Expressional and Biochemical Characterization of Rice Disease Resistance Gene Xa3/Xa26 Family

Abstract The rice (Oryza sativa L.) Xa3/Xa26 gene, conferring race‐specific resistance to bacterial blight disease and encoding a leucine‐rich repeat (LRR) receptor kinase‐like protein, belongs to a multigene family consisting of tandem clustered homologous genes, colocalizing with several uncharacterized genes for resistance to bacterial blight or fungal blast. To provide more information on the expressional and biochemical characteristics of the Xa3/Xa26 family, we analyzed the family members. Four Xa3/Xa26 family members in the indica rice variety Teqing, which carries a bacterial blight resistance gene with a chromosomal location tightly linked to Xa3/Xa26, and five Xa3/Xa26 family members in the japonica rice variety Nipponbare, which carries at least one uncharacterized blast resistance gene, were constitutively expressed in leaf tissue. The result suggests that some of the family members may be candidates of these uncharacterized resistance genes. At least five putative N‐glycosylation sites in the LRR domain of XA3/XA26 protein are not glycosylated. The XA3/XA26 and its family members MRKa and MRKc all possess the consensus sequences of paired cysteines, which putatively function in dimerization of the receptor proteins for signal transduction, immediately before the first LRR and immediately after the last LRR. However, no homo‐dimer between the XA3/XA26 molecules or hetero‐dimer between XA3/XA26 and MRKa or MRKc were formed, indicating that XA3/XA26 protein might function either as a monomer or a hetero‐dimer formed with other protein outside of the XA3/XA26 family. These results provide valuable information for further extensive investigation into this multiple protein family.