Low-lignin transgenic poplar: Is there a trade-off between enhanced fiber production and biomechanical stability?

Society depends heavily on wood for structural material, energy, and for its component fibers. Most end uses for wood fiber require the removal of lignin, an energy intensive process. Fermentation of wood to produce bioethanol, a growing source of transportation fuels, would benefit from reduction of lignin content.  In poplar (Populus spp.), antisense downregulation of the gene Pt4CL1 that encodes 4-coumarate:coenzyme A ligase (4CL), has been previously shown to inhibit the deposition of lignin by up to 45 %, while enhancing cellulose content by 15%, in greenhouse environments. To evaluate the effects of transgenic modification of lignin on wood properties in the field, we studied biomechanical properties of trees derived from 14 transgenic events of a hybrid white poplar (P. tremula × P. alba). We conducted >1,100 bending tests. The wood of the transgenic lines had a significantly lower modulus of elasticity (MOE), modulus of rupture (MOR), and material toughness than branches of similar size from non-transgenic trees (P<0.05). The effect was greater for larger branches (range in diameter 1.8-5.4 mm). If larger branches have lower mechanical properties, one could expect either poorer survival and/or productivity in transgenics due to wind and ice-related breakage, or altered tree form and tensionwood development to maintain similar stress distributions. These results suggest that evaluation of biomechanics during field trials of trees with transgenic modifications to wood chemistry is important to understand their value and risks. Studies of transgene expression, lignin content, and rate of growth during a second year of field evaluation are underway.