Unraveling the mystery behind the short-day-specific flowering of tobacco cultivar Maryland Mammoth

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. Highlight The previously unexplained gigantism of Maryland Mammoth tobacco is caused by a truncated major floral activator protein that results in weaker activation and the inability to flower under long-day conditions. ### Competing Interest Statement The authors have declared no competing interest. * (FT) : FLOWERING LOCUS T (LD) : long-day (MM) : Maryland Mammoth (SD) : short-day