A cellulose synthase-containing compartment moves rapidly beneath sites of secondary wall synthesis

The woody secondary walls of plants represent the major sites of cellulose deposition. The polymerization of cellulose occurs at the plasma membrane by the secondary wall cellulose synthase complex (CSC). In the long, cylindrical cells that make up the xylem, secondary wall deposition is confi ned to discrete regions of the cell, and yellow fl uorescent protein (YFP)-labeled CSCs are also localized to these regions. Using fl uorescence loss in photobleaching (FLIP) of complete hoops containing YFPCSCs, we demonstrate movement of the complexes beneath the nascent secondary wall in developing xylem vessels. We have devised a method for determining particle velocities for particles moving around a cylindrical object using data from FLIP. By applying this method to the hoops of YFPCSCs of the developing vessels, we have obtained the fi rst estimates of speed of these complexes. These speeds are calculated to be in excess of 7 m s 1 and are far higher than those speeds previously reported for the primary wall complex. These high speeds are unlikely to be consistent with CSC movement being attributed to cellulose synthesis alone, and suggest the existence of a highly motile compartment beneath the nascent secondary wall.