Mapping Of Sheath Blight Resistance QTLs And Construction Of Chromosome Segment Substitution Lines Using The Resistant Germplasm YSBR1

Rice is one of the important food crops in the world. Rice sheath blight (SB), caused by Rhizoctonia solani, has become the severest disease in some parts of the rice planting region in southern, causing severe yield loss each year. It is extremely urgent to explore new resistance germplasm and breed disease-resistant cultivars. A new rice line, YSBRl, from a cross of indica/japonica rice, showed excellent resistance to SB in field experiment in many years. In this study, we firstly developed an F2clonal population from the cross of YSBRl (pollen donor)/Lemont (pollen receiptor, susceptible) for mapping SB resistance (SBR) QTLs; secondly, we tried to construct a set of chromosome segment substitution lines (CSSLs) that cover the whole genome of YSBR1in the background of Lemont by using backcross and marker assisted selection (MAS). And thirdly, we evaluated the resistant phenotypes of some CSSLs to SB and identified SBR QTLs by using the inoculum insertion method developed by Pan et al.(1997) and the modified SBR rating system that improved by Zuo et al.(2006). The results were as follows:1. Total of216polymorphic markers out of2693pairs candidate markers between the two parents were developed. The polymorphism of the two parents is just around8.02%, which is mainly due to that both the parents are not belong to typical indica or japonica subspecies but are partial indica (YSBR1) and partial japonica (Lemont). The frequency of the polymorphic markers varied from chromosomes with the highest of18.02%and18.97%on chromosome2、3, respectively, and with lower frequency on chromosomes5、9and11with less than5%, and especially on chromosome11with only1.37%.The polymorphic frenquence of markers on the remaining7chromosomes are similar.2. Identification of SB resistance for this F2clonal population and the parents was performed in field in the years2009and2010. Results indicated that the YSBR1and the Lemont could be clearly considered as high resistant and susceptible to SB disease, respectively, and the disease rating of their hybrid F1plants was rated between the two parents. A large range on SB resistance of the F2clonal population, and the similar trend of each line on resistance between years were observed. The distribution of average SBR score on the population closed to normal distribution in both years, presenting the SB resistance of YSBR1also controlled by poly gene.3. Construction of genetic map and mapping of SBR QTLs. The genetic map covered the rice genome of3205cM, including18linkage groups and169polymorphic markers with the average marker intervasl of18.96cM. The program of composite interval mapping from public software WinQTLcart2.5was used to map SBR QTLs. Seven and5SBR QTLs were detected in2009and in2010, respectively. Among them, three (qSB-2Y, qSB-5Le, and qSB-12Y) were detected in both years at similar marker intervals. Except the QTL qSB-1Le、qSB-5Le及SB-8Le, in which the susceptible alleles were from susceptible variety Lemont, the remaining ones all contained the resistant alleles from the YSBR1resistant variety. In the intervals of most SBR QTLs were also detected containing QTLs controlling morphological traits.4. Construction of YSBR1whole genome chromosome segment substitution lines (CSSLs) in Lemont background and identification of SBR QTLs. From different backcross generations, we obtained a total of112CSSLs, in which the introgression YSBR1chromosome segments almost covered the whole genome of YSBR1except the chromosome11. In field, most CSSLs selected from BC4generation showed similar agronomic phenotype and developmental process. Genotypic analysis showed that every CSSLs contained more than one YSBR1segments. SB disease resistance of52CSSLs was firstly evaluated in field at adult stage. Results indicated that: the CSSLs have significant difference on SB resistance; the CSSLs that contained the segments covering qSB-2Y and qSB-12Y showed significatnly enhanced resistance levels than Lemont. Since the plant height of the CSSLs contaning qSB-12Y are higher than that of Lemont, and the morphological traits of the CSSLs contaning qSB-12Y are similar to those of Lemont, the qSB-2Y, was considered as a reliable major SBR QTL. In a total, due to the multiple segments in each CSSL and the CSSLs with preliminarly SBR phenotype without covering the whole YSBR1genome, the further study is obviously needed for identifying the SBR QTLs from YSBR1.