Hatakeyama Katsunori, Niwa Tomohisa, Kato Takeyuki, Ohara Takayoshi, Kakizaki Tomohiro, Matsumoto Satoru
Institute of Vegetable and Floriculture Science, NARO, 360 Kusawa, Ano, Tsu, Mie, 514-2392, Japan.
Faculty of Agriculture, Iwate University, 3-18-8 Ueda, Morioka, Iwate, 020-8550, Japan.
Mol Genet Genomics. 2017 Apr;292(2):397-405. doi: 10.1007/s00438-016-1281-1. Epub 2016 Dec 24.
To facilitate prevention of clubroot disease, a major threat to the successful cultivation of Chinese cabbage (Brassica rapa L.), we bred clubroot-resistant (CR) cultivars by introducing resistance genes from CR turnips via conventional breeding. Among 11 CR loci found in B. rapa, we identified CRb in Chinese cabbage cultivar 'CR Shinki' as a single dominant gene for resistance against Plasmodiophora brassicae pathotype group 3, against which the stacking of Crr1 and Crr2 loci was not effective. However, the precise location and pathotype specificity of CRb have been controversial, because CRa and Rcr1 also map near this locus. Previously, our fine-mapping study revealed that CRb is located in a 140-kb genomic region on chromosome A03. Here, we determined the nucleotide sequence of an approximately 64-kb candidate region in the resistant line; this region contains six open reading frames (ORFs) similar to NB-LRR encoding genes that are predicted to occur in tandem with the same orientation. Among the six ORFs present, only four on the genome of the resistant line showed a strong DNA sequence identity with each other, and only one of those four could confer resistance to P. brassicae isolate No. 14 of the pathotype group 3. These results suggest that these genes evolved through recent gene duplication and uneven crossover events that could lead to the acquisition of clubroot resistance. The DNA sequence of the functional ORF was identical to that of the previously cloned CRa gene; thus, we showed that the independently identified CRb and CRa are one and the same clubroot-resistance gene.
为促进对大白菜（Brassica rapa L.）成功种植构成重大威胁的根肿病的防治，我们通过常规育种从抗根肿病（CR）芜菁中导入抗性基因，培育出了抗根肿病品种。在芜菁中发现的11个CR位点中，我们在大白菜品种“CR Shinki”中鉴定出CRb是针对芸薹根肿菌致病型第3组的一个单一显性抗性基因，Crr1和Crr2位点的叠加对其无效。然而，CRb的确切位置和致病型特异性一直存在争议，因为CRa和Rcr1也定位在该位点附近。此前，我们的精细定位研究表明CRb位于A03染色体上一个140 kb的基因组区域。在此，我们确定了抗性品系中一个约64 kb候选区域的核苷酸序列；该区域包含六个与NB-LRR编码基因相似的开放阅读框（ORF），预计它们以相同方向串联出现。在存在的六个ORF中，抗性品系基因组上只有四个彼此具有很强的DNA序列同一性，并且这四个中只有一个能够赋予对致病型第3组芸薹根肿菌分离株14的抗性。这些结果表明，这些基因是通过近期的基因复制和不均匀交叉事件进化而来的，这些事件可能导致获得根肿病抗性。功能性ORF的DNA序列与先前克隆的CRa基因相同；因此，我们表明独立鉴定的CRb和CRa是同一个根肿病抗性基因。
