Quantitative Resistance: More Than Just Perception of a Pathogen

Using genome-wide associations and host-by-pathogen predictions to identify allelic interactions that control disease resistance

Characterizing the molecular mechanisms underlying disease symptom expression has been used to improve human health and disease resistance in crops and animal breeds. Quantitative trait loci and genome-wide association studies (GWAS) are widely used to identify genomic regions that are involved in disease progression. This study extends traditional GWAS significance tests of host and pathogen marker main effects by utilizing dual-genome reaction norm models to evaluate the importance of host-single nucleotide polymorphism (SNP) by pathogen-SNP interactions. Disease symptom severity data from Fusarium ear rot (FER) on maize (Zea mays L.) is used to demonstrate the use of both genomes in genomic selection models for breeding and the identification of loci that interact across organisms to impact FER disease development. Dual genome prediction models improved heritability estimates, error variances, and model accuracy while providing predictions for host-by-pathogen interactions that may be used to test the significance of SNP-SNP interactions. Independent GWAS for maize and Fusarium populations identified significantly associated loci and predictions that were used to evaluate the importance of interactions using two different association tests. Predictions from dual genome models were used to evaluate the significance of the SNP-SNP interactions that may be associated with population structure or polygenic effects. As well, association tests incorporating host and pathogen markers in models that also included genomic relationship matrices were used to account for population structure. Subsequent evaluation of protein-protein interactions from candidate genes near the interacting SNPs provides a further in silico evaluation method to expedite the identification of interacting genes.

Non-Invasive, Bioluminescence-Based Visualisation and Quantification of Bacterial Infections in Arabidopsis Over Time

Plant-pathogenic bacteria colonise their hosts using various strategies, exploiting both natural openings and wounds in leaves and roots. The vascular pathogen Xanthomonas campestris pv. campestris (Xcc) enters its host through hydathodes, organs at the leaf margin involved in guttation. Subsequently, Xcc breaches the hydathode-xylem barrier and progresses into the xylem vessels causing systemic disease. To elucidate the mechanisms that underpin the different stages of an Xcc infection, a need exists to image bacterial progression in planta in a non-invasive manner. Here, we describe a phenotyping setup and Python image analysis pipeline for capturing 16 independent Xcc infections in Arabidopsis thaliana plants in parallel over time. The setup combines an RGB camera for imaging disease symptoms and an ultrasensitive CCD camera for monitoring bacterial progression inside leaves using bioluminescence. The method reliably quantified bacterial growth in planta for two bacterial species, that is, vascular Xcc and the mesophyll pathogen Pseudomonas syringae pv. tomato (Pst). The camera resolution allowed Xcc imaging already in the hydathodes, yielding reproducible data for the first stages prior to the systemic infection. Data obtained through the image analysis pipeline was robust and validated findings from other bioluminescence imaging methods, while requiring fewer samples. Moreover, bioluminescence was reliably detected within 5 min, offering a significant time advantage over our previously reported method with light-sensitive films. Thus, this method is suitable to quantify the resistance level of a large number of Arabidopsis thaliana accessions and mutant lines to different bacterial strains in a non-invasive manner for phenotypic screenings.

Navigating Host Immunity and Concurrent Ozone Stress: Strain-Resolved Metagenomics Reveals Maintenance of Intraspecific Diversity and Genetic Variation in Xanthomonas on Pepper

The evolving threat of new pathogen variants in the face of global environmental changes poses a risk to a sustainable crop production. Predicting and responding to how climate change affects plant-pathosystems is challenging, as environment affects host-pathogen interactions from molecular to the community level, and with eco-evolutionary feedbacks at play. To address this knowledge gap, we studied short-term within-host eco-evolutionary changes in the pathogen, Xanthomonas perforans, on resistant and susceptible pepper in the open-top chambers (OTCs) under elevated Ozone (O3) conditions in a single growing season. We observed increased disease severity with greater variance on the resistant cultivar under elevated O3, yet no apparent change on the susceptible cultivar. Despite the dominance of a single pathogen genotype on the susceptible cultivar, the resistant cultivar supported a heterogeneous pathogen population. Altered O3 levels led to a strain turnover, with a relatively greater gene flux on the resistant cultivar. Both standing genetic variation and de novo parallel mutations contributed toward evolutionary modifications during adaptation onto the resistant cultivar. The presence of elevated O3, however, led to a relatively higher genetic polymorphism, with random and transient mutations. Population heterogeneity along with genetic variation, and the promotion of interdependency are mechanisms by which pathogen responds to stressors. While parallel mutations may provide clues to predicting long-term pathogen evolution and adaptive potential. And, a high proportion of transient mutations suggest less predictable pathogen evolution under climatic alterations. This knowledge is relevant as we study the risk of pathogen emergence and the mechanisms and constraints underlying long-term pathogen adaptation under climatic shifts.

The receptor MIK2 interacts with the kinase RKS1 to control quantitative disease resistance to Xanthomonas campestris

Molecular mechanisms underlying qualitative resistance have been intensively studied. In contrast, although quantitative disease resistance (QDR) is a common, durable, and broad-spectrum form of immune responses in plants, only a few related functional analyses have been reported. The atypical kinase Resistance related kinase 1 (RKS1) is a major regulator of QDR to the bacterial pathogen Xanthomonas campestris (Xcc) and is positioned in a robust protein-protein decentralized network in Arabidopsis (Arabidopsis thaliana). Among the putative interactors of RKS1 found by yeast two-hybrid screening, we identified the receptor-like kinase MDIS1-interacting receptor-like kinase 2 (MIK2). Here, using multiple complementary strategies including protein-protein interaction tests, mutant analysis, and network reconstruction, we report that MIK2 is a component of RKS1-mediated QDR to Xcc. First, by co-localization experiments, co-immunoprecipitation (Co-IP), and bimolecular fluorescence complementation, we validated the physical interaction between RKS1 and MIK2 at the plasma membrane. Using mik2 mutants, we showed that MIK2 is required for QDR and contributes to resistance to the same level as RKS1. Interestingly, a catalytic mutant of MIK2 interacted with RKS1 but was unable to fully complement the mik2-1 mutant phenotype in response to Xcc. Finally, we investigated the potential role of the MIK2-RKS1 complex as a scaffolding component for the coordination of perception events by constructing a RKS1-MIK2 centered protein-protein interaction network. Eight mutants corresponding to seven RKs in this network showed a strong alteration in QDR to Xcc. Our findings provide insights into the molecular mechanisms underlying the perception events involved in QDR to Xcc.

The emergence of nonlinear evolutionary trade-offs and the maintenance of genetic polymorphisms

Evolutionary models of quantitative traits often assume trade-offs between beneficial and detrimental traits, requiring modellers to specify a function linking trait values. The choice of trade-off function can be consequential; functions that assume diminishing returns (accelerating costs) typically lead to single equilibrium genotypes, while decelerating costs often lead to genetic polymorphisms. Despite their importance, our current theory has little to say on which trade-off functions are the most biologically plausible. To address this gap, we explored how the genetic determination of quantitative traits can lead to different trade-off functions, using resistance to infectious diseases as an example trait. We developed a model where alleles at separate loci pleiotropically increase resistance while decreasing fecundity. We then used this model to generate genotype landscapes and investigate how epistasis effects the trade-off function. Regardless of the strength of epistasis, our model consistently led to accelerating costs. We then incorporated our genotype model into an eco-evolutionary model of disease resistance. Unlike other models with accelerating costs, our approach often led to genetic polymorphisms. Our results suggest that accelerating costs are a strong null model for evolutionary trade-offs and that the eco-evolutionary conditions required for polymorphism may be more nuanced than previously thought.

Screening Method to Identify Watermelon Cultivars Resistant to Acidovorax citrulli, the Cause of Bacterial Fruit Blotch

Acidovorax citrulli is a causative pathogen for bacterial fruit blotch (BFB) in Cucurbitaceae, including watermelon. The most effective method to control this plant disease is to cultivate resistant cultivars. Herein, this study aimed to establish an efficient screening method to determine the resistance of watermelon cultivars against A. citrulli. To this end, we explored the virulence of seven A. citrulli isolates belonging to clonal complex group I or II based on gltA gene analysis. Furthermore, we evaluated the BFB occurrence in the seedlings of two arbitrarily selected watermelon cultivars according to the growth stage of the watermelon seedlings, inoculum concentration, and incubation temperature after inoculation in a humidity chamber. Taken together, we established the following method to determine the resistance of watermelon cultivars against A. citrulli: watermelon seedlings at the fully expanded two-leaf stage can be spray-inoculated with an A. citrulli bacterial suspension at a concentration of 1.0 × 106 cfu/ml; after incubation for 48 h at 28℃ in a humidity chamber, the plants were cultivated in a growth chamber at 25℃ with 80% relative humidity under a 12-h light/dark cycle; and BFB occurrence on the plants can be investigated at 7 dpi by visual estimation of the diseased leaf area (%). Based on these experimental methods, we investigated the resistance degree of 43 commercial watermelon cultivars against A. citrulli KACC 17005. Therefore, our results can provide important information for the development of resistant cultivars against A. citrulli.

The levels of pattern-triggered immunity in the root and stembase of tomato cultivars positively correlate with the resistance to Ralstonia solanacearum

BACKGROUND

Bacterial wilt (BW), caused by Ralstonia solanacearum (Rs), is one of the most destructive diseases impacting a wide range of crops globally. The infection process is complex involving intricate interactions between the plant and Rs. Managing BW is challenging, and crop breeding remains the most effective strategy for disease control. Resistance to BW in crops is primarily associated with quantitative trait loci (QTLs), which are believed to correlate with the simultaneous activation of multiple defense mechanisms against pathogens. This study aimed to clarify the nature of BW resistance and determine whether pattern-triggered immunity (PTI) plays a role in this resistance.

RESULTS

PTI can be triggered in tomato roots and stembases by an Rs hrpG- mutant and by the cell wall extract (PiCWE) from the root-infected beneficial fungus Piriformospora indica (Pi). Among tomato plants with varying resistance levels to Rs, BW-resistant (BWR) and moderate-resistant (BWMR) cultivars exhibited higher levels of root and stembase PTI in response to Rs hrpG- inoculation and PiCWE treatment than in BW-susceptible (BWS) cultivars. Additionally, BWR and BWMR cultivars showed enhanced leaf PTI after inoculation with a Pseudomonas syringae pv. tomato (Pst) hrcC- mutant. The BWR cultivar Hawaii 7996 (H7996) also demonstrated high tolerance to several leaf pathogens.

CONCLUSIONS

Efficient systems for the analyses of PTI responses in tomato roots, stembases and leaves in response to patterns derived from root-infected pathogenic and beneficial microorganisms have been established. The levels of PTI in roots, stembases, and leaves are positively correlated with BW resistance in tomato plants. The BWR cultivar H7996 also shows tolerance to various leaf pathogens. This study reveals a significant correlation between tomato PTI and resistance to Rs, provides valuable insights into the nature of BW resistance, and offers critical information for tomato breeding.

Identification of RppSLN from an Elite Landrace: A Major Locus Conferring Resistance to Southern Corn Rust in Maize (Zea mays L.)

Southern corn rust (SCR) is one of the most destructive foliar diseases in maize (Zea mays L.), resulting in significant yield losses. Therefore, the continuous identification of disease-resistant germplasm and the deployment of resistant hybrids is essential for durably controlling SCR. The objective of this research was to identify and characterize resistance loci against SCR in maize to expand disease management strategies. Here, we identified a maize landrace with high resistance to SCR 'Silunuo' (SLN) approaching complete immunity. We backcrossed it with a susceptible inbred line, N531, to generate a stable SCR-resistant introgression line N531_R. By crossing it with F35 (a susceptible inbred line), we created a large F2 segregating population and mapped a major SCR-resistant locus on chromosome 10, known as RppSLN. Based on the genome assembly and annotation, we found that RppSLN harbors two NBS-LRR (nucleotide binding site-leucine-rich repeat) genes, namely Zmays10G000430 and Zmays10G000440. These NBS-LRR genes were significantly induced during artificial inoculation with Puccinia polysora, suggesting that they might be candidate genes collectively contributing to the resistance level at this locus. In conclusion, this study identified a major SCR resistance locus directly isolated from a landrace, providing valuable support and information for expanding new disease-resistant germplasms and promoting the utilization of landraces.

Comparing Apples and Oranges: Advances in Disease Resistance Breeding of Woody Perennial Fruit Crops

Apple and citrus are perennial tree fruit crops that are vital for nutritional security and agricultural economy and to achieve the Sustainable Development Goals of the United Nations. Apple scab and fire blight, along with Huanglongbing, canker, and tristeza virus, stand out as their most notorious diseases and annually destabilize fruit supply. An environmentally sound approach to managing these diseases is improving tree resistance through breeding and biotechnology. Perennial fruit tree germplasm collections are distributed globally and offer untapped potential as sources of resistance. However, long juvenility, specific pollination and flowering habits, and extensive outcrossing hinder apple and citrus breeding. Advances in breeding approaches include trans- and cis-genesis, genome editing, and rapid-cycle breeding, which, in addition to conventional crossbreeding, can all facilitate accelerated integration of resistance into elite germplasm. In addition, the global pool of available sources of resistance can be characterized by the existing genetic mapping and gene expression studies for accurate discovery of associated loci, genes, and markers to efficiently include these sources in breeding efforts. We discuss and propose a multitude of approaches to overcome the challenges of breeding for resistance in woody perennials and outline a technical path to reduce the time required for the ultimate deployment of disease-resistant cultivars.

The complex relationship between disease resistance and yield in crops

In plants, growth and defence are controlled by many molecular pathways that are antagonistic to one another. This results in a 'growth-defence trade-off', where plants temporarily reduce growth in response to pests or diseases. Due to this antagonism, genetic variants that improve resistance often reduce growth and vice versa. Therefore, in natural populations, the most disease resistant individuals are often the slowest growing. In crops, slow growth may translate into a yield penalty, but resistance is essential for protecting yield in the presence of disease. Therefore, plant breeders must balance these traits to ensure optimal yield potential and yield stability. In crops, both qualitative and quantitative disease resistance are often linked with genetic variants that cause yield penalties, but this is not always the case. Furthermore, both crop yield and disease resistance are complex traits influenced by many aspects of the plant's physiology, morphology and environment, and the relationship between the molecular growth-defence trade-off and disease resistance-yield antagonism is not well-understood. In this article, we highlight research from the last 2 years on the molecular mechanistic basis of the antagonism between defence and growth. We then discuss the interaction between disease resistance and crop yield from a breeding perspective, outlining the complexity and nuances of this relationship and where research can aid practical methods for simultaneous improvement of yield potential and disease resistance.

High-Resolution Disease Phenotyping Reveals Distinct Resistance Mechanisms of Tomato Crop Wild Relatives against Sclerotinia sclerotiorum

Besides the well-understood qualitative disease resistance, plants possess a more complex quantitative form of resistance: quantitative disease resistance (QDR). QDR is commonly defined as a partial but more durable form of resistance and, therefore, might display a valuable target for resistance breeding. The characterization of QDR phenotypes, especially of wild crop relatives, displays a bottleneck in deciphering QDR's genomic and regulatory background. Moreover, the relationship between QDR parameters, such as infection frequency, lag-phase duration, and lesion growth rate, remains elusive. High hurdles for applying modern phenotyping technology, such as the low availability of phenotyping facilities or complex data analysis, further dampen progress in understanding QDR. Here, we applied a low-cost (<1.000 €) phenotyping system to measure lesion growth dynamics of wild tomato species (e.g., Solanum pennellii or Solanum pimpinellifolium). We provide insight into QDR diversity of wild populations and derive specific QDR mechanisms and their cross-talk. We show how temporally continuous observations are required to dissect end-point severity into functional resistance mechanisms. The results of our study show how QDR can be maintained by facilitating different defense mechanisms during host-parasite interaction and that the capacity of the QDR toolbox highly depends on the host's genetic context. We anticipate that the present findings display a valuable resource for more targeted functional characterization of the processes involved in QDR. Moreover, we show how modest phenotyping technology can be leveraged to help answer highly relevant biological questions.

Comparative Transcriptomics of Soybean Genotypes with Partial Resistance Toward Phytophthora sojae, Conrad, and M92-220 to Moderately Susceptible Fast Neutron Mutant Soybeans and Sloan

The breeding of disease-resistant soybeans cultivars to manage Phytophthora root and stem rot caused by the pathogen Phytophthora sojae involves combining quantitative disease resistance (QDR) and Rps gene-mediated resistance. To identify and confirm potential mechanisms of QDR toward P. sojae, we conducted a time course study comparing changes in gene expression among Conrad and M92-220 with high QDR to susceptible genotypes, Sloan, and three mutants derived from fast neutron irradiation of M92-220. Differentially expressed genes from Conrad and M92-220 indicated several shared defense-related pathways at the transcriptomic level but also defense pathways unique to each cultivar, such as stilbenoid, diarylheptanoid, and gingerol biosynthesis and monobactam biosynthesis. Gene Ontology pathway analysis showed that the susceptible fast neutron mutants lacked enrichment of three terpenoid-related pathways and two cell wall-related pathways at either one or both time points, in contrast to M92-220. The susceptible mutants also lacked enrichment of potentially important Kyoto Encyclopedia of Genes and Genomes pathways at either one or both time points, including sesquiterpenoid and triterpenoid biosynthesis; thiamine metabolism; arachidonic acid; stilbenoid, diarylheptanoid, and gingerol biosynthesis; and monobactam biosynthesis. Additionally, 31 genes that were differentially expressed in M92-220 following P. sojae infection were not expressed in the mutants. These 31 genes have annotations related to unknown proteins; valine, leucine, and isoleucine biosynthesis; and protein and lipid metabolic processes. The results of this study confirm previously proposed mechanisms of QDR, provide evidence for potential novel QDR pathways in M92-220, and further our understanding of the complex network associated with QDR mechanisms in soybean toward P. sojae.

Prevalence, pandemic, preventions and policies to overcome antimicrobial resistance

Antimicrobial resistance (AMR) is a growing concern in Asia, and it is essential to understand the prevalence, pandemic, prevention, and policies to overcome it. According to the World Health Organization (WHO), AMR is one of the main causes of death; in 2019, it was linked to 4.95 million fatalities and caused about 1.27 million deaths. A core package of actions has been provided by WHO to help countries prioritize their needs when creating, carrying out, and overseeing national action plans on antimicrobial resistance. Using a people-cantered approach to AMR, the interventions address the needs and obstacles that individuals and patients encounter when trying to obtain healthcare. The people-cantered core package of AMR treatments seeks to improve public and policymakers; awareness and comprehension of AMR by changing the narrative of AMR to emphasize the needs of people and systemic impairments. Additionally, it backs a more comprehensive and programmatic national response to AMR, which emphasizes the value of fair and inexpensive access to high-quality healthcare services for the avoidance, identification, and management of drug-resistant diseases. The report signals increasing resistance to antibiotics in bacterial infections in humans and the need for better data. In conclusion, the prevalence of AMR in Asia is a significant public health concern, and it is crucial to implement policies and interventions to overcome it.

Breeding and management of major resistance genes to stem canker/blackleg in Brassica crops

Blackleg (also known as Phoma or stem canker) is a major, worldwide disease of Brassica crop species, notably B. napus (rapeseed, canola), caused by the ascomycete fungus Leptosphaeria maculans. The outbreak and severity of this disease depend on environmental conditions and management practices, as well as a complex interaction between the pathogen and its hosts. Genetic resistance is a major method to control the disease (and the only control method in some parts of the world, such as continental Europe), but efficient use of genetic resistance is faced with many difficulties: (i) the scarcity of germplasm/genetic resources available, (ii) the different history of use of resistance genes in different parts of the world and the different populations of the fungus the resistance genes are exposed to, (iii) the complexity of the interactions between the plant and the pathogen that expand beyond typical gene-for-gene interactions, (iv) the incredible evolutionary potential of the pathogen and the importance of knowing the molecular processes set up by the fungus to "breakdown' resistances, so that we may design high-throughput diagnostic tools for population surveys, and (v) the different strategies and options to build up the best resistances and to manage them so that they are durable. In this paper, we aim to provide a comprehensive overview of these different points, stressing the differences between the different continents and the current prospects to generate new and durable resistances to blackleg disease.

Plant triterpenoid saponins function as susceptibility factors to promote the pathogenicity of Botrytis cinerea

The gray mold fungus Botrytis cinerea is a necrotrophic pathogen that causes diseases in hundreds of plant species, including high-value crops. Its polyxenous nature and pathogenic success are due to its ability to perceive host signals in its favor. In this study, we found that laticifer cells of Euphorbia lathyris are a source of susceptibility factors required by B. cinerea to cause disease. Consequently, poor-in-latex (pil) mutants, which lack laticifer cells, show full resistance to this pathogen, whereas lot-of-latex mutants, which produce more laticifer cells, are hypersusceptible. These S factors are triterpenoid saponins, which are widely distributed natural products of vast structural diversity. The downregulation of laticifer-specific oxydosqualene cyclase genes, which encode the first committed step enzymes for triterpene and, therefore, saponin biosynthesis, conferred disease resistance to B. cinerea. Likewise, the Medicago truncatula lha-1 mutant, compromised in triterpenoid saponin biosynthesis, showed enhanced resistance. Interestingly, the application of different purified triterpenoid saponins pharmacologically complemented the disease-resistant phenotype of pil and hla-1 mutants and enhanced disease susceptibility in different plant species. We found that triterpenoid saponins function as plant cues that signal transcriptional reprogramming in B. cinerea, leading to a change in its growth habit and infection strategy, culminating in the abundant formation of infection cushions, the multicellular appressoria apparatus dedicated to plant penetration and biomass destruction in B. cinerea. Taken together, these results provide an explanation for how plant triterpenoid saponins function as disease susceptibility factors to promote B. cinerea pathogenicity.

Integrated Assays of Genome-Wide Association Study, Multi-Omics Co-Localization, and Machine Learning Associated Calcium Signaling Genes with Oilseed Rape Resistance to Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Ss) is one of the most devastating fungal pathogens, causing huge yield loss in multiple economically important crops including oilseed rape. Plant resistance to Ss pertains to quantitative disease resistance (QDR) controlled by multiple minor genes. Genome-wide identification of genes involved in QDR to Ss is yet to be conducted. In this study, we integrated several assays including genome-wide association study (GWAS), multi-omics co-localization, and machine learning prediction to identify, on a genome-wide scale, genes involved in the oilseed rape QDR to Ss. Employing GWAS and multi-omics co-localization, we identified seven resistance-associated loci (RALs) associated with oilseed rape resistance to Ss. Furthermore, we developed a machine learning algorithm and named it Integrative Multi-Omics Analysis and Machine Learning for Target Gene Prediction (iMAP), which integrates multi-omics data to rapidly predict disease resistance-related genes within a broad chromosomal region. Through iMAP based on the identified RALs, we revealed multiple calcium signaling genes related to the QDR to Ss. Population-level analysis of selective sweeps and haplotypes of variants confirmed the positive selection of the predicted calcium signaling genes during evolution. Overall, this study has developed an algorithm that integrates multi-omics data and machine learning methods, providing a powerful tool for predicting target genes associated with specific traits. Furthermore, it makes a basis for further understanding the role and mechanisms of calcium signaling genes in the QDR to Ss.

The Emergence of Non-Linear Evolutionary Trade-offs and the Maintenance of Genetic Polymorphisms

Evolutionary models of quantitative traits often assume trade-offs between beneficial and detrimental traits, requiring modelers to specify a function linking costs to benefits. The choice of trade-off function is often consequential; functions that assume diminishing returns (accelerating costs) typically lead to single equilibrium genotypes, while decelerating costs often lead to evolutionary branching. Despite their importance, we still lack a strong theoretical foundation to base the choice of trade-off function. To address this gap, we explore how trade-off functions can emerge from the genetic architecture of a quantitative trait. We developed a multi-locus model of disease resistance, assuming each locus had random antagonistic pleiotropic effects on resistance and fecundity. We used this model to generate genotype landscapes and explored how additive versus epistatic genetic architectures influenced the shape of the trade-off function. Regardless of epistasis, our model consistently led to accelerating costs. We then used our genotype landscapes to build an evolutionary model of disease resistance. Unlike other models with accelerating costs, our approach often led to genetic polymorphisms at equilibrium. Our results suggest that accelerating costs are a strong null model for evolutionary trade-offs and that the eco-evolutionary conditions required for polymorphism may be more nuanced than previously believed.

Molecular complexity of quantitative immunity in plants: from QTL mapping to functional and systems biology

In nature, plants defend themselves against pathogen attack by activating an arsenal of defense mechanisms. During the last decades, work mainly focused on the understanding of qualitative disease resistance mediated by a few genes conferring an almost complete resistance, while quantitative disease resistance (QDR) remains poorly understood despite the fact that it represents the predominant and more durable form of resistance in natural populations and crops. Here, we review our past and present work on the dissection of the complex mechanisms underlying QDR in Arabidopsis thaliana. The strategies, main steps and challenges of our studies related to one atypical QDR gene, RKS1 (Resistance related KinaSe 1), are presented. First, from genetic analyses by QTL (Quantitative Trait Locus) mapping and GWAs (Genome Wide Association studies), the identification, cloning and functional analysis of this gene have been used as a starting point for the exploration of the multiple and coordinated pathways acting together to mount the QDR response dependent on RKS1. Identification of RKS1 protein interactors and complexes was a first step, systems biology and reconstruction of protein networks were then used to decipher the molecular roadmap to the immune responses controlled by RKS1. Finally, exploration of the potential impact of key components of the RKS1-dependent gene network on leaf microbiota offers interesting and challenging perspectives to decipher how the plant immune systems interact with the microbial communities' systems.

Three Decades of Rust Surveys in the United States Reveal Drastic Virulence Changes in Oat Crown Rust

To better understand how the pathogenicity of the oat crown rust pathogen Puccinia coronata f. sp. avenae (Pca) has changed in the United States, 30 years of United States Department of Agriculture (USDA) survey isolates (n = 5,456) tested on 30 to 40 differential lines were analyzed for overall and Pc-resistance-gene-specific virulence trends and correlations. Pca is incredibly pathologically diverse, with 88% of races represented by a single isolate. There are a slightly higher proportion of unique races from the Northern region of the United States, and for one fourth of the years, Northern region isolates were significantly more virulent than Southern isolates, which supports the idea that sexual recombination in this region is mediated by the alternate host as a major factor in creating new races. However, there is also support for regular isolate movement between North and South regions as isolates in the United States are steadily accumulating virulences at a rate of 0.35 virulences per year. Virulence significantly increased for 23 and decreased for four of the 40 differential lines. In the past few years, virulence has reached 90% or greater for 16 differential lines. There were also strong correlations in virulence for certain Pc genes that are likely identical, allelic, or target the same or closely linked pathogen effectors (e.g., Pc39, Pc55, and Pc71), and the results were largely in concordance with recent genome-wide association study (GWAS) effector studies using USDA isolate subsets. Understanding changes in Pca pathogenicity is essential for the responsible deployment and management of Pc resistance genes for sustainable and profitable oat production.

Identification of Faba bean genetic loci associated with quantitative resistance to the fungus Botrytis fabae, causal agent of chocolate spot

Introduction

Chocolate spot, caused by the ascomycete fungus Botrytis fabae, is a devastating foliar disease and a major constraint on the quality and yield of faba beans (Vicia faba). The use of fungicides is the primary strategy for controlling the disease. However, high levels of partial genetic resistance have been identified and can be exploited to mitigate the disease.

Methods

The partially resistant V. faba cultivar Maris Bead and susceptible Egyptian accession ig70726 were crossed, and a genetic mapping population of 184 individuals was genotyped in the F2 generation and screened for resistance to B. fabae infection in the F3, F5, and F6 generations in a series of field experiments. A high-density linkage map of V. faba containing 3897 DArT markers spanning 1713.7 cM was constructed.

Results

Multiple candidate quantitative trait loci (QTLs) in 11 separate regions of the V. faba genome were identified; some on chromosomes 2, 3, and 6 overlapped with loci previously linked to resistance to Ascochyta leaf and pod blight caused by the necrotrophic fungus Ascochyta fabae. A transcriptomics experiment was conducted at 18 h post-inoculation in seedlings of both parents of the mapping population, identifying several differentially expressed transcripts potentially involved in early stage defence against B. fabae, including cell-wall associated protein kinases, NLR genes, and genes involved in metabolism and response to reactive oxygen species.

Discussion

This study identified several novel candidate QTLs in the V. faba genome that contribute to partial resistance to chocolate spot, but differences between growing seasons highlighted the importance of multi-year phenotyping experiments when searching for candidate QTLs for partial resistance.

Comparative transcriptome analysis reveals nicotine metabolism is a critical component for enhancing stress response intensity of innate immunity system in tobacco

The pyridine alkaloid nicotine acts as one of best-studied plant resistant traits in tobacco. Previous research has shown that NtERF199 and NtERF189, acting as master regulators within the NIC1 and NIC2 locus, quantitatively contribute to nicotine accumulation levels in N. tabacum. Genome editing-created Nic1(Nterf199) and Nic2 (Nterf189) double mutant provides an ideal platform for precisely dissecting the defensive role of nicotine and the connection between the nicotine biosynthetic pathway with other putative metabolic networks. Taking this advantage, we performed a comparative transcriptomic analysis to reevaluate the potential physiological and metabolic changes in response to nicotine synthesis defect by comparing the nic1nic2 and NIC1NIC2 plants. Our findings revealed that nicotine reduction could systematically diminishes the expression intensities of genes associated with stimulus perception, signal transduction and regulation, as well as secondary metabolic flux. Consequently, this global expression reduction might compromise tobacco adaptions to environmental fitness, herbivore resistances, and plant growth and development. The up-regulation of a novel set of stress-responsive and metabolic pathway genes might signify a newly established metabolic reprogramming to tradeoff the detrimental effect of nicotine loss. These results offer additional compelling evidence regarding nicotine's critical defensive role in nature and highlights the tight link between nicotine biosynthesis and gene expression levels of quantitative resistance-related genes for better environmental adaptation.

Pathogen lifestyle determines host genetic signature of quantitative disease resistance loci in oilseed rape (Brassica napus)

KEY MESSAGE

Using associative transcriptomics, our study identifies genes conferring resistance to four diverse fungal pathogens in crops, emphasizing key genetic determinants of multi-pathogen resistance. Crops are affected by several pathogens, but these are rarely studied in parallel to identify common and unique genetic factors controlling diseases. Broad-spectrum quantitative disease resistance (QDR) is desirable for crop breeding as it confers resistance to several pathogen species. Here, we use associative transcriptomics (AT) to identify candidate gene loci associated with Brassica napus constitutive QDR to four contrasting fungal pathogens: Alternaria brassicicola, Botrytis cinerea, Pyrenopeziza brassicae, and Verticillium longisporum. We did not identify any shared loci associated with broad-spectrum QDR to fungal pathogens with contrasting lifestyles. Instead, we observed QDR dependent on the lifestyle of the pathogen-hemibiotrophic and necrotrophic pathogens had distinct QDR responses and associated loci, including some loci associated with early immunity. Furthermore, we identify a genomic deletion associated with resistance to V. longisporum and potentially broad-spectrum QDR. This is the first time AT has been used for several pathosystems simultaneously to identify host genetic loci involved in broad-spectrum QDR. We highlight constitutive expressed candidate loci for broad-spectrum QDR with no antagonistic effects on susceptibility to the other pathogens studies as candidates for crop breeding. In conclusion, this study represents an advancement in our understanding of broad-spectrum QDR in B. napus and is a significant resource for the scientific community.

The chromosome-scale genome and the genetic resistance machinery against insect herbivores of the Mexican toloache, Datura stramonium

Plant resistance refers to the heritable ability of plants to reduce damage caused by natural enemies, such as herbivores and pathogens, either through constitutive or induced traits like chemical compounds or trichomes. However, the genetic architecture-the number and genome location of genes that affect plant defense and the magnitude of their effects-of plant resistance to arthropod herbivores in natural populations remains poorly understood. In this study, we aimed to unveil the genetic architecture of plant resistance to insect herbivores in the annual herb Datura stramonium (Solanaceae) through quantitative trait loci mapping. We achieved this by assembling the species' genome and constructing a linkage map using an F2 progeny transplanted into natural habitats. Furthermore, we conducted differential gene expression analysis between undamaged and damaged plants caused by the primary folivore, Lema daturaphila larvae. Our genome assembly resulted in 6,109 scaffolds distributed across 12 haploid chromosomes. A single quantitative trait loci region on chromosome 3 was associated with plant resistance, spanning 0 to 5.17 cM. The explained variance by the quantitative trait loci was 8.44%. Our findings imply that the resistance mechanisms of D. stramonium are shaped by the complex interplay of multiple genes with minor effects. Protein-protein interaction networks involving genes within the quantitative trait loci region and overexpressed genes uncovered the key role of receptor-like cytoplasmic kinases in signaling and regulating tropane alkaloids and terpenoids, which serve as powerful chemical defenses against D. stramonium herbivores. The data generated in our study constitute important resources for delving into the evolution and ecology of secondary compounds mediating plant-insect interactions.

Traits linked to natural variation of sulfur content in Arabidopsis thaliana

Sulfur (S) is an essential mineral nutrient for plant growth and development; it is important for primary and specialized plant metabolites that are crucial for biotic and abiotic interactions. Foliar S content varies up to 6-fold under a controlled environment, suggesting an adaptive value under certain natural environmental conditions. However, a major quantitative regulator of S content in Arabidopsis thaliana has not been identified yet, pointing to the existence of either additional genetic factors controlling sulfate/S content or of many minor quantitative regulators. Here, we use overlapping information of two separate ionomics studies to select groups of accessions with low, mid, and high foliar S content. We quantify series of metabolites, including anions (sulfate, phosphate, and nitrate), thiols (cysteine and glutathione), and seven glucosinolates, gene expression of 20 genes, sulfate uptake, and three biotic traits. Our results suggest that S content is tightly connected with sulfate uptake, the concentration of sulfate and phosphate anions, and glucosinolate and glutathione synthesis. Additionally, our results indicate that the growth of pathogenic bacteria is enhanced in the A. thaliana accessions containing higher S in their leaves, suggesting a complex regulation between S homeostasis, primary and secondary metabolism, and biotic pressures.

Phenotyping, genetics, and "-omics" approaches to unravel and introgress enhanced resistance against apple scab (Venturia inaequalis) in apple cultivars (Malus × domestica)

Apple scab disease, caused by the fungus Venturia inaequalis, endangers commercial apple production globally. It is predominantly managed by frequent fungicide sprays that can harm the environment and promote the development of fungicide-resistant strains. Cultivation of scab-resistant cultivars harboring diverse qualitative Rvi resistance loci and quantitative trait loci associated with scab resistance could reduce the chemical footprint. A comprehensive understanding of the host-pathogen interaction is, however, needed to efficiently breed cultivars with enhanced resistance against a variety of pathogenic strains. Breeding efforts should not only encompass pyramiding of Rvi loci and their corresponding resistance alleles that directly or indirectly recognize pathogen effectors, but should also integrate genes that contribute to effective downstream defense mechanisms. This review provides an overview of the phenotypic and genetic aspects of apple scab resistance, and currently known corresponding defense mechanisms. Implementation of recent "-omics" approaches has provided insights into the complex network of physiological, molecular, and signaling processes that occur before and upon scab infection, thereby revealing the importance of both constitutive and induced defense mechanisms. Based on the current knowledge, we outline advances toward more efficient introgression of enhanced scab resistance into novel apple cultivars by conventional breeding or genetic modification techniques. However, additional studies integrating different "-omics" approaches combined with functional studies will be necessary to unravel effective defense mechanisms as well as key regulatory genes underpinning scab resistance in apple. This crucial information will set the stage for successful knowledge-based breeding for enhanced scab resistance.

Transcriptional landscape of pathogen-responsive lncRNAs in tomato unveils the role of hydrolase encoding genes in response to Botrytis cinerea invasion

LncRNAs have gained increasing attention owing to their important regulatory roles on growth and stress responses of plants. However, the mechanisms underlying the functions of lncRNAs in fruit-pathogen interaction are still largely unknown. In this study, a total of 273 lncRNAs responding to Botrytis cinerea infection were identified in tomato fruit, among which a higher percentage of antisense lncRNAs were targeted to the genes enriched in hydrolase activity. To ascertain the roles of these lncRNAs, seven hydrolase-related transcripts were transiently knocked-down by virus-induced gene silencing. Silencing of lncRNACXE20 reduced the expression level of a carboxylesterase gene, further enhancing the resistance of tomato to B. cinerea. In contrast, silencing of lncRNACHI, lncRNAMMP, lncRNASBT1.9 and lncRNAPME1.9 impaired the resistance to B. cinerea, respectively. Further RT-qPCR assay and enzymatic activity detection displayed that the attenuated resistance of lncRNAMMP and lncRNASBT1.9-silenced plants was associated with the inhibition on the expression of JA-related genes, while the decreased resistance of lncRNACHI-silenced plants resulted in reduced chitinase activity. Collectively, these results may provide references for deciphering the mechanisms underlying specific lncRNAs to interfere with B. cinerea infection by regulating the expression of defence-related genes or affecting hydrolase activity.

Quantitative disease resistance in wild Silene vulgaris to its endemic pathogen Microbotryum silenes-inflatae

The evolution of disease resistances is an expected feature of plant-pathogen systems, but whether the genetics of this trait most often produces qualitative or quantitative phenotypic variation is a significant gap in our understanding of natural populations. These two forms of resistance variation are often associated with differences in number of underlying loci, the specificities of host-pathogen coevolution, as well as contrasting mechanisms of preventing or slowing the infection process. Anther-smut disease is a commonly studied model for disease of wild species, where infection has severe fitness impacts, and prior studies have suggested resistance variation in several host species. However, because the outcome of exposing the individual host to this pathogen is binary (healthy or diseased), resistance has been previously measured at the family level, as the proportion of siblings that become diseased. This leaves uncertain whether among-family variation reflects contrasting ratios of segregating discrete phenotypes or continuous trait variation among individuals. In the host Silene vulgaris, plants were replicated by vegetative propagation in order to quantify the infection rates of the individual genotype with the endemic anther-smut pathogen, Microbotryum silenes-inflatae. The variance among field-collected families for disease resistance was significant, while there was unimodal continuous variation in resistance among genotypes. Using crosses between genotypes within ranked resistance quartiles, the offspring infection rate was predicted by the parental resistance values. While the potential remains in this system for resistance genes having major effects, as there were suggestions of such qualitative resistance in a prior study, here the quantitative disease resistance to the endemic anther-smut pathogen is indicated for S. vulgaris. The variation in natural populations and strong heritability of the trait, combined with severe fitness consequences of anther-smut disease, suggests that resistance in these host populations is highly capable of responding to disease-induced selection.

Molecular mechanisms underpinning quantitative resistance to Phytophthora sojae in Glycine max using a systems genomics approach

Expression of quantitative disease resistance in many host-pathogen systems is controlled by genes at multiple loci, each contributing a small effect to the overall response. We used a systems genomics approach to study the molecular underpinnings of quantitative disease resistance in the soybean-Phytophthora sojae pathosystem, incorporating expression quantitative trait loci (eQTL) mapping and gene co-expression network analysis to identify the genes putatively regulating transcriptional changes in response to inoculation. These findings were compared to previously mapped phenotypic (phQTL) to identify the molecular mechanisms contributing to the expression of this resistance. A subset of 93 recombinant inbred lines (RILs) from a Conrad × Sloan population were inoculated with P. sojae isolate 1.S.1.1 using the tray-test method; RNA was extracted, sequenced, and the normalized read counts were genetically mapped from tissue collected at the inoculation site 24 h after inoculation from both mock and inoculated samples. In total, more than 100,000 eQTLs were mapped. There was a switch from predominantly cis-eQTLs in the mock treatment to an almost entirely nonoverlapping set of predominantly trans-eQTLs in the inoculated treatment, where greater than 100-fold more eQTLs were mapped relative to mock, indicating vast transcriptional reprogramming due to P. sojae infection occurred. The eQTLs were organized into 36 hotspots, with the four largest hotspots from the inoculated treatment corresponding to more than 70% of the eQTLs, each enriched for genes within plant-pathogen interaction pathways. Genetic regulation of trans-eQTLs in response to the pathogen was predicted to occur through transcription factors and signaling molecules involved in plant-pathogen interactions, plant hormone signal transduction, and MAPK pathways. Network analysis identified three co-expression modules that were correlated with susceptibility to P. sojae and associated with three eQTL hotspots. Among the eQTLs co-localized with phQTLs, two cis-eQTLs with putative functions in the regulation of root architecture or jasmonic acid, as well as the putative master regulators of an eQTL hotspot nearby a phQTL, represent candidates potentially underpinning the molecular control of these phQTLs for resistance.

Evolutionary Epidemiology Consequences of Trait-Dependent Control of Heterogeneous Parasites

AbstractDisease control can induce both demographic and evolutionary responses in host-parasite systems. Foreseeing the outcome of control therefore requires knowledge of the eco-evolutionary feedback between control and system. Previous work has assumed that control strategies have a homogeneous effect on the parasite population. However, this is not true when control targets those traits that confer to the parasite heterogeneous levels of resistance, which can additionally be related to other key parasite traits through evolutionary trade-offs. In this work, we develop a minimal model coupling epidemiological and evolutionary dynamics to explore possible trait-dependent effects of control strategies. In particular, we consider a parasite expressing continuous levels of a trait-determining resource exploitation and a control treatment that can be either positively or negatively correlated with that trait. We demonstrate the potential of trait-dependent control by considering that the decision maker may want to minimize both the damage caused by the disease and the use of treatment, due to possible environmental or economic costs. We identify efficient strategies showing that the optimal type of treatment depends on the amount applied. Our results pave the way for the study of control strategies based on evolutionary constraints, such as collateral sensitivity and resistance costs, which are receiving increasing attention for both public health and agricultural purposes.

Tomato deploys defence and growth simultaneously to resist bacterial wilt disease

Plant disease limits crop production, and host genetic resistance is a major means of control. Plant pathogenic Ralstonia causes bacterial wilt disease and is best controlled with resistant varieties. Tomato wilt resistance is multigenic, yet the mechanisms of resistance remain largely unknown. We combined metaRNAseq analysis and functional experiments to identify core Ralstonia-responsive genes and the corresponding biological mechanisms in wilt-resistant and wilt-susceptible tomatoes. While trade-offs between growth and defence are common in plants, wilt-resistant plants activated both defence responses and growth processes. Measurements of innate immunity and growth, including reactive oxygen species production and root system growth, respectively, validated that resistant plants executed defence-related processes at the same time they increased root growth. In contrast, in wilt-susceptible plants roots senesced and root surface area declined following Ralstonia inoculation. Wilt-resistant plants repressed genes predicted to negatively regulate water stress tolerance, while susceptible plants repressed genes predicted to promote water stress tolerance. Our results suggest that wilt-resistant plants can simultaneously promote growth and defence by investing in resources that act in both processes. Infected susceptible plants activate defences, but fail to grow and so succumb to Ralstonia, likely because they cannot tolerate the water stress induced by vascular wilt.

Expression Analysis Reveals Differentially Expressed Genes in BPH and WBPH Associated with Resistance in Rice RILs Derived from a Cross between RP2068 and TN1

BPH (brown planthopper) and WBPH (white backed planthopper) are significant rice pests that often co-occur as sympatric species and cause substantial yield loss. Despite their genetic similarities, different host-resistance genes confer resistance against these two hoppers. The defense mechanisms in rice against these pests are complex, and the molecular processes regulating their responses remain largely unknown. This study used specific recombinant inbred lines (RILs) derived from a cross between rice varieties RP2068-18-3-5 (BPH- and WBPH-resistant) and TN1 (BPH- and WBPH-susceptible) to investigate the mechanisms of interaction between these planthoppers and their rice hosts. WBPH and BPH were allowed to feed on specific RILs, and RNA-Seq was carried out on WBPH insects. Transcriptome profiling and qRT-PCR results revealed differential expression of genes involved in detoxification, digestion, transportation, cuticle formation, splicing, and RNA processing. A higher expression of sugar transporters was observed in both hoppers feeding on rice with resistance against either hopper. This is the first comparative analysis of gene expressions in these insects fed on genetically similar hosts but with differential resistance to BPH and WBPH. These results complement our earlier findings on the differential gene expression of the same RILs (BPH- or WBPH-infested) utilized in this study. Moreover, identifying insect genes and pathways responsible for countering host defense would augment our understanding of BPH and WBPH interaction with their rice hosts and enable us to develop lasting strategies to control these significant pests.

Unraveling the role of effector proteins in Bipolaris oryzae infecting North East Indian rice cultivars through time-course transcriptomics analysis

Bipolaris oryzae, causing brown spot disease in rice, is one of the neglected diseases reducing rice productivity. Limited knowledge is available on the genetics of host-pathogen interaction. Here, we used time-course transcriptome sequencing to elucidate the differential transcriptional responses of the pathogen genes in two contradictory infection-responsive rice hosts. Evaluation of transcriptome data showed similar regulation of fungal genes within susceptible (1733) and resistant (1846) hosts at an early stage however, in the later stage, the number was significantly higher in susceptible (2877) compared to resistant (1955) hosts. GO enrichment terms for upregulated genes showed a similar pattern in both the hosts at an early stage, but in the later stage terms related to degradation of carbohydrates, carbohydrate transport, and pathogenesis are enriched extensively within the susceptible host. Likewise, similar expression responses were observed with the secretory and effector proteins. Plant pathogenic homologs genes such as those involved in appressorium and conidia formation, host cell wall degradative enzymes, etc. were reported to be highly upregulated within the susceptible host. This study predicts the successful establishment of B. oryzae BO1 in both the host surfaces at an early stage, while disease progression only occurs in the susceptible host in later stage.

Specific resistance prevents the evolution of general resistance and facilitates disease emergence

Host-shifts, where pathogens jump from an ancestral host to a novel host, can be facilitated or impeded by standing variation in disease resistance, but only if resistance provides broad-spectrum general resistance against multiple pathogen species. Host resistance comes in many forms and includes both general resistance, as well as specific resistance, which may only be effective against a single pathogen species or even genotype. However, most evolutionary models consider only one of these forms of resistance, and we have less understanding of how these two forms of resistance evolve in tandem. Here, we develop a model that allows for the joint evolution of specific and general resistance and asks if the evolution of specific resistance drives a decrease in the evolution of general resistance. We also explore how these evolutionary outcomes affect the risk of foreign pathogen invasion and persistence. We show that in the presence of a single endemic pathogen, the two forms of resistance are strongly exclusionary. Critically, we find that specific resistance polymorphisms can prevent the evolution of general resistance, facilitating the invasion of foreign pathogens. We also show that specific resistance polymorphisms are a necessary condition for the successful establishment of foreign pathogens following invasion, as they prevent the exclusion of the foreign pathogen by the more transmissible endemic pathogen. Our results demonstrate the importance of considering the joint evolution of multiple forms of resistance when evaluating a population's susceptibility to foreign pathogens.

Killing softly: a roadmap of Botrytis cinerea pathogenicity

Botrytis cinerea, a widespread plant pathogen with a necrotrophic lifestyle, causes gray mold disease in many crops. Massive secretion of enzymes and toxins was long considered to be the main driver of infection, but recent studies have uncovered a rich toolbox for B. cinerea pathogenicity. The emerging picture is of a multilayered infection process governed by the exchange of factors that collectively contribute to disease development. No plant shows complete resistance against B. cinerea, but pattern-triggered plant immune responses have the potential to significantly reduce disease progression, opening new possibilities for producing B. cinerea-tolerant plants. We examine current B. cinerea infection models, highlight knowledge gaps, and suggest directions for future studies.

Quantitative disease resistance: Multifaceted players in plant defense

In contrast to large-effect qualitative disease resistance, quantitative disease resistance (QDR) exhibits partial and generally durable resistance and has been extensively utilized in crop breeding. The molecular mechanisms underlying QDR remain largely unknown but considerable progress has been made in this area in recent years. In this review, we summarize the genes that have been associated with plant QDR and their biological functions. Many QDR genes belong to the canonical resistance gene categories with predicted functions in pathogen perception, signal transduction, phytohormone homeostasis, metabolite transport and biosynthesis, and epigenetic regulation. However, other "atypical" QDR genes are predicted to be involved in processes that are not commonly associated with disease resistance, such as vesicle trafficking, molecular chaperones, and others. This diversity of function for QDR genes contrasts with qualitative resistance, which is often based on the actions of nucleotide-binding leucine-rich repeat (NLR) resistance proteins. An understanding of the diversity of QDR mechanisms and of which mechanisms are effective against which classes of pathogens will enable the more effective deployment of QDR to produce more durably resistant, resilient crops.

Comparative transcriptome profiling reveals the importance of GmSWEET15 in soybean susceptibility to Sclerotinia sclerotiorum

Soybean sclerotinia stem rot (SSR) is a disease caused by Sclerotinia sclerotiorum that causes incalculable losses in soybean yield each year. Considering the lack of effective resistance resources and the elusive resistance mechanisms, we are urged to develop resistance genes and explore their molecular mechanisms. Here, we found that loss of GmSWEET15 enhanced the resistance to S. sclerotiorum, and we explored the molecular mechanisms by which gmsweet15 mutant exhibit enhanced resistance to S. sclerotiorum by comparing transcriptome. At the early stage of inoculation, the wild type (WT) showed moderate defense response, whereas gmsweet15 mutant exhibited more extensive and intense transcription reprogramming. The gmsweet15 mutant enriched more biological processes, including the secretory pathway and tetrapyrrole metabolism, and it showed stronger changes in defense response, protein ubiquitination, MAPK signaling pathway-plant, plant-pathogen interaction, phenylpropanoid biosynthesis, and photosynthesis. The more intense and abundant transcriptional reprogramming of gmsweet15 mutant may explain how it effectively delayed colonization by S. sclerotiorum. In addition, we identified common and specific differentially expressed genes between WT and gmsweet15 mutant after inoculation with S. sclerotiorum, and gene sets and genes related to gmsweet15_24 h were identified through Gene Set Enrichment Analysis. Moreover, we constructed the protein-protein interaction network and gene co-expression networks and identified several groups of regulatory networks of gmsweet15 mutant in response to S. sclerotiorum, which will be helpful for the discovery of candidate functional genes. Taken together, our results elucidate molecular mechanisms of delayed colonization by S. sclerotiorum after loss of GmSWEET15 in soybean, and we propose novel resources for improving resistance to SSR.

Recent developments in plant-downy mildew interactions

Downy mildews are obligate oomycete pathogens that attack a wide range of plants and can cause significant economic impacts on commercial crops and ornamental plants. Traditionally, downy mildew disease control relied on an integrated strategies, that incorporate cultural practices, deployment of resistant cultivars, crop rotation, application of contact and systemic pesticides, and biopesticides. Recent advances in genomics provided data that significantly advanced understanding of downy mildew evolution, taxonomy and classification. In addition, downy mildew genomics also revealed that these obligate oomycetes have reduced numbers of virulence factor genes in comparison to hemibiotrophic and necrotrophic oomycetes. However, downy mildews do deploy significant arrays of virulence proteins, including so-called RXLR proteins that promote virulence or are recognized as avirulence factors. Pathogenomics are being applied to downy mildew population studies to determine the genetic diversity within the downy mildew populations and manage disease by selection of appropriate varieties and management strategies. Genome editing technologies have been used to manipulate host disease susceptibility genes in different plants including grapevine and sweet basil and thereby provide new soucres of resistance genes against downy mildews. Previously, it has proved difficult to transform and manipulate downy mildews because of their obligate lifestyle. However, recent exploitation of RNA interference machinery through Host-Induced Gene Silencing (HIGS) and Spray-Induced Gene Silencing (SIGS) indicate that functional genomics in downy mildews is now possible. Altogether, these breakthrough technologies and attendant fundamental understanding will advance our ability to mitigate downy mildew diseases.

Pathogen-derived mechanical cues potentiate the spatio-temporal implementation of plant defense

BACKGROUND

The ongoing adaptation of plants to their environment is the basis for their survival. In this adaptation, mechanoperception of gravity and local curvature plays a role of prime importance in finely regulating growth and ensuring a dynamic balance preventing buckling. However, the abiotic environment is not the exclusive cause of mechanical stimuli. Biotic interactions between plants and microorganisms also involve physical forces and potentially mechanoperception. Whether pathogens trigger mechanoperception in plants and the impact of mechanotransduction on the regulation of plant defense remains however elusive.

RESULTS

Here, we found that the perception of pathogen-derived mechanical cues by microtubules potentiates the spatio-temporal implementation of plant immunity to fungus. By combining biomechanics modeling and image analysis of the post-invasion stage, we reveal that fungal colonization releases plant cell wall-born tension locally, causing fluctuations of tensile stress in walls of healthy cells distant from the infection site. In healthy cells, the pathogen-derived mechanical cues guide the reorganization of mechanosensing cortical microtubules (CMT). The anisotropic patterning of CMTs is required for the regulation of immunity-related genes in distal cells. The CMT-mediated mechanotransduction of pathogen-derived cues increases Arabidopsis disease resistance by 40% when challenged with the fungus Sclerotinia sclerotiorum.

CONCLUSIONS

CMT anisotropic patterning triggered by pathogen-derived mechanical cues activates the implementation of early plant defense in cells distant from the infection site. We propose that the mechano-signaling triggered immunity (MTI) complements the molecular signals involved in pattern and effector-triggered immunity.

Characterization of defense responses against bacterial pathogens in duckweeds lacking EDS1

ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) mediates the induction of defense responses against pathogens in most angiosperms. However, it has recently been shown that a few species have lost EDS1. It is unknown how defense against disease unfolds and evolves in the absence of EDS1. We utilize duckweeds; a collection of aquatic species that lack EDS1, to investigate this question. We established duckweed-Pseudomonas pathosystems and used growth curves and microscopy to characterize pathogen-induced responses. Through comparative genomics and transcriptomics, we show that the copy number of infection-associated genes and the infection-induced transcriptional responses of duckweeds differ from other model species. Pathogen defense in duckweeds has evolved along different trajectories than in other plants, including genomic and transcriptional reprogramming. Specifically, the miAMP1 domain-containing proteins, which are absent in Arabidopsis, showed pathogen responsive upregulation in duckweeds. Despite such divergence between Arabidopsis and duckweed species, we found conservation of upregulation of certain genes and the role of hormones in response to disease. Our work highlights the importance of expanding the pool of model species to study defense responses that have evolved in the plant kingdom independent of EDS1.

Climate change impedes plant immunity mechanisms

Rapid climate change caused by human activity is threatening global crop production and food security worldwide. In particular, the emergence of new infectious plant pathogens and the geographical expansion of plant disease incidence result in serious yield losses of major crops annually. Since climate change has accelerated recently and is expected to worsen in the future, we have reached an inflection point where comprehensive preparations to cope with the upcoming crisis can no longer be delayed. Development of new plant breeding technologies including site-directed nucleases offers the opportunity to mitigate the effects of the changing climate. Therefore, understanding the effects of climate change on plant innate immunity and identification of elite genes conferring disease resistance are crucial for the engineering of new crop cultivars and plant improvement strategies. Here, we summarize and discuss the effects of major environmental factors such as temperature, humidity, and carbon dioxide concentration on plant immunity systems. This review provides a strategy for securing crop-based nutrition against severe pathogen attacks in the era of climate change.

A Global Transcriptome and Co-expression Analysis Reveals Robust Host Defense Pathway Reprogramming and Identifies Key Regulators of Early Phases of Cicer-Ascochyta Interactions

Ascochyta blight (AB) caused by the filamentous fungus Ascochyta rabiei is a major threat to global chickpea production. The mechanisms underlying chickpea response to A. rabiei remain elusive to date. Here, we investigated the comparative transcriptional dynamics of AB-resistant and -susceptible chickpea genotypes upon A. rabiei infection, to understand the early host defense response. Our findings revealed that AB-resistant plants underwent rapid and extensive transcriptional reprogramming compared with a susceptible host. At the early stage (24 h postinoculation [hpi]), mainly cell-wall remodeling and secondary metabolite pathways were highly activated, while differentially expressed genes related to signaling components, such as protein kinases, transcription factors, and hormonal pathways, show a remarkable upsurge at 72 hpi, especially in the resistant genotype. Notably, our data suggest an imperative role of jasmonic acid, ethylene, and abscisic acid signaling in providing immunity against A. rabiei. Furthermore, gene co-expression networks and modules corroborated the importance of cell-wall remodeling, signal transduction, and phytohormone pathways. Hub genes such as MYB14, PRE6, and MADS-SOC1 discovered in these modules might be the master regulators governing chickpea immunity. Overall, we not only provide novel insights for comprehensive understanding of immune signaling components mediating AB resistance and susceptibility at early Cicer-Ascochyta interactions but, also, offer a valuable resource for developing AB-resistant chickpea. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Genome-wide association study for resistance to Pseudomonas syringae pv. garcae in Coffea arabica

Bacteria halo blight (BHB), a coffee plant disease caused by Pseudomonas syringae pv. garcae, has been gaining importance in producing mountain regions and mild temperatures areas as well as in coffee nurseries. Most Coffea arabica cultivars are susceptible to this disease. In contrast, a great source of genetic diversity and resistance to BHB are found in C. arabica Ethiopian accessions. Aiming to identify quantitative trait nucleotides (QTNs) associated with resistance to BHB and the influence of these genomic regions during the domestication of C. arabica, we conducted an analysis of population structure and a Genome-Wide Association Study (GWAS). For this, we used genotyping by sequencing (GBS) and phenotyping for resistance to BHB of a panel with 120 C. arabica Ethiopian accessions from a historical FAO collection, 11 C. arabica cultivars, and the BA-10 genotype. Population structure analysis based on single-nucleotide polymorphisms (SNPs) markers showed that the 132 accessions are divided into 3 clusters: most wild Ethiopian accessions, domesticated Ethiopian accessions, and cultivars. GWAS, using the single-locus model MLM and the multi-locus models mrMLM, FASTmrMLM, FASTmrEMMA, and ISIS EM-BLASSO, identified 11 QTNs associated with resistance to BHB. Among these QTNs, the four with the highest values of association for resistance to BHB are linked to g000 (Chr_0_434_435) and g010741 genes, which are predicted to encode a serine/threonine-kinase protein and a nucleotide binding site leucine-rich repeat (NBS-LRR), respectively. These genes displayed a similar transcriptional downregulation profile in a C. arabica susceptible cultivar and in a C. arabica cultivar with quantitative resistance, when infected with P. syringae pv. garcae. However, peaks of upregulation were observed in a C. arabica cultivar with qualitative resistance, for both genes. Our results provide SNPs that have potential for application in Marker Assisted Selection (MAS) and expand our understanding about the complex genetic control of the resistance to BHB in C. arabica. In addition, the findings contribute to increasing understanding of the C. arabica domestication history.

How the epidemiology of disease-resistant and disease-tolerant varieties affects grower behaviour

Population-scale effects of resistant or tolerant crop varieties have received little consideration from epidemiologists. When growers deploy tolerant crop, population-scale disease pressures are often unaffected. This only benefits growers using tolerant varieties, selfishly decreasing yields for others. However, resistant crop can reduce disease pressure for all. We coupled an epidemiological model with game theory to understand how this affects uptake of control. Each time a grower plants a new crop, they must decide whether to use an improved (i.e. tolerant/resistant) or unimproved variety. This decision is based on strategic-adaptive expectations in our model, with growers comparing last season's profit with an estimate of what is expected from the alternative crop. Despite the positive feedback loop promoting use of a tolerant variety whenever it is available, a mixed unimproved- and tolerant-crop equilibrium can persist. Tolerant crop can also induce bistability between a scenario in which all growers use tolerant crop and the disease-free equilibrium, where no growers do. However, due to 'free-riding' by growers of unimproved crop, resistant crop nearly always exists in a mixed equilibrium. This work highlights how growers respond to contrasting incentives caused by tolerant and resistant varieties, and the distinct effects on yields and population-scale deployment.

Identification of genetic loci in lettuce mediating quantitative resistance to fungal pathogens

KEY MESSAGE

We demonstrate genetic variation for quantitative resistance against important fungal pathogens in lettuce and its wild relatives, map loci conferring resistance and predict key molecular mechanisms using transcriptome profiling. Lactuca sativa L. (lettuce) is an important leafy vegetable crop grown and consumed globally. Chemicals are routinely used to control major pathogens, including the causal agents of grey mould (Botrytis cinerea) and lettuce drop (Sclerotinia sclerotiorum). With increasing prevalence of pathogen resistance to fungicides and environmental concerns, there is an urgent need to identify sources of genetic resistance to B. cinerea and S. sclerotiorum in lettuce. We demonstrated genetic variation for quantitative resistance to B. cinerea and S. sclerotiorum in a set of 97 diverse lettuce and wild relative accessions, and between the parents of lettuce mapping populations. Transcriptome profiling across multiple lettuce accessions enabled us to identify genes with expression correlated with resistance, predicting the importance of post-transcriptional gene regulation in the lettuce defence response. We identified five genetic loci influencing quantitative resistance in a F6 mapping population derived from a Lactuca serriola (wild relative) × lettuce cross, which each explained 5-10% of the variation. Differential gene expression analysis between the parent lines, and integration of data on correlation of gene expression and resistance in the diversity set, highlighted potential causal genes underlying the quantitative trait loci.

Identification of positive and negative regulators of antiviral RNA interference in Arabidopsis thaliana

Virus-host coevolution often drives virus immune escape. However, it remains unknown whether natural variations of plant virus resistance are enriched in genes of RNA interference (RNAi) pathway known to confer essential antiviral defense in plants. Here, we report two genome-wide association study screens to interrogate natural variation among wild-collected Arabidopsis thaliana accessions in quantitative resistance to the endemic cucumber mosaic virus (CMV). We demonstrate that the highest-ranked gene significantly associated with resistance from both screens acts to regulate antiviral RNAi in ecotype Columbia-0. One gene, corresponding to Reduced Dormancy 5 (RDO5), enhances resistance by promoting amplification of the virus-derived small interfering RNAs (vsiRNAs). Interestingly, the second gene, designated Antiviral RNAi Regulator 1 (VIR1), dampens antiviral RNAi so its genetic inactivation by CRISPR/Cas9 editing enhances both vsiRNA production and CMV resistance. Our findings identify positive and negative regulators of the antiviral RNAi defense that may play important roles in virus-host coevolution.

RNA-Seq and Gene Regulatory Network Analyses Uncover Candidate Genes in the Early Defense to Two Hemibiotrophic Colletorichum spp. in Strawberry

Two hemibiotrophic pathogens, Colletotrichum acutatum (Ca) and C. gloeosporioides (Cg), cause anthracnose fruit rot and anthracnose crown rot in strawberry (Fragaria × ananassa Duchesne), respectively. Both Ca and Cg can initially infect through a brief biotrophic phase, which is associated with the production of intracellular primary hyphae that can infect host cells without causing cell death and establishing hemibiotrophic infection (HBI) or quiescent (latent infections) in leaf tissues. The Ca and Cg HBI in nurseries and subsequent distribution of asymptomatic infected transplants to fruit production fields is the major source of anthracnose epidemics in North Carolina. In the absence of complete resistance, strawberry varieties with good fruit quality showing rate-reducing resistance have frequently been used as a source of resistance to Ca and Cg. However, the molecular mechanisms underlying the rate-reducing resistance or susceptibility to Ca and Cg are still unknown. We performed comparative transcriptome analyses to examine how rate-reducing resistant genotype NCS 10-147 and susceptible genotype ‘Chandler’ respond to Ca and Cg and identify molecular events between 0 and 48 h after the pathogen-inoculated and mock-inoculated leaf tissues. Although plant response to both Ca and Cg at the same timepoint was not similar, more genes in the resistant interaction were upregulated at 24 hpi with Ca compared with those at 48 hpi. In contrast, a few genes were upregulated in the resistant interaction at 48 hpi with Cg. Resistance response to both Ca and Cg was associated with upregulation of MLP-like protein 44, LRR receptor-like serine/threonine-protein kinase, and auxin signaling pathway, whereas susceptibility was linked to modulation of the phenylpropanoid pathway. Gene regulatory network inference analysis revealed candidate transcription factors (TFs) such as GATA5 and MYB-10, and their downstream targets were upregulated in resistant interactions. Our results provide valuable insights into transcriptional changes during resistant and susceptible interactions, which can further facilitate assessing candidate genes necessary for resistance to two hemibiotrophic Colletotrichum spp. in strawberry.

Quantitative Resistance Loci to Southern Rust Mapped in a Temperate Maize Diversity Panel

Southern rust is a severe foliar disease of maize (Zea mays) resulting from infection with the obligate biotrophic fungus Puccinia polysora. This disease reduces photosynthetic productivity, which in turn reduces yields, with the greatest yield losses (up to 50%) associated with earlier onset infections. P. polysora urediniospores overwinter only in tropical and subtropical regions but cause outbreaks when environmental conditions favor initial infection. Increased temperatures and humidity during the growing season combined with an increased frequency of moderate winters are likely to increase the frequency of severe southern rust outbreaks in the U.S. Corn Belt. In summer 2020, a severe outbreak of southern rust was observed in eastern Nebraska, United States. We scored a replicated maize association panel planted in Lincoln, NE for disease severity and found that disease incidence and severity showed significant variation among maize genotypes. Genome-wide association studies identified four loci associated with significant quantitative variation in disease severity. These loci were associated with candidate genes with plausible links to quantitative disease resistance. A transcriptome-wide association study identified additional genes associated with disease severity. Together, these results indicate that substantial diversity in resistance to southern rust exists among current temperate-adapted maize germplasm, including several candidate loci that may explain the observed variation in resistance to southern rust.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Efficacy of Chitosan Nanoparticle Loaded-Salicylic Acid and -Silver on Management of Cassava Leaf Spot Disease

Leaf spot is one of the most important cassava diseases. Nanotechnology can be applied to control diseases and improve plant growth. This study was performed to prepare chitosan (CS) nanoparticle (NP)-loaded salicylic acid (SA) or silver (Ag) by the ionic gelation method, and to evaluate their effectiveness on reducing leaf spot disease and enhancing the growth of cassava plants. The CS (0.4 or 0.5%) and Pentasodium triphosphate (0.2 or 0.5%) were mixed with SA varying at 0.05, 0.1, or 0.2% or silver nitrate varying at 1, 2, or 3 mM to prepare three formulations of CS-NP-loaded SA named N1, N2, and N3 or CS-NP-loaded Ag named N4, N5, and N6. The results showed that the six formulations were not toxic to cassava leaves up to 800 ppm. The CS-NP-loaded SA (N3) and CS-NP-loaded Ag (N6) were more effective than the remaining formulations in reducing the disease severity and the disease index of leaf spot. Furthermore, N3 at 400 ppm and N6 at 200, 400, and 800 ppm could reduce disease severity (68.9–73.6% or 37.0–37.7%, depending on the time of treatment and the pathogen density) and enhance plant growth more than or equal to commercial fungicide or nano-fungicide products under net-house conditions. The study indicates the potential to use CS-NP-loaded SA or Ag as elicitors to manage cassava leaf spot disease.

Coevolutionary theory of hosts and parasites

Host and parasite evolution are closely intertwined, with selection for adaptations and counter-adaptations forming a coevolutionary feedback loop. Coevolutionary dynamics are often difficult to intuit due to these feedbacks and are hard to demonstrate empirically in most systems. Theoretical models have therefore played a crucial role in shaping our understanding of host-parasite coevolution. Theoretical models vary widely in their assumptions, approaches and aims, and such variety makes it difficult, especially for non-theoreticians and those new to the field, to: (1) understand how model approaches relate to one another; (2) identify key modelling assumptions; (3) determine how model assumptions relate to biological systems; and (4) reconcile the results of different models with contrasting assumptions. In this review, we identify important model features, highlight key results and predictions and describe how these pertain to model assumptions. We carry out a literature survey of theoretical studies published since the 1950s (n = 219 papers) to support our analysis. We identify two particularly important features of models that tend to have a significant qualitative impact on the outcome of host-parasite coevolution: population dynamics and the genetic basis of infection. We also highlight the importance of other modelling features, such as stochasticity and whether time proceeds continuously or in discrete steps, that have received less attention but can drastically alter coevolutionary dynamics. We finish by summarizing recent developments in the field, specifically the trend towards greater model complexity, and discuss likely future directions for research.