The rice blast resistance (R) genes Pi2 and Piz-t confer broad-spectrum resistance against different sets of Magnaporthe grisea isolates. We first identified the Pi2 gene using a map-based cloning strategy. The Pi2 gene is a member of a gene cluster comprising nine gene members (named Nbs1-Pi2 to Nbs9-Pi2) and encodes a protein with a nucleotide-binding site and leucine-rich repeat (LRR) domain. Fine genetic mapping, molecular characterization of the Pi2 susceptible mutants, and complementation tests indicated that Nbs4-Pi2 is the Pi2 gene. The Piz-t gene, a Pi2 allele in the rice cultivar Toride 1, was isolated based on the Pi2 sequence information. Complementation tests confirmed that the family member Nbs4-Piz-t is Piz-t. Sequence comparison revealed that only eight amino-acid changes, which are confined within three consecutive LRR, differentiate Piz-t from Pi2. Of the eight variants, only one locates within the xxLxLxx motif. A reciprocal exchange of the single amino acid between Pi2 and Piz-t did not convert the resistance specificity to each other but, rather, abolished the function of both resistance proteins. These results indicate that the single amino acid in the xxLxLxx motif may be critical for maintaining the recognition surface of Pi2 and Piz-t to their respective avirulence proteins.
稻瘟病廣譜抗性基因（R）Pi2和Piz-t對(duì)多套稻瘟病菌株具有抗性。我們采用圖位克隆的方法確定了Pi2基因，它是一個(gè)含9個(gè)成員基因簇（分別命名為Nbs1-Pi2到Nbs9-Pi2）的一個(gè)，編碼一個(gè)含有核苷酸結(jié)合位點(diǎn)和富亮氨酸重復(fù)序列（LRR）結(jié)構(gòu)域的蛋白質(zhì)。通過精細(xì)定位，Pi2感病突變體的分子鑒定和互補(bǔ)實(shí)驗(yàn)表明，Nbs4-Pi2是Pi2基因。Piz-t是水稻品種Toride 1中Pi2的等位基因，根據(jù)Pi2的序列信息分離出來?；パa(bǔ)試驗(yàn)證明該家族成員的Nbs4-Piz-t是Piz-t。序列比對(duì)顯示Pi2與Piz-t在三個(gè)連續(xù)的LRR區(qū)間存在8個(gè)氨基酸的不同差異，8個(gè)突變氨基酸只有一個(gè)位于xxLxLxx結(jié)構(gòu)內(nèi)。在Pi2和Piz-t間互換這個(gè)突變氨基酸并不會(huì)交換兩者的抗性特征，而是使這兩個(gè)抗性蛋白失去功能。這些結(jié)果表明這個(gè)在xxLxLxx結(jié)構(gòu)內(nèi)的氨基酸是Pi2和Piz-t抗性蛋白表面結(jié)構(gòu)的關(guān)鍵，從而識(shí)別對(duì)應(yīng)的無毒蛋白。