Sum Frequency Generation Vibrational Spectroscopy and Imaging of Crystalline Cellulose Microfibrils in Plant Cell Walls*

In plant cell walls, cellulose microfibrils (CMFs), which are bundles of cellulose, are the key load-bearing components that are mixed with other matrix polymers to carry specific mechanical properties necessary to fulfill their functional roles. However, due to the structural complexity of plant cell walls, characterizing CMFs without interference from other components is significantly challenging. Sum frequency generation (SFG) vibrational spectroscopy has the capability to selectively characterize crystalline (noncentrosymmetric) materials dispersed in a three-dimensional amorphous medium due to its unique selection rules.

In this study, CMF orientation and directionality were determined using SFG spectroscopy with theoretical calculations, employing the polarizability and dipole derivative tensors obtained from density functional theory (DFT) calculations. Machine-learning-based data processing was employed to perform nonlinear regression for the comparison of experimental and simulated data to determine the structural parameters.

With this unique characterization method, we have discovered that interfascicular fiber (IFF) and xylem cell walls exhibit significantly different CMF orientations and directionality during cell wall synthesis. In the case of IFF, supporting body mass of the plant along the gravity direction; in the case of xylem, to support the negative pressure produced for evapotranspiration of water from root to leaf.