Bacterial blight caused by Xanthomonas oryzae pv. oryzae ( Xoo ) is an important disease constraining rice ( Oryza sativa L.) production worldwide. The XM6 line was induced by N-methyl-N-nitrosourea from IR24, an Indica cultivar that is susceptible to Philippine and Japanese Xoo races. XM6 was confirmed to carry a recessive gene named xa20 , resistant to six Philippine and five Japanese Xoo races. The chromosomal gene location was found using 10 plants with the shortest lesion length in an F 2 population consisting of 298 plants from a susceptible Japonica variety Koshihikari × XM6. Analysis using PCR-based DNA markers covering the whole rice genome indicated the gene as located on the distal region of the long arm of chromosome 3. The IKC3 line carries IR24 genetic background with Koshihikari fragment on chromosome 3 where a resistance gene was thought to be located. The F 2 population from IKC3 × XM6 clearly showed a bimodal distribution separating resistant and susceptible plants. Further linkage analysis conducted using this F 2 population revealed that xa20 is located within the 0.8 cM region flanked by DNA markers KIC3-33.88 (33.0 Mb) and KIC3-34.06 (33.2 Mb). This study yields important findings for resistance breeding and for the genetic mechanism of Xoo resistance.

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