Fine Mapping Of A Major Qtl For Resistance To Rice Bacterial Blight And Screening Of The Candidate Genes

Rice is an important crop. Bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most serious diseases of rice all over the world. Chemical control is temporarily effective but often causes environmental pollution. In contrast, methods that depend on resistance genes are more acceptable and environmentally friendly.Oryza meyeriana is highly resistant to BB diseases and the resistance genes from this wild rice can be used in breeding progams for improving BB resistance of rice. However, little progress has been made in resistance genes cloning from this wild rice mainly because of its sexual incompatibility with cultivated rice. In previous studies, we introduced the trait of BB resistance from O. meyeriana into cultivated rice using asymmetric somatic hybridization and obtained a hybrid line Y73with high resistance. Quantitative trait loci (QTL) for BB resistance were then mapped in Y73and a QTL on chromosome5, tentatively designated qR5, had the strongest effect on BB resistance, explaining about37%of the phenotypic variance.Based on previous reseach, fine mapping of the major QTL, qR5, and analysis of the candidate genes were conducted in this study. The main results are as follows:1. QTLs analysis has located qR5in the maker interval between R05D87and RM6360on chromosome5. A BC4F2population was constructed based on the continuous back-cross of Y73and IR24(a rice cultivar with high susceptibility to BB disease) with IR24as the recurrent parent. A chromosome segment substitution line (CSSL)2CK6-5was obtained through screening BC4F2population using molecular markers evenly distributing on12chromosomes.2CK6-5carries the particular chromosome segment containing qR5in the genetic background of IR24.2. Fine mapping of qR5was conducted in a segregating population derived from a cross between2CK6-5and IR24, and qR5was finally mapped in the marker interval between RM3476and RM18910. The target region was about220kb according to the genome sequence of Nipponbare (http://rapdb.dna.affrc.go.jp/).3. The genome of Y73was resequenced with the genome data of Nipponbare as reference using solexa resequencing technique.38single nucleotide polymorphism (SNP) mutations and38Insert/Deletion (InDel) mutations were obtained in the target region of qR5. Gene annotation according to the genome data of Nipponbare (http://rapdb.dna.affrc.go.jp/; http://rice.plantbiology.msu.edu/) suggested that24genes were associated with the76mutation sites.4. The DNA sequences containing putative mutations were cloned and sequenced to verify the mutation. Three mutations involved in three genes (Os05g0481500, Os05g0490300-01and Os05g0485000) were confirmed to be positive mutation. One of the three genes was then suggested to be the putative candidate gene of qR5.These results of this study provide the basis for cloning the novel gene for BB resistance and its utilization in marker-assisted breeding, and will help deduce the resistance mechanisms of the resistance gene.