A transposable element insertion in the susceptibility gene CsaMLO8 results in hypocotyl resistance to powdery mildew in cucumber.
A transposable element insertion in the susceptibility gene CsaMLO8 results in hypocotyl resistance to powdery mildew in cucumber.
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DOI：
10.1186/s12870-015-0635-x
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发表时间：
2015-10-09
影响因子：
5.3
通讯作者：
Schouten HJ
中科院分区：
文献类型：
--
作者：
Berg JA;Appiano M;Santillán Martínez M;Hermans FW;Vriezen WH;Visser RG;Bai Y;Schouten HJ
白粉病是黄瓜上的一种重要病害。CsaMLO8以前被认为是黄瓜PM的一个候选感病基因，原因有两个：1)该基因在系统发育上聚集在分支V上，该分支先前已被证明包含所有已知的在双子叶植物中发现的与MLO类似的PM感病基因；2)该基因与5号染色体上的一个QTL共定位，该QTL是下胚轴专化抗性的QTL克隆了感病和抗病黄瓜的CsaMLO8等位基因，并将其转化到mLO突变体番茄中。黄瓜幼苗接种Xanthii后，利用qRT-PCR技术检测接种后不同时间组织中CsaMLO8的表达。在电子计算机上研究了CsaMLO8功能缺失等位基因在重新测序的黄瓜材料中的出现情况。我们从感病和抗病的黄瓜品种中克隆了CsaMLO8等位基因，后者携带了抗下胚轴的QTL。我们发现，在抗性基因中插入一个非自主的Class LTR逆转座子元件会导致CsaMLO8 mRNA的异常剪接。CsaMLO8野生型等位基因在番茄MLO突变体中的异源表达恢复了番茄对PM的敏感性。然而，从抗病黄瓜中克隆的CsaMLO8等位基因的异源表达并不能恢复黄瓜纹枯病的感病。此外，我们还发现，接种黄瓜褐斑病病原菌后，CsaMLO8在下胚轴组织中的转录上调，而在子叶和叶片组织中不表达。这与观察到的CsaMLO8基因座上的QTL在下胚轴组织中引起完全抗性，而在子叶和真叶中只引起部分抗性的观察结果一致。我们利用115份黄瓜种质资源的测序数据，采用电子计算机方法研究了CsaMLO8功能缺失等位基因在黄瓜种质中的存在情况，发现在115份黄瓜种质中有31份存在该等位基因。CsaMLO8被鉴定为PM的一个功能性易感基因，特别是在下胚轴中，当接种PM病原菌P.xanthii时，它在转录上上调。CsaMLO8功能缺失突变是下胚轴对PM抗性的原因CsaMLO8的这一特殊等位基因在重新测序的黄瓜材料中出现了27%。本文的在线版本(doi：10.1186/s12870-0150635-x)包含补充材料，授权用户可以使用。
Powdery mildew (PM) is an important disease of cucumber (Cucumis sativus L.). CsaMLO8 was previously identified as a candidate susceptibility gene for PM in cucumber, for two reasons: 1) This gene clusters phylogenetically in clade V, which has previously been shown to harbour all known MLO-like susceptibility genes for PM identified in dicot species; 2) This gene co-localizes with a QTL on chromosome 5 for hypocotyl-specific resistance to PM. CsaMLO8 alleles from susceptible and resistant cucumber were cloned and transformed to mlo-mutant tomato. Cucumber seedlings were inoculated with Podosphaera xanthii, tissues were studied for CsaMLO8 expression at several timepoints post inoculation using qRT-PCR. The occurence of the observed loss-of-function allele of CsaMLO8 in resequenced cucumber accessions was studied in silico. We cloned CsaMLO8 alleles from susceptible and resistant cucumber genotypes, the latter carrying the QTL for hypocotyl resistance. We found that insertion of a non-autonomous Class LTR retrotransposable element in the resistant genotype leads to aberrant splicing of CsaMLO8 mRNA. Heterologous expression of the wild-type allele of CsaMLO8 in a tomato mlo-mutant restored PM susceptibility. However, heterologous expression of the CsaMLO8 allele cloned from the resistant cucumber genotype failed to restore PM susceptibility. Furthermore we showed that inoculation of susceptible cucumber with the PM pathogen Podosphaera xanthii induced transcriptional upregulation of CsaMLO8 in hypocotyl tissue, but not in cotyledon or leaf tissue. This coincides with the observation that the QTL at the CsaMLO8-locus causes full resistance in hypocotyl tissue, but only partial resistance in cotyledons and true leafs. We studied the occurrence of the loss-of-function allele of CsaMLO8 in cucumber germplasm by an in silico approach using resequencing data of a collection of 115 cucumber accessions, and found that this allele was present in 31 out of 115 accessions. CsaMLO8 was characterised as a functional susceptibility gene to PM, particularly in the hypocotyl where it was transcriptionally upregulated upon inoculation with the PM pathogen P. xanthii. A loss-of-function mutation in CsaMLO8 due to the insertion of a transposable element was found to be the cause of hypocotyl resistance to PM. This particular allele of CsaMLO8 was found to occur in 27 % of the resequenced cucumber accessions. The online version of this article (doi:10.1186/s12870-015-0635-x) contains supplementary material, which is available to authorized users.