A mutant allele of the long-day floral activator NtFT5 is associated with the short-day-specific flowering of tobacco cultivar Maryland Mammoth.

Flowering in day-neutral tobacco (Nicotiana tabacum) cultivars requires the photoperiod-dependent expression of FLOWERING LOCUS T (FT)-like floral activators, which compete with FT-like inhibitors to interact with FD proteins and induce the switch 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 overexpression of the major LD floral inducer (NtFT5) from the day-neutral cultivar Hicks induces flowering in MM plants under LD conditions. However, a 2-bp deletion near the end of the endogenous NtFT5MM gene generates a truncated NtFT5MM protein that still interacts with FD proteins but acts as a weaker floral inducer than NtFT5. The truncation does not impair NtFT5MM protein stability but may affect its binding to NtFD1, weakening the expression of target genes. Our results provide a potential explanation for the MM gigantism phenotype first observed more than 100 years ago.