Science 344, 90–93 (2014)

Conversations about biomass degradation often focus on oligosaccharides to be broken down or sugars that can be liberated during the process, but the plant cell wall also includes the complex aromatic polymer lignin, which complicates degradation. Previous work has shown that lignin biosynthetic pathways are promiscuous enough to incorporate alternative monomers, which can be used to create weak spots in the polymer matrix. Monolignol ferulate conjugates such as coniferyl ferulate are of particular interest as their structure will introduce labile ester bonds into the lignin backbone, and their insertion has been demonstrated in vitro. Wilkerson et al. now extend this strategy to intact plants. The authors first searched for an enzyme that could function orthogonally to the host plant, synthesizing the desired monomers instead of adding to existing monomer pools. Deep sequencing of the root tissue of a Chinese medicinal plant previously shown to contain coniferyl ferulate led to the discovery of a feruloyl coenzyme A monolignol transferase (FMT). Introduction of the encoding gene into a hybrid poplar using a xylem-specific promoter led to spatially and temporally controlled expression, with no obvious phenotypes in the growing plants. Chemical derivatization and characterization of the resulting lignin confirmed the incorporation (and thus production) of the monomers and yielded substantial improvements in digestibility using mild alkaline pretreatment. These results provide the first example of a mature plant designed to be degraded.