Bacterial leaf steak (BLS) is one of the most destructive diseases in rice. Studies have shown that BLS resistance in rice is quantitatively inherited, controlled by multiple quantitative trait loci (QTLs). A QTL with relatively large effect, qBlsr5a, was previously mapped in a region of ~380 kb on chromosome 5. To fine map qBlsr5a further, a set of overlapping sub-chromosome segment substitution lines (sub-CSSLs) were developed from a large secondary F2 population (containing more than 7000 plants), in which only the chromosomal region harboring qBlsr5a was segregated. By genotyping the sub-CSSLs with molecular markers covering the target region and phenotyping the sub-CSSLs with artificial inoculation, qBlsr5a was delimited to a 30.0-kb interval, in which only three genes were predicted. qRT-PCR analysis indicated that the three putative genes did not show significant response to the infection of BLS pathogen in both resistant and susceptible parental lines. However, two nucleotide substitutions were found in the coding sequence of gene LOC_Os05g01710, which encodes the gamma chain of transcription initiation factor IIA (TFIIAγ). The nucleotide substitutions resulted in a change of the 39th amino acid from valine (in the susceptible parent) to glutamic acid (in the resistant parent). Interestingly, the resistant parent allele of LOC_Os05g01710 is identical to xa5, a major gene resistant to bacterial leaf blight (another bacterial disease of rice). These results suggest that LOC_Os05g01710 is very possibly the candidate gene of qBlsr5a.
細菌性條斑病(BSL)是水稻上最具有破壞性病害之一。研究表明水稻對BLS的抗性是數(shù)量性狀遺傳，受多個數(shù)量性狀位點（QTLs）控制。一個具有相對大效應的QTL，qBlsr5a，之前被定位在5號染色體的~380 kb的區(qū)間內(nèi)。為了進一步精細定位qBlsr5a，通過一個包含7000多個單株、僅在qBlsr5a所在染色體區(qū)間出現(xiàn)分離的次級F2群體，構(gòu)建了一套具有部分重疊的次級染色體片段置換系（sub-CSSLs）。利用覆蓋目標區(qū)域的分子標記對sub-CSSLs進行基因分型，并對sub-CSSLs進行人工接種進行表型分型，qBlsr5a被限定在只含有3個預測基因的30 kb區(qū)間內(nèi)。qRT-PCR分析表明這3個預測基因在抗感兩個親本家系接種BSL病菌后均沒有出現(xiàn)顯著反應。但是，在LOC_Os05g01710基因的編碼區(qū)內(nèi)發(fā)現(xiàn)了兩個核苷酸的替代，該基因編碼了一個轉(zhuǎn)錄起始因子IIA的伽馬鏈（TFIIAγ）。核苷酸替換導致感病親本中第39位纈氨酸變成抗病親本中的谷氨酸。有意思的是，抗病親本LOC_Os05g01710等位基因與xa5相同，xa5是抗細菌性白葉枯（另一種水稻細菌性疾病）的主效基因。這些表明LOC_Os05g01710很肯能是qBlsr5a的候選基因。