Genetic modification of switchgrass (<i>Panicum virgatum</i> L.) for improvement of plant architecture, biomass productivity and sugar release efficiency for biofuel

Switchgrass (Panicum virgatum L.) is a leading candidate bioenergy crop for sustainable biofuel production. To ensure its economic viability, tremendous improvements in switchgrass biomass productivity and recalcitrance to enzymatic saccharification are needed. Genetic manipulation of lignin biosynthesis by targeting transcriptional regulators of higher level domains of lignin biosynthesis and other complex traits could alter several bioenergy-desirable traits at once. A three-pronged approach was made in the dissertation research to target one plant growth regulator and transcription factors to alter plant architecture and cell wall biosynthesis.

Gibberellin (GA) catabolic enzymes, GA 2-oxidases (GA2oxs), were utilized to alternatively modify the lignin biosynthesis pathway as GA is known to play a role in plant lignification. Constitutive overexpression of switchgrass C₂₀ [C20] GA2ox genes altered plant morphology and modified plant architecture by increasing the number of tillers. Moreover, transgenic plants exhibited reduced lignin especially in leaves accompanied by 15% increase in sugar release (glucose).

The Knotted1 (PvKN1) TF, a putative repressor of lignin biosynthesis genes, was identified and evaluated for improving biomass characteristics of switchgrass for biofuel. Its ectopic overexpression in switchgrass altered the expression of genes in the lignin, cellulose and hemicellulose biosynthesis, and GA signalling pathways. Consequently, transgenic lines displayed altered growth phenotypes particularly at early stages of vegetative development and moderate changes in lignin content accompanied by improved sugar release by up to 16%.

The APETALA2/ ethylene responsive factor (AP2/ERF) TFs are key putative targets for engineering plants not only so they can withstand adverse environmental factors but also confer modified cell wall characteristics. To facilitate this, a total of 207 switchgrass AP2/ERF TFs comprising 3 families (AP2, ERF and related to API3/VP (RAV)) were identified. Sequence analysis for conserved putative motifs and expression pattern analysis delimited key genes for manipulation of switchgrass. To that end, the PvERF001 TF gene was ectopically overexpressed resulting in improved biomass yield and sugar release efficiency.

The transgenic plants and knowledge produced in this research will be used to create new lines of switchgrass with combined novel traits to address needs in biofuel production and sustainable plant cultivation to enable the development of the bioeconomy.