Masters Theses

Date of Award

12-2017

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant Sciences

Major Professor

C. Neal Stewart Jr.

Committee Members

Charles Kwit, Feng Chen

Abstract

Genetic engineering of switchgrass (Panicum virgatum L.), an emerging cellulosic bioenergy feedstock, has been performed to alter cell walls for improved biofuel conversion. However, gene flow from transgenic switchgrass presents regulatory issues that may prevent commercialization of the genetically engineered crop in the eastern United States. Depending on its expression level, microRNA156 (miR156) can reduce, delay or eliminate flowering, which may be useful to mitigate transgene flow. However, flowering transition is dependent upon both environmental and genetic cues. In this study of transgenic switchgrass, two low (T14 and T35) and two medium (T27 and T37) miR156 overexpressing ‘Alamo’ lines and nontransgenic control plants were used. A two-year field experiment was performed to compare flowering, reproduction, and biomass yield in eastern Tennessee, U.S.A. Growth chamber studies assessed temperature and photoperiod effects on flowering and reproduction across a simulated latitudinal cline.

In the field, medium miR156 overexpression line T37 resulted in the best overall combination of bioconfinement and biomass production. Though line T37 did flower, not all plants produced panicles, and panicle production was delayed in both years. Line T37 also produced fewer panicles, with a 65.9% reduction in year one and 23.8% reduction in year two over controls. T37 panicles produced 70.6% less flowers than control panicles during the second field year with commensurate decreased seed yield: 1205 seeds per plant vs. 18,539 produced by each control. These results are notable given that line T37 produced equivalent vegetative aboveground biomass as controls.

In latitudinal simulation growth chambers, elevated temperatures and decreased daylength promoted flowering of the miR156 transgenic switchgrass lines. As temperatures increased and day lengths decreased, more plants in lines T35, T37, and controls produced panicles. The simulated (Ecuador) tropical conditions were the only chambers in which three of the four transgenic lines flowered.

These results suggest that miR156 overexpression levels found in transgenic line T37 can be useful for bioconfinement, and the plants can significantly reproduce in tropical conditions, which would enable plant breeding for line improvement. Furthermore, the study suggests additional ways that miR156 can be manipulated to improve both biomass production and bioconfinement.

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