Transcriptional profiling of Impatiens walleriana genes through different stages of downy mildew infection reveals novel genes involved in disease susceptibility

Abstract Impatiens downy mildew is a destructive disease of Impatiens walleriana, the top annual bedding plant in the United States. The disease is caused by a recently emerged pathogen Plasmopara obducens. Because of new emergence, there are few genomic resources available in regard to both I. walleriana and P. obducens. In this study, we have analyzed the transcriptional changes in I. walleriana in response to P. obducens infection during different stages of disease development. Our main goal was to identify candidate genes that may be involved in I. walleriana susceptibility to P. obducens. Since the genome of I. walleriana is not available publicly, we constructed and optimized a de novo transcriptome assembly comprising of 73,022 transcripts. Differential expression analysis based on this optimized de novo transcriptome assembly generated 3000 to 4500 differentially expressed transcripts (DETs) at each timepoint (0 hr, 12 hr, 48 hr, 120 hr, and 240 hr). Functional annotation of these DETs revealed the presence of numerous plant stress responsive genes activated and deactivated throughout the infection cycle. For example, calcium signaling genes, receptor-like kinases (RLKs) including eleven leaf rust 10 disease resistance receptor-like protein kinase transcripts, powdery mildew resistance genes (MLO) and many other plant stress related genes were found to be differentially expressed in I. walleriana in response to P. obducens. Analysis reported here lays out a strong foundation for future studies aimed at improving resistance of I. walleriana to downy mildew disease.