Control of cell size has long mystified cell biologists as it is a key biological process, inherent to unicellular and multicellular organisms. In multicellular organisms, cell size control is a component of normal organ growth, patterning and differentiation. One understudied aspect that positively correlates with cell size is ploidy. It is generally accepted that cell size positively correlates with DNA content, but why this occurs is still obscure. It has been proposed that increased ploidy releases biochemical constraints on cell size. Alternatively, changes in ploidy may directly affect a mechanism that links cell cycle progression to chromatin content (D'Ario M., et al., 2021). We sought to test both hypotheses by exploring the role of RETINOBLASTOMA RELATEDÂ 1 (RBR1) as a potential factor that links ploidy to cell size regulation in the stomatal lineage. RBR1 is an inhibitor of entry into and progression through the S phase, and in the stomatal lineage the rbr1 mutation causes cells to overproliferate. To test whether the loss of RBR1 function abolishes the effect of ploidy on cell size, we created a genetic resource where we can track the development of cell lineages that have lost RBR1 function. In this poster we will present the first results of these assays, discuss how cell size regulation in polyploid stomata might have implications in the selection of natural polyploids and explore the fine role of RBR1 during stomatal development.