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Localizing the Bacterial Blight Resistance Gene, Xa22(t), to a 100-Kilobase Bacterial Artificial Chromosome

The rice bacterial blight resistance gene, Xa22(t), provides resistance to a broad spectrum of Xanthomonas oryzae pv. oryzae isolates. Here, we localize the gene to a small 100-kb fragment of chromosome 11 by a combination of genetic recombination analysis and physical mapping. Mapping was done with two F₂ populations from the cross between Zhachanglong and Zhenzhuai. The first population consisted of 248 random individuals and 404 highly susceptible individuals selected from an F₂ population of more than 2,000 individuals and was used to construct a linkage map around the Xa22(t) locus. For the second F₂ population, 7,680 plants were examined with simple sequence repeat markers flanking the Xa22(t) locus to identify recombinants useful for fine-genetic mapping. Two large-insert bacterial artificial chromasome (BAC) libraries (from cvs. Teqing and Minghui63) were screened with a marker (R1506) which cosegregated perfectly with Xa22(t) in the first population. Restriction mapping of the resulting BAC clones enabled a physical map of the area to be constructed, and subclones from the BAC clones provided additional restriction fragment length polymorphism probes which could be placed on the fine-structure genetic map using the recombinants from the second mapping population. The Xa22(t) locus was mapped to a ≈ 100-kb interval delimited by the R1506 marker and a subclone from the M3H8 BAC clone.