Function, evolution, and interaction of the coupled genes responsible for the Pik-h encoded blast resistance of rice
Q. PAN (1), C. Zhai (1), W. Wu (1), Y. Zhang (1), L. Wang (1). (1) South China Agricultural University, Guangzhou, China
<i>Pik-h</i>, which is an allele of <i>Pik</i>, confers resistance against certain races of rice blast. Its positional cloning showed that it comprises a pair of NBS-LRR genes, <i>Pikh-1</i> and <i>Pikh-2.</i> The allele is distinguishable from other known blast resistance genes on the basis of key variable nucleotides, and SNP diagnosis among the five rice populations implies that it appears to be the most recently evolved of the set of <i>Pik</i> alleles. Comparisons between the sequences of <i>Pik-h </i>and other <i>Pik </i>alleles showed that the functional K haplotype exists as two sub-haplotypes, KM and KH, which both evolved prior to the domestication of rice. While <i>Pikh-1</i> appears to be constitutively transcribed, the transcript abundance of <i>Pikh-2</i> responds to pathogen challenge, suggesting that while Pikh-1 may well be involved in elicitor recognition, Pikh-2 is more likely to be responsible for downstream signalling. <i>In vitro</i>, the CC domain of Pikh-1 was shown interact directly with both AvrPik-h and Pikh-2. Transient expression assays demonstrated that Pikh-2 mediates the initiation of the defence response. Moreover, nucleocytoplasmic partitioning of both Pikh-1 and Pikh-2 is required for the functionality of <i>Pik-h</i>. In the proposed <i>Pik-h</i> resistance pathway, it is suggested that Pikh-1 acts as an adaptor between AvrPik-h and Pikh-2, while Pikh-2 transduces the signal to trigger <i>Pik-h-</i>specific resistance.
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