Background

Rice blast is generally considered the most devastating rice disease worldwide. The development of resistant varieties has been proven to be the most economical strategy to control the disease. A cluster of resistant (R) genes on rice chromosome 12 including Pita, Pita2 and Ptr has been studies for decades. However, the relationship between these R genes has not been well established.

Results

In this study, we compared the resistance spectra controlled by Pita2 and Pita by testing their monogenic lines (MLs) in four hotspots found in the Philippines and Burundi from 2014 to 2018. The reaction patterns were distinct in two countries and that Pita2-mediated field resistance was relatively prevalent. Pathogenicity tests using 328 single-spore isolates in greenhouse further verified that IRBLta2-Re for Pita2 conferred a relatively broader spectrum resistance than those of Pita. Rough and fine mapping of Pita2 were conducted using F₂ and F₃ populations derived from IRBLta2-Re [CO] and CO 39 consisting of 4344 progeny to delimit Pita2 in a genomic interval flanked by two markers 12 g18530 and 12 g18920 proximal to the centromere of chromosome 12. Alignment of the markers to the genomic sequence of IR64, which harbors Pita2 verified by genetic analysis, approximately delimited the candidate gene(s) within 313-kb genomic fragment. The two Pita2 suppressive mutants that contain mutations within Pita2 were verified and identified. Comparative sequence analysis in these two mutants further identified that each individual allele contains a single nucleotide substitution at a different position resulting in nonsense and missense mutations in the protein product of LOC_Os12g18729. On the contrary, no sequence mutation was detected in other candidate genes, indicating that mutations in LOC_Os12g18729 were responsible for the loss of function of Pita2. Pita2 encodes a novel R protein unique from Pita, which is exactly identical to the previously cloned Ptr. Moreover, based on the resistance gene analysis of rice varieties and mutants containing Pita, it was found that Pita2 rather than Pita was responsible for the specificity to some differential isolates with AvrPita. The diagnosis and survey of Pita2 in IRRI released varieties showed relatively low frequency, implying a high value of its application for breeding resistant varieties against rice blast via marker assisted selection.

Conclusion

Our study clarified the relationship between Pita, Pita2 and Ptr. Pita2 is identical to Ptr and distinct from Pita in both sequence and chromosomal location although Pita2 and Pita are genetically linked to each other. The loss of function of Pita2 but not Pita eliminate the specificity to some AvrPita containing isolates, however, the mechanism underlying the recognition between Pita2/Pita and AvrPita remains elusive.

中文翻译：

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广谱稻瘟病抗性基因Pita2编码一种独特的Pita新型R蛋白

背景

稻瘟病通常被认为是世界范围内最具毁灭性的水稻疾病。抗性品种的开发已被证明是控制该病最经济的策略。水稻第12号染色体上的抗性（R）基因簇包括Pita，Pita2和Ptr已有数十年的研究了。然而，这些R基因之间的关系尚未很好地建立。

结果

在这项研究中，我们通过测试Pita2和Pita在2014年至2018年在菲律宾和布隆迪发现的四个热点中的单基因系（ML），比较了它们的电阻谱。反应模式在两个国家和Pita2介导的领域中是不同的抵抗力相对普遍。使用温室328的单孢子分离致病性试验进一步验证了IRBLta2-Re的用于Pita2赋予比相对更广谱抗性皮塔。使用F ₂和F ₃进行Pita2的粗略和精细映射来源于IRBLta2-Re [CO]和CO 39的群体，由4344个后代组成，在一个基因组间隔内界定Pita2，其侧翼是两个靠近12号染色体着丝粒的标记12 g18530和12 g18920。 ，其中包含通过遗传分析验证的Pita2，大约在313 KB基因组片段内定界了候选基因。两个Pita2抑制性突变体，其中包含Pita2内的突变经过验证和识别。在这两个突变体中进行的比较序列分析进一步确定，每个等位基因在不同位置包含一个核苷酸取代，从而导致LOC_Os12g18729蛋白产物中的无义和错义突变。相反，在其他候选基因中未检测到序列突变，这表明LOC_Os12g18729中的突变是Pita2功能丧失的原因。Pita2编码一种独特于Pita的新R蛋白，与先前克隆的Ptr完全相同。此外，根据含Pita的水稻品种和突变体的抗性基因分析，发现Pita2而不是Pita负责AvrPita对一些分离株的特异性。对IRRI释放品种中Pita2的诊断和调查显示频率较低，这意味着通过标记辅助选择将其用于抗稻瘟病的育种品种具有很高的应用价值。

结论

我们的研究阐明了Pita，Pita2和Ptr之间的关系。Pita2是相同的PTR和从不同皮塔在两个序列和染色体位置虽然Pita2和皮塔被基因彼此连接。Pita2的功能丧失而不是Pita丧失了对某些含AvrPita分离株的特异性，但是，Pita2 / Pita和AvrPita之间识别的潜在机制仍然难以捉摸。