Browsing by Subject "Resistenzzüchtung"

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Analyzing resistance to ergot caused by Claviceps purpurea [Fr.] Tul. and alkaloid contamination in winter rye (Secale cereale L.)
(2022) Kodisch, Anna; Miedaner, Thomas
Ergot caused by Claviceps purpurea [Fr.] Tul. is one of the oldest well-known plant diseases leading already in medieval times to severe epidemic outbreaks. After the occurrence of honeydew, the characteristic ergot bodies called sclerotia are formed on the ear. These are containing toxic ergot alkaloids (EAs). Strict limits are set within the European Union. Rye (Secale cereale L.) as cross-pollinating crop is particularly vulnerable to ergot since the competitive situation of fungal spores and pollen during flowering. Nevertheless, even today the threat is real as agricultural practice is changing and screening studies revealed EAs in samples of the whole cereal value chain frequently. The aims were to establish a harmonized method to test ergot resistance and EA contamination in winter rye, to clarify major significant factors and their relevance and to reveal the suitability of one commercial ELISA test. Further, effort was paid to examine the covariation of ergot amount and EA content considering different factors because of prospective legislative changes. Genotypes showed significant variation for ergot severity and pollen-fertility restoration after natural infection as well as artificial inoculation whereas a high positive correlation could be found between both treatments. Additionally, variances of environment, general combining ability (GCA), specific combining ability (SCA), and interactions were significant. Although male pollen-fertility restoration was of utmost importance, the female component was also significant. This illustrates that apart from promising selection of high restoration ability the maternal restorability could be exploited in future breeding programs especially when a high pollen amount is already reached. A large-scale calibration study was performed to clarify the covariation of ergot severity, EA content (HPLC, ELISA) considering genotypes, locations, countries, years, and isolates. EA profile was rather stable across country-specific isolates although large differences regarding the EA content were detected. The moderate covariation between ergot severity and EA content (HPLC) indicates that a reliable prediction of the EA content based on ergot severity is not possible what can also not be explained by grouping effects of the factors. Further, EAs seem not to act as virulence factor in the infection process since EA content showed no relationship to disease severity. Additionally, the missing correlation of ELISA and HPLC leads to the conclusion that the ELISA is not an appropriate tool what can be used safely to screen samples regarding ergot in the daily life. The genetic variation of male-sterile CMS-single crosses was analysed in a special design without pollen in field and greenhouse to identify resistance mechanisms and to clarify whether ergot can be reduced in the female flower. At this, comparison of needle and spray inoculation revealed medium to high correlations illustrating that both methods were suitable for this research. Significant environment and genotype by environment interaction variances were detected. So, testing across several environments is necessary also without pollen. Further, small but significant genotypic variation and identification of one more ergot-resilient candidate revealed that selection of female lines could be promising to further reduce ergot. The EA content was lower for less susceptible genotypes. Thus, EA content can be considerably reduced by breeding. A strong positive correlation could be found for ergot severity and EA content when analysing 15 factorial single crosses. The male pollen-fertility restoration was also here the most relevant component but the female component contributed an obviously higher proportion for the EA content than for ergot severity. In conclusion, this thesis demonstrate that implementing of a high and environmental stable male fertility restoration ability via exotic Rf genes can effectively reduce ergot although also the female restorability enables great opportunities. The unpredictable covariation between ergot amount and EA content illustrates that both traits have to be assessed, in particular the EA content by a valid HPLC approach to guarantee food and feed safety.
Effiziente Verfahren für die Züchtung neuer Erdbeersorten (Fragaria x ananassa Duch.)
(2015) Bestfleisch, Markus; Wünsche, Jens Norbert
In this thesis, different approaches aimed on efficient breeding of new strawberry cultivars were investigated that comprise horticultural traits such as earliness and yield potential as well as resistance traits against two of the most important pathogens in strawberry cultivation, Botrytis cinerea and Xanthomonas fragariae. High yield potential and an extended ripening period in combination with disease resistance form the basis for economic success and contribute to a competitive and sustainable strawberry cultivation in Germany. In a traditional diallel cross breeding experiment, a set of 13 strawberry cultivars were crossed in a reciprocal way. The crossings resulted in a total of 144 F1-populations which were evaluated in a field trial over two years with regards to their horticultural traits in order to investigate the general and specific combining abilities of the parental cultivars concerning the traits earliness and marketable yield. Within this thesis, a statistical model was developed to explain the components of genetic variance and the breeding value of the parental cultivars based on their calculated combining abilities. It was demonstrated that there is no reciprocal effect on the progeny and it is practically irrelevant whether a cultivar is used as mother or father in the crossing experiment. The genetic variance in the breeding experiment is mainly based on the general combining ability (GCA) of the parental cultivars (additive effects). Specific and reciprocal combining abilities (non-additive effects) appeared less relevant. These findings are taken into account for the further development of the breeding approach which has also a main focus on resistance towards the grey mold disease and the angular leaf spot disease. The grey mold disease, caused by the necrotrophic fungus Botrytis cinerea Pers. [teleomorph Botryotinia fuckeliana (de Bary) Whetzel], is the most important fungal disease in strawberries and requires frequent applications of chemical plant protection products. The angular leafspot disease, caused by Xanthomonas fragariae Kennedy & King, is the most important bacterial disease in strawberry cultivation worldwide and there are currently no plant protection products available within the EU for an effective control of the disease. In a successful control strategy, the cultivation of resistant cultivars is of fundamental importance and the results of this thesis will contribute to the development of a targeted breeding approach. Initially, resistance tests were established for both diseases. Reproducible results concerning the resistance characteristics of different cultivars were achieved in artificial inoculation experiments and adapted evaluation scales. With this approach, more than 100 strawberry cultivars, wild-types and breeding clones from the collection of the German Fruit Gene Bank of the JKI in Dresden-Pillnitz were evaluated in three years of testing. Concerning B. cinerea, a total of five genotypes were identified as partially resistant. In parallel, six partially resistant genotypes were found towards X. fragariae. No completely resistant genotyped were identified until now. Additionally to the resistance test against X. fragariae, the systemic dispersal of the bacteria within the plant was further investigated with molecular-biological methods. On the one hand, bacterial DNA was detected by a sensitive nested-PCR method in different plant tissues of inoculated plants from partially resistant and susceptible cultivars at different time points. Additionally, a GFP-tagged virulent X. fragariae strain was produced and used for inoculation experiments in different strawberry genotypes as well. The systemic dispersal of the bacteria was visualized under the fluorescent microscope and the results were confirmed by the nested-PCR detection method. Already three days after the inoculation, X. fragariae spreads systemically throughout the entire host plant and can be detected in all plant tissue samples. The bacteria were detected in all plant parts of partially resistant cultivars after three days post-inoculation as well. The results from the resistance tests lead to direct recommendations for the choice of cultivars on the one hand and on the other hand, the identified partially resistant genotypes can be used in further targeted breeding approaches. The results of this thesis show new ways and strategies for the improvement of strawberry breeding programs. They increase the success for targeted breeding of new cultivars for strawberry cultivation in Germany in order to maintain a competitive and sustainable strawberry production in the future.
Genetic analysis of resistance to ear rot and mycotoxin contamination caused by Fusarium graminearum in European maize
(2012) Martin, Matthias; Melchinger, Albrecht E.
Maize is affected by a number of diseases. Among the various ear rots of maize, Gibberella ear rot (GER) caused by Fusarium graminearum is prevalent in Central Europe. This fungal pathogen produces secondary metabolites (mycotoxins), which adversely affect the health of humans and animals. Two important mycotoxins are the immunosuppressant deoxynivalenol (DON) and the mycoestrogen zearalenone (ZEA). The most efficient method to reduce mycotoxin contamination in maize is cultivation of resistant varieties. However, resistance breeding using classical phenotypic selection is laborious and time-consuming. Therefore, marker-assisted selection (MAS) may be a promising alternative to classical selection. Furthermore, for setting up a breeding program, knowledge about the relevance of the different modes of gene action and genotypic correlations among resistance and agronomic traits is required. The objectives of this study were to (1) estimate quantitative genetic parameters for GER severity and mycotoxin concentration in connected populations of doubled haploid (DH) lines, (2) map quantitative trait loci (QTL) for GER resistance and reduced mycotoxin contamination in these populations, (3) examine the congruency of QTL in these populations, (4) evaluate the prospects of using MAS to breed for GER resistance and reduced mycotoxin contamination, (5) estimate the genotypic correlation between the resistance of DH lines per se and the resistance of their testcrosses, (6) evaluate the influence of selection for increased resistance on agronomic performance of hybrids and (7) examine the relevance of different modes of gene action involved in the expression of the resistance in flint maize. Three field experiments were conducted, each of which comprised a different set of plant material. Experiment I comprised five DH line populations derived from the following F1 crosses among elite flint inbred lines: D152×UH006, D152×UH007, UH007×UH006, UH009×UH006 and UH009×UH007. Experiment II comprised testcross progenies of 94 DH lines and a dent single cross tester. Experiment III comprised the five F1 crosses, from which the DH populations had been derived, the F2 and the first backcross generations to the parents (BC1-P1, BC1-P2) as well as the two parent lines of each cross. Plants were artificially infected with spores of F. graminearum shortly after mid-silking using the silk channel inoculation technique. The DH lines were genotyped with simple sequence repeat (SSR) DNA markers, genetic linkage maps were constructed and QTL analyses were performed for resistance to GER, DON and ZEA contamination. Estimates of genotypic and genotype-by-environment interaction variances in Experiment I for GER severity and mycotoxin concentration were significant and heritabilities were moderately high to high in all populations. Thus, differences among DH lines for the resistance traits were mainly caused genetically and the resistance response varied depending on the environment. Owing to the effectiveness of artificial inoculation, the prospects are good to improve line resistance using a small number of test environments. QTL were detected in the four largest populations. Depending on the population, the mapped QTL together explained 21-51% of the genotypic variance for GER severity and 19-45% for DON concentration and 52% for ZEA concentration. Additive gene action was more important than digenic interactions of QTL, as indicated by the number of QTL having significant additive effects, their relative contributions to the total genotypic variance explained and the magnitude of their effects. Colocalized QTL for resistance to GER and mycotoxin contamination were identified in each mapping population. This was in agreement with strong genotypic correlations among these traits. QTL located at similar positions were detected across three populations in two chromosomal regions and across two populations in additional two regions. The results of this study indicated that a combination of classical phenotypic selection and MAS is a promising strategy for resistance breeding. In Experiment II, significant genotypic variation for resistance in lines and testcrosses showed that selection will be successful in both groups. Owing to low genotypic correlations between lines and testcrosses, however, resources should be mainly allocated to the evaluation of GER in testcrosses. Correlations of resistance with agronomic traits were weak or not significant. Therefore, selection for resistance and better agronomic performance can be carried out simultaneously. In Experiment III, generation means analysis indicated a prevalence of additive gene action for resistance. Significant dominance effects were found in only one cross for resistance to GER, but in four crosses for resistance to DON contamination. Owing to prevalence of additive gene action, the prospects are good to improve the resistance of the flint germplasm and to accumulate more favorable gene combinations in future breeding lines. Comparing the hybrid performance of flint×flint crosses of Experiment II and flint×dent crosses of Experiment III with their corresponding mid-parent performances indicated mid-parent heterosis for resistance. Therefore, prediction of hybrid performance based on performance of their parents will be possible only to a very limited extent. Future research should focus on fine mapping and validating of the detected QTL. For an efficient use of the QTL in a marker-assisted breeding program, knowledge about their effects in different genetic backgrounds is needed. Of particular importance are thereby the QTL effects in flint×dent crosses, which represent the preferred type of hybrid in Central European maize breeding programs.
Genetische und physiologische Einflußfaktoren sowie deren Wechselwirkungen auf die Trichothecenbildung bei Roggen, Triticale und Weizen nach Inokulation mit Fusarium culmorum (W. G. Sm.) Sacc.
(2002) Reinbrecht, Carsten; Geiger, Hartwig H.
Fusarium culmorum causes specific hazards of cereal quality by the producion of trichothecenes. Prophylaxis by plant breeding can be highly effective. The aim of this study was to investigate the accumulation of trichothecenes in cereals with regard to host and fungal genotype, to physiological factors and the resulting interactions. To determine the effects of 6 environments (E), 2 inoculation dates (ID), 2 fungal isolates (I), and 5 conidia concentrations (C) and their interactions, field trials with up to 12 rye, 6 triticale, and 8 wheat genotypes (G) were conducted in 1995-1997. Kinetics of trichothecene in the heads were described with 6 harvest dates (H) in 2 host genotypes each. In a growth chamber, 2 levels of temperature (T) and 2 of relative humidity (R) were investigated by using 2 host genotypes each. Average deoxynivalenol (DON) accumulation of rye, triticale, and wheat was 41, 46, and 82 mg kg-1, respectively. Genotypes differed significantly in rye and wheat. In all cereal species, GxE interactions were important. In wheat, DON content was highly correlated to all resistance traits, whereas in rye only a tight correlation existed to the relative specific grain weight. In triticale and wheat, inoculation at full anthesis resulted in higher DON contents than inoculation at heading. In rye, no effect of inoculation date was found. In contrast, GxID interaction was significant in rye. The nivalenol (NIV) producing isolate led to lower trichothecene contents than the DON producing isolate. This effect was found to be significant only in rye and triticale. Significant GxI interactions occurred in wheat only. Even one week after inoculation, considerable DON concentrations could be obtained in harvested heads, especially in wheat. Maximum DON contents were observed between 3 and 6 weeks after inoculation (in wheat: partially above 300 mg kg-1). NIV contents were always lower than DON contents. Until full ripening, DON contents slightly decreased, whereas NIV contents increased continuously. HxE and HxI interactions were most important. Trichothecene content in chaff and spindles was 2-4 fold higher than in the respective kernels at 6 and 8 weeks after inoculation. With higher conidia concentrations, increasing contents of DON+3-Acetyl-DON were measured. GxC interactions were highly significant. Highest heritabilities were found in the upper concentration levels. When the relative humidity was high, trichothecene concentrations of kernels were superior. With the temperature, an inverse effect was obtained. It seems that GxT interaction contributed most to GxE in rye and wheat, in triticale also the GxR interaction. In conclusion, assisting resistance traits may replace an expensive quantification of trichothecenes in early generations. In advanced generations, tests should be conducted in several environments with high conidia concentrations, and a toxin analysis should be carried out directly.
Genetische Variation für Resistenz gegen Mutterkorn (Claviceps purpurea [Fr.] Tul.) bei selbstinkompatiblen und selbstfertilen Roggenpopulationen
(2006) Mirdita, Vilson; Miedaner, Thomas
Ergot (Claviceps purpurea [Fr.] Tul.) is one of the most important diseases in rye. Infection during flowering results in the production of black, overwintering organs (sclerotia) instead of kernels, which contain harmful alcaloids. Three experiments were conducted to estimate quantitative-genetic parameters of the resistance of rye to ergot under the conditions of organic farming. The general aim was the estimation of genetic variation among and within self-incompatible rye populations and among CMS lines and their male-sterile testcrosses. In 2002 and 2004, genetic variation in resistance to ergot was tested among 65 rye populations at each of two locations (Experiment 1). Thirteen populations were registered rye varieties and the remaining 52 were genetic resources. To assess genetic variation within populations, 50 full-sib families (FSF) from each of five rye populations were developed and tested at four locations (Experiment 2). To test genetic differences in the susceptibility of ovaries towards fungal penetration in the absence of pollen, (i) 64 currently available CMS lines and (ii) their male-sterile crosses with three testers (=sets) were tested in 2003 and 2004, and in 2004, respectively. Inoculation was performed by spraying an aggressive mixture of isolates of Claviceps purpurea three times during the flowering period. The micro-plots were grown in a chess-board design separated by wheat plots to reduce the neighbouring effects. Traits of resistance were the proportion of infected spikes relative to the total number of spikes per plot, and the percentage by weight of ergot sclerotia in the grain. In Experiment 3, the weight of slcerotia per spike and per pair of spikelet were measured due to the absence of grain. Amount of pollen shedding was rated on the basis of the anther size and extrusion. Highly significant genotypic and genotype-environment interaction variances were found among rye populations in the percentage of ergot sclerotia in the grain. All genotypes were infected by ergot. No differences in mean among the registered rye varieties and genetic resources were detected. Because all populations were highly pollen shedding, the results indicate the existence of genetically determined resistance to ergot within the self-incompatible rye. Correlation between both resistance traits was significant (rp = 0.92). Genetic variation within populations was highly significant for all five populations. Individual progenies with resistance higher than the population mean were observed. The mean resistance of initial populations hardly differed from the mean of their progeny indicating a predominantly additive inheritance. Highly significant genetic variation in resistance to ergot was also detected among the currently available 64 CMS lines. Corresponding testcrosses mostly had a higher weight of sclerotia per spike than the lines. Considerable differences in the level of resistance were observed among testcrosses. Crosses with tester line 1 were substantially more susceptible, whereas those with tester 2 were hardly over the mean of the parental lines. The material showed a quantitative distribution of ergot resistance. Weak to medium-sized correlations (0.33 ? 0.47) between locations were detected among lines. The correlation between locations was even weaker in testcrosses. Weak correlations in ergot weight per spike were observed between CMS lines and their testcrosses in sets 2 and 3. In set 1, the estimated phenotypic correlation was higher (rp = 0.65). Estimates of error-corrected correlations were always higher than phenotypic correlations. No genetic difference was detected among the CMS lines for the amount of alkaloids in their sclerotia. This study shows that incompatible rye populations as well as self-fertile hybrid populations contain a substantial genetic variation for resistance to ergot that is inherited quantitatively. In both materials, mainly additive genetic variance was found. Because of a significant genotype-environment interaction, multi-environment trials are necessary to select for resistance. The results of this study nevertheless indicate good prospects to improve resistance to ergot in rye breeding in the long term.
Genomics-assisted breeding strategies for quantitative resistances to Northern corn leaf blight in maize (Zea mays L.) and Fusarium diseases in maize and in triticale (× Triticosecale Wittm.)
(2021) Galiano Carneiro, Ana Luísa; Miedaner, Thomas
Fusarium head blight (FHB) in triticale (× Triticosecale Wittm.), Gibberella ear rot (GER) and Northern corn leaf blight (NCLB) in maize (Zea mays L.) are devastating crop diseases causing yield losses and/or reducing grain quality worldwide. Resistance breeding is the most efficient and sustainable approach to reduce the damages caused by these diseases. For all three pathosystems, a quantitative inheritance based on many genes with small effects has been described in previous studies. Hence, this thesis aimed to assess the potential of genomics-assisted breeding strategies to reduce FHB, GER and NCLB in applied breeding programs. In particular, the objectives were to: (i) Dissect the genetic architecture underlying quantitative variation for FHB, GER and NCLB through different quantitative trait loci (QTL) and association mapping approaches; (ii) assess the potential of genomics-assisted selection to select superior triticale genotypes harboring FHB resistance; (iii) phenotype and characterize Brazilian resistance donors conferring resistance to GER and NCLB in multi-environment trials in Brazil and in Europe; and (iv) evaluate approaches for the introgression and integration of NCLB and GER resistances from tropical to adapted germplasm. The genome-wide association study (GWAS) conducted for FHB resistance in triticale revealed six QTL that reduced damages by 5 to 8%. The most prominent QTL identified in our study was mapped on chromosome 5B and explained 30% of the genotypic variance. To evaluate the potential of genomic selection (GS), we performed a five-fold cross-validation study. Here, weighted genomic selection increased the prediction accuracy from 0.55 to 0.78 compared to the non-weighted GS model, indicating the high potential of the weighted genomic selection approach. The successful application of GS requires large training sets to develop robust models. However, large training sets based on the target trait deoxynivalenol (DON) are usually not available. Due to the rather moderate correlation between FHB and DON, we recommend a negative selection based on genomic estimated breeding values (GEBVs) for FHB severity in early breeding stages. In the long-run, however, we encourage breeders to build and test GS calibrations for DON content in triticale. The genetic architecture of GER caused by Fusarium graminearum in maize was investigated in Brazilian tropical germplasm in multi-environment trials. We observed high genotype-by-environment interactions which requires trials in many environments for the identification of stable QTL. We identified four QTL that explained between 5 to 22% of the genotypic variance. Most of the resistance alleles identified in our study originated from the Brazilian tropical parents indicating the potential of this exotic germplasm as resistance source. The QTL located on chromosome bin 1.02 was identified both in Brazilian and in European trials, and across all six biparental populations. This QTL is likely stable, an important feature for its successful employment across different genetic backgrounds and environments. This stable QTL is a great candidate for validation and fine mapping, and subsequent introgression in European germplasm but possible negative linkage drag should be tackled. NCLB is another economically important disease in maize and the most devastating leaf disease in maize grown in Europe. Virulent races have already overcome the majority of known qualitative resistances. Therefore, a constant monitoring of S. turcica races is necessary to assist breeders on the choice of effective resistances in each target environment. We investigated the genetic architecture of NCLB in Brazilian tropical germplasm and identified 17 QTL distributed along the ten chromosomes of maize explaining 4 to 31% of the trait genotypic variance each. Most of the alleles reducing the infections originated from Brazilian germplasm and reduced NCLB between 0.3 to 2.5 scores in the 1-9 severity scale, showing the potential of Brazilian germplasm to reduce not only GER but also NCLB severity in maize. These QTL were identified across a wide range of environments comprising different S. turcica race compositions indicating race non-specific resistance and most likely stability. Indeed, QTL 7.03 and 9.03/9.04 were identified both in Brazil and in Europe being promising candidates for trait introgression. These major and stable QTL identified for GER and NCLB can be introgressed into elite germplasm by marker-assisted selection. Subsequently, an integration step is necessary to account for possible negative linkage drag. A rapid genomics-assisted breeding approach for the introgression and integration of exotic into adapted germplasm has been proposed in this thesis. Jointly, our results demonstrate the high potential of genomics-assisted breeding strategies to efficiently increase the quantitative resistance levels of NCLB in maize and Fusarium diseases in maize and in triticale. We identified favorable QTL to increase resistance levels in both crops. In addition, we successfully characterized Brazilian germplasm for GER and NCLB resistances. After validation and fine mapping, the introgression and integration of the QTL identified in this study might contribute to the release of resistant cultivars, an important pillar to cope with global food security.
Die Hypersensibilität der Europäischen Pflaume (Prunus domestica L.) gegenüber dem Scharkavirus (Plum pox virus)
(2005) Neumüller, Michael; Wünsche, Jens Norbert
In terms of economic damage, Sharka is the most important virus disease in stone fruit crops. The causative agent of this disease is the Plum pox virus (PPV). Some genotypes of European plum (Prunus domstica L.) show a hypersensitive response when infected with PPV. Results about the inheritance of this type of resistance mediated by hypersensitivity and different types of symptoms characteristic for the hypersensitive defence response in the interavtion PPV/Prunus domestica are presented. From the biochemichal point of view, it is shown that the defence response is indeed a hypersensitive response. A strategy for the development of DNA markers for hypersensitivity to PPV in European plum is developed. The potential use of hypersensitivity for solving the Sharka problem is discussed.
Inheritance of quantitative resistance and aggressiveness in the wheat/Fusarium pathosystem with emphasis on Rht dwarfing genes
(2010) Voß, Hans-Henning; Miedaner, Thomas
Fusarium head blight (FHB), or scab, is one of the most devastating fungal diseases affecting small-grain cereals and maize, causing severe yield losses and contamination of grain with mycotoxins such as deoxynivalenol (DON) worldwide. Fusarium graminearum (teleomorph Gibberella zeae) and Fusarium culmorum are the most prevalent Fusarium species in wheat production in Central and Northern Europe. Breeding for increased resistance to FHB in wheat is considered the most effective strategy for large scale disease management and mycotoxin reduction. Height reducing Rht genes are extensively used in wheat breeding programmes worldwide in order to improve lodging resistance and yield potential, with Rht-D1b being the most important Rht allele in Northern Europe. However, their individual effects on FHB resistance are yet unclear. Due to the incremental approach to increase host resistance the question arises whether the Fusarium pathogen has the capability to adapt by increased aggressiveness and/or increased mycotoxin production. Therefore, the objectives of the present study were to investigate the effects on FHB resistance of Rht-D1b and additional Rht alleles, the segregation variance for FHB resistance and identification of FHB resistance QTL in subsequent mapping analyses in three crossing populations segregating for the semi-dwarfing Rht-D1b allele and two sets of isogenic wheat lines. Regarding the pathogen, the study aims to determine the segregation variance in two F. graminearum crosses of highly aggressive parental isolates and to examine the stability of host FHB resistance, pathogen aggressiveness and the complex host-pathogen-environment interactions in a factorial field trial. All experiments were conducted on the basis of multienvironmental field trials including artificial inoculation of spores. The presence of Rht-D1b resulted in 7-18% reduction in plant height, but considerably increased FHB severity by 22-53% within progenies from three tested European elite winter wheat crosses. In the following QTL mapping analyses the QTL with the strongest additive effects was located at the Rht-D1 locus on chromosome arm 4DS and accordingly coincided with a major QTL for plant height in all three wheat populations. On total, a high number of 8 to 14 minor QTL for FHB reaction that were found in the three populations which emphasised the quantitative inheritance of FHB resistance in European winter wheat. The detected QTL mostly showed significant QTL-by-environment interactions and often coincided with QTL for plant height. By means of isogenic lines in the genetic background of the variety Mercia, Rht-D1b and Rht-B1d significantly increased mean FHB severity by 52 and 35%, respectively, compared to the wild-type (rht). Among the Maris Huntsman data set, the Rht alleles increased mean FHB severity by 22 up to 83%, but only the very short lines carrying Rht-B1c or Rht-B1b+Rht-D1b showed significance. The analyses of 120 progenies of the crosses from each of the highly aggressive parental F. graminearum isolates revealed significant genetic variation for aggressiveness, DON and fungal mycelium production following sexual recombination. This variation resulted in stable transgressive segregants towards increased aggressiveness in one of the two progeny. The factorial field trial, including eleven F. graminearum and F. culmorum isolates varying in aggressiveness and seven European elite winter wheat varieties, varying in their FHB resistance level, displayed no significant wheat variety × isolate interaction. Nevertheless, isolates possessing increased aggressiveness significantly increased FHB severity and DON production at a progressive rate on varieties with reduced FHB resistance. In conclusion, the analysed Rht alleles led to differently pronounced negative effects on FHB resistance that strongly depended on the genetic background. However, significant genetic variation for FHB resistance exists for selection and, thus, to largely counteract these effects by accumulating major and minor FHB resistance QTL. Significant genetic variation for aggressiveness among F. graminearum and the capability to increase its level of aggressiveness beyond yet known levels simply by sexual recombination may lead to long term erosion of FHB resistance. The rate at which increased aggressiveness develops will depend on the selection intensity and whether it is of constant, episodic or balanced nature. Consequently, the selection pressure imposed on the pathogen should be minimized by creating and maintaining a broad genetic base of FHB resistance that relies on more than one genetically unrelated resistance source by combining phenotypic and marker-assisted selection to achieve a sustainably improved FHB resistance in wheat breeding.
Quantitativ-genetische Untersuchungen zur Vererbung der Resistenz gegen Ährenfusarium bei Triticale (x Triticosecale Wittmack)
(2004) Heinrich, Nicole; Miedaner, Thomas
Fusarium head blight (FHB), caused by Fusarium culmorum (W.G. Smith) Sacc. and F. graminearum Schwabe, is recognized as one of the most destructive diseases of small-grain cereals. Fusarium infection can cause substantial yield losses. Infected grain may also be contaminated by mycotoxins that are harmful to humans and livestock. Agronomical measures and fungicides are only partly effective in controlling FHB. The development of disease-resistant cultivars together with appropriate crop management practices are effective strategies to control FHB. In this study, seven triticale cultivars and three breeding strains, representing a range of FHB resistances, their 45 diallel F1 crosses, progenies of 15 F2s from a six-parent diallel and their 30 backcrosses (BC, 15 to each parent), and five F2:3 bulks were investigated. Parents and their progenies were grown in several environments (years, locations) and tested for FHB resistance after artificial inoculation with Fusarium culmorum. Within the scope of this study, three experiments were conducted to estimate various quantitative-genetic parameters of several traits. In Experiment 1, the influence of FHB on yield-related traits of the ten parents was assessed. Compared to a non-inoculated variant, Fusarium reduced 1000-grain weight by 10.0%, spike weight by 9.3%, the number of kernels per spike by 4.3%, and test weight by 7.4%. Inoculation also increased deoxynivalenol (DON, 26.4 mg kg-1) and exoantigen (1.34 OD). content of the kernels. Genotypic variation and genotype-environment interaction were significant for all traits. The correlation between symptom ratings (spikes, kernels) and yield traits and between spike weight and kernels per spike were negative and high. The aim of Experiment 2 was to estimate combining ability, hybrid performance and heterosis for FHB ratings, DON and exoantigen content. Heterosis of FHB for spike and kernel rating was small. Across environments, the DON content in F1 crosses, however, was 15.5% higher than their mid-parent value. A high and significant (P = 0.01) correlation of r = 0.8 was found for both spike and kernel FHB symptom ratings between mid-parent and F1 performance. Except for exoantigen content, the general combining ability (GCA) was the main source of variation, suggesting additive gene effects for FHB resistance. Significant specific combining ability variance implies non-additive types of allelic interaction also. Therefore, in some crosses dominant effects can play an important role. The relationship between the GCA effect of a parent and its per se performance was close. In Experiment 3, genetic variation and effects for FHB resistance were estimated in segregating generations. The resistance level of the parents and their F2 progenies were similar. In contrast, the resistance of the BC progenies to the resistant parent was considerably higher than that of the backcrosses to the susceptible parent. Significant differences between the means of the 15 crosses and a high genetic variation within crosses were observed. Transgression could not be detected. F2:3 bulks and their parents had a comparable resistance level. For F2 and BC progenies, the additive effect was more important than the dominant effect. In contrast, the F1 crosses had a higher dominant effect, but with a large error. The study revealed considerable genetic variation in all generations for FHB resistance that can be exploited in a breeding programme. The mainly additive genetic effect makes it possible to select crossing parents on the basis of their per se performance. Due to the importance of genotype-environment interaction, resistance tests in various environments are strongly recommended. Screening for FHB resistance can best be accomplished by assessing symptom ratings of spikes and/or the spike weight relative to a non-inoculated variant. The high cross-environment interaction variance in the F2 generation points to the problem of selecting in unreplicated segregating material. Selection should be postponed to the F3 or later generations. The large genetic variation of FHB resistance and the preponderance of additive gene effects are encouraging to further increase resistance in triticale by recurrent selection.
Resistance breeding in maize (Zea mays L.) against the European corn borer (Ostrinia nubilalis Hübner) and the use of DNA-markers for marker-assisted selection
(2005) Papst, Christine; Melchinger, Albrecht E.
The European corn borer (Ostrinia nubilalis Hb., ECB) is an important pest in maize production. Feeding of ECB larvae causes grain yield losses of up to 30% and promotes ear and stalk rots caused by Fusarium spp.. Maize cultivars carrying the Bt gene are highly resistant to ECB larvae feeding. However, the use of transgenic cultivars is controversially discussed. In contrast, the natural host plant resistance (HPR) is regarded as more durable. The main objective of this study was to identify quantitative trait loci (QTL) for HPR against ECB and to draw conclusions about their usefulness in marker-assisted selection (MAS). The specific research questions were: (1) Where are QTL for ECB resistance and related agronomic traits located in the maize genome and what are their genetic effects? (2) How consistent are QTL detected across unrelated populations? (3) How consistent are QTL detected for line per se and testcross performance? (4) Which physiological mechanisms underlie the resistance against ECB larvae feeding? (5) What is the association between ECB resistance and mycotoxin concentrations in grain maize? Two unrelated dent populations (A and B) were developed. For Experiment 1 the F2:3 families were evaluated for line per se performance for ECB resistance. All F2:3 families of Population B were testcrossed with a susceptible tester line and also evaluated for ECB resistance (Experiment 2). Two sets of F2:3 families from Population B, each comprising the most resistant and the most susceptible lines, were selected (Experiment 3). In Experiment 4, 10 maize cultivars consisting of four pairs of transgenic hybrids and their isogenic counterparts were used to determine the association between mycotoxin concentration and ECB resistance. All entries in Experiment 4 were analyzed for mycotoxin concentration of deoxynivalenol (DON), fumonisin (FUM), fusarenon-X (FUS), moniliformin (MON) and nivalenol (NIV) in grain samples. In all four experiments, resistance to ECB larvae feeding was evaluated using manual infestation with ECB larvae. Furthermore agronomic and quality traits were recorded. In Experiment 1, two QTL for resistance were detected in Population A, both explaining about 25% of the genotypic variance. No common QTL for resistance traits was found across Populations A and B. Possible explanations for the low consistency of QTL across populations are a low power of QTL detection caused by small population sizes, sampling, and environmental effects. Furthermore, population-specific QTL regions cannot be ruled out. In Experiment 2, six QTL for resistance explaining 27% of the genotypic variance were found for testcross performance. Three common QTL for resistance were detected for line per se and testcross performance. Phenotypic as well as genotypic correlations between line per se and testcross performance were low for resistance, indicating a moderate consistency across the different types of progeny. The low consistency across both types of progeny is presumably attributable to the low power of QTL detection in TC progenies caused by a decreased genotypic variance and masking effects of the tester allele. Despite the low consistency of QTL across populations and progenies in the present study, a comparison with other reports from the literature revealed that most of the QTL occurred in clusters. Given the low percentage of genotypic variance explained by QTL-marker associations, we conclude that MAS will not be efficient for resistance breeding against ECB with the current molecular marker techniques. In Experiment 3, significant correlations were observed between resistance and quality traits, such as digestibility and stalk strength. These findings confirm the importance of increased cell-wall fortification for resistance against ECB larvae feeding, and support the hypothesis that candidate genes for resistance are involved in lignin biosynthesis. The analyses of mycotoxin concentrations in Experiment 4 showed that DON, FUM, and MON were the most prevalent mycotoxins in maize kernels. Differences between protected and infested plots were only significant for DON and FUM. Transgenic Bt hybrids showed lower mycotoxin concentrations in kernels than the other hybrids. However, only low correlations were found between ECB resistance and mycotoxin concentrations across all 10 hybrids. Therefore, selection for ECB resistance does not necessarily reduce mycotoxin concentration, suggesting that each complex of characters must be improved simultaneously by breeding. Even if MAS for resistance against the ECB does not seem promising at the moment, the information about QTL regions may be a first step for further research on possible candidate genes, e.g., brown midrib genes located in the common QTL regions with effects on the lignin biosynthesis. Genotypes with an improved digestibility, without impairing ECB resistance by reduced cell-wall strength, would be most promising.