Time Course Field Analysis of COMT-Downregulated Switchgrass: Lignification, Recalcitrance, and Rust Susceptibility
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Time Course Field Analysis of COMT-Downregulated Switchgrass: Lignification, Recalcitrance, and Rust Susceptibility Holly L. Baxter 1,2 & Mitra Mazarei 1,2 & Chunxiang Fu 2,3 & Qunkang Cheng 4 & Geoffrey B. Turner 2,5 & Robert W. Sykes 2,5 & Mark T. Windham 4 & Mark F. Davis 2,5 & Richard A. Dixon 2,6 & Zeng-Yu Wang 2,3 & C. Neal Stewart Jr. 1,2
Published online: 18 May 2016 # Springer Science+Business Media New York 2016
Abstract 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 reproductivestage 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 COMTdownregulated 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.
Electronic supplementary material The online version of this article (doi:10.1007/s12155-016-9751-1) contains supplementary material, which is available to authorized users.
Keywords Biomass . Caffeic acid O-methyltransferase (COMT) . Lignin . Lignocellulosic biofuel . Switchgrass
* C. Neal Stewart, Jr. [email protected]
Introduction
1
Department of Plant Sciences, University of Tennessee, Knoxville, TN 37996, USA
2
BioEnergy Science Center (BESC), Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
3
Samuel Roberts Noble Foundation, Ardmore, OK 73401, USA
4
Department of En
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