Functional Analysis of FT and TFL1 Orthologs from Orchid (Oncidium Gower Ramsey) that Regulate the Vegetative to Reproductive Transition

Abstract

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.