The MADS-box genes expressed in the inflorescence of Orchis italica (Orchidaceae)

The Orchidaceae family, which is one of the most species-rich flowering plant families, includes species with highly diversified and specialized flower shapes. The aim of this study was to analyze the MADS-box genes expressed in the inflorescence of Orchis italica , a wild Mediterranean orchid species. MADS-box proteins are transcription factors involved in various plant biological processes, including flower development. In the floral tissues of O . italica , 29 MADS-box genes are expressed that are classified as both class I and II. Class I MADS-box genes include one Mβ-type gene, thereby confirming the presence of this type of MADS-box genes in orchids. The class II MIKC* gene is highly expressed in the column, which is consistent with the conserved function of the MIKC* genes in gametophyte development. In addition, homologs of the SOC , SVP , ANR1 , AGL12 and OsMADS32 genes are expressed. Compared with previous knowledge on class II MIKC C genes of O . italica involved in the ABCDE model of flower development, the number of class B and D genes has been confirmed. In addition, 4 class A ( AP1/FUL) transcripts, 2 class E ( SEP) transcripts, 2 new class C ( AG) transcripts and 1 new AGL6 transcript have been identified. Within the AP1/FUL genes, the sequence divergence, relaxation of purifying selection and expression profiles suggest a possible functional diversification within these orchid genes. The detection of only two SEP transcripts in O . italica , in contrast with the 4 genes found in other orchids, suggests that only two SEP genes could be present in the subfamily Orchidoideae. The expression pattern of the MIKC C genes of O . italica indicates that low levels at the boundary of the domain of a given MADS-box gene can overlap with the expression of genes belonging to a different functional A-E class in the adjacent domain, thereby following a “fading borders” model.