True Love or Just a Surface Charge? FLOWERING LOCUS T Helps Arabidopsis Say It with Flowers

Did you order flowers for Valentine’s Day? If your Valentine’s Day flowers arrive at the wrong time, say on Groundhog Day, you might receive a chilly reception. Like your florist, plants make sure they deliver their flowers on time. In plants, FLOWERING LOCUS T (FT) promotes the initiation of flowering, and TERMINAL FLOWER (TFL) delays the initiation of flowering (reviewed in Turck et al., 2008). However, the FT and TFL proteins have similar sizes and only about 40 nonconservative amino acid sequence differences between them. It’s as if your florist employed two nearly identical twins—one who rushed your flowers to their destination and one who delayed them. If one of the twins called in sick, your flowers might arrive too early or too late. It would be an odd business model for a florist, but it works for most angiosperms (reviewed in Pin and Nilsson, 2012).

To examine the basis for the opposite functions of FT and TFL, Ho and Weigel (pages 552–564) combine random mutagenesis with a functional screen to identify mutants that affect (or do not affect) FT flower-promoting activity in Arabidopsis thaliana. The authors use PCR to generate random mutants of FT, make transgenic Arabidopsis with constructs overexpressing individual mutants, and measure the flowering time of the resulting plants. To eliminate mutants with stop codons, they required expression of a fluorescent tag at the C terminus of FT. Overexpression of wild-type FT causes early flowering, allowing the authors to distinguish mutants with loss-of-function, antimorphic, or TFL-like repressive phenotypes. To reveal the identity of each mutant, the authors sequenced the FT transgene from each line of interest. Examining the resulting sequences from early-flowering lines, the authors find that most mutations cause no change in phenotype, indicating that FT shows substantial resistance to disruption. Indeed, the authors examined mutants in 166 of the 175 codons of FT; mutants in 156 codons retained FT function, and mutants in 11 codons reduced FT activity. The authors also tested whether mutations reducing FT activity affected FT transport, by comparing the FT mutants expressed under the shoot apical meristem–specific FD promoter and the phloem-specific SUCROSE-PROTON SYMPORTER2 promoter. Indeed, they identified some mutants that function better when expressed in the meristem than when expressed in the phloem, indicating that these mutants might affect the regulated process of FT transport.

The authors also identified mutations that confer TFL-like activity; these mutations alter the FT surface charge near the potential ligand binding pocket (see figure). However, the authors showed by yeast two-hybrid and bimolecular fluorescence complementation that these mutations did not change the interaction of FT with 14-3-3/GENERAL REGULATORY FACTOR or FD. These mutations did alter the interaction of FT with TCP (for TEOSINTE BRANCHED1, CYCLOIDEA, PCF) transcription factors, indicating the importance of TCP factors in the timing of flowering. This work shows an interesting relationship between surface charge and function, but intriguing questions remain—specifically, identification of the postulated cofactors that help FT and TFL deliver the flowers on time.

Mutants that alter the surface charge give FT a TFL-like activity. Surface charges, with negative charges in red and positive charges in blue, of FT (left) and TFL (right), compared with three FT mutants that give TFL-like activities: E109K (top), Q140K (middle), and N152K (bottom). The asterisk indicates the ligand binding pocket. (Reprinted from Ho and Weigel [2014], Figure 4.)