Genetic Analysis And Fine Mapping Of Rice Bacterial Blight Resistance Gene Xa3

Bacterial blight, caused by Xanthomonas oryzae pv. Oryzae, is one of the most serious diseases of rice production worldwide. Until now, over 30 resistance genes have been identified for different Xoo races.A F₂ population from a cross between a resistance line IRBB3 carrying Xa3 and its susceptible near-isogenic parental line IR24 were used for fine genetic mapping of Xa3. The IRBB3 was high resistant to Xoo strain PXO86 with an average lesion length of 1.0 cm at 3 weeks after inoculation. The IR24 was highly susceptible to PXO86 with an average lesion length of 20.3 cm. Two hundred and sixty four individuals randomly chosen from the F₂ population were used for genetic mapping of Xa3. The distribution of lesion length for PXO86 inoculation in 264 F₂ plants was bimodal with an apparent valley at approximate 9 cm. Using a lesion length of 9 cm as the dividing point, the numbers of resistant (lesion length≤9 cm) and susceptible (lesion length＞9 cm) individuals fit the expected 3:1 ratio (x²=0.25, P>0.5), indicating that the resistance of IRBB3 to PXO86 was only controlled by Xa3.Xa3-mediated resistance for rice bacterial blight is infuenced by genetic background. Xa3 is genetically tightly linked to Xa26, another gene for bacterial blight resistance. Xa26 belongs to a clustered multigene family encoding leucine-rich repeat (LRR) receptor kinase-like proteins. To characterize Xa3, we fine mapped it using a population segregating for only one resistance gene and markers developed from Xa26 family. Genetic analysis showed that Xa3 co-segregated with the marker of Xa26 gene and segregated from the markers of other members of Xa26 family. DNA fingerprinting revealed that rice line IRBB3 carrying Xa3 had the same copy numbers of Xa26 family members as rice line Minghui 63 carrying Xa26. Phenotypic comparison showed that all the rice lines carrying either Xa3 or Xa26 developed dark brown deposition at the border between the lesion caused by incompatible- pathogen infection and health leaf tissue, while other rice lines did not show this dark brown deposition in either incompatible or compatible interactions. These results suggest that Xa3 and Xa26 is the same gene. We name it Xa3/Xa26 to indicate the relationship between the two gene symbols. The putative encoding products of Xa3/Xa26 and its susceptible allele xa3/xa26 shared 92% sequence identity. The sequence difference occurred in the LRR domains, specifically at the solvent-exposed amino acid residues, might be the major cause that differentiates the resistant and susceptible proteins.