B and E MADS‐box genes determine the perianth formation in Cymbidium goeringii Rchb.f

Cymbidium goeringii Rchb.f. is an important ornamental plant with a striking well‐differentiated lip. Its complex floral architecture presents an exciting opportunity to examine perianth development. In flowering plants, class A, B and E floral homeotic genes play key roles in the specification of perianth identity. In this study, we used a cDNA library of wild‐type C. goeringii flower buds for transcriptome sequencing. Eighteen candidate class A, B and E genes (including AP1/FUL ‐, AP2 ‐, DEF ‐, GLO ‐, SEP ‐ and AGL6 ‐like genes) were identified. Quantitative real time polymerase chain reaction (qRT‐PCR) results showed that CgDEF1 , CgSEP2 and CgAGL6‐1 were strongly detected only in the sepals and petals and were significantly downregulated in the lips. CgDEF3 , CgDEF4 and CgAGL6‐3 were highly expressed in the lips and lip‐like petals but were only minimally detected in the sepals. Yeast two‐hybrid analysis indicated that CgDEF1 and CgGLO formed a heterodimer. CgAGL6‐1/CgSEP2 and CgDEF1 formed higher‐order protein complexes with the assistance of the CgGLO protein, and both CgAGL6‐1 and CgSEP2 formed a heterodimer. CgDEF3/CgDEF4 could interact independently with CgGLO and CgAGL6‐3, respectively, while CgDEF3 and CgDEF4 also formed heterodimers with the assistance of the CgGLO. Based on a comprehensive analysis relating these gene expression patterns to protein interaction profiles, the mechanism of sepal/petal/lip determination was studied in C. goeringii . Furthermore, a hypothesis explaining the sepal/petal/lip determination of C. goeringii is proposed. The lip‐quartet (CgDEF3/CgDEF4/CgAGL6‐3/CgGLO) promoted lip formation, whereas the sepal/petal‐quartet (CgDEF1/CgAGL6‐1/CgSEP2/CgGLO) promoted sepal/petal formation. These results enrich the current knowledge regarding the mechanism and pathways of perianth formation in orchids.