Identification And Characterization Of Quantitative Trait Loci For Resistance To Curvularia Leaf Spot In Maize

Curvularia leaf spot, caused by the casual fungal pathogen Curvularia lunata, is a disease of corn firstly identified in Northern region of China in1980s. The leaf spot fungus mainly infects leaves. The infection could rapidly spread to the whole plant under favorable hot-wet weather, and causes severe yield loss. Resistance to the disease was reported to be quantitative in nature, and jointly controlled by additive, dominant, and epitastic genetic effects. The solution to genetically contain curvularia leaf spot could be normally accomplished by studying disease resistance mechanism, screening candidate resistance genes, identifying markers linked to disease-resistance genes, and applying molecular markers to assist breeding to improve disease resistance.The thesis mainly focuses on identification and chracterization of quantitative trait loci for resistance to curvularia leaf spot in maize, and the results were listed as follows:1. A segregating F2：3population was developed from a cross between the resistant inbred line Shen137and the susceptible inbred line Huangzao4, in which a selective F2:3population consisting of117F2:3families were subjected to both genotyping and phenotyping. Of the514SSR markers,150of them which were reliable and polymorphic between two parents were selected to construct a genetic linkage map. The map was1824.5cM in length with an average of12.2cM every two adjacent markers and in agreement with the published reference genetic linkage map.QTL analysis was conducted in years2008and2009(each year with two replicates) with Composite interval mapping (CIM) method. In2008, three QTL have been detected, and one of them, located on bin6.05, was consistently detected in two replicates. The QTL shared the common confidence interval and explained variance ranging from7.6～12.5%. In2008, six QTL were detected, and one QTL located on bin10.04was common in two replicates, flanked by two markers umc1061and umc1239. This resistance QTL accounted for12.1-28.9%of the total phenotypic variation.2. Four F1plants derived from the cross of Shen137×Huangzao4was self-pollinated to produce822F2plants. A selective genotyping population was formed by selecting the62extremely resistant and59completely susceptible F2plants. Totally,145SSR markers were used to genotype each selective F2plant. The genetic linkage map, constructed by Mapmaker3.0, was1545.8cM in length with an average distance of10.7cM every two adjacent markers. QTL analysis was performed using CIM to reveal five QTLs. One of these five QTL, located on chromosome bin10.04, accounted for19.5%of the total phenotypic variation, flanked by two markers umc1061and umc1239. Interestingly, this QTL was also detected in the previous F2:3population in2009.3. The F3BC4population was obtained by continuously backcrossing to the susceptible inbred line, Huangzao4. A combined whole-genome background screening method and AFLP technology was applied to dig those genome segments involved in maize resistance to curvularia leaf spot. A total of40individuals were used to investigate whole-genome background, and found that several chromosome segments were introduced from the donor parent. The donor fragments were scattered in bin2.08, bin3.04-3.05, bin4.0-4.02, bin5.01, bin7.05, bin8.03-8.05and bin9.07. The three extremely resistant and three completely susceptible individuals, together with two parents, were subjected to AFLP analysis. Totally,64AFLP primer combinations were used to search for polymorphic AFLP bands, resulting in eight AFLP bands polymorphic between resistant and susceptible individuals. Sequence analysis revealed that five polymorphic AFLP fragments, E51M14-331, E52M14-470, E52M20-273, E53M30-678and E59M41-263, were amplified from retrotransposons. Another three polymorphic AFLP bands were related to segments of coding sequences of three proteins. Among them, the AFLP fragment E59M14-331on bin2.02belongs to an S-locus receptor-like kinase. The other two AFLP fragments, E52M11-254(bin4.01) and E59M30-221(bin4.04), were amplified from two unknown proteins. Intriguely, E52M11-254overlapped with polymorphic region reveled by the whole genome screening. It was speculated that the AFLP band, E52M11-254, may be associated with maize resistance to curvularia leaf spot.