The FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1) genes play crucial roles in regulating the vegetative to reproductive phase transition. Orthologs of FT/TFL1 (OnFT and OnTFL1) were isolated and characterized from Oncidium Gower Ramsey. OnFT mRNA was detected in axillary buds, leaves, pseudobulb and flowers. In flowers, OnFT was expressed more in young flower buds than in mature flowers and was predominantly expressed in sepals and petals. The expression of OnFT was regulated by photoperiod, with the highest expression from the 8th to 12th hour of the light period and the lowest expression at dawn. In contrast, the expression of OnTFL1 was only detected in axillary bud and pseudobulb, and was not influenced by light. Ectopic expression of OnFT in transgenic Arabidopsis plants showed novel phenotypes by flowering early and losing inflorescence indeterminacy. In addition, ectopic expression of OnFT was able to partially complement the late flowering defect in transgenic Arabidopsis ft-1 mutants. In transgenic tfl1-11 mutant plants, 35S::OnTFL1 delayed flowering and rescued the phenotype of terminal flowers. Furthermore, substitution of the key single amino acid His85 by Tyr was able to convert the OnTFL1 function to OnFT by promoting flowering in 35S::OnTFL1-H85Y transgenic Arabidopsis plants. Further analysis indicated that the expression of APETALA1 (AP1) was significantly up-regulated in 35S::OnFT and 35S::OnTFL1-H85Y plants, and was down-regulated in 35S::OnTFL1 transgenic Arabidopsis plants. Our data indicated that OnFT and OnTFL1 are putative phosphatidylethanolamine-binding protein genes in orchids that regulate flower transition similar to their orthologs in Arabidopsis.
To study the evolution of phosphatidylethanolamine-binding protein (PEBP) gene families in non-flowering plants, we performed a functional analysis of the PEBP gene AcMFT of the MFT clade in the pteridophyte Adiantum capillus-veneris. The expression of AcMFT was regulated by photoperiod similar to that for FT under both long day and short day conditions. Ectopic expression of AcMFT in Arabidopsis promotes the floral transition and partially complements the late flowering defect in transgenic Arabidopsis ft-1 mutants, suggesting that AcMFT functions similarly to FT in flowering plants. Interestingly, a similar partial compensation of the ft-1 late flowering phenotype was observed in Arabidopsis ectopically expressing only exon 4 of the C terminus of AcMFT and FT. This result indicated that the fourth exon of AcMFT and FT plays a similar and important role in promoting flowering. Further analysis indicated that exons 1-3 in the N terminus specifically enhanced the function of FT exon 4 in controlling flowering in Arabidopsis. Protein pull-down assays indicated that Arabidopsis FD proteins interact with full-length FT and AcMFT, as well as peptides encoded by 1-3 exon fragments or the 4th exon alone. Furthermore, similar FRET efficiencies for FT-FD and AcMFT-FD heterodimer in nucleus were observed. These results indicated that FD could form the similar complex with FT and AcMFT. Further analysis indicated that the expression of AP1, a gene downstream of FT, was up-regulated more strongly by FT than AcMFT in transgenic Arabidopsis. Our results revealed that AcMFT from a non-flowering plant could interact with FD to regulate the floral transition and that this function was reduced due to the weakened ability of AcMFT-FD to activate the downstream gene AP1.
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