Insights into northern leaf blight disease : investigating the Exserohilum turcicum-Sorghum bicolor interaction

Abstract

Exserohilum turcicum is a foliar pathogen that causes the widespread and damaging disease, northern leaf blight (NLB), in both sorghum and maize. Even though E. turcicum is an economically important pathogen, limited information is available on the underlying molecular mechanisms of the sorghum-E. turcicum interaction and, more specifically, the sorghum response. This study aimed to gain insight into the sorghum response to E. turcicum infection. A real-time quantitative PCR (qPCR) assay was developed that specifically detects and accurately quantifies the relative E. turcicum DNA present in infected sorghum and maize leaf samples. The qPCR assay was applied to quantify E. turcicum DNA in sorghum and maize leaf samples across NLB disease stages. Pathogenicity trials in commercial sorghum and maize varieties with varying degrees of NLB resistance were undertaken to show that the sorghum E. turcicum isolate 73 is only pathogenic on sorghum. The pathogenic specialization of the sorghum E. turcicum isolate was based on visual disease symptoms and in planta fungal biomass that was determined through the E. turcicum qPCR assay. Transcriptomic characterization of the sorghum response to E. turcicum infection was done through RNA sequencing of two time points of a moderately resistant sorghum-E. turcicum interaction. Differential expression analysis of the RNA-Seq data revealed the significant downregulation of the Sb- THI1-1 and Sb-THI1-2 thiamine biosynthesis gene targets after E. turcicum inoculation. Sorghum putative Sb-THIC and Sb-COG0212 genes were also identified. The sorghum targets putatively involved in thiamine metabolism as well as sorghum putative resistance genes were analyzed through RT-qPCR analysis. Research from this study will positively impact NLB resistance breeding efforts for effective disease management.