In Arabidopsis thaliana, the genes FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1) have antagonistic roles in regulating the onset of flowering: FT activates and TFL1 represses flowering. Both encode small, closely related transcription cofactors of ∼175 amino acids. Previous studies identified a potential ligand binding residue as well as a divergent external loop as critical for the differences in activity of FT and TFL1, but the mechanisms for the differential action of FT and TFL1 remain unclear. Here, we took an unbiased approach to probe the importance of residues throughout FT protein, testing the effects of hundreds of mutations in vivo. FT is surprisingly robust to a wide range of mutations, even in highly conserved residues. However, specific mutations in at least four different residues are sufficient to convert FT into a complete TFL1 mimic, even when expressed from TFL1 regulatory sequences. Modeling the effects of these mutations on the surface charge of FT protein suggests that the affected residues regulate the docking of an unknown ligand. These residues do not seem to alter the interaction with FD or 14-3-3 proteins, known FT interactors. Potential candidates for differential mediators of FT and TFL1 activities belong to the TCP (for TEOSINTE BRANCHED1, CYCLOIDEA, PCF) family of transcription factors.