Four Orchid (Oncidium Gower Ramsey) AP1/AGL9-like MADS Box Genes Show Novel Expression Patterns and Cause Different Effects on Floral Transition and Formation in Arabidopsis thaliana

Four AP1/AGL9 functional MADS box genes were characterized from the orchid (Oncidium Gower Ramsey). OMADS6 is a SEP3 ortholog, OMADS11 is a SEP1/2 ortholog, OMADS7 is an AGL6-like gene and OMADS10 is a putative paleoAP1 ortholog. The identity of these four genes was further supported by the presence of conserved motifs in the C-terminal regions of the proteins. OMADS6 showed an expression pattern different from SEP3 orthologs, with expression in the sepal, petal, lip and carpel, and was barely detected in the stamen. The expression pattern for OMADS11 was similar to OMADS6 and different from SEP1/2 orthologs since its expression was undetectable in the stamen. The expression pattern for OMADS7 was nearly identical to OMADS6. The similarities in the expression patterns of the SEP/AGL6-like genes OMADS6, 11 and 7 indicated that their transcriptional regulation is highly evolutionarily conserved in the orchid. Unlike OMADS6/11/7, OMADS10 was only expressed in vegetative leaves and in the lip and carpel of mature flowers, which distinguishes it from most genes in the SQUA subfamily. Ectopic expression of OMADS6, 11 or 7 caused extremely early flowering, whereas 35S::OMADS10 only caused moderately early flowering in transgenic Arabidopsis plants. In addition, flower organ conversions, such as carpelloid sepals and staminoid petals, were observed in 35S::OMADS6 and carpelloid sepals were produced in 35S::OMADS7, while flower organ conversions were not observed in 35S::OMADS11 or 35S::OMADS10 transgenic flowers. This result reveals possible functional diversification of the orchid AP1/AGL9 genes OMADS6, 11, 7 and 10 in regulating flower transition and formation.