Spatially and Temporally Regulated Expression of Rice MADS Box Genes with Similarity to Arabidopsis Class A, B and C Genes

The floral morphology of grass species is distinct from that of typical dicot plants. In order to achieve a better understanding of the molecular basis for this diversion, we isolated RAPIA, RAP1B and RAG, putative rice orthologs for the Arabidopsis class A gene APETALA1 (AP1) and class C gene AGAMOUS (AG). The expression patterns of RAP1A, RAG and OsMADS2, a rice ortholog of the class B gene, were analyzed by in situ hybridization. RAP1A mRNA was expressed in the apical region of the floral meristem at an early stage of spikelet development, and then its expression was localized in developing lemma, palea and lodicules. The OsMADS2 transcript was first observed in the region where stamen primordia are formed. Soon after, OsMADS2 mRNA appeared in the lodicule primordia as well as the stamen primordia, and this RNA accumulation pattern persisted until late stages of floral development. The expression of RAG was observed in stamens and pistils of wild-type young spikelets. These RNA accumulation patterns are mostly similar to those of Arabidopsis class A, B, C genes, supporting the notion that the ABC model may be extended to rice.