Lipid Metabolism Plays a Role in the Resistance Response of Barley to Infection by the Spot Blotch Pathogen Cochliobolus sativus

Spot blotch, caused by Cochliobolus sativus, is a serious disease of barley in the Midwestern United States capable of causing up to 30% yield loss. Spot blotch has been controlled for the past 50 years by three quantitative trait loci (QTL) on chromosomes 1H, 3H, and 7H contributed by the breeding line NDB112 and termed the “Midwest Six-rowed Durable Resistance Haplotype” (MSDRH). We identified a gamma radiation-induced susceptibility mutant in the field and are using it to further explore the genetic basis of spot blotch resistance. Genetic analysis of F₂ and F₃ families from a cross of Morex wild type (WT) x Morex mutant (MUT) suggested a single major gene was controlling the seedling spot blotch resistance in the population as well as an association between seedling susceptibility and the development of large necrotic lesions at the adult plant stage. RNA-seq analysis was conducted to better understand the transcriptional changes underlying the early defense response. Differential expression revealed 18 up- and 27 down-regulated genes in the WT compared to the MUT across three time points (12, 24, and 36 hours after inoculation) when challenged with C. sativus isolate. Two genes were identified computationally as deletion candidates. One of these, MLOC_17240, encodes a putative long-chain fatty acid CoA ligase and was selected for further investigation. Primers designed to amplify regions across the entire genomic sequence of MLOC_17240 revealed that the products match the intended sequence in the WT, but the primers also amplified products from MUT DNA. These results demonstrate the importance of temporal regulation of cell death pathways for resistance to spot blotch, but the exact location and nature of the mutation in Morex MUT remains unknown.