Fine mapping of the rice bacterial blight resistance gene Xa-4 and its co-segregation marker

An F-2 population developed from the Xa-4 near isogenic lines, IR24 and IRB B4, was used for fine mapping of the rice bacterial blight resistance gene, Xa-4. Some restriction fragment length polymorphism (RFLP) markers on the high-density map constructed by Harushima et at. and the amplified DNA frag ments homologous to the conserved domains of plant disease resistance (R) g enes were used to construct the genetic linkage map around the gene Xa-4 by scoring susceptible individuals in the population. Xa-4 was mapped between the RFLP marker G181 and the polymerase chain reaction (PCR) marker M55. T he R gene homologous fragment marker RS13 was found co-segregating with Xa- 4 by analyzing all the plants in the population. This result opened an appr oach to map-based cloning of this gene, and marker RS13 can be applied to m olecular marker-assisted selection of Xa-4 in rice breeding programs.