Field and saccharification performances of poplars severely downregulated in CAD1

o Lignin is one of the main factors causing lignocellulosic biomass recalcitrance to enzymatic hydrolysis. Greenhouse-grown poplars severely downregulated for CINNAMYL ALCOHOL DEHYDROGENASE 1 (CAD1), the enzyme catalyzing the last step in the monolignol-specific branch of lignin biosynthesis, have increased saccharification yields and normal growth. o Here, we assess the performance of these hpCAD poplars in the field under short rotation coppice culture for two consecutive rotations of one and three years. o While one-year-old hpCAD wood had 10% less lignin, three-year-old hpCAD wood had wild-type lignin levels. Because of their altered cell wall composition, including elevated levels of cinnamaldehydes, both one- and three-year-old hpCAD wood showed enhanced saccharification yields upon harsh alkaline pretreatments (up to +85% and +77%, respectively). In contrast to previous field trials with poplars less severely downregulated for CAD, the hpCAD poplars displayed leaning phenotypes, early bud set, early flowering and yield penalties. Moreover, hpCAD wood had enlarged vessels, decreased wood density, and reduced relative and free water contents. o Our data show that the phenotypes of CAD-deficient poplars are strongly dependent on the environment and underpin the importance of field trials in translating basic research towards applications.