| Fusarium Head Blight is a devastating wheat disease worldwide. This thesis describes the early events in the F. graminearum infection process in wheat spikes, characterizes the early response of wheat spikes to the infection by assessment of transcript accumulation of defense response genes and tests the association between the wheat Type II resistance mechanism and a systemic induced expression of defense response genes in F. graminearum point-inoculated wheat spikes.;Microscopy of inoculated glumes revealed that in both Sumai 3 (resistant) and Wheaton (susceptible) genotypes the fungus penetrated through stomata, exhibited subcuticular growth along stomatal rows, colonized glume parenchyma cells and sporulated within 48--76 hai. Assessment of temporal expression patterns of defense response genes encoding peroxidase, PR1, PR2 (beta-1,3-glucanase), PR3 (chitinase), PR4 and PR5 (thaumatin-like protein) from F. graminearum spray-inoculated wheat spikes showed that in both genotypes, transcripts for the six defense response genes accumulated as early as 6--12 hai during F. graminearum infection and peaked at 36--48 hai. Greater and earlier PR4 and PR5 transcript accumulation was observed in Sumai 3 as compared to Wheaton indicating a potential role in a resistant-dependent defense response.;The association between Type II resistance mechanism and an induction of defense response genes in uninfected regions of F. graminearum point-inoculated spikes was studied. Point-inoculation was done at middle spike regions. At lower, middle and upper spike regions, the extent of fungal colonization and transcript accumulation of peroxidase, PR-1, PR-3 and PR-5 were assessed within 48 hours after inoculation (hai) A transgenic isolate of F. graminearum expressing GUS activity proved extremely useful in determining pathogen spread in the spike and, GUS staining was readily detected both in- and ex planta. At 48 hai, the fungus was restricted in middle spike regions; however, enhanced transcript accumulation of the four defense response genes was observed in uninfected spike regions indicating that direct contact with the pathogen was not required for this response. These studies document the systemic activation of defense response marker genes in wheat spikes. However, this systemic response is not exclusive of Type II resistance. |
