Characterization of C- and D-Class MADS-Box Genes in Orchids.

Orchids (members of the Orchidaceae family) possess unique flower morphology and adaptive reproduction strategies. Although the mechanisms underlying their perianth development have been intensively studied, the molecular basis of reproductive organ development in orchids remains largely unknown. Here, we report the identification and functional characterization of two ()-like MADS-box genes, 'Orchid' () and , which are putative C- and D-class genes, respectively, from the orchid 'Chao Praya Smile'. Both and are highly expressed in the reproductive organ, known as the column, compared to perianth organs, while expression gradually increases in pace with pollination-induced ovule development and is localized in ovule primordia. Ectopic expression of , but not , rescues floral defects in the Arabidopsis () mutant, including reiteration of stamenoid perianth organs in inner whorls and complete loss of carpels. Downregulation of and in orchids by artificial microRNA interference using l-Met sulfoximine selection-based gene transformation systems shows that both genes are essential for specifying reproductive organ identity, yet they, exert different roles in mediating floral meristem determinacy and ovule development, respectively, in spp. orchids. Notably, knockdown of and also affects perianth organ development in orchids. Our findings suggest that and not only act as evolutionarily conserved C- and D-class genes, respectively, in determining reproductive organ identity, but also play hitherto unknown roles in mediating perianth organ development in orchids.