Abstract
The oomycete pathogen, Pseudoperonospora cubensis, is the causal agent of downy mildew on cucurbits, and at present, no effective resistance to this pathogen is available in cultivated cucumber (Cucumis sativus). To better understand the host response to a virulent pathogen, we performed expression profiling throughout a time course of a compatible interaction using whole transcriptome sequencing. As described herein, we were able to detect the expression of 15,286 cucumber genes, of which 14,476 were expressed throughout the infection process from 1 day post-inoculation (dpi) to 8 dpi. A large number of genes, 1,612 to 3,286, were differentially expressed in pair-wise comparisons between time points. We observed the rapid induction of key defense related genes, including catalases, chitinases, lipoxygenases, peroxidases, and protease inhibitors within 1 dpi, suggesting detection of the pathogen by the host. Co-expression network analyses revealed transcriptional networks with distinct patterns of expression including down-regulation at 2 dpi of known defense response genes suggesting coordinated suppression of host responses by the pathogen. Comparative analyses of cucumber gene expression patterns with that of orthologous Arabidopsis thaliana genes following challenge with Hyaloperonospora arabidopsidis revealed correlated expression patterns of single copy orthologs suggesting that these two dicot hosts have similar transcriptional responses to related pathogens. In total, the work described herein presents an in-depth analysis of the interplay between host susceptibility and pathogen virulence in an agriculturally important pathosystem.
Highlights
Cucumber (Cucumis sativus L.) is an economically important vegetable crop cultivated in over 80 countries, with more than 66 million tons produced annually for both fresh use and processing
Comparative analyses revealed correlated gene expression patterns in cucumber and Arabidopsis thaliana leaves infected with downy mildew, suggesting orthologous host responses in these two dicotyledonous hosts
The first visible symptoms of Ps. cubensis infection were apparent at 1 dpi, in the form of water soaking on the abaxial leaf surface at the inoculation site (Figure 1A). These symptoms correspond to zoospore encystment and initial penetration through the stomata into the host [36]. Systems such as Hyaloperonospora arabidopsidis, the causal agent of downy mildew on A. thaliana, analogous processes occur in the early stages of infection
Summary
Cucumber (Cucumis sativus L.) is an economically important vegetable crop cultivated in over 80 countries, with more than 66 million tons produced annually for both fresh use and processing (http://faostats.fao.org). We performed expression profiling, using the susceptible cucumber cultivar ‘Vlaspik’, over a time course of infection with the downy mildew pathogen Ps. cubensis to identify genes, pathways, and systems that are altered during a compatible interaction.
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