MicroRNA408 Enhances Growth and Saccharification Efficiency Through Altering the Balance between Vascular Development and Lignification in Hybrid Poplar

Saccharification, the conversion of cellulose to sugar for fermentation-based biofuels, remains problematic for lignocellulosic feedstocks such as wood. Genetic modifications to reduce the amount or change the structure of lignin can improve saccharification efficiency but often result in dwarf plants. Here, we report that overexpression of Pag-MIR408 in hybrid poplar results in a near doubling saccharification efficiency, with no requirement for acid-pretreatment, increased accessibility of secondary cell walls to fungal cellulase and scaffoldin cellulose-binding modules, accompanied by enhanced photosynthesis, and increased number of cambium zone layers, xylem width and vessel diameter. Conversely, Pag-MIR408 loss-of-function plants by CRISPR/Cas9 showed decreased cell wall accessibility. Moreover, miR408 targets LACCASE 19/25, leading to delayed lignification, and modestly reduced lignin content, S/G ratio, as well as the degree of lignin polymerization. This study provides a unique insight into enhanced saccharification efficiency altered wood structure through modulation of the balance between secondary development and lignification.