Abstract
Flowering is a rigorously timed and morphologically complex shift in plant development. This change depends on endogenous as well as environmental factors. FLOWERING LOCUS T (FT) integrates several cues from different pathways acting as a flowering promoter. Contrary to the role of FT, its paralog TERMINAL FLOWER 1 (TFL1) delays floral transition. Although FT/TFL1 homologs have been studied in model eudicots and monocots, scarce studies are available in non-model monocots like the Orchidaceae. Orchids are very diverse and their floral complexity is translated into a unique aesthetic display, which appeals the ornamental plant market. Nonetheless, orchid trade faces huge limitations due to their long vegetative phase and intractable indoor flowering seasons. Little is known about the genetic basis that control reproductive transition in orchids and, consequently, manipulating their flowering time remains a challenge. In order to contribute to the understanding of the genetic bases that control flowering in orchids we present here the first broad-scale analysis of FT/TFL1-like genes in monocots with an expanded sampling in Orchidaceae. We also compare expression patterns in three selected species and propose hypotheses on the putative role of these genes in their reproductive transition. Our findings show that FT-like genes are by far more diversified than TFL1-like genes in monocots with six subclades in the former and only one in the latter. Within MonFT1, the comparative protein sequences of MonFT1A and MonFT1B suggest that they could have recruited functional roles in delaying flowering, a role typically assigned to TFL1-like proteins. On the other hand, MonFT2 proteins have retained their canonical motifs and roles in promoting flowering transition. This is also shown by their increased expression levels from the shoot apical meristem (SAM) and leaves to inflorescence meristems (IM) and floral buds (FBs). Finally, TFL1-like genes are retained as single copy and often times are lost. Their loss could be linked to the parallel recruitment of MonFT1A and MonFT1B homologs in delaying flowering and maintaining indeterminacy of the inflorescence meristem. These hypotheses lay the foundation for future functional validation in emerging model orchid species and comparative analyses in orchids with high horticultural potential in the market.
Highlights
The transformation of a vegetative shoot apical meristem (SAM) into an inflorescence meristem (IM) forming bracts and floral buds (FBs) is a rigorously timed and morphologically complex shift in plant development (Henderson and Dean, 2004; Benlloch et al, 2007; Liu et al, 2009; Thouet et al, 2012)
All routes converge in the floral integrators, a group of genes that control the vegetative-to-reproductive transition which include FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), LEAFY (LFY), FLOWERING LOCUS D (FD), and AGAMOUS like 24 (AGL24) (Boss et al, 2004; Wellmer and Riechmann, 2010)
The resulting topology shows a duplication event prior to angiosperm diversification (UFBS = 100), which results in the FT-like (UFBS = 100) and BROTHER OF FT AND TFL1 (BFT)/TERMINAL FLOWER 1 (TFL1)-like (UFBS = 100) clades (Supplementary Figure S1)
Summary
The transformation of a vegetative shoot apical meristem (SAM) into an inflorescence meristem (IM) forming bracts and floral buds (FBs) is a rigorously timed and morphologically complex shift in plant development (Henderson and Dean, 2004; Benlloch et al, 2007; Liu et al, 2009; Thouet et al, 2012). In the model species Arabidopsis thaliana (Brassicaceae) floral transition depends on endogenous as well as environmental factors, which have been dissected into different pathways. These include the photoperiod, vernalization, hormonal and autonomous agerelated pathways (Mouradov et al, 2002; Blázquez et al, 2003; Mutasa-Göttgens and Hedden, 2009; Amasino, 2010). In Arabidopsis, overexpression of TFL1 results in delayed flowering and indeterminate inflorescences (Ratcliffe et al, 1998, 1999). As a result, during the vegetative-to-reproductive transition a negative feedback loop is maintained between the inflorescence regulator TFL1 and the floral meristem genes
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