Abstract text: The deposition of cell wall material and enzymes at the cell surface via exocytosis is essential for growth. How exocytosis is acting in cell wall expansion is not currently clear. It relies on the cytoskeleton and a membrane-associated machinery controlling the formation of vesicles and their fusion with the plasma membrane. To investigate the role of exocytosis in growth, we use Intracellular Membrane Light-Activated Reversible Inhibition by Assembled Trap (IM-LARIAT) to optogenetically manipulate the activity of RabA4b, a GTPase required for targeting vesicles to the root hair tip. Contrary to knock-out mutants, this system is dominant, circumventing genetic redundancies. Optogenetics allows triggering and observations of events before compensatory responses start. Because IM-LARIAT had not been used in plants, we tested the system in Nicotiana benthamiana leaf epidermis cells. Co-expression of CRY2clust-mCitrine and CRY2-binding protein CIB1 fused to mScarlet3-RabA4b allow us to control the aggregation of RabA4b-containing vesicles with blue light. Using two-photon excitation, we show that light-activated aggregation of CRY2clust/CIB1 allows spatio-temporal control of exocytosis. We show that the light-unresponsive mutant CRY2clustD387A failed to aggregate upon light stimulation. Having provided the proof of concept, we are generating stable Arabidopsis lines to study the role of exocytosis in root hairs growth.