¹ National Research Laboratory of Plant Functional Genomics, Department of Life Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea

² Department of Plant Pathology, University of California Davis, CA 95616

³ Department of Plant and Microbial Biology , 361 Koshland Hall, University of California, Berkeley, Berkeley CA 94720-3102

⁴ Department of Biology, Korea University, Seoul 136-701, Korea

Genetic and molecular analyses in Arabidopsis and Antirrhinum have provided the basis for the ABC model of floral development. Three classes of homeotic genes (A, B and C groups) that encodes proteins with a well conserved MADS box motif regulate floral organ development in single or two adjacent whorls among the four whorls of flowers. The class B genes that include APETALA3 (AP3) and PISTILLATA (PI) are required to specify petal and stamen identities. In rice, we have previously isolated the PI family gene, OsMADS4. The AP3 and PI proteins bind DNA only as an AP3/PI heterodimer. OsMADS16 was isolated by the yeast-two hybrid screening method with OsMADS4 as a bait. The deduced amino acid sequences and the expression pattern of the OsMADS16 are similar to those of the AP3 gene in Arabidopsis. We constructed transgenic lines that ectopically express OsMADS16 under the control of the maize ubiquitin promoter. The resulting flowers exhibited replacement of the fourth whorl carpels by stamen-like organs, which resemble flowers of the previously described Arabidopsis mutation superman and the transgenic plants ectopically expressing the AP3 gene. To understand the function of the OsMADS16 gene more precisely, we also generated transgenic plants expressing the C-terminal truncated protein. The transgenic plants displayed the alterations of second and third whorl. OsMADS16 interacted with OsMADS4 that belongs to the PI gene family, OsMADS6 and OSMADS8 that are homologous to SEPALLATA genes required for the proper function of the class B and C genes.