Does a vascular fungus of tomato induce a defence response or a change in host plant quality that also affects the oviposition of spider mites?

It has been suggested that previous infection by a vascular fungus causes induced resistance against two-spotted spider mites. To test the generality of this phenomenon, a series of experiments was carried out using two lines of tomato, differing only in resistance againstFusarium. In addition, tests were done in order to see whether the defense response against the fungus also affects the phytophagous mite directly. Inoculation of tomato plants with a vascular fungus (Fusarium oxysporum f.sp.lycopersici race 1) prior to infestation with spider mites caused a decrease in the rate of oviposition of two-spotted spider mites (Tetranychus urticae) on aFusarium-susceptible line, but only when plants were moderately to severely wilted. Spider mite oviposition did not change significantly of a previously inoculatedFusarium-resistant line.AsFusarium causes vascular occlusion and wilting of the plants, drought stress was experimentally induced to determine its influence on the reduction of oviposition. Drought caused a significant reduction in spider mite oviposition. We conclude that the effect of previousFusarium-inoculation on spider mite oviposition is primarily due to the fungus affecting the quality of the host plant (including the effect it may have on the composition of defensive compounds), rather than due to the stimulation of the defense system of the plant. SinceFusarium seals off the xylem vessels, thereby causing wilting of susceptible plants, the reduction in mite oviposition may well be due to drought stress in the leaves, rather than due to the production of phytoalexins.     

Host plant resistance among tomato accessions to the spider mite <Emphasis Type="Italic">Tetranychus evansi</Emphasis> in Kenya

The spider mite Tetranychus evansi has a broad range of host plants. Control of T. evansi has been a big challenge to tomato farmers due to its fast rate of reproduction, development of resistance to chemical pesticides and its ability to use weeds as alternative hosts when the tomato plants are not available. The aim of the current study was to determine the host plant acceptance and the relative contributions of trichomes in the control of the red spider mite by comparing the survival, development and oviposition rates of the red spider mite on eight tomato accessions. Leaflets from eight tomato varieties were assayed with the spider mites to determine the egg laying capacity and developmental time of the spider mites on the tomato accessions as well as the trichome densities. Densities of trichome types I, IV, V and VI varied among the tomato accessions. Variation in types I, IV and VI accounted for most of the variation in mite responses. The varieties with high densities of types IV and VI had the highest fecundity and mite development did not go beyond the larval stage. The developmental time varied significantly among the tomato accessions. The results indicated that the higher the density of trichome type I the lower the adult survival. The findings indicated possible resistance of some of the tested tomato accessions against T. evansi which is partially associated with trichomes types and density.     

Quantitative trait loci increasing acylsugars in tomato breeding lines and their impacts on silverleaf whiteflies

Solanum pennellii LA716, a wild relative of tomato, produces acylsugars, an insect resistance compound with activity against many tomato insect pests. Breeding of cultivated tomato using S. pennellii LA716 as a donor parent has led to the development of the elite acylsugar-producing tomato breeding line CU071026. CU071026 contains five introgressed S. pennellii genomic regions, and produces acylsugars at moderate levels that are effective against insect pests. A BC1F1 population was created by crossing the F1 CU071026?×?S. pennellii LA716 with CU071026 as the recurrent parent; this BC1F1 population was used to identify additional regions of the S. pennellii genome important for further improvement of acylsugar production. This population was genotyped with 94 markers in the segregating regions and phenotyped for level of acylsugar production. Using QTLNetwork 2.1 for the detection of quantitative trait loci (QTL) and epistatic interactions, this study identified five QTL for total acylsugar level. Additionally, two epistatic interactions between QTL were found to control significant levels of total acylsugar production. Two of the QTL identified were further evaluated in silverleaf whitefly (Bemisia tabaci) field cage trials using acylsugar breeding lines that differ for the presence/absence of these QTL. While high levels of silverleaf whitefly resistance were observed in all acylsugar breeding lines, lines containing the additional QTL on either chromosomes 6 or 10 had increased levels of total acylsugar production and reduced incidence of whitefly. Acylsugar lines containing the chromosome 6 QTL also had increased density of the type IV glandular trichomes which produce and exude acylsugars.     

Development of a mite bioassay to evaluate plant resistance and its use in determining regeneration of spider mite resistance

The zonal geranium (Pelargonium xhortorum) possesses tall glandular trichomes that secrete anacardic acids, a viscous, sticky exudate which has been suggested as the primary mechanism in two-spotted spider mite (Tetranychus urticae Koch) resistance. A new bioassay was developed using small Plexiglas^® cylinders as chambers for evaluating the resistance of geranium leaves to the two-spotted spider mite. This bioassay was easy to prepare, required only 24 hours to conduct, exhibited no problems with desiccation, condensation, or mite accountability, and yielded reproducible results. This bioassay was then used to study the regeneration of resistance of attached geranium leaves after they were made mite-susceptible by removing the excreted anacardic acids with water. Washed leaves regained full resistance after 14 days.     

Resistance of cultivated tomato to cell content-feeding herbivores is regulated by the octadecanoid-signaling pathway

The octadecanoid signaling pathway has been shown to play an important role in plant defense against various chewing insects and some pathogenic fungi. Here, we examined the interaction of a cell-content feeding arachnid herbivore, the two-spotted spider mite (Tetranychus urticae Koch), with cultivated tomato (Lycopersicon esculentum) and an isogenic mutant line (defenseless-1 [def-1]) that is deficient in the biosynthesis of the octadecanoid pathway-derived signal, jasmonic acid (JA). Spider mite feeding and fecundity on def-1 plants was significantly greater than on wild-type plants. Decreased resistance of def-1 plants was correlated with reduced JA accumulation and expression of defensive proteinase inhibitor (PI) genes, which were induced in mite-damaged wild-type leaves. Treatment of def-1 plants with methyl-JA restored resistance to spider mite feeding and reduced the fecundity of female mites. Plants expressing a 35S::prosystemin transgene that constitutively activates the octadecanoid pathway in a Def-1-dependent manner were highly resistant to attack by spider mites and western flower thrips (Frankliniella occidentalis), another cell-content feeder of economic importance. These findings indicate that activation of the octadecanoid signaling pathway promotes resistance of tomato to a broad spectrum of herbivores. The techniques of amplified fragment length polymorphism (AFLP) and bulk segregant analysis were used to map the Def-1 gene to a region on the long arm of chromosome 3 that is genetically separable from the map position of known JA biosynthetic genes. Tight linkage of Def-1 to a T-DNA insertion harboring the maize (Zea mays) Dissociation transposable element suggests a strategy for directed transposon tagging of the gene.     

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 F_2:3 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 F_2:3 mapping population, which allowed the identification of a new late blight resistant QTL in tomato.     

High-resolution analysis of a QTL for resistance to Stagonospora nodorum glume blotch in wheat reveals presence of two distinct resistance loci in the target interval

Stagonospora nodorum glume blotch (SNG), caused by the necrotrophic fungus Stagonospora nodorum, is one of the economically important diseases of bread wheat (Triticum aestivum L.). Resistance to SNG is known to be quantitative and previous studies of a recombinant inbred line (RIL) population identified a major quantitative trait locus (QTL) for resistance to SNG on the short arm of chromosome 3B. To localize this QTL (QSng.sfr-3BS) with high resolution, we constructed a genetic map for the QTL target region using information from sequenced flow-sorted chromosomes 3B of the two parental cultivars ‘Arina’ and ‘Forno’, the physical map of chromosome 3B of cultivar ‘Chinese Spring’ and BAC-clone sequences. The mapping population of near-isogenic lines (NIL) was evaluated for SNG resistance in field infection tests. NILs segregated for disease resistance as well as for plant height; additionally, we observed a high environmental influence on the trait. Our analysis detected a strong negative correlation of SNG resistance and plant height. Further analysis of the target region identified two linked loci associated with SNG resistance. One of them was also associated with plant height, revealing an effect of QSng.sfr-3BS on plant height that was hidden in the RIL population. This result demonstrates an unexpectedly high genetic complexity of resistance controlled by QSng.sfr-3BS and shows the importance of the study of QTL in mendelized form in NILs.     

An anchored linkage map for sugar beet based on AFLP,SNP and RAPD markers and QTL mapping of a new source of resistance to <Emphasis Type="Italic">Beet necrotic yellow vein virus</Emphasis>

Rhizomania, caused by Beet necrotic yellow vein virus (BNYVV), is an important sugar-beet disease worldwide and can result in severe losses of root yield and sugar content. We have identified a major QTL for BNYVV resistance from a new source in a segregating population of 158 individuals. The QTL explained an estimated 78% of the observed phenotypic variation and the gene conferring the partial resistance is referred to as Rz4. AFLP was used in combination with bulked segregant analysis (BSA) to develop markers linked to the resistance phenotype. AFLP marker analysis was extended to produce a linkage map that was resolved into nine linkage groups. These were anchored to the nine sugar-beet chromosomes using previously published SNP markers. This represents the first anchored sugar-beet linkage map to be published with non-anonymous markers. The final linkage map comprised 233 markers covering 497.2 cM, with an average interval between markers of 2.1 cM. The Rz4 QTL and an Rz1 RAPD marker were mapped to chromosome III, the known location of the previously identified BNYVV resistance genes Rz1, Rz2 and Rz3. The availability to breeders of new resistance sources such as Rz4 increases the potential for breeding durable disease resistance.     

Functional response of Nesidiocoris tenuis (Hemiptera: Miridae) to the two-spotted spider mite,Tetranychus urticae

We examined the functional response of Nesidiocoris tenuis to varying densities of the two-spotted spider mite, Tetranychus urticae and found that the data fit a Type II response, with the attack rate (a) increasing with increased predator size. N. tenuis can be considered as a potential biological control agent of this pest in tomato.     

QTL mapping for downy mildew resistance in cucumber via bulked segregant analysis using next-generation sequencing and conventional methods

Key message

QTL mapping using NGS-assisted BSA was successfully applied to an F ₂ population for downy mildew resistance in cucumber. QTLs detected by NGS-assisted BSA were confirmed by conventional QTL analysis.

Abstract

Downy mildew (DM), caused by Pseudoperonospora cubensis, is one of the most destructive foliar diseases in cucumber. QTL mapping is a fundamental approach for understanding the genetic inheritance of DM resistance in cucumber. Recently, many studies have reported that a combination of bulked segregant analysis (BSA) and next-generation sequencing (NGS) can be a rapid and cost-effective way of mapping QTLs. In this study, we applied NGS-assisted BSA to QTL mapping of DM resistance in cucumber and confirmed the results by conventional QTL analysis. By sequencing two DNA pools each consisting of ten individuals showing high resistance and susceptibility to DM from a F₂ population, we identified single nucleotide polymorphisms (SNPs) between the two pools. We employed a statistical method for QTL mapping based on these SNPs. Five QTLs, dm2.2, dm4.1, dm5.1, dm5.2, and dm6.1, were detected and dm2.2 showed the largest effect on DM resistance. Conventional QTL analysis using the F₂ confirmed dm2.2 (R ² = 10.8–24 %) and dm5.2 (R ² = 14–27.2 %) as major QTLs and dm4.1 (R ² = 8 %) as two minor QTLs, but could not detect dm5.1 and dm6.1. A new QTL on chromosome 2, dm2.1 (R ² = 28.2 %) was detected by the conventional QTL method using an F₃ population. This study demonstrated the effectiveness of NGS-assisted BSA for mapping QTLs conferring DM resistance in cucumber and revealed the unique genetic inheritance of DM resistance in this population through two distinct major QTLs on chromosome 2 that mainly harbor DM resistance.

     

QTL mapping of fruit mineral contents provides new chances for molecular breeding of tomato nutritional traits

Key message

Agronomical characterization of a RIL population for fruit mineral contents allowed for the identification of QTL controlling these fruit quality traits, flanked by co-dominant markers useful for marker-assisted breeding.

Abstract

Tomato quality is a multi-variant attribute directly depending on fruit chemical composition, which in turn determines the benefits of tomato consumption for human health. Commercially available tomato varieties possess limited variability in fruit quality traits. Wild species, such as Solanum pimpinellifolium, could provide different nutritional advantages and can be used for tomato breeding to improve overall fruit quality. Determining the genetic basis of the inheritance of all the traits that contribute to tomato fruit quality will increase the efficiency of the breeding program necessary to take advantage of the wild species variability. A high-density linkage map has been constructed from a recombinant inbred line (RIL) population derived from a cross between tomato Solanum lycopersicum and the wild-relative species S. pimpinellifolium. The RIL population was evaluated for fruit mineral contents during three consecutive growing seasons. The data obtained allowed for the identification of main QTL and novel epistatic interaction among QTL controlling fruit mineral contents on the basis of a multiple-environment analysis. Most of the QTL were flanked by candidate genes providing valuable information for both tomato breeding for new varieties with novel nutritional properties and the starting point to identify the genes underlying these QTL, which will help to reveal the genetic basis of tomato fruit nutritional properties.

     

Pesticidal activity of ingenane diterpenes isolated from Euphorbia kansui against Nilaparvata lugens and Tetranychus urticae

The methanol extract of dried roots of Euphorbia kansui, known as “Gan Sui” in oriental medicine, showed pesticidal activity against brown plant hopper (Nilaparvata lugens Stal) and two-spotted spider mite (Tetranychus urticae Koch). Bioassay-guided fractionation led to the isolation of two active compounds from the dried roots of E. kansui. The substances were identified as 3-O-(2,3-dimethylbutanoyl)-13-O-dodecanoylingenol (1) and 3-O-(2′E,4′Z-decadienoyl)-ingenol (2) by MS and NMR spectral data. Both compounds exhibited insecticidal activity against the brown plant hopper and compound 1 was active against the two-spotted spider mite. Compared to anise oil and eugenol, the two ingenane diterpenes showed greater activity against brown plant hopper. The results suggest that 3-O-(2,3-dimethylbutanoyl)-13-O-dodecanoylingenol and 3-O-(2′E,4′Z-decadienoyl)-ingenol could be used directly as natural pesticides or as lead principles for the control of brown plant hopper and two-spotted spider mite.     

Possible ecological consequences of heterospecific mating behavior in two tetranychid mites

Interspecific mating between the two-spotted spider mite,Tetranychus urticae Koch, and the Banks grass mite,Oligonychus pratensis (Banks), was documented using laboratory populations. The incidence of mating betweenT. urticae males andO. pratensis females was 26.0%, while that for the reciprocal mating was 18.8%. The incidence of mating was affected by both male and female species. Such matings may have several important ecological consequences. Interspecific matings resulted in all-male progenies. Thus, progeny sex ratios may be distorted by misdirected mating behavior. In addition, heterospecific mating resulted in lower fecundity than conspecific matings in the two-spotted spider mite, although not in the Banks grass mite. Aerial dispersal behavior of the two-spotted spider mite was also affected. Under crowded conditions and deteriorating resource quality, female mites exhibit an aerial dispersal posture that helps them to become airborne, and allows them to disperse long distances. Forty-two percent ofT. urticae females that mated with conspecific males exhibited this dispersal behavior, compared to only 3.6% for virgin females. The incidence of aerial dispersal behavior for females that mated with heterospecific males was intermediate (27.3%). The effects of these behavioral alterations on male and female fitness may depend on the population structure and resource distribution.     

Rapid identification of QTLs underlying resistance to <Emphasis Type="Italic">Cucumber mosaic virus</Emphasis> in pepper (<Emphasis Type="Italic">Capsicum frutescens</Emphasis>)

Key message

Next-generation sequencing enabled a fast discovery of QTLs controlling CMV resistant in pepper. The gene CA02g19570 as a possible candidate gene of qCmr2.1 was identified for resistance to CMV in pepper.

Abstract

Cucumber mosaic virus (CMV) is one of the most important viruses infecting pepper, but the genetic basis of CMV resistance in pepper is elusive. In this study, we identified a candidate gene for CMV resistance QTL, qCmr2.1 through SLAF-seq. Segregation analysis in F₂, BC₁ and F_2:3 populations derived from a cross between two inbred lines ‘PBC688’ (CMV-resistant) and ‘G29’ (CMV-susceptible) suggested quantitative inheritance of resistance to CMV in pepper. Genome-wide comparison of SNP profiles between the CMV-resistant and CMV-susceptible bulks constructed from an F₂ population identified two QTLs, designated as qCmr2.1 on chromosome 2 and qCmr11.1 on chromosome 11 for resistance to CMV in PBC688, which were confirmed by InDel marker-based classical QTL mapping in the F₂ population. As a major QTL, joint SLAF-seq and traditional QTL analysis delimited qCmr2.1 to a 330 kb genomic region. Two pepper genes, CA02g19570 and CA02g19600, were identified in this region, which are homologous with the genes LOC104113703, LOC104248995, LOC102603934 and LOC101248357, which were predicted to encode N-like protein associated with TMV-resistant in Solanum crops. Quantitative RT-PCR revealed higher expression levels of CA02g19570 in CMV resistance genotypes. The CA02g19600 did not exhibit obvious regularity in expression patterns. Higher relative expression levels of CA02g19570 in PBC688 and F₁ were compared with those in G29 during days after inoculation. These results provide support for CA02g19570 as a possible candidate gene of qCmr2.1 for resistance to CMV in pepper.

     

The Effects of Timing and Rates of Release of Phytoseiulus persimilis against Two-spotted Spider Mite Tetranychus urticae on Dwarf Hops

Single inoculative releases of the phytoseiid mite Phytoseiulus persimilis were made against the two-spotted spider mite, Tetranychus urticae, on two varieties of dwarf hops in 1996 and 1997 at means of 20, 10, 5, 2.5 and nil per plant, and at up to three timings. The numbers of spider mites recorded on leaves after the predators were released were related inversely to the rates of release. The earliest releases of the predator maintained spider mites at lower population densities than did those made later in the year. In all treatments the numbers of spider mites decreased when the prey:predator ratio reached approximately 10:1.     

Herbivory-associated degradation of tomato trichomes and its impact on biological control of Aculops lycopersici

Tomato plants have their leaves, petioles and stems covered with glandular trichomes that protect the plant against two-spotted spider mites and many other herbivorous arthropods, but also hinder searching by phytoseiid mites and other natural enemies of these herbivores. This trichome cover creates competitor-free and enemy-free space for the tomato russet mite (TRM) Aculops lycopersici (Acari: Eriophyidae), being so minute that it can seek refuge and feed inbetween the glandular trichomes on tomato cultivars currently used in practice. Indeed, several species of predatory mites tested for biological control of TRM have been reported to feed and reproduce when offered TRM as prey in laboratory experiments, yet in practice these predator species appeared to be unable to prevent TRM outbreaks. Using the phytoseiid mite, Amblydromalus limonicus, we found exactly the same, but also obtained evidence for successful establishment of a population of this predatory mite on whole plants that had been previously infested with TRM. This successful establishment may be explained by our observation that the defensive barrier of glandular plant trichomes is literally dropped some time after TRM infestation of the tomato plants: the glandular trichome heads first rapidly develop a brownish discoloration after which they dry out and fall over onto the plant surface. Wherever TRM triggered this response, predatory mites were able to successfully establish a population. Nevertheless, biological control was still unsuccessful because trichome deterioration in TRM-infested areas takes a couple of days to take effect and because it is not a systemic response in the plant, thereby enabling TRM to seek temporary refuge from predation in pest-free trichome-dense areas which continue to be formed while the plant grows. We formulate a hypothesis unifying these observations into one framework with an explicit set of assumptions and predictions to be tested in future experiments.     

Multi-environment QTL mapping reveals genetic architecture of fruit cracking in a tomato RIL <Emphasis Type="Italic">Solanum lycopersicum</Emphasis> × <Emphasis Type="Italic">S. pimpinellifolium</Emphasis> population

Key message

QTL and codominant genetic markers for fruit cracking have been identified in a tomato genetic map derived from a RIL population, providing molecular tools for marker-assisted breeding of this trait.

Abstract

In tomato, as well as in other fleshy fruits, one of the main disorders that widely limit quality and production is fruit cracking or splitting of the epidermis that is observed on the fruit skin and flesh at any stage of fruit growth and maturation. To elucidate the genetic basis of fruit cracking, a quantitative trait loci (QTL) analysis was conducted in a recombinant inbred line (RIL) population derived from a cross between tomato (Solanum lycopersicum) and the wild-relative species S. pimpinellifolium. The RIL population was evaluated for fruit cracking during three consecutive growing seasons. Construction of a high-density linkage map based on codominant markers, covering more than 1000 cM of the whole genome, led to the identification of both main and epistatic QTL controlling fruit cracking on the basis of a single-environment as well as multiple-environment analysis. This information will enhance molecular breeding for novel cracking resistant varieties and simultaneously assist the identification of genes underlying these QTL, helping to reveal the genetic basis of fruit cracking in tomato.

     

Dissection and fine mapping of a major QTL for preharvest sprouting resistance in white wheat Rio Blanco

Preharvest sprouting (PHS) is a major constraint to white wheat production. Previously, we mapped quantitative trait loci (QTL) for PHS resistance in white wheat by using a recombinant inbred line (RIL) population derived from the cross Rio Blanco/NW97S186. One QTL, QPhs.pseru-3A, showed a major effect on PHS resistance, and three simple sequence repeat (SSR) markers were mapped in the QTL region. To determine the flanking markers for the QTL and narrow down the QTL to a smaller chromosome region, we developed a new fine mapping population of 1,874 secondary segregating F₂ plants by selfing an F6 RIL (RIL25) that was heterozygous in the three SSR marker loci. Segregation of PHS resistance in the population fitted monogenic inheritance. An additive effect of the QTL played a major role on PHS resistance, but a dominant effect was also observed. Fifty-six recombinants among the three SSR markers were identified in the population and selfed to produce homozygous recombinants or QTL near-isogenic lines (NIL). PHS evaluation of the recombinants delineated the QTL in the region close to Xbarc57 flanked by Xbarc321 and Xbarc12. To saturate the QTL region, 11 amplified fragment length polymorphism (AFLP) markers were mapped in the QTL region with 7 AFLP co-segregated with Xbarc57 by using the NIL population. Dissection of the QTL as a Mendelian factor and saturation of the QTL region with additional markers created a solid foundation for positional cloning of the major QTL.