Date Published: March 1, 2019
Publisher: Public Library of Science
Author(s): Maria Carmen Valoroso, Maria Concetta Censullo, Serena Aceto, Keqiang Wu.
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 MIKCC 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 MIKCC 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.
Among the flowering plants, the monocot family Orchidaceae is one of the most species-rich and widespread; this family has adapted to different habitats and exhibits highly specialized reproductive strategies . The Orchidaceae family includes five subfamilies (Apostasioideae, Cypripedioideae, Vanilloideae, Epidendroideae and Orchidoideae) and numerous tribes and subtribes . One of the most attractive orchid structures is the flower that assumes an enormous variety of shapes and colors among the species although it has a generally conserved structural organization. Three outer tepals, two lateral inner tepals and an inner median tepal (labellum or lip) define a zygomorphic perianth. Male and female reproductive tissues are fused to form the gynostemium or column, and pollen grains are located at the top of this structure. The ovary is located at the base of the column, and its maturation is triggered by pollination .
The inflorescence transcriptome of O. italica  includes twenty-nine transcripts encoding for MADS-box proteins. This number is lower than that of the MADS-box genes present in the genome of P. equestris (51), D. catenatum (63) and A. shenzenica (36) [35, 36, 38] because it does not include the transcripts specifically expressed in not-floral tissues (e.g., leaf, root, stem, etc.). BLAST analysis and phylogenetic reconstruction (Fig 2) demonstrated that the MADS-box genes expressed in the inflorescence of O. italica belong to both class I and II. Excluding three class I and three class II genes (SOC, ANR and one class A), they show floral specific expression or are expressed in floral tissues at levels higher than in leaves (Fig 3).
Current advances in transcriptome and genome sequencing are highlighting the molecular programs that underlie floral evolution and development of non-model species. For many species belonging to basal angiosperms, magnoliids and basal eudicots, the “fading borders model” proposes a gradient of the expression levels of floral homeotic genes to explain flower development . In particular, low expression levels at the boundary of the domain of a given MADS-box gene overlap with the expression of a different homeotic gene in the adjacent domain. The analysis of all the MADS-box genes expressed in the inflorescence of O. italica demonstrate that they follow a “fading borders” scheme and that their expression is generally conserved among orchids of different subfamilies. Given that class I and MIKC* genes are understudied compared with MIKCC genes, it will be interesting to focus forthcoming studies on these classes of MADS-box genes in orchids to understand their developmental role and evolution.