Monolignol Ferulate Transferase Introduces Chemically Labile Linkages into the Lignin Backbone

Constructed for Deconstruction Lignin provides strength to wood but also impedes efficient degradation when wood is used as biofuel. Wilkerson et al. (p. 90) engineered poplar to produce lignin that is more amenable to degradation. From a handful of plants that contain more digestible lignin monomers, Angelica sinensis was selected and its monolignol transferase activities analyzed. The enzyme involved, coniferyl ferulate feruloyl-CoA monolignol transferase, was then expressed in poplar. The resulting poplar trees showed no difference in growth habit under greenhouse conditions, but their lignin showed improved digestibility. Engineered poplar lignin with readily cleavable ester bonds in the polymer backbone improves wood degradability. Redesigning lignin, the aromatic polymer fortifying plant cell walls, to be more amenable to chemical depolymerization can lower the energy required for industrial processing. We have engineered poplar trees to introduce ester linkages into the lignin polymer backbone by augmenting the monomer pool with monolignol ferulate conjugates. Herein, we describe the isolation of a transferase gene capable of forming these conjugates and its xylem-specific introduction into poplar. Enzyme kinetics, in planta expression, lignin structural analysis, and improved cell wall digestibility after mild alkaline pretreatment demonstrate that these trees produce the monolignol ferulate conjugates, export them to the wall, and use them during lignification. Tailoring plants to use such conjugates during cell wall biosynthesis is a promising way to produce plants that are designed for deconstruction.