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Curd is a specialized organ as well as the most important product organ of cauliflower (Brassica oleracea L.var.botrytis).However, the mechanism underlying the regulation of curd formation and development remains largely unknown.In the present study, a novel homologous gene containing the Organ Size Related (OSR) domain, namely, CDAG1 (Curd development associated gene 1) was identified in cauliflower.Quantitative analysis indicated that CDAG1 showed significantly high transcript levels in young tissues.Functional analysis demonstrated that ectopic overexpression of CDAG1 in Arabidopsis and cauliflower could significantly promote organ growth and result in bigger organ size then increase biomass.The enlarged organs were demonstrated predominantly due to increase of cell number.In addition, 228 genes involved in CDAG1-mediated regulatory network were discovered by transcriptome analysis.Among these genes, CDAG1 was confirmed to inhibit the transcriptional expression of endogenous OSR genes, ARGOS and ARL, while a series of ethylene-responsive transcription factors (ERFs) were identified to increased expression in 35S:CDAG1 transgenic Arabidopsis plants.It implied that CDAG1 may function in ethylene-mediated signal pathway.These findings provide new insights into the function of OSR genes, and suggest the potential applications of CDAG1 in breeding high-yielding crops.