Acyl-CoA monolignol transferases couple a CoA thioester substrate to a monolignol, a hydroxycinnamyl alcohol, creating an ester-linked monolignol conjugate for lignification. p-Coumaroyl-CoA monolignol transferases (PMTs) increase the production of monolignol p-coumarates, thereby increasing the value of lignin with the p-coumarate “clip-off” and its byproducts. Feruloyl-CoA monolignol transferases (FMTs) improve cell wall saccharification, after mild pretreatments, by catalyzing the production of monolignol ferulate conjugates that create readily-cleavable bonds in the polymer backbone, and also add ferulate to the lignin as a clip-off product. Optimization of monolignol conjugate production in the bioenergy crop plant Sorghum bicolor can lead to improvements in digestibility and lignin value. The optimal bioenergy sorghum plant would have one type of monolignol conjugate to prevent competition between the acyltransferases for substrates and to decrease the cost associated with clipping off and purifying the p-coumaric acid or ferulic acid from the cell wall. One strategy we are employing to meet this goal is to knock out the native SbPMT using CRISPR-Cas9 and to concurrently overexpress the native SbFMT, which will significantly decrease or eliminate monolignol p-coumarates from the lignin while increasing the level of ferulates on the lignin. Together with the high native levels of ferulate on sorghum hemicelluloses, these lines have the potential to be rich in a single valuable clip-off product, ferulic acid, and may also have improved cell-wall saccharification.
