Resistance to Phytophthora infestans in Lycopersicon pennellii

Identification and mapping of late blight resistance QTLs in the wild tomato accession PI 224710 (Solanum pimpinellifolium)

Late blight (LB), caused by oomycete Phytophthora infestans, is one of the most destructive diseases of the cultivated tomato, Solanum lycopersicum. Since new and aggressive clonal lineages of P. infestans, many of which overcoming formerly effective fungicides or host resistance genes, have continued to emerge, it is crucial to identify, characterize, and utilize new sources of host resistance in tomato breeding. A recent screening of tomato germplasm identified Solanum pimpinellifolium accession PI 224710 with very strong resistance to several current P. infestans clonal lineages. The present study aimed to identify and characterize QTLs associated with LB resistance in PI 224710. Disease screening of a large F2 population (n = 1721), derived from a cross between PI 224710 and LB-susceptible tomato breeding line Fla. 8059, followed by F3 progeny testing, resulted in the identification of 43 highly-resistant and 27 highly-susceptible F2 individuals. A selective genotyping approach, using 469 non-identical SNP markers, resulted in the construction of a genetic linkage map and identification of three LB-resistance QTLs on chromosomes 6, 9 and 10 of PI 224710. A comparison of the QTLs genomic locations with the tomato physical map resulted in the identification of several candidate genes, which might be underpinning the LB-resistance QTLs in PI 224710. The identified markers associated with the LB-resistance QTLs can be utilized in breeding programs to transfer resistance from PI 224710 into tomato breeding lines and hybrid cultivars via marker-assisted breeding; they also can be used to develop near-isogenic lines for fine mapping of the QTLs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01498-1.

Unraveling the genetic basis of quantitative resistance to diseases in tomato: a meta-QTL analysis and mining of transcript profiles

KEY MESSAGE

Whole-genome QTL mining and meta-analysis in tomato for resistance to bacterial and fungal diseases identified 73 meta-QTL regions with significantly refined/reduced confidence intervals. Tomato production is affected by a range of biotic stressors, causing yield losses and quality reductions. While sources of genetic resistance to many tomato diseases have been identified and characterized, stability of the resistance genes or quantitative trait loci (QTLs) across the resources has not been determined. Here, we examined 491 QTLs previously reported for resistance to tomato diseases in 40 independent studies and 54 unique mapping populations. We identified 29 meta-QTLs (MQTLs) for resistance to bacterial pathogens and 44 MQTLs for resistance to fungal pathogens, and were able to reduce the average confidence interval (CI) of the QTLs by 4.1-fold and 6.7-fold, respectively, compared to the average CI of the original QTLs. The corresponding physical length of the CIs of MQTLs ranged from 56 kb to 6.37 Mb, with a median of 921 kb, of which 27% had a CI lower than 500 kb and 53% had a CI lower than 1 Mb. Comparison of defense responses between tomato and Arabidopsis highlighted 73 orthologous genes in the MQTL regions, which were putatively determined to be involved in defense against bacterial and fungal diseases. Intriguingly, multiple genes were identified in some MQTL regions that are implicated in plant defense responses, including PR-P2, NDR1, PDF1.2, Pip1, SNI1, PTI5, NSL1, DND1, CAD1, SlACO, DAD1, SlPAL, Ph-3, EDS5/SID1, CHI-B/PR-3, Ph-5, ETR1, WRKY29, and WRKY25. Further, we identified a number of candidate resistance genes in the MQTL regions that can be useful for both marker/gene-assisted breeding as well as cloning and genetic transformation.

Deep learning the cis-regulatory code for gene expression in selected model plants

Elucidating the relationship between non-coding regulatory element sequences and gene expression is crucial for understanding gene regulation and genetic variation. We explored this link with the training of interpretable deep learning models predicting gene expression profiles from gene flanking regions of the plant species Arabidopsis thaliana, Solanum lycopersicum, Sorghum bicolor, and Zea mays. With over 80% accuracy, our models enabled predictive feature selection, highlighting e.g. the significant role of UTR regions in determining gene expression levels. The models demonstrated remarkable cross-species performance, effectively identifying both conserved and species-specific regulatory sequence features and their predictive power for gene expression. We illustrated the application of our approach by revealing causal links between genetic variation and gene expression changes across fourteen tomato genomes. Lastly, our models efficiently predicted genotype-specific expression of key functional gene groups, exemplified by underscoring known phenotypic and metabolic differences between Solanum lycopersicum and its wild, drought-resistant relative, Solanum pennellii.

Distinct Phyllosphere Microbiome of Wild Tomato Species in Central Peru upon Dysbiosis

Plants are colonized by myriads of microbes across kingdoms, which affect host development, fitness, and reproduction. Hence, plant microbiomes have been explored across a broad range of host species, including model organisms, crops, and trees under controlled and natural conditions. Tomato is one of the world's most important vegetable crops; however, little is known about the microbiota of wild tomato species. To obtain insights into the tomato microbiota occurring in natural environments, we sampled epiphytic microbes from leaves of four tomato species, Solanum habrochaites, S. corneliomulleri, S. peruvianum, and S. pimpinellifolium, from two geographical locations within the Lima region of Peru over 2 consecutive years. Here, a high-throughput sequencing approach was applied to investigate microbial compositions including bacteria, fungi, and eukaryotes across tomato species and geographical locations. The phyllosphere microbiome composition varies between hosts and location. Yet, we identified persistent microbes across tomato species that form the tomato microbial core community. In addition, we phenotypically defined healthy and dysbiotic samples and performed a downstream analysis to reveal the impact on microbial community structures. To do so, we compared microbial diversities, unique OTUs, relative abundances of core taxa, and microbial hub taxa, as well as co-occurrence network characteristics in healthy and dysbiotic tomato leaves and found that dysbiosis affects the phyllosphere microbial composition in a host species-dependent manner. Yet, overall, the present data suggests an enrichment of plant-promoting microbial taxa in healthy leaves, whereas numerous microbial taxa containing plant pathogens occurred in dysbiotic leaves.Concluding, we identify the core phyllosphere microbiome of wild tomato species, and show that the overall phyllosphere microbiome can be impacted by sampling time point, geographical location, host genotype, and plant health. Future studies in these components will help understand the microbial contribution to plant health in natural systems and can be of use in cultivated tomatoes.

Late blight in tomato: insights into the pathogenesis of the aggressive pathogen Phytophthora infestans and future research priorities

This review provides insights into the molecular interactions between Phytophthora infestans and tomato and highlights research gaps that need further attention. Late blight in tomato is caused by the oomycota hemibiotroph Phytophthora infestans, and this disease represents a global threat to tomato farming. The pathogen is cumbersome to control because of its fast-evolving nature, ability to overcome host resistance and inefficient natural resistance obtained from the available tomato germplasm. To achieve successful control over this pathogen, the molecular pathogenicity of P. infestans and key points of vulnerability in the host plant immune system must be understood. This review primarily focuses on efforts to better understand the molecular interaction between host pathogens from both perspectives, as well as the resistance genes, metabolomic changes, quantitative trait loci with potential for improvement in disease resistance and host genome manipulation via transgenic approaches, and it further identifies research gaps and provides suggestions for future research priorities.

Fine Mapping of the Ph-2 Gene Conferring Resistance to Late Blight (Phytophthora infestans) in Tomato

Late blight is a devastating tomato disease. Breeding new varieties with multiple resistance (R) genes is highly effective for preventing late blight. The Ph-2 gene mediates resistance to Phytophthora infestans race T₁ in tomato. In this study, we used an F₂ population derived from a cross between Solanum lycopersicum Moboline (resistant) and LA3988 (susceptible) cultivars for the fine mapping of Ph-2. Two flanking markers, CAPS-1 and CC-Ase, mapped Ph-2 to a 141-kb genomic region containing 21 projected genes, 5 of which were identified as putative R genes. The Solyc10g085460 coding sequence varied significantly between the parents. The markers developed and candidate genes identified in this study shall be useful for the molecular breeding of tomato exhibiting increased late blight resistance and for the cloning of the Ph-2 gene.

Identification and Mapping of Late Blight Resistance Quantitative Trait Loci in Tomato Accession PI 163245

Late blight (LB), caused by the oomycete (Mont.) de Bary, is one of the most devastating diseases of tomato ( L.) and potato ( tuberosum L. worldwide. The importance of LB on tomato has increased due to the occurrence of aggressive and fungicide-resistant clonal lineages of . Consequently, identification and characterization of new sources of genetic resistance to LB has become a priority in tomato breeding. Previously, we reported accession PI 163245 as a promising source of highly heritable LB resistance for tomato breeding. The purpose of this study was to identify and map quantitative trait loci (QTLs) associated with LB resistance in this accession using a trait-based marker analysis (a.k.a. selective genotyping). An F mapping population ( = 560) derived from a cross between a LB-susceptible tomato breeding line (Fla. 8059) and PI 163245 was screened for LB resistance, and the most resistant ( = 39) and susceptible ( = 35) individuals were selected for genotyping. Sequencing and comparison of the reduced representation libraries (RRLs) derived from genomic DNA of the two parents resulted in the identification of 33,541 putative single nucleotide polymorphism (SNP) markers, of which, 233 genome-wide markers were used to genotype the 74 selected F individuals. The marker analysis resulted in the identification of four LB resistance QTLs conferred by PI 163245, located on chromosomes 2, 3, 10, and 11. Research is underway to develop near-isogenic lines (NILs) for fine mapping the QTLs and develop tomato breeding lines with LB resistance introduced from PI 163245.

Expression profiling across wild and cultivated tomatoes supports the relevance of early miR482/2118 suppression for Phytophthora resistance

Plants possess a battery of specific pathogen resistance (R-)genes. Precise R-gene regulation is important in the presence and absence of a pathogen. Recently, a microRNA family, miR482/2118, was shown to regulate the expression of a major class of R-genes, nucleotide-binding site leucine-rich repeats (NBS-LRRs). Furthermore, RNA silencing suppressor proteins, secreted by pathogens, prevent the accumulation of miR482/2118, leading to an upregulation of R-genes. Despite this transcriptional release of R-genes, RNA silencing suppressors positively contribute to the virulence of some pathogens. To investigate this paradox, we analysed how the regulation of NBS-LRRs by miR482/2118 has been shaped by the coevolution between Phytophthora infestans and cultivated and wild tomatoes. We used degradome analyses and qRT-PCR to evaluate and quantify the co-expression of miR482/2118 and their NBS-LRR targets. Our data show that miR482/2118-mediated targeting contributes to the regulation of NBS-LRRs in Solanum lycopersicum. Based on miR482/2118 expression profiling in two additional tomato species-with different coevolutionary histories with P. infestans-we hypothesize that pathogen-mediated RNA silencing suppression is most effective in the interaction between S. lycopersicum and P. infestans Furthermore, an upregulation of miR482/2118 early in the infection may increase susceptibility to P. infestans.

Identification of tomato introgression lines with enhanced susceptibility or resistance to infection by parasitic giant dodder (Cuscuta reflexa)

The parasitic flowering plant genus Cuscuta (dodder) is a parasitic weed that infects many important crops. Once it winds around the shoots of potential host plants and initiates the development of penetration organs, called haustoria, only a few plant species have been shown to deploy effective defense mechanisms to ward off Cuscuta parasitization. However, a notable exception is Solanum lycopersicum (tomato), which exhibits a local hypersensitive reaction when attacked by giant dodder (Cuscuta reflexa). Interestingly, the closely related wild desert tomato, Solanum pennellii, is unable to stop the penetration of its tissue by the C. reflexa haustoria. In this study, we observed that grafting a S. pennellii scion onto the rootstock of the resistant S. lycopersicum did not change the susceptibility phenotype of S. pennellii. This suggests that hormones, or other mobile substances, produced by S. lycopersicum do not induce a defense reaction in the susceptible tissue. Screening of a population of introgression lines harboring chromosome fragments from S. pennellii in the genome of the recurrent parent S. lycopersicum, revealed that most lines exhibit the same defense reaction as shown by the S. lycopersicum parental line. However, several lines showed different responses and exhibited either susceptibility, or cell death that extended considerably beyond the infection site. These lines will be valuable for the future identification of key loci involved in the perception of, and resistance to, C. reflexa and for developing strategies to enhance resistance to infection in crop species.

Rapid identification of candidate genes for resistance to tomato late blight disease using next-generation sequencing technologies

Tomato late blight caused by Phytophthora infestans (Mont.) de Bary, also known as the Irish famine pathogen, is one of the most destructive plant diseases. Wild relatives of tomato possess useful resistance genes against this disease, and could therefore be used in breeding to improve cultivated varieties. In the genome of a wild relative of tomato, Solanum habrochaites accession LA1777, we identified a new quantitative trait locus for resistance against blight caused by an aggressive Egyptian isolate of P. infestans. Using double-digest restriction site-associated DNA sequencing (ddRAD-Seq) technology, we determined 6,514 genome-wide SNP genotypes of an F2 population derived from an interspecific cross. Subsequent association analysis of genotypes and phenotypes of the mapping population revealed that a 6.8 Mb genome region on chromosome 6 was a candidate locus for disease resistance. Whole-genome resequencing analysis revealed that 298 genes in this region potentially had functional differences between the parental lines. Among of them, two genes with missense mutations, Solyc06g071810.1 and Solyc06g083640.3, were considered to be potential candidates for disease resistance. SNP and SSR markers linking to this region can be used in marker-assisted selection in future breeding programs for late blight disease, including introgression of new genetic loci from wild species. In addition, the approach developed in this study provides a model for identification of other genes for attractive agronomical traits.

Mapping Quantitative Trait Loci (QTL) for Resistance to Late Blight in Tomato

Late blight caused by Phytophthora infestans (Montagne, Bary) is a devastating disease of tomato worldwide. There are three known major genes, Ph-1, Ph-2, and Ph-3, conferring resistance to late blight. In addition to these three genes, it is also believed that there are additional factors or quantitative trait loci (QTL) conferring resistance to late blight. Precise molecular mapping of all those major genes and potential QTL is important in the development of suitable molecular markers and hence, marker-assisted selection (MAS). The objective of the present study was to map the genes and QTL associated with late blight resistance in a tomato population derived from intra-specific crosses. To achieve this objective, a population, derived from the crossings of NC 1CELBR × Fla. 7775, consisting of 250 individuals at F2 and F2-derived families, were evaluated in replicated trials. These were conducted at Mountain Horticultural Crops Reseach & Extension Center (MHCREC) at Mills River, NC, and Mountain Research Staion (MRS) at Waynesville, NC in 2011, 2014, and 2015. There were two major QTL associated with late blight resistance located on chromosomes 9 and 10 with likelihood of odd (LOD) scores of more than 42 and 6, explaining 67% and 14% of the total phenotypic variation, respectively. The major QTLs are probably caused by the Ph-2 and Ph-3 genes. Furthermore, there was a minor QTL on chromosomes 12, which has not been reported before. This minor QTL may be novel and may be worth investigating further. Source of resistance to Ph-2, Ph-3, and this minor QTL traces back to line L3707, or Richter's Wild Tomato. The combination of major genes and minor QTL may provide a durable resistance to late blight in tomato.

Phytophthora infestans: New Tools (and Old Ones) Lead to New Understanding and Precision Management

New tools have revealed that migrations of Phytophthora infestans have been a dominant feature of the population biology of this pathogen for the past 50 years, and maybe for the past 170 years. We now have accurate information on the composition of many P. infestans populations. However, migration followed by selection can lead and has led to dramatically rapid changes in populations over large regions. Except for the highlands of central Mexico, many populations of P. infestans have probably been in flux over the past several decades. There is some evidence that this pathogen has different characteristics in the field than it does in the lab, and early field phenotypic analyses of hypotheses concerning fitness and pathogenicity would be beneficial. The newly available capacity to acquire and process vast amounts of weather and weather forecast data in combination with advancements in molecular diagnostics enables much greater precision in late blight management to produce recommendations that are site, host, and pathogen specific.

Pooled Enrichment Sequencing Identifies Diversity and Evolutionary Pressures at NLR Resistance Genes within a Wild Tomato Population

Nod-like receptors (NLRs) are nucleotide-binding domain and leucine-rich repeats containing proteins that are important in plant resistance signaling. Many of the known pathogen resistance (R) genes in plants are NLRs and they can recognize pathogen molecules directly or indirectly. As such, divergence and copy number variants at these genes are found to be high between species. Within populations, positive and balancing selection are to be expected if plants coevolve with their pathogens. In order to understand the complexity of R-gene coevolution in wild nonmodel species, it is necessary to identify the full range of NLRs and infer their evolutionary history. Here we investigate and reveal polymorphism occurring at 220 NLR genes within one population of the partially selfing wild tomato species Solanum pennellii. We use a combination of enrichment sequencing and pooling ten individuals, to specifically sequence NLR genes in a resource and cost-effective manner. We focus on the effects which different mapping and single nucleotide polymorphism calling software and settings have on calling polymorphisms in customized pooled samples. Our results are accurately verified using Sanger sequencing of polymorphic gene fragments. Our results indicate that some NLRs, namely 13 out of 220, have maintained polymorphism within our S. pennellii population. These genes show a wide range of πN/πS ratios and differing site frequency spectra. We compare our observed rate of heterozygosity with expectations for this selfing and bottlenecked population. We conclude that our method enables us to pinpoint NLR genes which have experienced natural selection in their habitat.

Identification of QTLs controlling resistance to Pseudomonas syringae pv. tomato race 1 strains from the wild tomato, Solanum habrochaites LA1777

Screening of wild tomato accessions revealed a source of resistance to Pseudomonas syringe pv. tomato race 1 from Solanum habrochaites and facilitated mapping of QTLs controlling disease resistance. Pseudomonas syringae pv. tomato (Pst) causes bacterial speck of tomato, which is one of the most persistent bacterial diseases in tomato worldwide. Existing Pst populations have overcome genetic resistance mediated by the tomato genes Pto and Prf. The objective of this study was to identify sources of resistance to race 1 strains and map quantitative trait loci (QTLs) controlling resistance in the wild tomato Solanum habrochaites LA1777. Pst strains A9 and 407 are closely related to current field strains and genome sequencing revealed the lack of the avrPto effector as well as select mutations in the avrPtoB effector, which are recognized by Pto and Prf. Strains A9 and 407 were used to screen 278 tomato accessions, identifying five exhibiting resistance: S. peruvianum LA3799, S. peruvianum var. dentatum PI128655, S. chilense LA2765, S. habrochaites LA2869, and S. habrochaites LA1777. An existing set of 93 introgression lines developed from S. habrochaites LA1777 was screened for resistance to strain A9 in a replicated greenhouse trial. Four QTLs were identified using composite interval mapping and mapped to different chromosomes. bsRr1-1 was located on chromosome 1, bsRr1-2 on chromosome 2, and bsRr1-12a and bsRr1-12b on chromosome 12. The QTLs detected explained 10.5-12.5% of the phenotypic variation. Promising lines were also subjected to bacterial growth curves to verify resistance and were analyzed for general horticultural attributes under greenhouse conditions. These findings will provide useful information for future high-resolution mapping of each QTL and integration into marker-assisted breeding programs.

Differential Susceptibility of 39 Tomato Varieties to Phytophthora infestans Clonal Lineage US-23

During the summers of 2012 and 2013, 39 tomato (Solanum lycopersicum) lines or varieties were evaluated for resistance to late blight in three separate field trials. In each trial, late blight was caused by field isolates of Phytophthora infestans clonal lineage US-23. Varieties with the late blight resistance genes Ph-1, Ph-2, Ph-3, and Ph-2 + Ph-3 were included, along with several heirloom varieties with grower-reported resistance and varieties with no known resistance. All six varieties with Ph-2 + Ph-3, along with NC25P, which is homozygous for Ph-3 only, showed a high level of resistance. Plum Regal F1, which is heterozygous for Ph-3 only, showed moderate resistance. Legend, the only variety with Ph-2 alone, also showed moderate resistance. Three heirloom varieties, Matt's Wild Cherry, Lemon Drop, and Mr. Stripey, showed a high level of resistance comparable with that of varieties with Ph-2 + Ph-3. New Yorker, possessing Ph-1 only, showed no resistance. Indeterminate varieties had significantly less disease than determinate varieties in two of the three trials. Overall, this study suggests that tomato varieties with both Ph-2 and Ph-3 can be used to effectively manage late blight caused by P. infestans clonal lineage US-23. Varieties possessing only Ph-2, or heterozygous for Ph-3, were better protected than those without any late blight resistance but might still require supplemental fungicide applications, while the variety that was homozygous for Ph-3 was highly resistant. Several heirloom varieties were also highly resistant, and the unknown mechanism of their resistance warrants further research. Finally, the plasticity observed in United States P. infestans populations over the past several decades necessitates continued monitoring for genetic changes within P. infestans that could lead to the breakdown of resistance reported here.

Appraisal of artificial screening techniques of tomato to accurately reflect field performance of the late blight resistance

Late blight (LB) caused by the oomycete Phytophthora infestans continues to thwart global tomato production, while only few resistant cultivars have been introduced locally. In order to gain from the released tomato germplasm with LB resistance, we compared the 5-year field performance of LB resistance in several tomato cultigens, with the results of controlled conditions testing (i.e., detached leaflet/leaf, whole plant). In case of these artificial screening techniques, the effects of plant age and inoculum concentration were additionally considered. In the field trials, LA 1033, L 3707, L 3708 displayed the highest LB resistance, and could be used for cultivar development under Polish conditions. Of the three methods using controlled conditions, the detached leaf and the whole plant tests had the highest correlation with the field experiments. The plant age effect on LB resistance in tomato reported here, irrespective of the cultigen tested or inoculum concentration used, makes it important to standardize the test parameters when screening for resistance. Our results help show why other reports disagree on LB resistance in tomato.

The Ph-3 gene from Solanum pimpinellifolium encodes CC-NBS-LRR protein conferring resistance to Phytophthora infestans

KEY MESSAGE

Ph-3 is the first cloned tomato gene for resistance to late blight and encodes a CC-NBS-LRR protein. Late blight, caused by Phytophthora infestans, is one of the most destructive diseases in tomato. The resistance (R) gene Ph-3, derived from Solanum pimpinellifolium L3708, provides resistance to multiple P. infestans isolates and has been widely used in tomato breeding programmes. In our previous study, Ph-3 was mapped into a region harbouring R gene analogues (RGA) at the distal part of long arm of chromosome 9. To further narrow down the Ph-3 interval, more recombinants were identified using the flanking markers G2-4 and M8-2, which defined the Ph-3 gene to a 26 kb region according to the Heinz1706 reference genome. To clone the Ph-3 gene, a bacterial artificial chromosome (BAC) library was constructed using L3708 and one BAC clone B25E21 containing the Ph-3 region was identified. The sequence of the BAC clone B25E21 showed that only one RGA was present in the target region. A subsequent complementation analysis demonstrated that this RGA, encoding a CC-NBS-LRR protein, was able to complement the susceptible phenotype in cultivar Moneymaker. Thus this RGA was considered the Ph-3 gene. The predicted Ph-3 protein shares high amino acid identity with the chromosome-9-derived potato resistance proteins against P. infestans (Rpi proteins).

Reassessment of QTLs for late blight resistance in the tomato accession L3708 using a restriction site associated DNA (RAD) linkage map and highly aggressive isolates of Phytophthora infestans

Tomato late blight caused by the oomycete pathogen Phytophthora infestans (Mont.) de Bary is a major threat to tomato production in cool and wet environments. Intensified outbreaks of late blight have been observed globally from the 1980s, and are associated with migration of new and more aggressive populations of P. infestans in the field. The objective of this study was to reassess late blight resistance in the wild tomato accession L3708 (Solanum pimpinellifolium L.) against pathogens of different aggressiveness. An F2:3 genetic mapping population was developed using L3708 as the paternal parent. Two isolates of P. infestans, Pi39A and Pi733, were used for inoculation. Pi733 is a highly aggressive genotype that defeats three known late blight resistance genes, Ph-1, Ph-2, and Ph-5t in tomato. In contrast, Pi39A is a less aggressive genotype that defeats only Ph-1. Restriction site Associated DNA Sequencing (RAD-Seq) technology was used to massively sequence 90 bp nucleotides adjacent to both sides of PstI restriction enzyme cutting sites in the genome for all individuals in the genetic mapping population. The RAD-seq data were used to construct a genetic linkage map containing 440 single nucleotide polymorphism markers. Quantitative trait locus (QTL) analysis identified a new disease-resistant QTL specific to Pi733 on chromosome 2. The Ph-3 gene located on chromosome 9 could be detected whichever isolates were used. This study demonstrated the feasibility and efficiency of RAD-Seq technology for conducting a QTL mapping experiment using an F2:3 mapping population, which allowed the identification of a new late blight resistant QTL in tomato.

Fine mapping of the Ph-3 gene conferring resistance to late blight (Phytophthora infestans) in tomato

Late blight, caused by the oomycete pathogen Phytophthora infestans (Mont.) de Bary, is a devastating disease for tomato and potato crops. In the past decades, many late blight resistance (R) genes have been characterized in potato. In contrast, less work has been conducted on tomato. The Ph-3 gene from Solanum pimpinellifolium was introgressed into cultivated tomatoes and conferred broad-spectrum resistance to P. infestans. It was previously assigned to the long arm of chromosome 9. In this study, a high-resolution genetic map covering the Ph-3 locus was constructed using an F2 population of a cross between Solanum lycopersicum CLN2037B (containing Ph-3) and S. lycopersicum LA4084. Ph-3 was mapped in a 0.5 cM interval between two markers, Indel_3 and P55. Eight putative genes were found in the corresponding 74 kb region of the tomato Heinz1706 reference genome. Four of these genes are resistance gene analogs (RGAs) with a typical nucleotide-binding adaptor shared by APAF-1, R proteins, and CED-4 domain. Each RGA showed high homology to the late blight R gene Rpi-vnt1.1 from Solanum venturii. Transient gene silencing indicated that a member of this RGA family is required for Ph-3-mediated resistance to late blight in tomato. Furthermore, this RGA family was also found in the potato genome, but the number of the RGAs was higher than in tomato.

Gene profiling in partially resistant and susceptible near-isogenic tomatoes in response to late blight in the field

In order to better understand resistance to Phytophthora infestans in tomato, we compared the global gene expression of the susceptible tomato, M82, with its more resistant near-isogenic line, 6-2 (IL6-2), under field conditions using a microarray with more than 12 800 tomato expressed sequence tags (ESTs). Because variance in the field was a major concern, we investigated the likelihood of false positives or false negatives and demonstrated that either probability was very low. The two isolines had indistinguishable constitutive gene expressions prior to inoculation. However, a few genes were particularly prone to variation in both isolines in the absence of P. infestans. Included among these genes were catalase, genes coding for pathogenesis-related proteins, endochitinase and cytochrome P450. In response to inoculation with P. infestans, a time course of gene expression identified 1248 transcripts that were similarly induced or repressed in each line, and 991 that were differentially expressed between the two lines. These differences provide hypotheses to explain the difference in resistance between the two isolines. For example, one hypothesis is that genes up-regulated in IL6-2 in response to inoculation with P. infestans, but not up-regulated in M82, contribute to the resistance in IL6-2. Using virus-induced gene silencing (VIGS), we were able to partially silence two such genes-one encoded a protein with homology to an R gene with the Toll/interleukin-1 receptor-nucleotide-binding site-leucine-rich repeat (TIR-NBS-LRR) motif (37O19) and the other encoded a peroxisomal membrane protein (35P7). Partial silencing of 37O19 reduced the resistance in IL6-2 (P = 0.001), but had no effect on the response of M82. Partial silencing of 35P7 reduced the resistance in IL6-2 moderately significantly (P = 0.067), but had no effect in M82. We expect that hypotheses developed from this gene expression study performed under field conditions will provide an important avenue to an accurate understanding of the genes involved in resistance.

Fractionation, stability, and isolate-specificity of QTL for resistance to Phytophthora infestans in cultivated tomato (Solanum lycopersicum)

Cultivated tomato (Solanum lycopersicum) is susceptible to late blight, a major disease caused by Phytophthora infestans, but quantitative resistance exists in the wild tomato species S. habrochaites. Previously, we mapped several quantitative trait loci (QTL) from S. habrochaites and then introgressed each individually into S. lycopersicum. Near-isogenic lines (NILs) were developed, each containing a single introgressed QTL on chromosome 5 or 11. NILs were used to create two recombinant sub-NIL populations, one for each target chromosome region, for higher-resolution mapping. The sub-NIL populations were evaluated for foliar and stem resistance to P. infestans in replicated field experiments over two years, and in replicated growth chamber experiments for resistance to three California isolates. Each of the original single QTL on chromosomes 5 and 11 fractionated into between two and six QTL for both foliar and stem resistance, indicating a complex genetic architecture. The majority of QTL from the field experiments were detected in multiple locations or years, and two of the seven QTL detected in growth chambers were co-located with QTL detected in field experiments, indicating stability of some QTL across environments. QTL that confer foliar and stem resistance frequently co-localized, suggesting that pleiotropy and/or tightly linked genes control the trait phenotypes. Other QTL exhibited isolate-specificity and QTL × environment interactions. Map-based comparisons between QTL mapped in this study and Solanaceae resistance genes/QTL detected in other published studies revealed multiple cases of co-location, suggesting conservation of gene function.