Factorial combinations of protein interactions generate a multiplicity of florigen activation complexes in wheat and barley

Abstract

SUMMARYThe FLOWERING LOCUS T (FT) protein is a central component of a mobile flowering signal (florigen) that is transported from leaves to the shoot apical meristem (SAM). Two FT monomers and two DNA-binding bZIP transcription factors interact with a dimeric 14-3-3 protein bridge to form a hexameric protein complex. This complex, designated as the 'florigen activation complex' (FAC), plays a critical role in flowering. The wheat homologue of FT, designated FT1 (= VRN3), activates expression of VRN1 in the leaves and the SAM, promoting flowering under inductive long days. In this study, we show that FT1, other FT-like proteins, and different FD-like proteins, can interact with multiple wheat and barley 14-3-3 proteins. We also identify the critical amino acid residues in FT1 and FD-like proteins required for their interactions, and demonstrate that 14-3-3 proteins are necessary bridges to mediate the FT1-TaFDL2 interaction. Using in vivo bimolecular fluorescent complementation (BiFC) assays, we demonstrate that the interaction between FT1 and 14-3-3 occurs in the cytoplasm, and that this complex is then translocated to the nucleus, where it interacts with TaFDL2 to form a FAC. We also demonstrate that a FAC including FT1, TaFDL2 and Ta14-3-3C can bind to the VRN1 promoter in vitro. Finally, we show that relative transcript levels of FD-like and 14-3-3 genes vary among tissues and developmental stages. Since FD-like proteins determine the DNA specificity of the FACs, variation in FD-like gene expression can result in spatial and temporal modulation of the effects of mobile FT-like signals.

Antagonistic Transcription Factor Complexes Modulate the Floral Transition in Rice

“…The resulting ternary complex, named florigen activation complex (FAC), is targeted to the nucleus where it further dimerizes, forming a heterohexameric complex tethered by OsFD1 on target DNA sequences (Zhao et al, 2015;Taoka et al, 2011). Similar interactions take place in many plant species, including tomato (Solanum lycopersicum; Park et al, 2014), potato (Solanum tuberosum; Teo et al, 2017), wheat (Triticum aestivum) and barley (Hordeum vulgare; Li et al, 2015), maize (Zea mays; Danilevskaya et al, 2008), and ½AQ2 hybrid aspen (Tylewicz et al, 2015), suggesting that this molecular module is widely conserved among angiosperms. This conservation is further corroborated by interspecific interactions demonstrated to occur between Hd3a/RFT1 and FD (Jang et al, 2017).…”

“…Given the high number of OsbZIP-coding genes in rice, the combinatorial interactions possibly leading to different florigencontaining complexes are very high (Tylewicz et al, 2015;Park et al, 2014;Tsuji et al, 2013;Li et al, 2015). Additionally, the floral transition in rice is associated with both induction and repression of gene expression at the SAM, and different complexes could operate by promoting or repressing expression of specific targets .…”

“…In rice and other plant species, many bZIP TFs have been already described that form alternative FACs with the florigens and control different developmental processes (Tylewicz et al, 2015;Tsuji et al, 2013;Li et al, 2015). We evaluated whether other TFs abundant in leaves might form alternative FACs with a flowering repressive function.…”

Night-Break Experiments Shed Light on the Photoperiod1-Mediated Flowering

“…Upon arrival at the shoot apical meristem, FT-like proteins form part of a hexameric floral activation complex that binds the promoters of MADS-box meristem identity genes, inducing flowering development (Taoka et al, 2011;Li et al, 2015). Overexpression of FT1 in transgenic wheat plants results in an earlyflowering phenotype, even under noninductive SD conditions, whereas plants carrying loss-of-function mutations in FT1 exhibit a late-flowering phenotype (Lv et al, 2014).…”

Identification of the VERNALIZATION 4 gene reveals the origin of spring growth habit in ancient wheats from South Asia

“…The photoperiod and vernalization pathways converge at the regulation of FT1 (= VRN3 in wheat) (10), which encodes a mobile protein that travels from leaves to the shoot apical meristem (SAM) (11,12). Once in the SAM, FT1 becomes part of a florigen activation complex that binds to the VRN1 promoter, inducing its transcription (13)(14)(15). In common wheat, most genes exist in three copies (homeologs), which are designated using their respective genomes (e.g., VRN-A1, VRN-B1, and VRN-D1).…”