BackgroundPowdery 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.MethodsCsaMLO8 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.ResultsWe 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.ConclusionsCsaMLO8 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0635-x) contains supplementary material, which is available to authorized users.


  • Powdery mildew (PM) is an important disease of cucumber (Cucumis sativus L.)

  • Transposon insertion in CsaMLO8 leads to aberrant splicing and to loss of the susceptibility gene (S-gene) function By cloning CsaMLO8 from cDNA of a PM resistant cucumber genotype that is homozygous for the hypocotyl resistance Quantitative trait locus (QTL), we found evidence for aberrant splicing of CsaMLO8 in this genotype, leading to products with deletions of respectively 72 and 174 bp in exon 11, compared to the Wild type (WT) gene

  • We show that complementation by CsaMLO8 overexpression in Slmlo1 mutant tomato background restores PM susceptibility

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Powdery mildew (PM) is an important disease of cucumber (Cucumis sativus L.). Cucumber (Cucumis sativus L.) is an economically important crop, with an annual global production of over 65 megatons [1]. Powdery mildew (PM) is one of the most widespread diseases in cucurbits, and a limiting factor for cucumber production. Two species of fungi have been reported to cause PM in cucumber, i.e., Podosphaera xanthii (synonymous with P. fusca, previously named Sphaerotheca fuliginea) and Golovinomyces cichoracearum (previously named Erysiphe cichoracearum). As the genome of cucumber (‘Chinese long’ inbred line 9930) was published in 2009 [7], and several other cucumber accessions have been resequenced [8, 9], the time is ripe to identify causal genes for cucumber resistance to mildew diseases


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