Assessing the Flowering Genetic Regulatory Network in Neotropical Orchids

Abstract

During the reproductive transition in flowering plants, a vegetative apical meristem (SAM) transforms into an inflorescence meristem (IM) that forms bracts and flowers. In grasses such as rice, a genetic regulatory network (GRN) controlling reproductive transitions has been identified. It includes the integration of promoters and repressors from different gene lineages with active duplication events during angiosperm diversification. With the objective to understand the evolution and expression of flowering GRN in Orchidaceae, we performed comprehensive phylogenetic analyses of all genes from the flowering GRN and analyzed by RT-PCR the expression of targeted homologs in key developmental stages. Our ML results indicate that the FT/TFL1, FD, FLC/FUL, SOC1 and AGL24/SVP gene lineages have been subject to multiple duplications in monocots, as well as in Orchidaceae. Conversely, FLC genes are lost in Orchidaceae, suggesting major changes in the repression of flowering. Our studies also show active expression of many target genes in Elleanthus aurantiacus (Orchidoideae) in the SAM and in IM, indicating important functions in the reproductive transition. We describe how the flowering GRN in orchids has significant variations in copy number and expression patterns when compared to the canonical rice flowering GRN.

Evolution of major flowering pathway integrators in Orchidaceae

“…7 a). Because our goal was to record the expression patterns in different developmental stages during the vegetative to reproductive transition in this biannual flowering species, we dissected the SAM, IM, leaves and floral buds following Madrigal et al, ( 2021 ) (Fig. 7 a).…”

“…Comprehensive phylogenetic studies aimed to characterize the genetic complements of the FGRN in the Orchidaceae have been done for the FT and AGL24/SVP gene lineages. In both cases, the occurrence of several independent duplication events in monocots and eudicots has hindered a straightforward (or a one to one) comparison of the well-characterized FGRNs to orchids (Madrigal et al 2021 ; Ospina-Zapata et al 2020 ; Ramirez-Ramirez et al 2021 ). Nevertheless, these analyses have been instrumental to tackle the complexity of the FGRN in terms of copy number, expression patterns, and the ecological links to flowering inductive conditions.…”

“…FLC genes are present in Poales ( MonFLC ) where they can also repress flowering. However, the monocot FLC genes are characterized by divergent, short protein sequences (Madrigal et al 2021 ; Ruelens et al 2013 ). Our data confirm that MonFLC genes have important losses in the C-terminal region motifs in comparison con EudiFLC , accompanied by characteristic signals of relaxed purifying selection, yet repressive roles in flowering are conserved in distantly related species (Fig.…”

“…For SEM analysis, SAM, IM and FBs from E. aurantiacus were collected in 70% ethanol and stored for one month or longer. Apices and buds were dissected in ethanol 90% following (Madrigal et al 2021 ; Ospina-Zapata et al 2020 ). The dissected samples were dehydrated in a progressive ethanol series.…”

“…We have previously implemented large-scale phylogenetic reconstructions for the FT and the AGL24/SVP genes across angiosperms, with a special focus in orchids. Such approach has proven valuable in narrowing down the putative genes more likely to control reproductive transitions and to assign them a hypothetical role as promoters or repressors (Madrigal et al 2021 ; Ospina-Zapata et al 2020 ; Ramirez-Ramirez et al 2021 ). In order to understand the evolution of the FGRN in Orchidaceae, here we performed comprehensive ML analyses of the CO/COL4, FD, FLC/FUL and SOC1 gene lineages in angiosperms, with a special focus on Orchidaceae.…”