Wednesday, May 2, 2007 - 8:00 AM

Tracking lignin coalescence and migration through plant cell walls during pretreatment

Bryon S. Donohoe1, Melvin P. Tucker2, Mark Davis2, Stephen R. Decker1, Michael E. Himmel1, and Todd B. Vinzant1. (1) Chemical and Biosciences Center, National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401, (2) National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401

The abundance and distribution of lignin within plant cell walls is thought to be a critical determinant of biomass digestibility. It remains unclear the exact role process modified lignin plays in enzyme accessibility to cellulose microfibrils. In this study, we have directly visualized lignin from its origin in the cell wall matrix to its emergence as droplets at the cell wall surface by TEM, SEM, and electron tomography. We have also employed NMR, FTIR, immuno-cytochemical staining, and immuno-electron microscopy to confirm the identity of extracted and in situ droplets as lignin derived. In native material, lignin is a homogeneously distributed component of the lignified cell wall. During pretreatment that exceeds the in planta glass transition temperature of lignin, the lignin appears to coalesce into small (10-100 nm) droplets within the cell wall and migrate out of the cell wall. Lignin is seen to emerge from the cell wall into pits, cell corners, and the luminal cell wall surface. Finally, discrete droplets (50-5000 nm) of lignin appear coating cell wall surfaces. We propose a mechanism whereby melted lignin coalesces because of a phase separation from water and is driven out of the cell wall largely due to microfibril collapse during the redistribution of matrix material. Further testing of this hypothesis will provide insight into the optimal manipulation of lignin to enable maximum enzyme accessibility.