Development of specialized epidermal cells and structures plays a key role in plant tolerance to biotic and abiotic stresses. In the paddy field, the bright green leaf (bgl) mutants of rice (Oryza sativa) exhibit a luminous green color that is clearly distinguishable from the normal green of wild-type plants. Transmission and scanning electron microscopy revealed that small cuticular papillae (or small papillae; SP), nipple-like structures, are absent on the adaxial and abaxial leaf surfaces of bgl mutants, leading to more direct reflection and less diffusion of green light. Map-based cloning revealed that the bgl locus encodes OsRopGEF10, one of eleven OsRopGEFs in rice. RopGEFs (guanine nucleotide exchange factors for Rop) activate Rop/Rac GTPases, acting as molecular switches in eukaryotic signal transduction by replacing the bound GDP (inactive form) with GTP (active form) in response to external or internal cues. In agreement with the timing of SP initiation on the leaf epidermis, OsRopGEF10 is most strongly expressed in newly developing leaves before emergence from the leaf sheath. In yeast two-hybrid assays, OsRopGEF10 interacts with OsRac1, one of seven OsRac proteins; consistent with this, both proteins are localized in the plasma membrane. These results suggest that OsRopGEF10 activates OsRac1 to turn on the molecular signaling pathway for SP development. Together, our findings provide new insights into the molecular genetic mechanism of SP formation during early leaf morphogenesis.