Theses and Dissertations at Montana State University (MSU)
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Item Understanding resistance and transcriptional responses to potato virus Y infection in potato plants(Montana State University - Bozeman, College of Agriculture, 2021) Ross, Brian Thomas; Chairperson, Graduate Committee: Michelle Flenniken; Nina Zidack and Michelle L. Flenniken were co-authors of the article, 'Extreme resistance to viruses in potato and soybean' in the journal 'Frontiers in plant science' which is contained within this dissertation.; Nina Zidack and Michelle L. Flenniken were co-authors of the article, 'Transcriptional responses to potato virus Y infection in resistant and susceptible potato cultivars' submitted to the journal 'Cultivars' which is contained within this dissertation.The potato is one of the world's most important crops. Cultivation of potatoes occurs on every continent except Antarctica and in a wide variety of climates. Potatoes are susceptible to a multitude of pathogens that can decrease yield and market quality. Viruses are particularly problematic for potato growers, as most potato production involves the replanting of tubers grown the previous year. Because virus-infected potato plants can harbor virus in their tubers, these tubers can in turn be the source of infection in the next generation of plants. Strains of Potato virus Y are the most economically burdensome viruses for potato growers worldwide. In field settings, Potato virus Y is primarily transmitted to plant by aphids feeding on leaves, but PVY can also be transmitted mechanically through infected plant sap. The use of insecticides and the application of mineral oil to leaves can help limit aphid populations and prevent infection to an extent but are generally both less effective and more environmentally impactful than genetic antiviral resistance mechanisms. The incorporation of genes that provide durable resistance to Potato virus Y into commercial potatoes is a major focus of potato breeders. One form of resistance, called extreme resistance, is characterized by a lack of symptoms and little to no virus replication occurring at the site of infection, but the molecular mechanisms of this response are not well understood. A comprehensive analysis of the extreme resistance literature indicates that movement of the resistance protein from the cytoplasm to the nucleus of the cell directly after virus infection may be a key aspect of this immune response. The downstream, transcriptional aspects of the extreme resistance response are also not well understood. We analyzed the gene expression from a Potato virus Y-resistant potato variety, Payette Russet, and a commonly grown susceptible variety, Russet Burbank, at a series of time points after virus infection using RNA sequencing. Results of these analyses indicate that an immune response likely occurs in Payette Russet quickly after virus inoculation. These analyses also indicate that the virus-susceptible variety, Russet Burbank, exhibits changes in gene expression that are similar to other susceptible potato varieties during asymptomatic or tolerant infection. Furthering our understanding of the molecular mechanisms controlling resistance and severity of virus infections will help inform future breeding and genetic engineering efforts, which require detailed knowledge of the mechanisms of virus resistance.Item Effect of the WRKY76 transcription factor on starch biosynthesis and plant growth(Montana State University - Bozeman, College of Agriculture, 2016) Estabrooks, Hannah Margaret; Chairperson, Graduate Committee: Jack MartinStarch is an important contributor to plant growth as excess photosynthate is stored in leaves as starch during the day to be mobilized at night and re-synthesized in sink tissues. Starch is also the principal constituent of cereal seeds and its variation greatly influence crop yields. The starch pathway is complex and its regulation is not fully understood. Transcription Factors (TFs) are known to act as master regulators of whole biosynthetic pathways and the work presented here was aimed at gaining a better understanding of starch production in leaves by identifying a TF which specifically regulates the leaf starch biosynthetic pathway. Leaf starch levels are regulated in part by ADP-glucose pyrophosphorylase (AGPase), the rate limiting step of starch biosynthesis. Transgenic rice event (NR16+) with increased leaf AGPase activity (due to overexpression of the AGPase large and small subunit transgenes Sh2r6hs and Bt2) was subjected to RNA-sequencing. The results indicated that the leaf specific AGP transgene, which increased leaf starch, also had upregulation of the WRKY76 TF. Another regulatory protein, F-Box, was chosen as a candidate due to the F-box family's involvement in plant development. The current study examines the potential of these gene products for increasing starch biosynthesis in leaves via leaf specific overexpression. Results indicate that overexpression of WRKY76 increases leaf starch an average of 39% at both the one month and anthesis growth stages in comparison to the Varietal Control Nipponbare. WRKY76 transgenic lines have enhanced phenotype with an improved harvest index due to biomass and immature panicles trending down by 4% and 21% respectively, while seed weight trended 12% higher. Events overexpressing WRKY76 were also found to upregulate important starch biosynthetic and carbon metabolism genes including AGPL1, AGPS2, SSIIIb, GBSII, Rbcs, PRK, and GS2 as well as leading to a general upregulation of leaf tissue carbohydrates. Events 1, 2, and 12 additionally had on average 13% increased photosynthetic rate at the one month growth stage. The findings of this study support WRKY76 as a dynamic regulator of the starch biosynthetic pathway conferring more efficient carbon assimilation leading to an increased harvest index.