Controlling Disease Resistance Research Articles

Quantitative Trait Analysis Shows the Potential for Alleles from the Wild Species Arachis batizocoi and A. duranensis to Improve Groundnut Disease Resistance and Yield in East Africa

Diseases are the most important factors reducing groundnut yields worldwide. In East Africa, late leaf spot (LLS) and groundnut rosette disease (GRD) are the most destructive diseases of groundnut. Limited resistance is available in pure pedigree cultivated groundnut lines and novel sources of resistance are required to produce resistant new varieties. In this work, 376 interspecific lines from 3 different populations derived from crosses with the wild species A. duranensis, A. ipaënsis, A. batizocoi and A. valida were phenotyped for 2 seasons and across 2 locations, Serere and Nakabango, in Uganda. Several genotypes showed a higher yield, a larger seed, an earlier flowering, and similar resistance to the local cultivar checks. Genotypic data was used to construct a linkage map for the AB-QTL population involving the cross between Fleur11 and [A. batizocoi x A. duranensis]4x. This linkage map, together with the phenotypic data was used to identify quantitative trait loci controlling disease resistance. These lines will be useful in combining good agronomic traits and stacking disease resistance to improve the groundnut crop in sub-Saharan Africa.

A reference-anchored oat linkage map reveals quantitative trait loci conferring adult plant resistance to crown rust (Puccinia coronata f. sp. avenae).

We mapped three adult plant resistance (APR) loci on oat chromosomes 4D and 6C and developed flanking KASP/PACE markers for marker-assisted selection and gene pyramiding. Using sequence orthology search and the available oat genomic and transcriptomic data, we surveyed these genomic regions for genes that may control disease resistance. Sources of durable disease resistance are needed to minimize yield losses in cultivated oat caused by crown rust (Puccinia coronata f. sp. avenae). In this study, we developed five oat recombinant inbred line mapping populations to identify sources of adult plant resistance from crosses between five APR donors and Otana, a susceptible variety. The preliminary bulk segregant mapping based on allele frequencies showed two regions in linkage group Mrg21 (Chr4D) that are associated with the APR phenotype in all five populations. Six markers from these regions in Chr4D were converted to high-throughput allele specific PCR assays and were used to genotype all individuals in each population. Simple interval mapping showed two peaks in Chr4D, named QPc.APR-4D.1 and QPc.APR-4D.2, which were detected in the OtanaA/CI4706-2 and OtanaA/CI9416-2 and in the Otana/PI189733, OtanaD/PI260616, and OtanaA/CI8000-4 populations, respectively. These results were validated by mapping two entire populations, Otana/PI189733 and OtanaA/CI9416, genotyped using Illumina HiSeq, in which polymorphisms were called against the OT3098 oat reference genome. Composite interval mapping results confirmed the presence of the two quantitative trait loci (QTL) located on oat chromosome 4D and an additional QTL with a smaller effect located on chromosome 6C. This mapping approach also narrowed down the physical intervals to between 5 and 19Mb, and indicated that QPc.APR-4D.1, QPc.APR-4D.2, and QPc.APR-6C explained 43.4%, 38.5%, and 21.5% of the phenotypic variation, respectively. In a survey of the gene content of each QTL, several clusters of disease resistance genes that may contribute to APR were found. The allele specific PCR markers developed for these QTL regions would be beneficial for marker-assisted breeding, gene pyramiding, and future cloning of resistance genes from oat.

Rice extra-large G proteins play pivotal roles in controlling disease resistance and yield-related traits.

To better explore the potential of rice extra-large G (XLG) proteins in future breeding, we characterised the function of OsXLG1, OsXLG2 and OsXLG3 in disease resistance. Loss-of-function Osxlg2 and Osxlg3 mutants showed reduced resistance to the fungal pathogen Magnaporthe oryzae, whereas Osxlg1 mutants were specifically compromised in resistance to the bacterial pathogen Xanthomonas oryzae pv oryzae. Consistent with their effects on rice blast resistance, mutations in OsXLG2 and OsXLG3 caused greater defects than did mutations in OsXLG1 for chitin-induced defence responses. All three OsXLGs interacted with components of a surface immune receptor complex composed of OsCERK1, OsRLCK176 and OsRLCK185. Further characterisation of yield-related traits showed that the Osxlg3 mutants displayed reduced plant height, panicle length and 1000grain weight, whereas Osxlg1 mutants exhibited increased plant height, panicle length and 1000-grain weight. Together the study shows the differential contributions of the three OsXLG proteins to disease resistance to fungal and bacterial pathogens, their yield-related traits and provides insights for future improvement of rice production.

Sequencing and analysis of gerbera daisy leaf transcriptomes reveal disease resistance and susceptibility genes differentially expressed and associated with powdery mildew resistance

BackgroundRNA sequencing has been widely used to profile genome-wide gene expression and identify candidate genes controlling disease resistance and other important traits in plants. Gerbera daisy is one of the most important flowers in the global floricultural trade, and powdery mildew (PM) is the most important disease of gerbera. Genetic improvement of gerbera PM resistance has become a crucial goal in gerbera breeding. A better understanding of the genetic control of gerbera resistance to PM can expedite the development of PM-resistant cultivars.ResultsThe objectives of this study were to identify gerbera genotypes with contrasting phenotypes in PM resistance and sequence and analyze their leaf transcriptomes to identify disease resistance and susceptibility genes differentially expressed and associated with PM resistance. An additional objective was to identify SNPs and SSRs for use in future genetic studies. We identified two gerbera genotypes, UFGE 4033 and 06–245-03, that were resistant and susceptible to PM, respectively. De novo assembly of their leaf transcriptomes using four complementary pipelines resulted in 145,348 transcripts with a N50 of 1124 bp, of which 67,312 transcripts contained open reading frames and 48,268 were expressed in both genotypes. A total of 494 transcripts were likely involved in disease resistance, and 17 and 24 transcripts were up- and down-regulated, respectively, in UFGE 4033 compared to 06–245-03. These gerbera disease resistance transcripts were most similar to the NBS-LRR class of plant resistance genes conferring resistance to various pathogens in plants. Four disease susceptibility transcripts (MLO-like) were expressed only or highly expressed in 06–245-03, offering excellent candidate targets for gene editing for PM resistance in gerbera. A total of 449,897 SNPs and 19,393 SSRs were revealed in the gerbera transcriptomes, which can be a valuable resource for developing new molecular markers.ConclusionThis study represents the first transcriptomic analysis of gerbera PM resistance, a highly important yet complex trait in a globally important floral crop. The differentially expressed disease resistance and susceptibility transcripts identified provide excellent targets for development of molecular markers and genetic maps, cloning of disease resistance genes, or targeted mutagenesis of disease susceptibility genes for PM resistance in gerbera.

Phosphate excess increases susceptibility to pathogen infection in rice.

Phosphorus (P) is an essential nutrient for plant growth and productivity. Due to soil fixation, however, phosphorus availability in soil is rarely sufficient to sustain high crop yields. The overuse of fertilizers to circumvent the limited bioavailability of phosphate (Pi) has led to a scenario of excessive soil P in agricultural soils. Whereas adaptive responses to Pi deficiency have been deeply studied, less is known about how plants adapt to Pi excess and how Pi excess might affect disease resistance. We show that high Pi fertilization, and subsequent Pi accumulation, enhances susceptibility to infection by the fungal pathogen Magnaporthe oryzae in rice. This fungus is the causal agent of the blast disease, one of the most damaging diseases of cultivated rice worldwide. Equally, MIR399f overexpression causes an increase in Pi content in rice leaves, which results in enhanced susceptibility to M. oryzae. During pathogen infection, a weaker activation of defence‐related genes occurs in rice plants over‐accumulating Pi in leaves, which is in agreement with the phenotype of blast susceptibility observed in these plants. These data support that Pi, when in excess, compromises defence mechanisms in rice while demonstrating that miR399 functions as a negative regulator of rice immunity. The two signalling pathways, Pi signalling and defence signalling, must operate in a coordinated manner in controlling disease resistance. This information provides a basis to understand the molecular mechanisms involved in immunity in rice plants under high Pi fertilization, an aspect that should be considered in management of the rice blast disease.

Involvement of Jasmonic Acid in the Induced Systemic Resistance of Tomato against Ralstonia syzigiisub sp. indonesiensis by Indigenous Endophyte Bacteria

Endophyte bacteria colonize plant roots and exert a beneficial effect on plant growth are well known for their potential to reduce plant pathogen populations in the soil, thereby suppressing diseases. Elucidation of signaling pathways controlling disease resistance is a major objective in research on plant-pathogen interactions. It has been suggested that jasmonic acid (JA) could be an integral part of a general signal transduction system regulating inducible defense genes in plants. Recent studies had shown that Endophyte bacteria could elicited induce systemic resistance (ISR) of plants by jasmonic acid pathway. This research purposed to study the involvement of JA in ISR of tomato plants against Ralstonia syzigiisubsp. Indonesiensis by indigenous endophyte bacteria. This research assayed the JA contained in the leaves and roots of tomato plants that treated with indigenous endophyte bacteria (seed and seedlings treatment) and inoculated with R. syzigiisub sp. Indonesiensis with Gas Chromatography-Mass Spectrophotometry (GC-MS) in interval between 0 to 30 days after pathogen innoculation. Results shown that all selected indigenous endophyte bacteria can suppress R. syzigiisub sp. Indonesiensis attack and increase JA contained in leaves and roots of tomato significantly until 12 days after pathogen innoculations. Strain Bacillus cereus EPL1.1.3 and Serratianematodiphila TLE1.1 respectively had the highest JA activivity both in roots and leaves of tomato. This indicated that one of the selected indigenous endophyte bacteria abilities to suppress pathogens attack mechanism related to JA pathway.

Genome-Wide Association Studies Identified Resistance Loci to Orange Rust and Yellow Leaf Virus Diseases in Sugarcane (Saccharum spp.).

Sugarcane (Saccharum spp.) supplies globally ∼80% of table sugar and 60% of bioethanol. Sugarcane orange rust and Sugarcane yellow leaf virus (SCYLV) are major sugarcane diseases, causing up to 50 and 40% yield losses, respectively. Sugarcane cultivars resistant to these diseases are needed to sustain sugarcane production in several regions. Dissecting DNA sequence variants controlling disease resistance provides a valuable tool for fulfilling a breeding strategy to develop resistant cultivars. In this study, we evaluated disease reactions to orange rust and SCYLV of a sugarcane diversity panel in repeated trials. We conducted a genome-wide association study between high-density markers and disease resistance reactions. We identified 91 putative DNA markers and 82 candidate genes significantly associated with resistance to one of the two diseases. These provide an important genetic resource for finding genes and molecular markers for disease resistance. Our results emphasized the importance of utilizing a wide germplasm collection for breeding resistant sugarcane cultivars.

Introgression and inheritance of charcoal rot (Macrophomina phaseolina) resistance from silver sunflower (Helianthus argophyllus Torr. & A. Gray) into cultivated sunflower (Helianthus annuus L.)

Charcoal rot is a devastating disease of sunflower causing significant yield losses. There is a lack of high-level resistance in cultivated germplasm for this disease. The purpose of this study was to incorporate resistance against charcoal rot into cultivated sunflower lines. The study indicated that sunflower elite cultivated germplasm was susceptible which showed complete stunting of growth and wilting when compared with wild accessions of Helianthus argophyllus. H. argophyllus was identified as immune for charcoal rot and has the potential to diversify the initial source of disease resistance in sunflower elite germplasm. The segregating ratios in the F2 generation showed the presence of two major genes having an epistatic interaction to control immunity while a single gene was found to control disease resistance or partial resistance. Stem color showed the highest correlation with disease scoring which may be used to differentiate resistant and susceptible plant progenies. The F3 progenies selected on the basis of this trait resulted in the selection of all types of resistance in the F4 generation with an immune class having a minimal reduction in or deterioration of morphological traits. Stem color also correlated with stem toughness (r = 0.46**), and as a result, plant genotypes with tough stems were selected. The selected progenies will be registered as a source of charcoal rot resistance.

Inheritance of resistance to downy mildew by F1 and F2 sunflower hybrids

Inheritance of resistance to downy mildew by F1 and F2 sunflower hybrids

Study of resistance to leaf and stem rust in Triticum aestivum–Aegilops speltoides lines

Presently, the use of bread wheat introgression lines resistant to pathogens in practical breeding is hampered by the lack of their cytogenetic characteristics, data on the genetic control of disease resistance, and influence of alien genetic material on the grain productivity and quality. For the solution of these problems, two wheat–Aegilops speltoides lines, L195 and L200, developed at ARISER and resistant to leaf and stem rust were studied. These lines were produced by crossing spring bread wheat cultivars with line L26b-4. Cytogenetic analysis of the lines involved C-banding, meiotic analyses, and FISH with pAs1 and Fat showed that the rust resistance genes, efficient against both rust types, to be mapped to the 2D-2S translocation in both lines. Genetic analysis revealed tight linkage of leaf rust resistance genes from Ae. speltoides to gametocidal genes and the absence of susceptible plants from the F2 hybrids and subsequent generations. Exceptions were found only in hybrid combinations with lines L2032 and L583: occasional susceptible plants were noted in the F2 and subsequent generations. Evaluation of lines L195 and L200 revealed high resistance to Ug99 + Lr24 (TTKST) and a local Saratov population of stem rust. The prebreeding studies of lines L195 and L200 showed their benefits in breeding for grain productivity in comparison to the recipient cultivar L503 and good bread-making quality. Due to the complex of agronomical traits and high resistance to leaf and stem rust, lines L195 and L200 can be considered promising donors for commercial bread wheat breeding.

Chicken major histocompatibility complex polymorphism and its association with production traits.

Major histocompatibility complex (MHC) is the best characterized genetic region controlling disease resistance and immune responses in chicken. MHC genes are also involved in various non-immune functions such as productive traits and reproductive success. The genetic diversity of MHC in an Iranian indigenous chicken (Khorasan) was studied, and association of the MHC alleles with production traits was determined. The MHC polymorphism was ascertained by genotyping the LEI0258 microsatellite locus by PCR-based fragment analysis. LEI0258 microsatellite marker is a genetic indicator for MHC, which is located on microchromosome 16 and strongly associated with serologically defined MHC haplotypes. A total of 25 different LEI0258 alleles (185-493 bp) and 76 genotypes were identified in 313 chickens. An allele of 361 bp had the highest frequency (26.44%), and alleles of 207 and 262 bp had the lowest (0.16%). High level of heterozygosity (87%) and good genotype frequency fit to the Hardy-Weinberg equilibrium was observed in this population (P = 0.238). The association study also revealed a significant influence of MHC alleles on body weight, egg weight, egg laying intensity, and weight of sexual maturity in Khorasan population (P < 0.05). The information obtained from this study indicates a high MHC genetic diversity and the association of MHC alleles with important production traits in Khorasan chicken. These data would be applicable in designing breeding and genetic resource conservation for indigenous chicken populations.

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.

English

Verticillium wilt disease causes significant losses to cotton yield and varietal development for resistance is critical in combating this threat. A study was carried out to determine the underlying genetic pattern controlling disease resistance and identify suitable parental lines to use in varietal tolerance development. Five cotton varieties were crossed in a half diallel mating system to produce ten crosses and five selfed parental lines which were screened against Verticillium wilt by artificial inoculation. There were significant variations on the severity scores, vascular brown index scores, morphological and agronomical traits under Verticillium wilt pressure. The Verticillium wilt severity scores ranged from 1.22 to 3.07 and Vascular Brown Index scores from 0.78 to 2.77. The mean squares of general combining ability (GCA) for the various characteristics of the parents were significant and also for the specific combining ability (SCA) of the vascular brown index score which was 0.27 was also significant. Three parental lines were identified as breeding material with good GCA, morphological and agronomical performance and these were CRI-MS-1, SZ9314 and BC853. This study implies that varietal tolerance can be developed by incorporating genotypes with resistance genes in breeding programmes. Future work should focus on developing resistant varieties suitable for production in Verticillium wilt prone areas. &nbsp; Key words: Verticillium wilt, general combining ability, specific combing ability, cotton.&nbsp

First Report of Foxtail Millet Seedling Damping-Off Caused by Binucleate Rhizoctonia AG-A in China.

Foxtail millet (Setaria italica) is an important food and fodder grain crop in China. In June 2013, foxtail millet seedlings of cultivar Jigu19 showing symptoms of damping-off were observed with about 30% incidence in Handan, Hebei Province. Infected plants showed pale brown lesions on the stems and brown discoloration of the roots, which later turned into root rot. As a consequence, infected seedlings wilted and died prematurely. Segments of the diseased root tissue (5 mm long) were washed with sterile water, disinfected with 0.5% NaOCl for 2 min, rinsed with sterile water, placed on potato dextrose agar (PDA), and incubated at 25°C in the dark. Three isolates were obtained and designated HD-1, HD-3, and HD-5. Morphological characteristics of the three isolates were similar with white colonies bearing large amounts of floccose aerial hyphae and no production of sclerotia after 14 days. Hyphal cells were stained with DAPI and all the three isolates were binucleate. Total genomic DNA was extracted from the mycelial mat using CTAB (4). The rDNA internal transcribed spacer (ITS) region was amplified using the universal primer pair ITS1 and ITS4 (1). PCR amplicons were purified and sequenced. A BLASTn search revealed that the resulting sequences (GenBank Accession Nos. KM017960, KM017961, and KM017962 for HD-1, HD-3, and HD-5, respectively) shared 99% identity with other Ceratobasidium sp. AG-A isolates (JX913824, FJ440197). In addition, sequence identity of HD-1, HD-3, and HD-5 with each other was 99.8%, 99.7%, and 99.8%, respectively. Thus, the isolates were identified as Ceratobasidium sp. AG-A, i.e., binucleate Rhizoctonia (BNR) AG-A. Pathogenicity tests were conducted by placing autoclaved wheat seeds, each colonized with Rhizoctonia isolates at inoculum density of 25 propagules per gram (ppg), on the surface of a sterilized mix of soil, sand, and nutrient soil (1:1:1, v/v/v) in pots (3). Each pot contained five healthy seedlings of the foxtail millet cultivar Yugu 1, and every seedling was inoculated by placing three colonized wheat seeds under the sheath of foxtail millet at the 2- or 3-leaf stage; non-inoculated sterilized wheat seeds were used as a control. Plants were incubated at 25°C with 14 h light and 10 h dark in a growth room for 10 days, and then assessed for disease. Damping-off symptoms similar to those in the field appeared on inoculated plants; control plants were asymptomatic. BNR Rhizoctonia were re-isolated from diseased plants and confirmed to be AG-A based on morphological characteristics and rDNA-ITS sequence. BNR AG-A has been reported in China as a pathogen of Chinese mustard, Chinese cabbage, potato, and sugar beet, but there are no previous reports of its presence on foxtail millet. R. solani AG-1 and AG-4 are usually regarded as pathogenic to foxtail millet (2). To our knowledge, this is the first report of BNR AG-A causing foxtail millet damping-off in China. We found binucleate Rhizoctonia AG-A, the anamorph of Ceratobasidium sp., can infect foxtail millet at seeding stage, which may cause serious losses of the crop. Therefore, we should pay particular attention to it when considering seeding disease control and breeding disease resistance varieties of foxtail millet in the future.

Newly developed SNP markers related to genes for leaf morphological traits and disease resistance in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

Based on the sequences of Brassica rapa ssp. pekinensis, we carried out a single-nucleotide polymorphism (SNP) study for genes controlling disease resistance (clubroot, turnip mosaic virus and soft rot) and leaf traits in B. rapa leafy genotypes ‘VC40’ and ‘SR5’. In total, 7645 SNP markers were obtained based on the annotation of a B. rapa database for disease resistance. Among these 7645 SNP markers, 141 were related to 125 genes linked to leaf traits. From these 141 SNP markers, 63 were screened and High Resolution Melt (HRM) primers were designed for genomic studies. A total of 20 polymorphic SNP primers were finally obtained. Eventually, these markers will be further used for the detection of quantitative trait loci and mapping studies.

Tyrosine phosphorylation of monocyte-derived macrophage proteins in buffalo (Bubalus bubalis): A potential phenotype of natural resistance

The aim of this work was to explore the possibility of using the presence of tyrosine-phosphorylated macrophage proteins as a phenotype of natural resistance. Tyrosine-phosphorylation of macrophage proteins was investigated in 18 buffaloes, that carried either the resistant, or the non-resistant, Natural Resistance-Associated Macrophage Protein one (NRAMP1) genotype, that various authors have associated with susceptibility to intracellular bacterial diseases. Monocyte-derived macrophages were Interferon-gamma (IFN-γ) stimulated and tyrosine-phosphorylation was assessed by Western blotting. Evidence of phosphorylation after IFN-γ stimulation was shown by 75% of the buffaloes carriers of the resistant genotype, and by 20% of the carriers of the non-resistant genotype (Chisquare value between the groups = 5.44; P = 0.02). The study of the Proteoma of monocyte-derived macrophages might open the way to the genetic control of disease resistance.

Phytopathogen Effectors Subverting Host Immunity: Different Foes, Similar Battleground

Phytopathogenic bacteria, fungi, and oomycetes invade and colonize their host plants through distinct routes. These pathogens secrete diverse groups of effector proteins that aid infection and establishment of different parasitic lifestyles. Despite this diversity, a comparison of different plant-pathogen systems has revealed remarkable similarities in the host immune pathways targeted by effectors from distinct pathogen groups. Immune signaling pathways mediated by pattern recognition receptors, phytohormone homeostasis or signaling, defenses associated with host secretory pathways and pathogen penetrations, and plant cell death represent some of the key processes controlling disease resistance against diverse pathogens. These immune pathways are targeted by effectors that carry a wide range of biochemical functions and are secreted by completely different pathogen groups, suggesting that these pathways are a common battleground encountered by many plant pathogens.

Inheritance of resistance to anthracnose stalk rot (Colletotrichum graminicola) in tropical maize inbred lines

Generation means was used to study the mode of inheritance of resistance to anthracnose stalk rot in tropical maize. Each population was comprised of six generations in two trials under a randomized block design. Inoculations were performed using a suspension of 10(5) conidia mL-1 applied into the stalk. Internal lesion length was directly measured by opening the stalk thirty days after inoculation. Results indicated contrasting modes of inheritance. In one population, dominant gene effects predominated. Besides, additive x dominant and additive x additive interactions were also found. Intermediate values of heritability indicated a complex resistance inheritance probably conditioned by several genes of small effects. An additive-dominant genetic model sufficed to explain the variation in the second population, where additive gene effects predominated. Few genes of major effects control disease resistance in this cross. Heterosis widely differed between populations, which can be attributed to the genetic background of the parental resistant lines.

The Major Histocompatibility Complex in Bovines: A Review

Productivity in dairy cattle and buffaloes depends on the genetic factors governing the production of milk and milk constituents as well as genetic factors controlling disease resistance or susceptibility. The immune system is the adaptive defense system that has evolved in vertebrates to protect them from invading pathogens and also carcinomas. It is remarkable in the sense that it is able to generate an enormous variety of cells and biomolecules which interact with each other in numerous ways to form a complex network that helps to recognize, counteract, and eliminate the apparently limitless number of foreign invading pathogens/molecules. The major histocompatibility complex which is found to occur in all mammalian species plays a central role in the development of the immune system. It is an important candidate gene involved in susceptibility/resistance to various diseases. It is associated with intercellular recognition and with self/nonself discrimination. It plays major role in determining whether transplanted tissue will be accepted as self or rejected as foreign.

Construction of a genetic linkage map with SSR, AFLP and morphological markers to locate QTLs controlling pathotype-specific powdery mildew resistance in diploid roses

Disease resistance is a sought-after trait in plant breeding programmes. One strategy to make resistance more durable is to increase the number of resistance genes, thereby increasing the number of pathotypes withstood. One of the most important diseases on roses is powdery mildew (PM) (Podosphaera pannosa). Recent studies show that pathotypes of PM and different types of resistances in roses exist. The results of this study aim to contribute to PM resistance in roses by the development of pathotype-specific markers on a genetic map. A diploid rose population (90 genotypes) derived from a cross between Rosa wichurana and Rosa ‘Yesterday’ was used to construct a genetic linkage map encompassing 20 AFLP primer combinations, 43 SSR, and 2 morphological markers. By applying the F1 pseudo test cross population strategy, two parental linkage maps were constructed (parent ‘Yesterday’ 536 cM; parent R. wichurana 526 cM). Both parental maps consisted of seven linkage groups with an average length of 70 cM (Kosambi) corresponding to the seven haploid rose chromosomes. These new maps were used to identify QTLs controlling disease resistance. The offspring population was screened for resistance to two PM pathotypes, R–E and R–P. QTLs for controlling pathotype-specific disease resistance were mapped by applying Kruskal–Wallis rank-sum tests and simple interval mapping. With two pathotypes analysed, nine QTL loci were detected on linkage groups 2, 3, 5 and 6, explaining 15–73% of the phenotypic variance for pathotype-specific disease response. The genetic maps developed here will be useful for future rose breeding, pathotype-specific resistance research and development of a consensus map for roses.