The MADS-box transcription factor-encoding genes are expressed mainly during plant reproductive development, where they play important roles in controlling floral organ initiation and identity. Few previous reports have investigated the functions of MADS-box transcription factors expressed in vegetative tissues. Here we describe the functional characterization of a rice AG-like MADS-box protein, OsMDP1 (Oryza sativa MADS-domain-containing protein 1). A partial cDNA encoding a MADS-box domain was identified via high-throughput screening of rice brassinolide-regulated genes, and the full-length cDNA was subsequently isolated via screening of a cDNA library constructed from rice materials at tillering stage. Expression pattern analyses indicated that OsMDP1 is transcribed mainly in vegetative tissues, including the mature leaf, coleoptile, root-elongation zone, culm internode, and especially the joint region between the leaf blade and sheath. Further studies revealed that transcription of OsMDP1 is stimulated by darkness and suppressed by brassinolide treatment. OsMDP1 deficiency resulted in shortened primary roots, elongated coleoptiles and enhanced lamina joint inclinations. Moreover, transgenic plants showed hypersensitivities to exogenous brassinolide in terms of lamina joint inclination and coleoptile elongation. OsMDP1 deficiency resulted in enhanced expression of OsXTR1, which encodes xyloglucan endotransglycosylase, the cell-wall loosening enzyme necessary for cell elongation, and modulated expressions of multiple genes involved in cell signalling and gene transcription, indicating the key negative regulatory role of OsMDP1 in BR signalling.