Ectopic expression of rice OsMADS1 reveals a role in specifying the lemma and palea, grass floral organs analogous to sepals

MADS-domain-containing transcription factors play diverse roles in plant de velopment. The prototypic members of this gene family are the floral organ identity genes of the model dicotyledonous plant, Arabidopsis thaliana. Seq uence relatedness and function ascribe them to AP1/AGL9, AG, AP3 and PI gen e groups. The rice MADS-box gene, OsMADS1, is a member of the AP1/AGL9 sub- group. Tomato and Petunia members of this sub-group specify floral meristem identity and control organ development in three inner whorls. Reported her e are phylogenetic analyses that show OsMADS1 to form a distinct clade with in the AGL9 gene family. This sub-group currently has only three other mono cot genes. We have studied the expression pattern of OsMADS1 and determined the consequences of its ectopic expression in transgenic rice plants. OsMA DS1 is not expressed during panicle branching; earliest expression is in sp ikelet meristems where it is excluded from the outer rudimentary/sterile gl umes. During organogenesis, OsMADS1 expression is confined to the lemma and palea, with weak expression in the carpel. Ectopic OsMADS1 expression resu lts in stunted panicles with irregularly positioned branches and spikelets. Additionally, in spikelets, the outer rudimentary glumes are transformed t o lemma/palea-like organs. Together, these data suggest a distinct role for OsMADS1 and its monocot relatives in assigning lemma/palea identity.