Photoperiod is an important external stimulus governing the precise timing of the floral transition in plants. Members of the FLOWERING LOCUS T (FT)-like clade of phosphatidylethanolamine-binding proteins induce this developmental process in numerous species by forming regulatory protein complexes with FD-like bZIP transcription factors. We identified several thus far unknown FT-like and FD-like genes in the genus Nicotiana and found that, even in the day-neutral species Nicotiana tabacum, floral initiation requires the photoperiod-dependent expression of several FT-like genes. Furthermore, floral promotion under long-day (LD) and short-day (SD) conditions is mediated by an FT-like protein (NtFT5) that originates from the genome of the paternal, facultative SD ancestor Nicotiana tomentosiformis. In contrast, its ortholog of the maternal LD ancestor Nicotiana sylvestris is not present in the genome of N. tabacum cv. SR1. Expression profiling in N. tabacum and its ancestors confirmed the relevance of these FT and FD orthologs in the context of polyploidization. We also found that floral inhibition by tobacco FT-like proteins is not restricted to SD conditions, highlighting the coincident expression of tobacco FT-like genes encoding floral activators and floral inhibitors. Multicolor bimolecular fluorescence complementation analysis revealed the preferential formation of FT/FD complexes that promote rather than inhibit flowering, which in concert with the regulation of NtFT and NtFD expression could explain how floral promotion overcomes floral repression during the floral transition in tobacco.

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Flowering in day-neutral tobacco (Nicotiana tabacum) plants requires the photoperiod-dependent expression of members of the FLOWERING LOCUS T (FT) like clade of phosphatidylethanolamine-binding proteins. FT-like floral activators and inhibitors compete for interaction with FD proteins to shift from vegetative to reproductive growth. In the short-day (SD) cultivar Maryland Mammoth (MM), vegetative growth persists under long-day (LD) conditions, generating unusually tall plants. We found that the major floral inducer under long-days (NtFT5) was expressed in MM and that NtFT5 overexpression induced flowering in MM plants under LD conditions. However, sequence analysis revealed a 2-bp deletion near the 3′ end of NtFT5 in MM plants resulting in a truncated C-terminus with an altered amino acid sequence. We found that the truncated NtFT5MM protein was still able to interact with tobacco FD proteins. However, constitutive overexpression under LD conditions in SD-specific flowering tobacco plants showed that NtFT5MM is a weaker floral inducer than NtFT5. Our data indicate that the truncation does not impair the stability of the NtFT5MM protein but affects its binding affinity for NtFD1, probably resulting in the weaker expression of target genes. Our results therefore provide a potential explanation for the MM gigantism phenotype first observed more than 100 years ago.