Background: Utilization of broad-spectrum resistance (R) genes is an effective and economical strategy to control the fungal pathogen Magnaporthe oryzae, the causal agent of the rice blast disease. Among the cloned blast resistance genes, Pi9, Pi2 and Piz-t confer broad-spectrum resistance to diverse M. oryzae isolates and were isolated from the Pi2/9 locus on chromosome 6. Identification and isolation of additional R genes with different resistance spectra from this locus will provide novel genetic resources for better control of this important rice disease.
Results: In this study, we identified a dominant R gene, Pi2-2, at the Pi2/9 locus from Jefferson, an elite U.S. rice cultivar, through genetic and physical mapping. Inoculation tests showed that Jefferson has different resistant specificities to M. oryzae isolates compared rice lines with the Pi9, Pi2 and Piz-t genes. Fine mapping delimited Pi2-2 to a 270-kb interval between the markers AP5659-3 and RM19817, and this interval contains three nucleotide-binding site-leucine-rich repeat (NBS-LRR) genes in the Nipponbare genome. Five bacterial artificial chromosome (BAC) clones spanning the region were identified, and a BAC contig covering the Pi2-2 locus was constructed.
Conclusions: We identified a new allelic gene at the Pi2/9 locus and fine-mapped the gene within a 270-kb region. Our results provide essential information for the isolation of the Pi2-2 gene and tightly linked DNA markers for rice blast resistance breeding.
背景：利用廣譜抗性基因（R）是控制水稻真菌性病害稻瘟病的一種經(jīng)濟有效的策略。作為已克隆的抗稻瘟病基因，Pi9、Pi2和Piz-t都具有廣譜抗性，Pi2/9區(qū)域位于6號染色體上。從這個位點識別出含不同抗譜的R基因?qū)⑻峁┬碌倪z傳資源，從而更好地控制這一重要的水稻病害。
結果：本研究中我們通過遺傳分析和圖位克隆從水稻品種Jefferson（美國原種）中鑒定出了一個顯性R基因Pi2-2，位于Pi2/9區(qū)域，接種試驗表明，Jefferson與帶有Pi9、Pi2和Piz-t的水稻品系相比，具有不同的抗病特異性。Pi2-2精細定位于第6染色體上，標記RM19817與AP5659-3之間大約270kb的區(qū)域內(nèi)，根據(jù)日本晴基因組這個區(qū)段含有3個核苷酸結合位點 - 富含亮氨酸的重復序列（NBS-LRR）。5個覆蓋該區(qū)段的BAC克隆已鑒定，其中一個BAC重疊覆蓋了Pi2-2位點。
結論：我們在Pi2/9區(qū)域確定了一個新的等位基因并精細定位到270 kb的區(qū)域內(nèi)。研究結果為Pi2-2基因的分離和對稻瘟病抗性育種緊密連鎖的DNA標記提供了重要信息。