The MADS box genes that encode regulatory proteins play important roles in both the formation of flower meristem and the determination of floral organ identity. We have characterized a flower-specific cDNA that belongs to the AGL2 gene family from rice, designated OsMADS5. The cDNA displays the structure of a typical plant MADS box gene, which consists of a MADS domain, K domain, a short I region between the MADS and K domain, and the C-terminal region downstream of the K domain. The gene was classified as a member of the AGL2 gene family from sequence homology analysis. The OsMADS5 protein is the most similar to OsMADS1 of rice (72% identity). In spatial and temporal RNA blot analyses, the OsMADS5 gene was expressed preferentially in anthers and weakly in carpels. During flower development, the gene was more highly expressed in the early floral stage than in the late. To study the functions of the gene, the cDNA clone was expressed ectopically using the CaMV 35S promoter in a heterologous tobacco plant system. Transgenic plants of OsMADS5 exhibited the phenotype of weak dwarfism and early flowering. These results indicate that OsMADS5 is structurally related to the AGL2 family and may be involved in controlling flowering time.
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