Stomata, the epidermal valves that regulate gas exchange between the plant and its environment, consist of paired guard cells flanking a central pore. They are an accessible model system for the study of asymmetric division regulation in flowering plants. In this study, Dominique Bergmann and colleagues use mutant screening, CRISPR/Cas9 gene editing and live-cell imaging of reporters to characterise stomatal patterning in the grass Brachypodium distachyon. They show that mutations in the BdYDA1 gene, a grass orthologue of the Arabidopsis MAPKKK gene YODA (AtYDA), result in an overabundance and clustering of stomata, confirming the essential role of YODA-dependent MAPK signalling in asymmetric cell divisions. However, unlike in Arabidopsis, BdYDA1 mutants undergo the asymmetric divisions required for stomata development, but fail to establish and maintain the cell fates required for correct epidermal patterning. This suggests that YODA1 signalling is essential for the reinforcement of cell fates in this system. Remarkably, BdYDA1 mutations also affect the development of other cell types, such as hair and silica cells. Taken together, this study demonstrates that BdYDA1 is a general regulator of both cell fate establishment and enforcement throughout the grass leaf epidermis, two processes that are crucial for correct epidermal patterning.
