Enhanced characteristics of genetically modified switchgrass (Panicum virgatum L.) for high biofuel production

Abstract

BackgroundLignocellulosic biomass is one of the most promising renewable and clean energy resources to reduce greenhouse gas emissions and dependence on fossil fuels. However, the resistance to accessibility of sugars embedded in plant cell walls (so-called recalcitrance) is a major barrier to economically viable cellulosic ethanol production. A recent report from the US National Academy of Sciences indicated that, “absent technological breakthroughs”, it was unlikely that the US would meet the congressionally mandated renewable fuel standard of 35 billion gallons of ethanol-equivalent biofuels plus 1 billion gallons of biodiesel by 2022. We here describe the properties of switchgrass (Panicum virgatum) biomass that has been genetically engineered to increase the cellulosic ethanol yield by more than 2-fold.ResultsWe have increased the cellulosic ethanol yield from switchgrass by 2.6-fold through overexpression of the transcription factor PvMYB4. This strategy reduces carbon deposition into lignin and phenolic fermentation inhibitors while maintaining the availability of potentially fermentable soluble sugars and pectic polysaccharides. Detailed biomass characterization analyses revealed that the levels and nature of phenolic acids embedded in the cell-wall, the lignin content and polymer size, lignin internal linkage levels, linkages between lignin and xylans/pectins, and levels of wall-bound fucose are all altered in PvMYB4-OX lines. Genetically engineered PvMYB4-OX switchgrass therefore provides a novel system for further understanding cell wall recalcitrance.ConclusionsOur results have demonstrated that overexpression of PvMYB4, a general transcriptional repressor of the phenylpropanoid/lignin biosynthesis pathway, can lead to very high yield ethanol production through dramatic reduction of recalcitrance. MYB4-OX switchgrass is an excellent model system for understanding recalcitrance, and provides new germplasm for developing switchgrass cultivars as biomass feedstocks for biofuel production.

Silencing CHALCONE SYNTHASE in Maize Impedes the Incorporation of Tricin into Lignin and Increases Lignin Content

“…Therefore, at least part of the lower Glc release is due to the reduced cellulose content in the leaves of C2-Idf plants. It has been established that saccharification efficiency is negatively correlated with lignin amount in both monocot and dicot species (Chen and Dixon, 2007;Sattler et al, 2010;Xu et al, 2011;Bouvier d'Yvoire et al, 2013;Jung et al, 2013;Van Acker et al, 2013;Vanholme et al, 2013a) and that a lower degree of polymerization of lignin eases its extraction from the biomass (Eudes et al, 2012;Vanholme et al, 2012;Shen et al, 2013). The alkaline pretreatment hydrolyzes ester bonds, thereby releasing (di)ferulates that acylate the hemicellulose (and of which a part is also coupled to lignin) and p-coumarates that additionally acylate the lignin backbone (Ralph, 2010).…”

Transgenic switchgrass (Panicum virgatum L.) targeted for reduced recalcitrance to bioconversion: a 2‐year comparative analysis of field‐grown lines modified for target gene or genetic element expression

“…Field‐grown switchgrass transgenic lines with high sugar release or good growth phenotypes included those silencing or OE: (i) GAUT4, GAUT4 ‐knockdown (KD) lines down‐regulating expression of galacturonosyltransferase4 , a gene encoding an enzyme involved in pectin biosynthesis (Biswal et al ., Biswal, A.K., Atmodjo, M.A., Li, M., Yoo, C.G., Pu, Y., Lee, Y.‐C., Zhang, J.Y., Bray, A., King, Z., LaFayette, P., Mohanty, S.S., Ryno, D., Yee, K., Thompson, O.A., Rodriguez Jr, M., Winkeler, K., Collins, C., Yang, X., Tan, L., Sykes, R.W., Gjersing, E., Ziebell, A., Turner, G.B., Decker, S.R., Parrot, W., Udvardi, M.K., Mielenz, J., Davis, M.F., Nelson, R.S., Ragauskas, A.J., and Mohnen, D.); (ii) miRNA, miRNA156 ‐overexpression (OE) lines OE miRNA156, a regulator of plant developmental processes (Fu et al ., 2012); (iii) MYB4, MYB4 ‐OE lines OE PvMYB4, an R2R3‐type MYB repressor of the lignin biosynthetic pathway (Shen et al ., 2012, 2013); (iv) COMT, COMT ‐KD lines down‐regulating expression of caffeic acid O ‐methyltransferase, a lignin biosynthetic gene (Fu et al ., 2011); and (v) FPGS, FPGS ‐KD lines down‐regulating expression of folylpolyglutamate synthase 1 , a gene encoding a C1 metabolism enzyme believed to provide methyl groups for lignin biosynthesis (Srivastava et al ., 2015). …”

Biofuels Get in the Fast Lane: Developments in Plant Feedstock Production and Processing

“…In switchgrass, a great number of investigations have focused on improving bioethanol relevant traits by the aforementioned approaches of classical breeding or genomic technologies [209,210]. Notably, crosses between tetraploid upland and lowland switchgrass resulted in hybrid vigor and progeny displayed increase of biomass yields in the range of 30-60% pointing to exploiting heterosis for generating elite cultivars [211].…”

“…Recently a genetic linkage map was developed by crossing upland and lowland switchgrass ecotypes which will provide a complete switch grass map and enable the identification of quantitative trait loci associated with biomass quantity and quality characters as well as cold tolerance and abiotic stress traits [212]. At the level of specific gene targeting, efforts to reduce lignin content focused on the down regulation of the 4CL1, COMT and CAD genes that are involved in lignin biosynthesis [213][214][215] and the over expression of the R2R3-MYB transcription factor [216,217]. Interestingly, a transgenic switch grass that over expresses the maize Corngrass1 (Cg1) microRNA, which is a micro RNA that locks the plant in the juvenile stage has reduced lignin levels, 250% more starch and higher saccharification efficiency [218].…”