Objective  A codominance functional marker of the broad-spectrum bacterial blight resistance gene,Xa7, of rice was identified for accurate detection, generation tracking, and differentiation between homozygous and hemizygous genotypes of the gene.

  Methods  A potential functional marker containing 4 primers was designed using Premier 5 software and based on the differences on the sequences of Xa7, xa7, and allele-free genome.The molecular distinctness of the marker in different materials was verified by PCR.Three crossbreed lines of Xa7 and their parents were inoculated with 7 bacterial blight pathogens at booting stage to examine the affected agronomic traits at maturation.

  Results  The homozygous R084 of Xa7 could be amplified into a 91 bp band and the Nip free of allele with a 153 bp band; while the heterozygote Nip/R084, 91 bp and 153 bp bands.The candidate codominance marker, Xa7fun, amplified fragments that matched the predicted target bands.No 91 bp fragment was amplified from 18 germplasms of varied types indicating a lack of Xa7 in them.Whereas Ry1, Ry2, and Ry3 had 91 bp band suggesting the inclusion of homozygous Xa7.Under an elevated temperature, Huazhan responded to the 7 bacterial blight pathogens as highly sensitive (HS), intermediate sensitive (MS), or sensitive (S); R084 to 6 of the 7 pathogens (HNA1-4, FuJ, GDA2, GD1358, PX086, and YN24) as highly resistant (HR), intermediate resistant (MR) or resistant (R); Ry-1 to 5 pathogens (GDA2, HNA1-4, FuJ, GD1358, and YN24) as HR or MR; Ry-2 to 5 pathogens (GDA2, GD1358, HNA1-4, PXO86, and YN24) as HR or R; and Ry-3 to 6 pathogens (HNA1-4, FuJ, GDA2, GD1358, PXO86, and YN24) as HR or MR.Therefore, the infiltration of Xa7 in the crossbred and improved lines RY-1, RY-2, and RY-3 significantly accentuated the blight resistance of Huazhan.

  Conclusion  The homozygous or hemizygous Xa7 could be accurately differentiated by the currently identified codominance functional marker Xa7fun.The Xa7 introgression did not significantly alter the critical agronomic traits in the hybridization from generation to generation and could be safely applied in breeding bacterial leaf blight resistant rice varieties．