12-09: Chemical Imaging of Lignin in Plant Cell Walls Using CARS Microscopy

Lignins are considered to be one of the primary contributors in the resistance of plant biomass to be deconstructed to fermentable sugars for biofuels production. To improve biomass conversion processes, deeper understanding of the structural architectures of lignins in plant cell walls is required.  We have developed microscopic tools that have in situ capability to preferably visualize lignin distribution in cell walls with high spatial resolution. The Coherent Anti-Stokes Raman Scattering (CARS) microscopy selectively images a specific chemical structure via its unique chemical bond vibration. Because the contrast mechanism is based on molecular vibrations, which are intrinsic to the samples, no extra labeling and sample preparation are needed.  For lignins, the aromatic ring stretch has a signature Raman mode at 1600 cm-1 that can be used for CARS measurement. We have applied CARS to measure the lignin distribution in transgenic alfalfa plant cell walls, as well as raw and pretreated corn stover. Compared to traditional Raman microscopy of imaging lignin, CARS provides much better signal contrast. Our results have also showed that CARS is an ideal tool to semi-quantitatively measure lignin content in situ.