Gene-for-gene tolerance to bacterial wilt in Arabidopsis

Identification of a Major QTL (qRRs-10.1) That Confers Resistance to Ralstonia solanacearum in Pepper (Capsicum annuum) Using SLAF-BSA and QTL Mapping

The soilborne pathogen Ralstonia solanacearum is the causal agent of bacterial wilt (BW), a major disease of pepper (Capsicum annuum). The genetic basis of resistance to this disease in pepper is not well known. This study aimed to identify BW resistance markers in pepper. Analysis of the dynamics of bioluminescent R. solanacearum colonization in reciprocal grafts of a resistant (BVRC 1) line and a susceptible (BVRC 25) line revealed that the resistant rootstock effectively suppressed the spreading of bacteria into the scion. The two clear-cut phenotypic distributions of the disease severity index in 440 F2 plants derived from BVRC 25 × BVRC 1 indicated that a major genetic factor as well as a few minor factors that control BW resistance. By specific-locus amplified fragment sequencing combined with bulked segregant analysis, two adjacent resistance-associated regions on chromosome 10 were identified. Quantitative trait (QTL) mapping revealed that these two regions belong to a single QTL, qRRs-10.1. The marker ID10-194305124, which reached a maximum log-likelihood value at 9.79 and accounted for 19.01% of the phenotypic variation, was located the closest to the QTL peak. A cluster of five predicted R genes and three defense-related genes, which are located in close proximity to the significant markers ID10-194305124 or ID10-196208712, are important candidate genes that may confer BW resistance in pepper.

The eggplant transcription factor MYB44 enhances resistance to bacterial wilt by activating the expression of spermidine synthase

Bacterial wilt (BW) caused by Ralstonia solanacearum is a serious disease affecting the production of Solanaceae species, including eggplant (Solanum melongena). However, few resistance genes have been identified in eggplant, and therefore the underlying mechanism of BW resistance remains unclear. Hence, we investigated a spermidine synthase (SPDS) gene from eggplant and created knock-down lines with virus-induced gene silencing. After eggplant was infected with R. solanacearum, the SmSPDS gene was induced, concurrent with increased spermidine (Spd) content, especially in the resistant line. We speculated that Spd plays a significant role in the defense response of eggplant to BW. Moreover, using the yeast one-hybrid approach and dual luciferase-based transactivation assay, an R2R3-MYB transcription factor, SmMYB44, was identified as directly binding to the SmSPDS promoter, activating its expression. Overexpression of SmMYB44 in eggplant induced the expression of SmSPDS and Spd content, increasing the resistance to BW. In contrast, the SmMYB44-RNAi transgenic plants showed more susceptibility to BW compared with the control plants. Our results provide insight into the SmMYB44-SmSPDS-Spd module involved in the regulation of resistance to R. solanacearum. This research also provides candidates to enhance resistance to BW in eggplant.

Quantitative Disease Resistance under Elevated Temperature: Genetic Basis of New Resistance Mechanisms to Ralstonia solanacearum

In the context of climate warming, plants will be facing an increased risk of epidemics as well as the emergence of new highly aggressive pathogen species. Although a permanent increase of temperature strongly affects plant immunity, the underlying molecular mechanisms involved are still poorly characterized. In this study, we aimed to uncover the genetic bases of resistance mechanisms that are efficient at elevated temperature to the Ralstonia solanacearum species complex (RSSC), one of the most harmful phytobacteria causing bacterial wilt. To start the identification of quantitative trait loci (QTLs) associated with natural variation of response to R. solanacearum, we adopted a genome wide association (GWA) mapping approach using 176 worldwide natural accessions of Arabidopsis thaliana inoculated with the R. solanacearum GMI1000 strain. Following two different procedures of root-inoculation (root apparatus cut vs. uncut), plants were grown either at 27 or 30°C, with the latter temperature mimicking a permanent increase in temperature. At 27°C, the RPS4/RRS1-R locus was the main QTL of resistance detected regardless of the method of inoculation used. This highlights the power of GWA mapping to identify functionally important loci for resistance to the GMI1000 strain. At 30°C, although most of the accessions developed wilting symptoms, we identified several QTLs that were specific to the inoculation method used. We focused on a QTL region associated with response to the GMI1000 strain in the early stages of infection and, by adopting a reverse genetic approach, we functionally validated the involvement of a strictosidine synthase-like 4 (SSL4) protein that shares structural similarities with animal proteins known to play a role in animal immunity.

Quantitative Methods to Assess Differential Susceptibility of Arabidopsis thaliana Natural Accessions to Dickeya dadantii

Among the most devastating bacterial diseases of plants, soft rot provoked by Dickeya spp. cause crop yield losses on a large range of species with potato being the most economically important. The use of antibiotics being prohibited in most countries in the field, identifying tolerance genes is expected to be one of the most effective alternate disease control approaches. A prerequisite for the identification of tolerance genes is to develop robust disease quantification methods and to identify tolerant plant genotypes. In this work, we investigate the feasibility of the exploitation of Arabidopsis thaliana natural variation to find tolerant genotypes and to develop robust quantification methods. We compared different quantification methods that score either symptom development or bacterial populations in planta. An easy to set up and reliable bacterial quantification method based on qPCR amplification of bacterial DNA was validated. This study demonstrates that it is possible to conduct a robust phenotyping of soft rot disease, and that Arabidopsis natural accessions are a relevant source of tolerance genes.

Mechanisms of quantitative disease resistance in plants

Quantitative disease resistance (QDR) causes the reduction, but not absence, of disease, and is a major type of disease resistance for many crop species. QDR results in a continuous distribution of disease scores across a segregating population, and is typically due to many genes with small effects. It may also be a source of durable resistance. The past decade has seen significant progress in cloning genes underlying QDR. In this review, we focus on these recently cloned genes and identify new themes of QDR emerging from these studies.

Epidemiological and Evolutionary Outcomes in Gene-for-Gene and Matching Allele Models

Gene-for-gene (GFG) and matching-allele (MA) models are qualitatively different paradigms for describing the outcome of genetic interactions between hosts and pathogens. The GFG paradigm was largely built on the foundations of Flor's early work on the flax-flax rust interaction and is based on the concept of genetic recognition leading to incompatible disease outcomes, typical of host immune recognition. In contrast, the MA model is based on the assumption that genetic recognition leads to compatible interactions, which can result when pathogens require specific host factors to cause infection. Results from classical MA and GFG models have led to important predictions regarding various coevolutionary phenomena, including the role of fitness costs associated with resistance and infectivity, the distribution of resistance genes in wild populations, patterns of local adaptation and the evolution and maintenance of sexual reproduction. Empirical evidence (which we review briefly here), particularly from recent molecular advances in understanding of the mechanisms that determine the outcome of host-pathogen encounters, suggests considerable variation in specific details of the functioning of interactions between hosts and pathogens, which may contain elements of both models. In this regard, GFG and MA scenarios likely represent endpoints of a continuum of potentially more complex interactions that occur in nature. Increasingly, this has been recognized in theoretical studies of coevolutionary processes in plant host-pathogen and animal host-parasite associations (e.g., departures from strict GFG/MA assumptions, diploid genetics, multi-step infection processes). However, few studies have explored how different genetic assumptions about host resistance and pathogen infectivity might impact on disease epidemiology or pathogen persistence within and among populations. Here, we use spatially explicit simulations of the basic MA and GFG scenarios to highlight qualitative differences between these scenarios with regard to patterns of disease and impacts on host demography. Given that such impacts drive evolutionary trajectories, future theoretical advances that aim to capture more complex genetic scenarios should explicitly address the interaction between epidemiology and different models of host-pathogen interaction genetics.

Opening the Ralstonia solanacearum type III effector tool box: insights into host cell subversion mechanisms

Effectors delivered to host cells by the Type III secretion system are essential to Ralstonia solanacearum pathogenicity, as in several other plant pathogenic bacteria. The establishment of exhaustive effector repertoires in multiple R. solanacearum strains drew a first picture of the evolutionary dynamics of the pathogen effector suites. Effector repertoires are diversified, with a core of 20-30 effectors present in most of the strains and the obtention of mutants lacking one or more effector genes revealed the functional overlap among this effector network. Recent functional studies have provided insights into the ability of single effectors to manipulate the host proteasome, elicit cell death, trigger the expression of plant genes, and/or display biochemical activities on plant protein targets.

Mapping QTL conferring resistance in maize to gray leaf spot disease caused by Cercospora zeina

BACKGROUND

Gray leaf spot (GLS) is a globally important foliar disease of maize. Cercospora zeina, one of the two fungal species that cause the disease, is prevalent in southern Africa, China, Brazil and the eastern corn belt of the USA. Identification of QTL for GLS resistance in subtropical germplasm is important to support breeding programmes in developing countries where C. zeina limits production of this staple food crop.

RESULTS

A maize RIL population (F7:S6) from a cross between CML444 and SC Malawi was field-tested under GLS disease pressure at five field sites over three seasons in KwaZulu-Natal, South Africa. Thirty QTL identified from eleven field trials (environments) were consolidated to seven QTL for GLS resistance based on their expression in at least two environments and location in the same core maize bins. Four GLS resistance alleles were derived from the more resistant parent CML444 (bin 1.10, 4.08, 9.04/9.05, 10.06/10.07), whereas the remainder were from SC Malawi (bin 6.06/6.07, 7.02/7.03, 9.06). QTLs in bin 4.08 and bin 6.06/6.07 were also detected as joint QTLs, each explained more than 11% of the phenotypic variation, and were identified in four and seven environments, respectively. Common markers were used to allocate GLS QTL from eleven previous studies to bins on the IBM2005 map, and GLS QTL "hotspots" were noted. Bin 4.08 and 7.02/7.03 GLS QTL from this study overlapped with hotspots, whereas the bin 6.06/6.07 and bin 9.06 QTLs appeared to be unique. QTL for flowering time (bin 1.07, 4.09) in this population did not correspond to QTL for GLS resistance.

CONCLUSIONS

QTL mapping of a RIL population from the subtropical maize parents CML444 and SC Malawi identified seven QTL for resistance to gray leaf spot disease caused by C. zeina. These QTL together with QTL from eleven studies were allocated to bins on the IBM2005 map to provide a basis for comparison. Hotspots of GLS QTL were identified on chromosomes one, two, four, five and seven, with QTL in the current study overlapping with two of these. Two QTL from this study did not overlap with previously reported QTL.

Breeding for resistances to Ralstonia solanacearum

Ralstonia solanacearum is one of the most devastating bacterial plant pathogens due to its large host range, worldwide geographic distribution and persistence in fields. This soilborne pathogen is the causal agent of bacterial wilt and it can infect major agricultural crops thereby reducing significantly their yield. To favor infection, the bacterium delivers, through the type three secretion system, effectors that manipulate plant immunity. In this review, the relative efficiency of control strategies and existing resistances to R. solanacearum will be presented. Then, the genetic and molecular insights gained from the study of bacterial wilt in model plants will be described. Finally, I will explore how the knowledge gathered from unraveling avirulence and virulence mechanisms of R. solanacearum effectors could help to develop more durable resistances in crop plants toward this destructive pathogen.

Arabidopsis dual resistance proteins, both RPS4 and RRS1, are required for resistance to bacterial wilt in transgenic Brassica crops

Bacterial wilt phytopathogen Ralstonia solanacearum is a serious soil-borne disease that attacks several economically important plants worldwide, including Brassicaceae. Previous studies indicate that recognition of avirulence (Avr)-effector PopP2 by resistance (R) protein, RRS1-R, and physical interaction between RRS1-R and PopP2 in the nucleus are required for resistance. Of late, we showed that a pair of Arabidopsis thaliana TIR-NLR proteins, RRS1 and RPS4, function together in disease resistance against multiple pathogen isolates. Here, we report that dual R proteins, RRS1 and RPS4, from A. thaliana ecotype Wassilewskija confer resistance to bacterial wilt in transgenic Brassica crops. For practical applications, this finding may provide a new strategy for developing disease resistant plants that express R genes from other plants.

Ralstonia solanacearum, a widespread bacterial plant pathogen in the post-genomic era

Ralstonia solanacearum is a soil-borne bacterium causing the widespread disease known as bacterial wilt. Ralstonia solanacearum is also the causal agent of Moko disease of banana and brown rot of potato. Since the last R. solanacearum pathogen profile was published 10 years ago, studies concerning this plant pathogen have taken a genomic and post-genomic direction. This was pioneered by the first sequenced and annotated genome for a major plant bacterial pathogen and followed by many more genomes in subsequent years. All molecular features studied now have a genomic flavour. In the future, this will help in connecting the classical field of pathology and diversity studies with the gene content of specific strains. In this review, we summarize the recent research on this bacterial pathogen, including strain classification, host range, pathogenicity determinants, regulation of virulence genes, type III effector repertoire, effector-triggered immunity, plant signalling in response to R. solanacearum, as well as a review of different new pathosystems.

TAXONOMY

Bacteria; Proteobacteria; β subdivision; Ralstonia group; genus Ralstonia.

DISEASE SYMPTOMS

Ralstonia solanacearum is the agent of bacterial wilt of plants, characterized by a sudden wilt of the whole plant. Typically, stem cross-sections will ooze a slimy bacterial exudate. In the case of Moko disease of banana and brown rot of potato, there is also visible bacterial colonization of banana fruit and potato tuber.

DISEASE CONTROL

As a soil-borne pathogen, infected fields can rarely be reused, even after rotation with nonhost plants. The disease is controlled by the use of resistant and tolerant plant cultivars. The prevention of spread of the disease has been achieved, in some instances, by the application of strict prophylactic sanitation practices.

USEFUL WEBSITES

Stock centre: International Centre for Microbial Resources-French Collection for Plant-associated Bacteria CIRM-CFBP, IRHS UMR 1345 INRA-ACO-UA, 42 rue Georges Morel, 49070 Beaucouzé Cedex, France, http://www.angers-nantes.inra.fr/cfbp/. Ralstonia Genome browser: https://iant.toulouse.inra.fr/R.solanacearum. GMI1000 insertion mutant library: https://iant.toulouse.inra.fr/R.solanacearumGMI1000/GenomicResources. MaGe Genome Browser: https://www.genoscope.cns.fr/agc/microscope/mage/viewer.php?

MtQRRS1, an R-locus required for Medicago truncatula quantitative resistance to Ralstonia solanacearum

Ralstonia solanacearum is a major soilborne pathogen that attacks > 200 plant species, including major crops. To characterize MtQRRS1, a major quantitative trait locus (QTL) for resistance towards this bacterium in the model legume Medicago truncatula, genetic and functional approaches were combined. QTL analyses together with disease scoring of heterogeneous inbred families were used to define the locus. The candidate region was studied by physical mapping using a bacterial artificial chromosome (BAC) library of the resistant line, and sequencing. In planta bacterial growth measurements, grafting experiments and gene expression analysis were performed to investigate the mechanisms by which this locus confers resistance to R. solanacearum. The MtQRRS1 locus was localized to the same position in two recombinant inbred line populations and was narrowed down to a 64 kb region. Comparison of parental line sequences revealed 15 candidate genes with sequence polymorphisms, but no evidence of differential gene expression upon infection. A role for the hypocotyl in resistance establishment was shown. These data indicate that the quantitative resistance to bacterial wilt conferred by MtQRRS1, which contains a cluster of seven R genes, is shared by different accessions and may act through intralocus interactions to promote resistance.