Ovule-specific MADS-box proteins have conserved protein-protein interactions in monocot and dicot plants

OsMADS13 is a rice MADS-box gene that is specifically expressed in developing ovules. The amino acid sequence of OsMADS13 shows 74% similarity to those of FLORAL BINDING PROTEIN 7 (FBP7) and FBP11, the products of two MADS-box genes that are necessary and sufficient to determine ovule identity in Petunia. To assess whether OsMADS13, the putative rice ortholog of FBP7 and FBP11, has an equivalent function, several analyses were performed. Ectopic expression of FBP7 and FBP11 in Petunia results in ectopic ovule formation on sepals and petals. Here we show that ectopic expression of OsMADS13 in rice and Arabidopsis does not result in the formation of such structures. Furthermore, ectopic expression of FBP7 and FBP11 in Arabidopsis also fails to induce ectopic ovule formation. To determine whether protein-protein interactions involving putative class D MADS-box proteins have been conserved, yeast two-hybrid assays were performed. These experiments resulted in the identification of three putative partners of OsMADS13, all of them encoded by AGL2-like genes. Interestingly the Petunia FBP7 protein also interacts with AGL2-like proteins. The evolutionary conservation of the MADS-box protein partners of these ovule-specific factors was confirmed by exchange experiments which showed that the protein partners of OsMADS13 interact with FBP7 and vice versa.