Leaf growth is a coordinated process fine-tuned by endogenous signals and environmental cues. In Arabidopsis, leaf growth is driven by the epidermis cell layers, composed of pavement cells, stomata and trichomes. Stomata are generated by the well-studied stomatal lineage, in which a Stomatal Lineage Ground Cell (SLGC) divides to form a meristemoid, which ultimately gives rise to stomatal guard cells. Interestingly, after subsequent divisions the SLGC can become a pavement cell, a pathway that generates half of the leaf pavement cells. While the molecular players regulating this SLGC-to-pavement cell transition are currently unknown, our unpublished data demonstrates a key role for the cell cycle inhibitory protein SIAMESE-RELATED1 (SMR1) in this process. In the proposed project, we will use innovative techniques and robust genetics to draw the molecular network around SMR1 during SLGC-to-pavement cell transition. Genes acting up- and downstream of SMR1 in this process will be identified by single-cell RNA sequencing and a forward genetics screen. In addition, SMR1 has previously been linked to osmotic stress and drought, two stresses affecting the numbers of stomata and pavement cells. We will investigate whether the SMR1-network is important in controlling the balance between stomata and pavement cells under drought stress. Altogether, we will reveal the molecular network around an important cell cycle regulator, SMR1, and explore its function in leaf growth under drought.