Molecular Pathogenesis of Viral and Subviral Agents in Model and Crop Grasses
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Viral diseases cause significant agricultural yield losses globally. Because grasses constitute most of our food, forage, and bioenergy sources, the resulting economic losses caused by grass-infecting viruses are particularly devastating. However, the lack of an established genetic model for grasses has generally hindered investigation of grass-virus molecular interactions, leading to a gap in our knowledge of monocot virology. In this study, the issue of the monocot virology knowledge gap is addressed at the levels of both the laboratory and the field. Two candidate grasses, Brachypodium distachyon (Brachypodium, a C3 grass) and Setaria viridis (Setaria, a C4 grass), were established as monocot model hosts for seven small RNA viruses in diverse genera. Aspects of the host disease response were characterized, including agronomically relevant phenotypic perturbations and expression profiles of defense hormone marker genes in salicylic acid, jasmonic acid, and ethylene signaling pathways. This comparative viromics approach revealed conserved and host-dependent defense hormone signaling responses to the diverse viral agents, as well as virus-specific responses between the C3 and C4 model hosts. Further, Brachypodium and the food crop Panicum miliaceum (proso millet) were utilized as laboratory models for investigating novel molecular features of Panicum mosaic virus (PMV), its associated satellite virus (SPMV), and satellite RNAs (satRNAs), the collective causal agents of the turfgrass disease known as St. Augustine Decline. A satRNA of PMV isolated from Stenotaphrum secundatum (St. Augustinegrass), satS, attenuates the normal disease phenotype induced by its PMV helper virus and actively acquires ~100-200 nucleotides from the 3′-end of the PMV helper virus RNA genome. This symptom attenuation and sequence acquisition is associated with host-dependent reductions in the systemic accumulation of helper virus RNA and capsid protein. Brachypodium and proso millet were also used to characterize the de novo polyadenylation of PMV and its subviral agents. The polyadenylated PMV RNAs resemble byproducts of a poly(A)-mediated RNA degradation pathway. Lastly, we report on the re-emergence of PMV and SPMV as the predominant viral pathogens of cultivated switchgrass in Nebraska. The Summer-based switchgrass varieties were more susceptible to PMV and PMV+SPMV infections, compared to Kanlow-based varieties. The susceptible varieties were more severely affected by the disease. Overall this study investigates questions of host-virus interactions, both in the laboratory and the field, and presents new findings on the topic of grass RNA virus biology.
SubjectPanicum mosaic virus
Pyle, Jesse Dylan (2015). Molecular Pathogenesis of Viral and Subviral Agents in Model and Crop Grasses. Master's thesis, Texas A & M University. Available electronically from