Plant cells are surrounded by rigid walls restricting their movement, and therefore oriented cell division is a central mechanism in defining shape during growth and development. Oriented divisions underlie formative events during embryonic and post-embryonic development to generate cell diversity. Despite detailed knowledge of the intrinsic cell division machinery, it is entirely unclear how orientation of cell divisions is controlled by developmental regulators. Classical models in the 19th century predicted that cell geometry constrains division plane through a simple physical rule. It was recently demonstrated that this geometric default rule can indeed explain many divisions in the early Arabidopsis embryo in 3D. However, all asymmetric divisions disobeyed this rule. It was shown that transcriptional response to the auxin hormone is required to suppress default division. Further analysis on the molecular regulation for this suppression revealed multiple Actin regulators. In this project, I will use a combined genetic, cell biological, molecular and computational approach to address 1) how Actin patterns change during embryo development, and do they correlate with oriented cell division, and 2) which proteins regulate Actin dynamics in the early embryo. Furthermore, I will 3) explore whether Actin regulation mediates auxin action in controlling oriented cell division. I expect the outcomes of this project to connect developmental regulators to the cell division machinery and thus to fill a major gap in our knowledge on how supracellular development is implemented into individual cell behaviour.