PhD Candidate Pennsylvania State University State College, Pennsylvania
Body of Abstract: The highly extensible epidermal cell walls limit cell growth and enable plants to comply with external loads such as wind and accumulated snow. Characterizing how wall polymers respond to high strains will offer insights into dynamic wall structures and deformation mechanisms that allow extensive wall extension. The onion outer epidermal cell wall, a thin sheet (~7 µm) consisting of only a single layer of primary cell walls, was used for mechanical and structural characterization. We fixed the wall stretched longitudinally to different levels of strains (0-50%) and examined their structures at different molecular scales using microscopy and X-ray scattering techniques. The optical microscopy showed that upon stretching, the wall area began decreasing with significant lateral shrinkage at strains of 20%. Further examination by atomic force microscopy showed distinct surface cellulose fibril structures at strains beyond 20%, with straightened and aligned longitudinal fibrils and curved and kinked transverse fibrils, indicating orientation-dependent cellulose deformation modes. Small-angle X-ray scattering showed that once cellulose fibrils longitudinally aligned at 20% strain, they packed with a center-to-center spacing of 7.4 nm, where matrix polymers, mainly pectin, may act as a space filler that prevents irreversible fibril bundling or collapsing. Wide-angle X-ray scattering revealed the nanoscale structural change of cellulose at 45% strain: the backbone of longitudinal cellulose was stretched by 0.4% while the transverse cellulose remained unstretched. The stretching of the cellulose backbone was observed for the first time in the primary cell walls, indicating that cellulose fibrils oriented towards the stretching direction bear large stresses in highly stretched walls. These observations showed dynamic rearrangement of wall polymers upon extensive mechanical stretching, shedding light on the structure-mechanical property relationships of cell wall polymers and refining our models of cell wall microstructure.