Plant Genome IV Conference Plant Genome IV Conference
Bacterial blight is a major rice disease caused by Xanthomonas oryzae pv. oryzae. Map-based cloning methods were applied for isolation of Xa-1, one of the bacterial blight resistance genes. cDNA libraries were constructed by rice line IR-BB1 which contains Xa-1. Candidate cDNAs for Xa-1 of approx. 25 clones were isolated with plaque hybridization by probing with a YAC clone Y5212 which contains the Xa-1 locus. To reduce the number of candidate clones, linkage analysis among Xa-1 and all the candidate cDNA clones was done using 4000 F3 plants with a pooled-sampling method. Thirteen cDNA clones were closely linked to Xa-1 without recombination. Southern analysis by using partially digested Y5212 DNA showed that the physical distance of Xa-1 region was about 150 kb. Although the 13 cDNAs were sequenced, structures similar to known disease resistance genes could not be found. To obtain the genomic DNA fragment carrying the Xa-1 gene, a cosmid library of IR-BB1 was screened by Xa-1-linked RFLP and cDNA markers. So far, 26 cosmid clones have been isolated. We now are transforming these cosmid clones into susceptible rice varieties Nipponbare and Kitaake by co-transformation with a plasmid carrying a hygromycine resistance gene. Transformants will be soon analyzed by inoculation test with X. oryzae pv. oryzae and by Southern hybridization with transformed cosmid clones.