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Background  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.    Methods   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 .    Results  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.    Conclusions   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.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-015-0635-x) contains supplementary material, which is available to authorized users.

A transposable element insertion in the susceptibility gene CsaMLO8 results in hypocotyl resistance to powdery mildew in cucumber