Regions outside the leucine-rich repeat domain determine the distinct resistance specificities of the rice blast resistance genes Pik and Pik-m

Two alleles of the rice blast resistance (R) Pik locus, Pik-m and Pik, are each composed of a pair of nucleotide-binding site-leucine-rich repeat (NBS-LRR) genes, referred to as the first gene and the second gene. Pik-m and Pik are unique in that many of the amino acid substitutions between them are distributed in or near the N-terminal coiled-coil (CC) domain of the first gene, suggesting that the CC domain of the first gene plays an important role in determinating their R specificity. To examine this hypothesis, I investigated resistance phenotypes of transgenic plants carrying each of two kinds of domain-swapped Pik-m-based recombinant first genes. Replacement of the LRR domain of Pik-m with the equivalent region of Pik did not change the Pik-m-type specificity, indicating that regions outside the LRR domain are responsible for differentiating the R specificity of Pik-m from Pik. In contrast, replacement of both the NBS and LRR domains of Pik-m with the corresponding region of Pik resulted in loss of blast resistance, suggesting that co-adaptation of polymorphisms in the CC and NBS domains is necessary to maintain resistance.