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Plant cell morphogenesis relies on the organization and function of two polymer arrays separated by the plasma membrane: the cortical microtubule cytoskeleton and cellulose microfibrils in the cell wall. Studies using in vivo markers confirmed that one function of the cortical microtubule array is to drive organization of cellulose microfibrils by guiding the trajectories of active cellulose synthase (CESA) complexes in the plasma membrane, thus orienting nascent microfibrils. Here we provide evidence that cortical microtubules also position the delivery of CESA complexes to the plasma membrane and interact with small CESA-containing compartments by a mechanism that permits motility driven by microtubule depolymerization. The association of CESA compartments with cortical microtubules was greatly enhanced during osmotic stress and other treatments that limit cellulose synthesis. On recovery from osmotic stress, delivery of CESA complexes to the plasma membrane was observed in association with microtubule-tethered compartments. These results reveal multiple functions for the microtubule cortical array in organizing CESA in the cell cortex.</p>