Cellulose microfibril orientation in onion (Allium cepa L.) epidermis studied by atomic force microscopy (AFM) and vibrational sum frequency generation (SFG) spectroscopy

Cellulose microfibril orientation in plant cell walls changes during cell expansion and development. The cellulose microfibril orientation in the abaxial epidermis of onion scales was studied by atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy. Onion epidermal cells in all scales are elongated along the onion bulb axis. AFM images showed that cellulose microfibrils exposed at the innermost surface of the abaxial epidermis are oriented perpendicular to the bulb axis in the outer scales and more dispersed in the inner scales of onion bulb. SFG analyses can determine the orientation of cellulose microfibrils averaged over the entire thickness of the cell wall. We found that the average orientation of cellulose microfibrils inside onion abaxial epidermal cell walls as revealed by SFG is similar to the orientation observed at the innermost cell wall surface by AFM. The capability to determine the average orientation of cellulose microfibrils in intact cell walls will be useful to study how cellulose microfibril orientation is related to biomechanical properties and the growth mechanism of plant cell walls.