Developing xylem‐preferential expression of PdGA20ox1, a gibberellin 20‐oxidase 1 from Pinus densiflora, improves woody biomass production in a hybrid poplar

Summary Woody biomass has gained popularity as an environmentally friendly, renewable and sustainable resource for liquid fuel production. Here, we demonstrate biotechnological improvement of the quantity and quality of woody biomass by employing developing xylem ( DX )‐preferential production of gibberellin ( GA ), a phytohormone that positively regulates stem growth. First, for the proof of concept experiment, we produced transgenic Arabidopsis plants expressing GA 20‐oxidase, a key enzyme in the production of bioactive GA s, from Pinus densiflora ( Pd GA 20ox1 ) under the control of either a constitutive 35S promoter, designated 35S::Pd GA 20ox1, or a DX ‐specific promoter (originated from poplar), designated DX 15::Pd GA 20ox1. As we hypothesized, both transgenic Arabidopsis plants ( 35S::Pd GA 20ox1 and DX 15::Pd GA 20ox1 ) exhibited an accelerated stem growth that resulted in a large increase of biomass, up to 300% compared to wild‐type control plants, together with increased secondary wall thickening and elongation of fibre cells. Next, we applied our concept to the production of transgenic poplar trees. Both transgenic poplar trees ( 35S::Pd GA 20ox1 and DX 15::Pd GA 20ox1 ) showed dramatic increases in biomass, up to 300%, with accelerated stem growth and xylem differentiation. Cell wall monosaccharide composition analysis revealed that in both Arabidopsis and poplar, glucose and xylose contents were significantly increased. However, undesirable phenotypes of 35S::Pd GA 20ox1 poplar, including poor root growth and leaf development, were found. Interestingly, DX 15::Pd GA 20ox1 poplar resulted in a reduction of undesirable phenotypes. Our results indicate that the controlled production of GA s through a tissue‐specific promoter can be utilized as an efficient biotechnological tool for producing enhanced plant biomass, minimizing unwanted effects.