Insight into alkaline deconstruction of genetically modified Zip-poplar by AFEXTM and EA processes

Hardwoods are recalcitrant to existing deconstruction processes that use the sugar platform for biofuels production. Ammonia pretreatments are highly effective for grasses, due to the abundance of ester bonds which crosslink lignin and carbohydrates in the plant cell wall. Also, lignin from grasses tends to be more amenable to ammonia solubilization and is more easily displaced from cell walls, thereby exposing the structural carbohydrates to enzymes. To have similar effects on hardwoods, without having apply high severity conditions to break lignin β-ether linkages, more readily-cleavable ester bonds were incorporated in to the backbone of the lignin polymer from poplar. Poplar lignin was altered by incorporation of coniferyl/synapyl ferulate (CA/SA-FA) conjugates through ferulate monolignol transferase (FMT) expression. This redesigned “Zip-poplar” contains easily cleavable (under mild alkaline conditions) ester linkages in the cell wall structure. Four different Zip-poplar lines with varied CA/SA-FA incorporation were subjected to ammonia fiber expansion (AFEX™) and extractive ammonia (EA) pretreatment processes under varied pretreatment conditions. The pretreated solids were further hydrolyzed with the same range of total enzyme loadings to provide comparative data on glucose and xylose yields of monomers and oligomers. These redesigned Zip-poplars were compared with wild type material in terms of enzymatic saccharification performance. These findings improve our understanding of the mechanisms of ammonia pretreatment of hardwoods involving Zip lignin and native lignin and the effects of lignin chemical structures on hardwood recalcitrance under ammonia treatment.