The Nanomechanics of Cellulose Synthesis

Despite its ubiquity and technological importance, cellulose is synthesized in plants by a process that is poorly understood. Cellulose is a crystalline fiber made up of glucan chains synthesized by a large enzymatic complex that resides in the plant cell membrane. Decisive progress was made more than a decade ago when fluorescently-tagged complexes were first observed moving in the cell membranes of living plants. Complexes move with a speed between 100 and 400 nm/min and this motion is presumed to be related to the rate of cellulose synthesis. We present the results of a careful study of CESA motion in two different plant species over a wide range of timescales. We show several surprising aspects of this motion and discuss models to explain it. After accounting for localization error, the mean squared displacement (MSD) of the moving cellulose synthase complexes shows subdiffusive timescales below 10 seconds suggesting short-range trapping. Over longer timescales, an extant model and particle MSDs support a Brownian ratchet mechanism. However, initial indications are that step-size distributions are non-Gaussian, indicating that a Brownian ratchet alone may not be sufficient to explain CESA mechanics.