Cortical Microtubules Regulate the Insertion of Cellulose Synthase Complexes in the Plasma Membrane

detected GFP-CESA3 in the trans-Golgi network and in a novel compartment, termed the microtubule-associated cellulose syn- thase compartment (MASC), which was found to depend on a dynamic microtubule cyto- skeleton. MASCs were more abundant in the fully elongated cells at the base of the hypocotyl than in the young cells of the apical hook. The fully elongated cells were also found to have a low density of plasma membrane CSCs. Interestingly, the authors were able to trigger MASC formation by exposing elongating hypocotyl cells to osmotic stress, cyclohexi- mide (a protein synthesis inhibitor), or CGA (a cellulose synthesis inhibitor) and noticed that such treatments rapidly decreased the density of CSCs at the plasma membrane. Thus, plasma membrane CSCs appear to be internalized into MASCs under certain con- ditions, and the authors suggest that this internalization may be a means of regulating cellulose synthesis. The authors examined Arabidopsis cotyle- don cells expressing both GFP-CESA3 and the microtubule marker mRFP-MBD. Using high-frame-rate, high-resolution imaging tech- niques, they found that CESA3-labeled Golgi bodies paused for up to 73 s at discrete positions on cortical microtubules (see figure). Numerous Golgi bodies frequently paused successively at the same position. Further- more, they determined that insertion of a CSC into the plasma membrane was always pre- ceded by pausing of a GFP-CESA3-labeled Golgi body just beneath the site of insertion and that pausing sometimes gave rise to the insertion of multiple CSCs. They present a model for CSC trafficking in which actin microfilaments distribute CSC-containing Golgi bodies throughout the cell and cortical microtubules determine the sites of CSC insertion in the plasma membrane, guide their trajectories during cellulose synthesis, and regulate their internalization under certain conditions.