Role of the actin cytoskeleton in cellular morphogenesis at the shoot apical meristem of Arabidopsis thaliana The morphogenesis of sessile plants is mainly driven by directional cell growth and cell division. The organization of their cytoskeleton and the mechanical properties of the cell wall greatly influence morphogenetic events in plants. It is well known that cortical microtubules (CMTs) contribute to directional growth by regulating the deposition of the cellulose microfibrils as major cell wall fortifying elements. More recent findings demonstrate that mechanical stresses existing in cells and tissues influence microtubule organization. Also in dividing cells mechanical stress directions contribute to the orientation of the new cell wall. In comparison to the microtubule cytoskeleton the role of the actin cytoskeleton in regulating shoot meristem morphogenesis has not been extensively studied. This thesis focuses on the functional relevance of the actin cytoskeleton during cell and tissue scale morphogenesis in the shoot apical meristem (SAM) of Arabidopsis thaliana. [...]