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 highly destructive disease of Impatiens walleriana, and economically important bedding ornamental crop. This disease is caused by a recently emerged pathogen Plasmopara obducens. Since both the host and pathogen are relatively less studied, there are only a few genomic resources available for both I. walleriana and P. obducens. In this study, we have analyzed 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 consisting of 73,022 transcripts. Differential expression analysis based on this optimized de novo transcriptome assembly revealed 3,000 to 4,500 differentially expressed transcripts (DETs) at 0 hr, 12 hr, 48 hr, 120 hr, and 240 hr time points after infection. Functional annotation of these DETs revealed that numerous plant stress responsive genes are activated and deactivated throughout the infection cycle. Genes in the calcium signaling pathways, receptor-like kinases (RLKs) including 10 disease resistance associated RLK transcripts, powdery mildew resistance genes (MLO), and many other plant stress related genes were predominantly differentially expressed in I. walleriana in response to P. obducens. Analyses reported here provides molecular insights into the disease susceptibility mechanism of the Impatiens downy mildew, and lays out a strong foundation for future studies aimed at improving downy mildew resistance in I. walleriana.