Assessing the Flowering Genetic Regulatory Network in Neotropical Orchids

During the reproductive transition in flowering plants a vegetative apical meristem (SAM) forming leaves, becomes an inflorescence meristem (IM) that forms bracts and flowers. In the monocot model Oryza sativa, the core flowering genetic regulatory network (GRN) relies on early activation of four major promoters, namely Heading date 3a (Hd3a, a FLOWERING LOCUS T-FT), Heading date 1 (Hd1, a CONSTANS-CO), FLOWERING LOCUS D (OsFD1), and 14–3–3 proteins, which form a florigen activation complex (FAC) to regulate the transition from SAM to IM. This is followed by the transcription of OsMADS22, OsMADS47 (an AGAMOUS-like 24/ SHORT VEGETATIVE PHASE-AGL24/SVP), OsMADS50 and OsMADS51 (a SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1-SOC1) that activate floral meristem identity genes like OsMADS14 and OsMADS15 (FRUITFUL-like-FUL homologs). Other genes like TERMINAL FLOWER LOCUS 1 (TFL1) and OsMADS55 (AGL24/SVP) repress flowering and maintain the vegetative phase. Our goal is to evaluate the flowering GRN in the Orchidaceae (ca. 25,000 species), one of the most diverse groups of ornamental angiosperms. In order to understand the molecular mechanisms that trigger flowering in Orchidaceae, we sequenced 13 reference transcriptomes of neotropical orchid species representing different phylogenetic positions, having ornamental potential and diversity of growth and floral forms. We isolated homologs and performed phylogenetic analyses of all genes from the flowering GRN to understand the evolution of these gene lineages. Our ML results indicate that FT/TFL1, FD, AGL24/SVP, SOC1 and FUL gene lineages have been subject to multiple duplications in monocots as well as in Orchidaceae as a result of recorded whole genome duplication events. In particular, we emphasize the recovery of six subclades of FT, three of FUL-like, three of AGL24/SVP, three of FD and three of SOC1. Conversely, fewer TFL1 homologs are found and some genes like FLC are lost in Orchidaceae, which suggests major changes in the repression of flowering. Our studies also show active expression of many representative genes in Cattleya trianae and Elleanthus aurantiacus in the SAM and in IM indicating important functions in the reproductive transition. We hypothesize that the flowering GRN in orchids has significant variations in copy number and expression patterns when compared to the canonical rice flowering GRN