Root hair development involves asymmetric cell division in Brachypodium distachyon and symmetric division in Oryza sativa.

• The root epidermis of most angiosperms comprises hair (H) cells and nonhair (N) cells. H cells are shorter than N cells in grasses (Poaceae). • The aim of this study was to determine the developmental basis for differences in H and N cell size in the grasses Brachypodium distachyon and Oryza sativa. • We show that cytokinesis in the last cell division in each epidermal file is asymmetric in B. distachyon. The smaller daughter cell becomes an H cell and the larger cell forms an N cell. By contrast, asymmetric cytokinesis does not occur during H cell and N cell development in O. sativa and the differences in size arise because there is more cell expansion in N cells than in H cells after root hair initiation. • The different sizes of mature H and N cells result from cell division asymmetry in B. distachyon but different rates of cell expansion in O. sativa. We hypothesize that the mechanism that includes asymmetric cytokinesis during the development of H and N cells evolved among the Pooideae or ancestors of this subfamily.