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Time Course Field Analysis of COMT-Downregulated Switchgrass: Lignification, Recalcitrance, and Rust Susceptibility

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Modifying plant cell walls by manipulating lignin biosynthesis can improve biofuel yields from lignocellulosic crops. For example, transgenic switchgrass lines with downregulated expression of caffeic acid O-methyltransferase, a lignin biosynthetic enzyme, produce up to 38 % more ethanol than controls. The aim of the present study was to understand cell wall lignification over the second and third growing seasons of COMT-downregulated field-grown switchgrass. COMT gene expression, lignification, and cell wall recalcitrance were assayed for two independent transgenic lines at monthly intervals. Switchgrass rust (Puccinia emaculata) incidence was also tracked across the seasons. Trends in lignification over time differed between the 2 years. In 2012, sampling was initiated in mid-growing season on reproductive-stage plants and there was little variation in the lignin content of all lines (COMT-downregulated and control) over time. COMT-downregulated lines maintained 11–16 % less lignin, 33–40 % lower S/G (syringyl-to-guaiacyl) ratios, and 15–42 % higher sugar release relative to controls for all time points. In 2013, sampling was initiated earlier in the season on elongation-stage plants and the lignin content of all lines steadily increased over time, while sugar release expectedly decreased. S/G ratios increased in non-transgenic control plants as biomass accumulated over the season, while remaining relatively stable across the season in the COMT-downregulated lines. Differences in cell wall chemistry between transgenic and non-transgenic lines were not apparent until plants transitioned to reproductive growth in mid-season, after which the cell walls of COMT-downregulated plants exhibited phenotypes consistent with what was observed in 2012. There were no differences in rust damage between transgenics and controls at any time point. These results provide relevant fundamental insights into the process of lignification in a maturing field-grown biofuel feedstock with downregulated lignin biosynthesis.

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Acknowledgments

We thank Angela Ziebell, Erica Gjersing, Crissa Doeppke, and Melvin Tucker for assistance with the cell wall characterization. We also thank Ben Wolfe, Marcus Laxton, and the UT field staff for general field maintenance and assistance with sample collection, Reggie Millwood for assistance with the USDA APHIS BRS permit regulations, Erika Barton for assistance with sample preparation, and Susan Holladay for assistance with data entry into LIMS. This work was supported by funding from the Southeastern Sun Grant Center and the BioEnergy Science Center. The BioEnergy Science Center is a U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological and Environmental Research in the DOE Office of Science.

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Authors and Affiliations

  1. Department of Plant Sciences, University of Tennessee, Knoxville, TN, 37996, USA

    Holly L. Baxter, Mitra Mazarei & C. Neal Stewart Jr.

  2. BioEnergy Science Center (BESC), Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA

    Holly L. Baxter, Mitra Mazarei, Chunxiang Fu, Geoffrey B. Turner, Robert W. Sykes, Mark F. Davis, Richard A. Dixon, Zeng-Yu Wang & C. Neal Stewart Jr.

  3. Samuel Roberts Noble Foundation, Ardmore, OK, 73401, USA

    Chunxiang Fu & Zeng-Yu Wang

  4. Department of Entomology and Plant Pathology, University of Tennessee, Knoxville, TN, 37996, USA

    Qunkang Cheng & Mark T. Windham

  5. National Renewable Energy Laboratory, Golden, CO, 80401, USA

    Geoffrey B. Turner, Robert W. Sykes & Mark F. Davis

  6. Department of Biological Sciences, University of North Texas, Denton, TX, 76203, USA

    Richard A. Dixon

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  1. Holly L. Baxter
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Baxter, H.L., Mazarei, M., Fu, C. et al. Time Course Field Analysis of COMT-Downregulated Switchgrass: Lignification, Recalcitrance, and Rust Susceptibility. Bioenerg. Res. 9, 1087–1100 (2016). https://doi.org/10.1007/s12155-016-9751-1

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