Development of wheat lines with complex resistance to rusts and Fusarium head blight
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Wheat is the second most important food crop worldwide. The world human population is increasing daily. Sustainable food production is necessary to ensure sufficient future food supply. Wheat production is negatively affected by various pathogens that challenge breeders to develop more disease resistant wheat cultivars. Leaf-, stem- and stripe rust as well as Fusarium head blight (FHB) are considered some of the most destructive fungal pathogens of wheat. Resistance breeding is considered the most economic and eco-friendly solution to control wheat diseases in the long-term. Combining of several effective resistance genes/quantitative trait loci (QTL) into a single wheat line is necessary to increase the probability of obtaining durable resistance. Molecular markers linked to resistance genes/QTL can assist breeders in selecting the best offspring throughout their breeding scheme. The main aim of this study was to develop a wheat line with combined resistance to the rusts and FHB. Both wheat rust and FHB resistant lines used in this study were developed in previous studies. The best resistant lines developed in these studies were identified using marker-assisted selection (MAS) and were then used in a breeding scheme to combine wheat rust and FHB resistance genes/QTL. Offspring from crosses was evaluated for the presence of six rust resistance genes/QTL: Lr19, Lr34/Yr18/Sr57, Sr2/Yr30, Sr26, Sr39 and QYr.sgi.2B-1, as well as for three FHB resistance genes/QTL: Fhb1, Qfhs.ifa-5A-1 and Qfhs.ifa-5A-2. The best identified lines obtained from a series of crosses were selected to develop two double-cross populations as well as self-pollinated populations. Selected pre-breeding lines were also used to optimise and apply the doubled haploid (DH) technique to develop lines with fixed genotypes. Molecular markers linked to the high-molecular-weight glutenin subunits (HMW-GS) and the 1BL.1RS translocation were used for screening the final populations developed because the primary focus for selection was rust and FHB resistance genes/QTL and not HMWGS and the 1BL.1RS translocation. Validation of the phenotypic expression of the rust and FHB resistance genes/QTL was done through greenhouse inoculation of the parental, control and experimental lines/cultivars containing different resistance gene/QTL combinations. Seedling and adult plants were inoculated with the three rusts respectively, firstly for the timeline study to determine the optimal day’s post inoculation (dpi) to collect sample material for quantitative-polymerase chain reaction (qPCR) analysis and secondly to determine fungal gene expression in the experimental lines. Both point and spray inoculations for FHB were used and disease development was evaluated at 4, 7, 10, 14, 18 and 21 dpi. Sample collection for qPCR analysis was done at 21 dpi. Good correlations between the data obtained from the phenotypic evaluations, qPCR and MAS were obtained for both rust and FHB resistance evaluations. Therefore, markers linked to rust and FHB resistance genes were phenotypically expressed. Lines with combined wheat rust and FHB resistance were obtained from the selfpollinated populations but are still segregating for some of the genes/QTL. Lines with combined rust and/or FHB resistance genes/QTL were also developed through the DH technique. No markers linked to stripe rust resistance were used in this study except for genes with multiple disease resistance such as Lr34/Yr18/Sr57 and Sr2/Yr30. Additional stripe rust resistance genes are however present in the genetic background of the experimental lines as they were present in the parental cultivars/lines originally used to develop these lines like Kariega, AvocetYrSP, 2S#/163 and CM-82036. Valuable prebreeding wheat lines with combined rust and FHB resistance have been developed which can be used in future disease resistance breeding programmes.