MARKERS FOR SELECTION OF THE RICE XA21 DISEASE RESISTANCE GENE

Six molecular markers were mapped to a 7.4-cM region of rice chromosom e 11 containing the Xa21 gene, which confers resistance to the pathoge n Xanthomonas oryzae pv oryzae. Three markers, RG103, 248 and 818, co- segregated with Xa21 in a population of 1141 plants. Multiple copies o f all marker loci were present within the region that was introgressed from Oryza longistaminata into O. sativa. The marker loci were cloned and primers were designed that defined sequence-tagged sites. Physica l mapping of the three tightly linked central markers revealed that RG 103, the marker that hybridizes to the Xa21 gene, resides on a separat e DNA fragment from the other two markers.