An orchid flower exhibits a zygomorphic corolla with a well-differentiated labellum. In Cymbidium sinense, many varieties with peloric or pseudopeloric flowers have been bred during centuries of domestication. However, little is known about the molecular basis controlling orchid floral zygomorphy and the origin of these varieties. Here, we studied the floral morphogenesis of C. sinense and transcriptome-wide enriched differentially expressed genes among different varieties. The floral zygomorphy of C. sinense is established in the early developmental process. Out of 27 MIKCC-MADS factors, we found two homeotic MADS genes whose expression was down-regulated in peloric varieties but up-regulated in pseudopeloric varieties. CsAP3-2 expressed in the inner floral organs co-operates with a labellum-specific factor CsAGL6-2, asymmetrically promoting the differentiation of inner tepals. Interestingly, we detected exon deletions on CsAP3-2 in peloric varieties, indicating that loss of B-function results in the origin of peloria. Additional petaloid structures developed when we ectopically expressed these genes in Arabidopsis, suggesting their roles in floral morphogenesis. These findings indicate that the interplay among MADS factors would be crucial for orchid floral zygomorphy, and mutations in these factors may have maintained during artificial selection.

Cymbidium has been artificially domesticated for centuries in Asia, which produced numerous cultivated varieties. Flowers with stamenoid tepals or those with multiple tepals have been found in different species of Cymbidium; however, the molecular basis controlling the formation of these phenotypes is still largely unknown. Previous work demonstrated that AGAMOUS/AG lineage MADS genes function in floral meristem determinacy as well as in reproductive organs development in both dicots and monocots, indicating a possible relationship with the origin of two flower varieties in Cymbidium. Here, we characterized and analyzed two AG lineage paralogues, CsAG1 and CsAG2, from Cymbidium sinense, both of which were highly expressed in the gynostemium column of a standard C. sinense. Interestingly, we detected ectopic expression of CsAG1 rather than CsAG2 in all floral organs of a stamenoid-tepal variety and significant down-regulation of CsAG1 in a variety with multiple tepals. Over-expression of CsAG1 in wild type Arabidopsis resulted in petal-to-stamen homeotic conversion, suggesting a conserved C-function of CsAG1 in the development of Cymbidium flower. Altogether, our results supported a hypothesis that disruption of a single AG-like factor would be associated with the formation of two domesticated varieties in C. sinense.