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 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.

兰科（Orchidaceae）是物种最为丰富的被子植物类群之一，其类群内的花卉形态呈现高度多样化与特化特征。本研究旨在分析地中海野生意大利红门兰（Orchis italica）花序中表达的MADS-box基因。MADS-box蛋白是参与多种植物生物学过程（包括花发育）的转录因子。在意大利红门兰的花组织中，共鉴定出29个表达的MADS-box基因，可划分为I类与II类两大类群。I类MADS-box基因包含1个Mβ型基因，这证实了兰科植物中存在该类型的MADS-box基因。II类MIKC*基因在合蕊柱（column）中高表达，这与MIKC*基因在配子体发育中的保守功能相符。此外，还检测到SOC、SVP、ANR1、AGL12及OsMADS32基因的同源物表达。相较于此前针对意大利红门兰参与花发育ABCDE模型的II类MIKC C类基因的研究认知，本次研究确认了其B类与D类基因的数量。此外，本研究还鉴定出4个A类（AP1/FUL）转录本、2个E类（SEP）转录本、2个新的C类（AG）转录本以及1个新的AGL6转录本。在AP1/FUL基因家族中，序列分化、纯化选择压力的松弛以及表达谱特征提示，这些兰科基因可能发生了功能分化。与其他兰科植物中发现的4个SEP基因不同，意大利红门兰仅检测到2个SEP转录本，这表明兰亚科（Orchidoideae）中可能仅存在2个SEP基因。意大利红门兰MIKC C类基因的表达模式显示，特定MADS-box基因的结构域边界处的低表达水平，可与相邻结构域内属于不同功能A-E类别的基因的表达产生重叠，这符合“边界渐隐”（fading borders）模型。