stress responses are induced in the necrotic line. The locus on chromosome 2 encodes ChiA1, a bifunctional GH18 chitinase/lysozyme. The enzymatic activity of ChiA1 is dispensable for the development of necrosis. We propose that the extremely high expression of ChiA1 involves a positive feedback loop between the loci on chromosomes 2 and 7. ChiA1 is tightly linked to major genes involved in the adaptation to different pollinators, a form of pre-zygotic isolation. This linkage of pre- and post-zygotic barriers strengthens reproductive isolation and probably contributes to rapid diversification and speciation.

Assassination tango: an NLR/NLR-ID immune receptors pair of rapeseed co-operates inside the nucleus to activate cell death

Plant immunity largely relies on intracellular nucleotide-binding domain leucine-rich repeat (NLR) immune receptors. Some plant NLRs carry integrated domains (IDs) that mimic authentic pathogen effector targets. We report here the identification of a genetically linked NLR-ID/NLR pair: BnRPR1 and BnRPR2 in Brassica napus. The NLR-ID carries two ID fusions and the mode of action of the pair conforms to the proposed "integrated sensor/decoy" model. The two NLRs interact and the heterocomplex localizes in the plant-cell nucleus and nucleolus. However, the BnRPRs pair does not operate through a negative regulation as it was previously reported for other NLR-IDs. Cell death is induced only upon co-expression of the two proteins and is dependent on the helper genes, EDS1 and NRG1. The nuclear localization of both proteins seems to be essential for cell death activation, while the IDs of BnRPR1 are dispensable for this purpose. In summary, we describe a new pair of NLR-IDs with interesting features in relation to its regulation and the cell death activation.

Incompatibility and Interchangeability in Molecular Evolution

There is remarkable variation in the rate at which genetic incompatibilities in molecular interactions accumulate. In some cases, minor changes-even single-nucleotide substitutions-create major incompatibilities when hybridization forces new variants to function in a novel genetic background from an isolated population. In other cases, genes or even entire functional pathways can be horizontally transferred between anciently divergent evolutionary lineages that span the tree of life with little evidence of incompatibilities. In this review, we explore whether there are general principles that can explain why certain genes are prone to incompatibilities while others maintain interchangeability. We summarize evidence pointing to four genetic features that may contribute to greater resistance to functional replacement: (1) function in multisubunit enzyme complexes and protein-protein interactions, (2) sensitivity to changes in gene dosage, (3) rapid rate of sequence evolution, and (4) overall importance to cell viability, which creates sensitivity to small perturbations in molecular function. We discuss the relative levels of support for these different hypotheses and lay out future directions that may help explain the striking contrasts in patterns of incompatibility and interchangeability throughout the history of molecular evolution.

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.

Mating system and speciation I: Accumulation of genetic incompatibilities in allopatry

Self-fertilisation is widespread among hermaphroditic species across the tree of life. Selfing has many consequences on the genetic diversity and the evolutionary dynamics of populations, which may in turn affect macroevolutionary processes such as speciation. On the one hand, because selfing increases genetic drift and reduces migration rate among populations, it may be expected to promote speciation. On the other hand, because selfing reduces the efficacy of selection, it may be expected to hamper ecological speciation. To better understand under which conditions and in which direction selfing affects the build-up of reproductive isolation, an explicit population genetics model is required. Here, we focus on the interplay between genetic drift, selection and genetic linkage by studying speciation without gene flow. We test how fast populations with different rates of selfing accumulate mutations leading to genetic incompatibilities. When speciation requires populations to pass through a fitness valley caused by underdominant and compensatory mutations, selfing reduces the depth and/or breadth of the valley, and thus overall facilitates the fixation of incompatibilities. When speciation does not require populations to pass through a fitness valley, as for Bateson-Dobzhanzky-Muller incompatibilities (BDMi), the lower effective population size and higher genetic linkage in selfing populations both facilitate the fixation of incompatibilities. Interestingly, and contrary to intuitive expectations, local adaptation does not always accelerate the fixation of incompatibilities in outcrossing relative to selfing populations. Our work helps to clarify how incompatibilities accumulate in selfing vs. outcrossing lineages, and has repercussions on the pace of speciation as well as on the genetic architecture of reproductive isolation.

On the impermanence of species: The collapse of genetic incompatibilities in hybridizing populations

Species pairs often become genetically incompatible during divergence, which is an important source of reproductive isolation. An idealized picture is often painted where incompatibility alleles accumulate and fix between diverging species. However, recent studies have shown both that incompatibilities can collapse with ongoing hybridization, and that incompatibility loci can be polymorphic within species. This paper suggests some general rules for the behavior of incompatibilities under hybridization. In particular, we argue that redundancy of genetic pathways can strongly affect the dynamics of intrinsic incompatibilities. Since fitness in genetically redundant systems is unaffected by introducing a few foreign alleles, higher redundancy decreases the stability of incompatibilities during hybridization, but also increases tolerance of incompatibility polymorphism within species. We use simulations and theories to show that this principle leads to two types of collapse: in redundant systems, exemplified by classical Dobzhansky-Muller incompatibilities, collapse is continuous and approaches a quasi-neutral polymorphism between broadly sympatric species, often as a result of isolation-by-distance. In nonredundant systems, exemplified by co-evolution among genetic elements, incompatibilities are often stable, but can collapse abruptly with spatial traveling waves. As both types are common, the proposed principle may be useful in understanding the abundance of genetic incompatibilities in natural populations.

Ammonium and nitric oxide condition the establishment of Arabidopsis Ler/Kas-2 immune-related hybrid incompatibility

High ammonium suppresses hybrid incompatibility between Ler and Kas-2 accessions through lowering nitric oxide levels and nitrate reductase activity required for autoimmunity. The immune-related hybrid incompatibility (HI) between Landsberg erecta (Ler) and Kashmir-2 (Kas-2) accessions is due to a deleterious genetic interaction between the RPP1 (RECOGNITION OF PERONOSPORA PARASITICA1)-like Ler locus and Kas-2 alleles of the receptor-like kinase SRF3 (STRUBBELIG RECEPTOR FAMILY 3). The genetic incompatibility is temperature-dependent and leads to constitutive activation of the salicylic acid (SA) pathway, dwarfism and cell death at 14-16 °C. Here we investigated the effect of nutrition on the occurrence of Ler/Kas-2 HI and found that high ammonium suppresses Ler/Kas-2 incompatible phenotypes independently of the ammonium/nitrate ratio. Ammonium feeding leads to compromised disease resistance to Pseudomonas syringae pv. tomato DC3000, lower total SA, nitric oxide and nitrate reductase activity in Ler/Kas-2 incompatible hybrids. In addition, we find that Ler/Kas-2 incompatibility is dependent on NPR1 (NONEXPRESSER OF PR GENES 1) and nitric oxide production. Overall, this work highlights the effect of nutrition on the expression of incompatible phenotypes independently of temperature.

Immunoglobulin genes, reproductive isolation and vertebrate speciation

Reproductive isolation drives the formation of new species, and many genes contribute to this through Dobzhansky–Muller incompatibilities (DMIs). These incompatibilities occur when gene divergence affects loci encoding interacting products such as receptors and their ligands. We suggest here that the nature of vertebrate immunoglobulin (IG) genes must make them prone to DMIs. The genes of these complex loci form functional genes through the process of recombination, giving rise to a repertoire of heterodimeric receptors of incredible diversity. This repertoire, within individuals and within species, must defend against pathogens but must also avoid pathogenic self‐reactivity. We suggest that this avoidance of autoimmunity is only achieved through a coordination of evolution between heavy‐ and light‐chain genes, and between these genes and the rest of the genome. Without coordinated evolution, the hybrid offspring of two diverging populations will carry a heavy burden of DMIs, resulting in a loss of fitness. Critical incompatibilities could manifest as incompatibilities between a mother and her divergent offspring. During fetal development, biochemical differences between the parents of hybrid offspring could make their offspring a target of the maternal immune system. This hypothesis was conceived in the light of recent insights into the population genetics of IG genes. This has suggested that antibody genes are probably as susceptible to evolutionary forces as other parts of the genome. Further repertoire studies in human and nonhuman species should now help determine whether antibody genes have been part of the evolutionary forces that drive the development of species.

Three problems in the genetics of speciation by selection

Speciation is the process by which barriers to gene flow evolve between populations. Although we now know that speciation is largely driven by natural selection, knowledge of the agents of selection and the genetic and genomic mechanisms that facilitate divergence is required for a satisfactory theory of speciation. In this essay, we highlight three advances/problems in our understanding of speciation that have arisen from studies of the genes and genomic regions that underlie the evolution of reproductive isolation. First, we describe how the identification of “speciation” genes makes it possible to identify the agents of selection causing the evolution of reproductive isolation, while also noting that the link between the genetics of phenotypic divergence and intrinsic postzygotic reproductive barriers remains tenuous. Second, we discuss the important role of recombination suppressors in facilitating speciation with gene flow, but point out that the means and timing by which reproductive barriers become associated with recombination cold spots remains uncertain. Third, we establish the importance of ancient genetic variation in speciation, although we argue that the focus of speciation studies on evolutionarily young groups may bias conclusions in favor of ancient variation relative to new mutations.

Cavity surface residues of PAD4 and SAG101 contribute to EDS1 dimer signaling specificity in plant immunity

Arabidopsis pathogen effector-triggered immunity (ETI) is controlled by a family of three lipase-like proteins (EDS1, PAD4, and SAG101) and two subfamilies of HET-S/LOB-B (HeLo)-domain "helper" nucleotide-binding/leucine-rich repeats (ADR1s and NRG1s). EDS1-PAD4 dimers cooperate with ADR1s, and EDS1-SAG101 dimers with NRG1s, in two separate defense-promoting modules. EDS1-PAD4-ADR1 and EDS1-SAG101-NRG1 complexes were detected in immune-activated leaf extracts but the molecular determinants for specific complex formation and function remain unknown. EDS1 signaling is mediated by a C-terminal EP domain (EPD) surface surrounding a cavity formed by the heterodimer. Here we investigated whether the EPDs of PAD4 and SAG101 contribute to EDS1 dimer functions. Using a structure-guided approach, we undertook a comprehensive mutational analysis of Arabidopsis PAD4. We identify two conserved residues (Arg314 and Lys380) lining the PAD4 EPD cavity that are essential for EDS1-PAD4-mediated pathogen resistance, but are dispensable for the PAD4-mediated restriction of green peach aphid infestation. Positionally equivalent Met304 and Arg373 at the SAG101 EPD cavity are required for EDS1-SAG101 promotion of ETI-related cell death. In a PAD4 and SAG101 interactome analysis of ETI-activated tissues, PAD4^R314A and SAG101^M304R EPD variants maintain interaction with EDS1 but lose association, respectively, with helper nucleotide-binding/leucine-rich repeats ADR1-L1 and NRG1.1, and other immune-related proteins. Our data reveal a fundamental contribution of similar but non-identical PAD4 and SAG101 EPD surfaces to specific EDS1 dimer protein interactions and pathogen immunity.

Evolutionary footprint of plant immunity

There are pieces of evidence from genomic footprints and fossil records indicating that plants have co-evolved with microbes after terrestrialization for more than 407 million years. Therefore, to truly comprehend plant evolution, we need to understand the co-evolutionary process and history between plants and microbes. Recent developments in genomes and transcriptomes of a vast number of plant species as well as microbes have greatly expanded our knowledge of the evolution of the plant immune system. In this review, we summarize recent advances in the co-evolution between plants and microbes with emphasis on the plant side and point out future research needed for understanding plant-microbial co-evolution. Knowledge of the evolution and variation of the plant immune system will better equip us on designing crops with boosted performance in agricultural fields.

Indirect recognition of pathogen effectors by NLRs

To perceive pathogen threats, plants utilize both plasma membrane-localized and intracellular receptors. Nucleotide-binding domain leucine-rich repeat containing (NLR) proteins are key receptors that can recognize pathogen-derived intracellularly delivered effectors and activate downstream defense. Exciting recent findings have propelled our understanding of the various recognition and activation mechanisms of plant NLRs. Some NLRs directly bind to effectors, but others can perceive effector-induced changes on targeted host proteins (guardees), or non-functional host protein mimics (decoys). Such guarding strategies are thought to afford the host more durable resistance to quick-evolving and diverse pathogens. Here, we review classic and recent examples of indirect effector recognition by NLRs and discuss strategies for the discovery and study of new NLR-decoy/guardee systems. We also provide a perspective on how executor NLRs and helper NLRs (hNLRs) provide recognition for a wider range of effectors through sensor NLRs and how this can be considered an expanded form of indirect recognition. Furthermore, we summarize recent structural findings on NLR activation and resistosome formation upon indirect recognition. Finally, we discuss existing and potential applications that harness NLR indirect recognition for plant disease resistance and crop resilience.