C/D Class MADS Box Genes from Two Monocots, Orchid (Oncidium Gower Ramsey) and Lily (Lilium longiflorum), Exhibit Different Effects on Floral Transition and Formation in Arabidopsis thaliana

We have characterized three CID class MADS box genes from an orchid (Oncidium Gower Ramsey) and a lily (Lilium longiflorum). OMADS4 of orchid and LMADS10 of lily are C class gene orthologs, whereas OMADS2 of orchid is a putative D class gene ortholog. The identity of these three genes is further supported by the presence of conserved motifs in the C-terminal regions of the proteins. The mRNA for these three genes can be detected in flowers and is absent in vegetative leaves. In flowers, OMADS4 and LMADS10 show similar expression patterns, being specifically expressed in the stamens and carpels. The expression of OMADS2 is restricted to the stigmatic cavity and ovary of the carpel. The similarities of the expression patterns of OMADS4/LMADS10 and OMADS2 to those of C and D class genes, respectively, indicate that their transcriptional regulation is highly evolutionarily conserved in these monocot species. Yeast two-hybrid analysis indicates that both OMADS2 and OMADS4 form homodimers and heterodimers with each other. Similar interactions are observed for LMADS2 and LMADS10. Ectopic expression of LMADS10 causes extremely early flowering, terminal flower formation and conversion of the sepals into carpel-like structures, similar to ectopic expression of the lily D class gene LMADS2. In contrast, 35S::OMADS2 and 35S::OMADS4 cause only early or moderately early flowering in transgenic Arabidopsis plants without floral organ conversion. This result indicates that C/D class genes from the lily have stronger effects than those from the orchid in transgenic Arabidopsis, revealing possible functional diversification of C/D class genes from the two monocots in regulating floral transition and formation.