The leaf epidermis is the outermost cell layer forming the interface between plants and the atmosphere that must both provide a robust barrier against (a)biotic stressors and facilitate carbon dioxide uptake and leaf transpiration.1 To achieve these opposing requirements, the plant epidermis developed a wide range of specialized cell types such as stomata and hair cells. Although factors forming these individual cell types are known,2-5 it is poorly understood how their number and size are coordinated. Here, we identified a role for BdPRX76/BdPOX, a class III peroxidase, in regulating hair cell and stomatal size in the model grass Bra-chypodium distachyon. In bdpox mutants, prickle hair cells were smaller and stomata were longer. Because stomatal density remained unchanged, the negative correlation between stomatal size and density was dis-rupted in bdpox and resulted in higher stomatal conductance and lower intrinsic water-use efficiency. BdPOX was exclusively expressed in hair cells, suggesting that BdPOX cell-autonomously promotes hair cell size and indirectly restricts stomatal length. Cell-wall autofluorescence and lignin stainings indicated a role for BdPOX in the lignification or crosslinking of related phenolic compounds at the hair cell base. Ectopic expres-sion of BdPOX in the stomatal lineage increased phenolic autofluorescence in guard cell (GC) walls and restricted stomatal elongation in bdpox. Together, we highlight a developmental interplay between hair cells and stomata that optimizes epidermal functionality. We propose that cell-type-specific changes disrupt this interplay and lead to compensatory developmental defects in other epidermal cell types.