Fine Mapping Of Tiller Angle Gene And A Novel Blast Resistance Gene In Rice

Rice (Oryza sativa L.) is the staple food of half of the world’s human population. In feeding more people, we need to explore further increases in yield potential. As the limition of water and area, breeding super hybrid rice with ideal rice plant type is regarded as the effective method for further promoting the yield of rice. We need varieties with higer yield potential, multiple resistance to diseases and insects, the basis of which is to discover useful genes in rice genome which are responsible for traits of interest. In this thesis, we identitied new genes of rice tiller angle and blast resistance which were important for breeders to develop new cultivars.A relatively wide tiller angle may help the plant to escape some diseases by decreasing humidity, but it occupies more space and also increases the extent of shade, thereby decreasing the photosynthetic efficiency of the leaves. Rice cultivars with an optimum tiller angle that makes plants more efficient in trapping light, and thus more productive, continue to be favored in most rice breeding programs today. Tiller angle is an important component of plant architecture in rice even though little is known of its mode of inheritance. In the part 2 of this thesis, a quantitative trait loci (QTL) analysis based on a set of recombinant inbred lines extracted from an indica X japonica cross identified a major effect locus (designated qTA-9a) on chromosome 9, which was expressed in two independent environments. The qTA-9a allele derived from the indica parent IR24 increased tiller angle. This QTL was validated in a set of chromosome segment substitution lines involving the same two parental lines. A high resolution genetic map was constructed from a large secondary F2 population and anchored to the genomic sequence of rice. The QTL was narrowed to a 65-kb physical region, inclusive of eight putative genes. We have provided the basis for positional cloning of qTA-9a and have developed markers for a major tiller angle QTL.Rice blast, caused by the fungal pathogen Magnaporthe grisea, is one of the most devastating diseases in rice worldwide. The exploitation and utilization of broad-spectrum or durable resistance genes is considered to be the most important and effective method for rice blast control.YNG, a japonica landrice from Yunnan province of China, showed high-level resistance to at least 140 Magnaporthe grisea isolates and had a broader resistance-spectrum than the well-known broad-spectrum resistance genes Pi9, Pil,Pi2, etc.. In this study, we conducted the genetic analysis of YNG for its blast resistance by inoculating F2 and F3 populations derived from a cross LTH X YNG with four differential isolates 99-20-2 (437.5),98095 (007.2), Sichuan 26 (217.3) and 97-3-2 (077.5), which showed broad-spectrum virulence to 31 monogenic lines jointly developed by IRRI and Japan. The result indicated that the resistance of YNG to isolates 99-20-2 and 98095 could be controlled by one same dominant resistance gene; and that to isolates 97-3-2 and Sichuan 26 by two and three dominant resistance genes, respectively. Therefore, it could be concluded that the resistance donor YNG had at least three resistance genes effective to Magnaporthe grisea isolates.We developed two mapping populations consisting of 876 F2 plants susceptible to isolate 99-20-2 and 732 F2 plants susceptible to isolate 98095 respectively, and mapped the gene effective to isolates 99-20-2 and 98095 within a 2.6 cM interval flanked by two SSR markers RM16368 (1.8cM) and RM6770 (0.85cM) on chromosome 4 in both of the two populations by using BSA-RCA (bulked-segregate analysis and recessive-class analysis) strategy and SSR markers, confirming that the resistance of YNG to the two isolates is indeed controlled by one same gene, tentatively named Pi-ym1(t) herein. We enlarged the mapping population to 3617 susceptible F2 plants, and finally fine-mapped the gene within a 55kb interval flanked by two marker ID2736 and ID2791. Due to no blast resistance genes located on the short arm of chromosome 4, we deduced the gene Pi-ym1(t) is a novel gene. Based on the DNA sequence of Nipponbare, we predicted two candidate genes of Pi-ym1(t) by using several gene prediction programs, which lays good foundation for cloning of this gene.