Phytochrome-interacting factors(PIFs)regulate an array of developmental responses ranging from seed germination to vegetational architecture in Arabidopsis.However,information regarding the functions of the PIF family in monocots has not been widely reported.Here,we investigate the roles of OsPIL15,a member of the rice(Oryza sativa L.cv.Nipponbare)PIF family,in regulating seedling growth.OsPIL15encodes a basic helix-loop-helix factor localized in the nucleus.OsPIL15-OX seedlings exhibit an exaggerated shorter aboveground part and undeveloped root system relative to wild-type seedlings,suggesting that OsPIL15 represses seedling growth in the dark.Microarray analysis combined with gene ontology analysis revealed that OsPIL15 represses a set of genes involved in auxin pathways and cell wall organization or biogenesis.Given the important roles of the auxin pathway and cell wall properties in controlling plant growth,we speculate that OsPIL15 represses seedling growth likely by regulating the auxinpathway and suppressing cell wall organization in etiolated rice seedlings.Additionally,exposure to red light or far-red light relieved growth retardation and promoted seedling elongation in the OsPIL15-OX lines,despite higher levels of OsPIL15transcripts under red light and far-red light than in the dark.These results suggest that light regulation of OsPIL15 expression is probably involved in photomorphogenesis in rice. Phytochrome-interacting factors (PIFs) regulate an array of developmental responses ranging from seed germination to vegetational architecture in Arabidopsis. However, information regarding the functions of the PIF family in monocots has not been widely reported. Here, we investigate the roles of OsPIL 15, a member of the rice (Oryza sativa L. cv. Nipponbare) PIF family, in regulating seedling growth. OsPIL15encodes a basic helix-loop-helix factor localized in the nucleus. OsPIL15-OX seedlings exhibit an exaggerated shorter aboveground part and undeveloped root system relative to wild-type seedlings, suggesting that OsPIL15 represses seedling growth in the dark. Microarray analysis combined with gene ontology analysis revealed that OsPIL15 represses a set of genes involved in auxin pathways and cell wall organization or biogenesis. Giving the important roles of the auxin pathway and cell wall properties in controlling plant growth, we speculate that OsPIL15 represses seedling growth likely by regulating t he auxinpathway and suppressing cell wall organization in etiolated rice seedlings.Additionally, exposure to red light or far-red light relieving growth retardation and promoted seedling elongation in the OsPIL15-OX lines, despite higher levels of OsPIL15transcripts under red light and far-red light than in the dark. These results suggest that light regulation of OsPIL15 expression is probably involved in photomorphogenesis in rice.