Pik-p

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Annotated Information

Function

The blast resistance gene Pik-p, mapping to the Pik locus on the long arm of rice chromosome 11, was isolated by map-based in silico cloning. Four NBS-LRR genes are present in the target region of cv. Nipponbare, and a presence/absence analysis in the Pik-p carrier cv. K60 excluded two of these as candidates for Pik-p. The other two candidates (KP3 and KP4) were expressed in cv. K60. A loss-of-function experiment by RNAi showed that both KP3 and KP4 are required for Pik-p function, while a gain- of-function experiment by complementation test revealed that neither KP3 nor KP4 on their own can impart resistance, but that resistance was expressed when both were introduced simultaneously. Both Pikp-1 (KP3) and Pikp-2 (KP4) encode coiled-coil NBS-LRR proteins and share, respectively, 95 and 99% peptide identity with the two alleles, Pikm1-TS and Pikm2-TS. The Pikp-1 and Pikp-2 sequences share only limited homology. Their sequence allowed Pik-p to be distinguished from Pik, Pik-s, Pik-m and Pik-h. Both Pikp-1 and Pikp-2 were constitutively expressed in cv. K60 and only marginally induced by blast infection. Pik-p-Fig. 1.png

Twenty-one R genes have been mapped to date onto rice chromosome 11, including the six rice blast resistance genes Pik, Pik-s, Pik-m, Pik-p, Pik-h and Pik-g, all of which have been shown by classical genetic analysis to be allelic to one another (Ballini et al. 2008; Yang et al. 2009). With regard to the race speciWcities and/or resistance spectra of those alleles, Kiyosawa (1987) found that there are “stair- type” resistances among the alleles in the Japanese blast pathogen population, and ranked the strength of these alleles in the order Pik-m > Pik > Pik-p > Pik-s. This, in turn, indicated that Pik, Pik-p and Pik-s all form part of the larger or stronger allele, Pik-m (Kiyosawa 1987). Among Chinese isolates, the same ranking was found in Fujian, Yunnan, Jiangsu and Heilongjiang, but not in Guandong, Hunan, Guizhou, Sichuan, Jiangsu, Liaoning or Jilin. In the latter regions, many isolates that were virulent against Pik-m were avirulent against Pik and Pik-p, which has been taken to indicate that the Pik alleles Pik-m, Pik and Pik-p (and perhaps other Pik alleles as well) are, indeed, independent R genes able to condition differential reactions against various isolates (Wang et al. 2009). The isolation of Pik- m and Pik-p has revealed that they are allelic and independent. They can be distinguished from one another on the basis of both their gene structure and their sequence. Thus, Pikp-1 and Pikm1-TS differ from one another with respect to the length of their introns and a three base pair indel (Fig. 2), while Pikp-2 carries an intron in its 5? UTR, which is not present in Pikm2-TS (Fig. 3). Furthermore, Pik-p could be distinguished from other alleles, as well as from R genes at other loci with a combination of two SNP assays .

Pik-p-Fig. 2.png Pik-p-Fig.3.png

Expression

The transcription level of both genes was assessed in cv. K60 at four time points after blast inoculation. Both Pikp-1 and Pikp-2 transcripts were detected prior to exposure to the pathogen. The effects of mock- and pathogen inoculation were to reduce the transcription level over the first 24 h, implying a response to stresses resulting from both inoculation and incubation in a humidity chamber. Expression of Pikp-1 and Pikp-2 appeared to increase only marginally during the 72 h following either mock or genuine inoculation. The expression patterns of both Pikm1-TS and Pikm2-TS (the alleles present in cv. Tsuyuake) were similar to those in cv. K60, but in contrast that of the pathogen-inducible gene, PBZ1 (Ryu et al. 2006), in cv. Tsuyuake differed in several respects.