Significant Increases in Pulping Efficiency in C4H-F5H-Transformed Poplars: Improved Chemical Savings and Reduced Environmental Toxins

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Abstract
The gene encoding ferulate 5-hydroxylase (F5H) was overexpressed in poplar (Populus tremula × Populus alba) using the cinnamate-4-hydroxylase (C4H) promoter to drive expression specifically in cells involved in the lignin biosynthetic pathway and was shown to significantly alter the mole percentage of syringyl subunits in the lignin, as determined by thioacidolysis. Analysis of poplar transformed with a C4H-F5H construct demonstrated significant increases in chemical (kraft) pulping efficiency from greenhouse-grown trees. Compared to wild-type wood, decreases of 23 kappa units and increases of >20 ISO brightness units were observed in trees exhibiting high syringyl monomer concentrations. These changes were associated with no significant modification in total lignin content and no observed phenotypic differences. C4H-F5H-transformed trees could increase pulp throughputs at mills by >60% while concurrently decreasing chemicals employed during processing (chemical pulping and bleaching) and, consequently, the amount of deleterious byproducts released into the environment. Keywords: Ferulate 5-hydroxylase (F5H); lignin biosynthesis; poplar; lignin degradation; syringyl lignin; lignin monomers; pulping; thioacidolysis; DFRC