Date of this Version
A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science
Major: Plant Pathology
Under the supervision of Professors Stephen Wegulo and Deanna Funnell-Harris
Lincoln, Nebraska, May 2023
Fusarium graminearum is a devastating pathogen of wheat that causes Fusarium head blight (FHB) and contaminates the grain with the mycotoxin deoxynivalenol (DON). Resistance to FHB is quantitative and it is important to identify additional genes conferring resistance against it. The goal of this thesis was to examine if the constitutive expression of two sorghum phenylpropanoid pathway genes, SbCCoAOMT (encoding caffeoyl-CoA O-methyltransferase) and SbC3’H (encoding p-coumarate 3-hydroxylase), in the moderately-susceptible spring wheat CB037 can provide Type-I and Type-II resistance to F. graminearum and determine the underlying mechanisms of the enhanced resistance. The constitutive expression lines (CCoAOMT413, CCoAOMT421, C3H112, C3H222) were screened for Type-I (initiation of infection) and Type-II (spread of infection) resistance by determining the area under the disease progress curve (AUDPC), percent of Fusarium-damaged kernels (FDK), and/or DON accumulation. In Type-II resistance experiments, when the experimental unit was 12 heads, the CCoAOMT413 line had significantly reduced AUDPC than the recipient line, CB037 (P ≤ 0.05) and FDK% and DON levels were also significantly lower than those in CB037 (P ≤ 0.05). Our results demonstrate that the constitutive expression of SbCCoAOMT can confer Type-II resistance to F. graminearum in wheat. To elucidate the resistance mechanism, global gene expression analysis of CCoAOMT413, CCoAOMT421 and CB037 at 12 and 72 hours post-inoculation (hpi) was performed. A total of 24,813 genes were differentially expressed. At 72 hpi, 1,482 genes were uniquely upregulated, and 1,047 genes were uniquely downregulated in CCoAOMT413. Gene ontology enrichment and heatmap analysis of genes showed that several defense-related biological processes had the highest expression in the CCoAOMT413 line inoculated with F. graminearum at 72 hpi, including phenylpropanoid and jasmonic acid biosynthesis pathway, WRKY transcription factors, and glutathione-S-transferases. Phloroglucinol staining of the rachis cross-sections showed that F. graminearum-inoculated CCoAOMT413 and CCoAOMT421 were more darkly stained than their mock counterparts, suggesting potential secondary metabolite deposition. Information from this study can be used to aid future breeding efforts to improve F. graminearum resistance in wheat.
Advisors: Stephen Wegulo and Deanna Funnell-Harris