Deciphering the Function and Regulation of SbCAD2: a Key Lignin Gene to Improve Sorghum Biomass Degradability

Genetic modification of lignin biosynthesis in the cell wall of biofuel feedstocks is likely one of the most effective ways to improve the conversion efficiency of cellulosic biomass to biofuel for the bioenergy industry. As a key enzyme that catalyzes the last step of monolignol synthesis, cinnamyl alcohol dehydrogenase (CAD) has become an attractive object in lignin modification to overcome the recalcitrance of biomass conversion. Transgenic down-regulation of CAD gene in bioenergy crops can decrease the lignin biosynthesis and improve their digestibility and saccharification efficiency. However, disrupting expression of this key gene in plants is frequently associated with stunt growth and other developmental abnormalities. To alleviate the negative effects on plant fitness caused by single gene disruption, the present study focuses on the coordinated regulation of a sorghum CAD gene (SbCAD2) at the transcriptional level, leading to an optimized modification strategy for biomass improvement. Specifically, it reports the identification and characterization of a promoter region in SbCAD2 responsible for the down-regulation of lignin production in a sorghum mutant and provides more insights on the rational modification of lignin composition for effective biomass conversion of bioenergy feedstocks.