sciEntific committEE

Downy mildew is an economically relevant pathogen of cucumbers and other cucurbits caused by the obligate oomycete, Pseudoperonospora cubensis. In recent years, host resistance has been overcome by this pathogen leaving many crops, namely cucumber, vulnerable to this foliar disease. In order to better understand the cucumber-downy mildew interaction at a molecular level, recent efforts have been made to sequence the genome of Ps. cubensis as well as to sequence the transcriptome of both Ps. cubensis and the susceptible cucumber cultivar “Vlaspik” over the time course of infection from 0-8 days post inoculation. Current work is focused on uncovering the transcription-related regulation of resistance by comparing the transcriptome of a susceptible host (Vlaspik) to a resistance cucumber host (PI 197088) over the time course of Ps. cubensis infection from 0–6 days post inoculation. Previous research has shown that PI 197088 shows resistance to downy mildew in field-based studies, and work in this study confirms this resistance through the use of controlled time course studies, which include leaf phenotype, microscopy, and real-time PCR data to show the level of resistance. Through this study, significantly differentially regulated genes in the resistant line were identified, and future work will examine the relationship between the susceptible and resistant cucumber transcriptomes and the roles that SNPs and small RNAs play in mediating resistance. Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew, is an obligate pathogen of Cucumis sativus (cucumber) and many other hosts in the Cucurbitaceae. Similar to many other plant pathogenic oomycetes, Ps. cubensis has a broad host range (>20 Genera), a wide geographical distribution, exhibits increasing tolerance to chemical management, and is highly adapted to variations in environment (Savory et al., 2010). For decades, cucurbit-producing regions in Europe and Asia have faced production pressures from Ps. cubensis. Historically, in the U.S. however, Ps. cubensis was successfully managed through plant resistance, conferred by the recessive locus, dm-1. In recent years, increasing virulence and environmental tolerance of Ps. cubensis has proven to be a primary factor limiting U.S. cucurbit production; indeed, in 2004, resistance in cucumber to Ps. cubensis was overcome (Savory et al., 2010). Downy mildew symptoms on cucumber include angular chlorotic lesions on the upper leaf surface that expand and coalesce, with sporulation occurring on the lower leaf surface. Wind-dispersed sporangia germinate in the residual water on leaf surfaces, and biflagellate zoospores swim to stomata. The zoospores encyst, produce appressoria, and form hyphae throughout host tissue. During infection, virulence factors are secreted through developing haustoria, which in turn function to manipulate the physiology and primary defense processes of their host. In total, this process functions to subvert immune signaling such that the pathogen can persist, develop, and acquire nutrients. In the later stages of the life cycle, the pathogen will sporulate and release asexually produced sporangia into the air to spread and infect new hosts (Burkhardt & Day, 2013). Recently, a draft genome assembly of Ps. cubensis isolate MSU-1 inventoried the virulence capacity of Ps. cubensis, as well as to began functional characterization of host range and adaptation (Tian et al., 2011). From these initial studies, we have developed a foundation to support the further study of this important obligate oomycete-plant interaction. Protein coding genes in the assembly were annotated using a combination of ab initio gene predictions, including protein and transcript evidence from other sequenced oomycete genomes. Through this work, we approximated 23,500 gene loci and models. The size of the Ps. cubensis genome is similar to other oomycetes with more streamlined genomes; however, the number of genes exceeds predictions of other comparable oomycete genome annotations. In two follow-up publications, RNA-Seq was used to profile genome-wide changes in gene expression during various stages in the development of Ps. cubensis on susceptible cucumber ̒ Vlaspik̓ (Adhikari et al., 2012; Savory et al., 2012). Using a gene correlation network analysis, more than 3,000 differentially expressed genes were grouped into six modules that correlate to important stages of infection. Most of the candidate RXLR-containing effectors were expressed between 2–4 dpi and the candidate host-targeting hydrolytic enzyme-encoding genes were expressed between 4–8 dpi. Additionally, we identified 440 putative transcription-factor encoding genes. Of these, ~250 were expressed throughout the course of infection, suggesting an extensive transcriptional module during host infection. Current work is focused on expanding upon the bioinformatic resources previously established in the Day lab in order to address the challenge of uncovering the transcriptomic networks *Corresponding author; phone: 517-884-4067; email: [email protected].