Wood forming tissue-specific bicistronic expression of PdGA20ox1 and PtrMYB221 improves both the quality and quantity of woody biomass production in a hybrid poplar.

With the exponential growth of the human population and industrial developments, research on renewable energy resources is required to alleviate environmental and economic impacts caused by the consumption of fossil fuels. In this study, we present a synthetic biological application of a wood forming tissue-specific bicistronic gene expression system to improve both the quantity and quality of woody biomass to minimize undesirable growth penalties. Our transgenic poplars, designed to express both PdGA20ox1 (a GA20-oxidase from Pinus densiflora producing bioactive gibberellin, GA) and PtrMYB221 (a MYB transcription factor negatively regulating lignin biosynthesis) under the developing xylem (DX) tissue-specific promoter (i.e., DX15::PdGA20ox1-2A-PtrMYB221 poplar), resulted in a 2-fold increase in biomass quantity compared to wild-type (WT), without undesirable growth defects. A similar phenotype was observed in transgenic Arabidopsis plants harboring the same gene constructs. These phenotypic consequences were further verified in the field experiments. Importantly, our transgenic poplars exhibited an improved quality of biomass with reduced lignin content (~16.0 wt%) but increased holocellulose content (~6.6 wt%). Furthermore, the saccharification efficiency of our transgenic poplar increased significantly by up to 8%. Our results demonstrate that the controlled production of both GA and a secondary wall modifying regulator in the same spatio-temporal manner can be utilized as an efficient biotechnological tool for producing the desired multi-purpose woody biomass.