Abstract
FLOWERING LOCUS T (FT) is the key flowering integrator in Arabidopsis (Arabidopsis thaliana), and its homologs encode florigens in many plant species regardless of their photoperiodic response. Two FT homologs, GmFT2a and GmFT5a, are involved in photoperiod-regulated flowering and coordinately control flowering in soybean. However, the molecular and genetic understanding of the roles played by GmFT2a and GmFT5a in photoperiod-regulated flowering in soybean is very limited. In this study, we demonstrated that GmFT2a and GmFT5a were able to promote early flowering in soybean by overexpressing these two genes in the soybean cultivar Williams 82 under noninductive long-day (LD) conditions. The soybean homologs of several floral identity genes, such as GmAP1, GmSOC1 and GmLFY, were significantly upregulated by GmFT2a and GmFT5a in a redundant and differential pattern. A bZIP transcription factor, GmFDL19, was identified as interacting with both GmFT2a and GmFT5a, and this interaction was confirmed by yeast two-hybridization and bimolecular fluorescence complementation (BiFC). The overexpression of GmFDL19 in soybean caused early flowering, and the transcription levels of the flowering identity genes were also upregulated by GmFDL19, as was consistent with the upregulation of GmFT2a and GmFT5a. The transcription of GmFDL19 was also induced by GmFT2a. The results of the electrophoretic mobility shift assay (EMSA) indicated that GmFDL19 was able to bind with the cis-elements in the promoter of GmAP1a. Taken together, our results suggest that GmFT2a and GmFT5a redundantly and differentially control photoperiod-regulated flowering in soybean through both physical interaction with and transcriptional upregulation of the bZIP transcription factor GmFDL19, thereby inducing the expression of floral identity genes.
Highlights
Plants integrate various environmental signals, such as photoperiod and temperature, to ensure flowering under those conditions that optimize seed production
Arabidopsis FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF) proteins produced in the phloem [7], [11] and are transported to the shoot apex, where they dimerize with the bZIP transcription factor FD to activate the expression of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) [9], [12] and the floral meristem identity genes APETALA1 (AP1) and LEAFY (LFY) [13], [14]
Our results suggest that the putative flowering model FT/FD-AP1 is well conserved in the legume soybean and that GmFDL19 may act as the key component in the photoperiodregulated flowering pathway controlled by GmFT2a and GmFT5a
Summary
Plants integrate various environmental signals, such as photoperiod and temperature, to ensure flowering under those conditions that optimize seed production. In Arabidopsis thaliana (Arabidopsis), multiple pathways converge on a small number of floral integrator genes, which include the floral promoters FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF), to integrate photoperiod, temperature, vernalization, and light quality signaling [1]. In addition to the FT-like proteins, the plant PEBP family consists of two other phylogenetically distinct groups of proteins, the TERMINAL FLOWER 1 (TFL1)-like proteins and the MOTHER OF FT AND TFL (MFT)-like proteins [4]–[8]. FT-like proteins from various species function in a manner similar to that of FT regarding the induction of flowering, transport in the phloem, and interaction with FD-like proteins [15]–[18], suggesting that this general mechanism is likely widely conserved across flowering plants. The rice FT ortholog Hd3a interacts with OsFD1 indirectly through a 14-3-3 protein to form a ternary trimer known as the florigen activation complex in the nuclei of the shoot apex, where it activates the expression of OsMADS15, an AP1 homolog that regulates flowering [19], [20]
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