A lesson in Complexity: Regulation of FLOWERING LOCUS T
Florigen and photoperiod
Flowering in many plant species is controlled by photoperiod, which represents the most reliable seasonal change in nature. The gene network controlling flowering is well studied in the model plant Arabidopsis thaliana, a facultative long day plant that flowers earlier if days are longer than a critical threshold of approximately 10 hours light per day. The FLOWERING LOCUS T (FT) gene is crucial for the accelerated flowering in response to long days . Interestingly, relatives of FT are also important to accelerate flowering in short day plants such as rice indicating that the role of FT-related genes is very general in controlling flowering although it may be differently hardwired within the regulatory network.
FT is tissue-specifically expressed, an import aspect its role in regulating flowering . Messenger RNA of FT is only produced and translated to protein in the phloem of leaves. The protein moves with the assimilate flow towards the shoot apex where it acts as transcriptional co-regulator to switch the developmental program from vegetative to reproductive [1, 3]. FT fulfills all criteria that were postulated for the flowering hormone Florigen in 1937 by Mikhail Chailakhyan.
Figure 2. FT fulfills the criteria for the flowering hormone Florigen. FT and its closest relative TSF are expressed in the leaf veins from where the protein moves to the shoot apex. The proteins interact directly with the transcription factor FD and trigger transcriptional reprogramming that switches the vegetative apical meristem into an inflorescence meristem . Expression of FT is stronger than that of TSF explaining that the effect of a mutation in the TSF gene is best visible in a ft mutant background.[less]
Figure 2. FT fulfills the criteria for the flowering hormone Florigen. FT and its closest relative TSF are expressed in the leaf veins from where the protein moves to the shoot apex. The proteins interact directly with the transcription factor FD and trigger transcriptional reprogramming that switches the vegetative apical meristem into an inflorescence meristem . Expression of FT is stronger than that of TSF explaining that the effect of a mutation in the TSF gene is best visible in a ft mutant background.
The regulatory network controlling flowering time
Regulation of FT expression is highly complex making the gene an ideal model for our studies. Gene expression is positively influenced by long days because only in these conditions the direct upstream activator CONSTANS (CO) is stabilized . CO binds DNA sequence-specifically but seems to require co-factors that belong to the CCAAT-box binding/NF-Y complex for activity [5-8].
FT expression is also controlled by biotic stress and abiotic factors such as temperature. High ambient temperature breaks the photoperiod control of FT, mostly because a transcription factor of the bHLH family, PHYTOCHROM INTERACTING FACTOR 4 (PIF4) can activate FT even in the absence of CO . On the other hand, delayed flowering in cold ambient temperatures requires the presence of a MADS-domain transcription factor named SHORT VEGETATIVE PHASE (SVP) that represses FT .
In some Arabidopsis thaliana accessions, a long exposure to cold temperatures, called vernalization, is required to get over a block provided by an inhibitory transcription factor called FLOWERING LOCUS C, yet another MADS-domain transcription factor .
Developmental age is important for the level of FT’s response to long days, which could be one of many reasons why older plants flower faster in response to long days than very young plants.
Most regulation of FT depends on the presence of chromatin-mediated gene regulation by the Polycomb Group protein (link to Project 2) pathway [4, 8, 11-12]. Arabidopsis mutants with a defective Polycomb Group pathway express FT without a requirement of CO and therefore flower independent of day length . In contrast, chromatin-mediated repression is not required to restrict FT expression to the phloem .
Figure 3. Many pathways converge to regulate the expression of the floral integrator gene FT. The juvenility and autonomous pathways provide an input from developmental cues, whereas the photoperiod, vernalization and ambient temperature pathways are connected to signal from the environment. The Polycomb pathway provides the structural scaffold to enable control by the regulatory pathways.[less]
Figure 3. Many pathways converge to regulate the expression of the floral integrator gene FT. The juvenility and autonomous pathways provide an input from developmental cues, whereas the photoperiod, vernalization and ambient temperature pathways are connected to signal from the environment. The Polycomb pathway provides the structural scaffold to enable control by the regulatory pathways.
Ongoing Research in our group
We map cis-regulatory regions at the FT gene and identify the corresponding upstream regulators. This allows us to learn more about the cross-talk between regulatory pathways and to understand the hierarchy between them.
To identify regulatory regions, we make use of phylogenetic analyses. In the past, comparing FT genes from different Brassicacea pinpointed a distal enhancer as highly conserved phylogenetic shadow . Now, comparing the sequence diversity within the Arabidopsis thaliana species helps us to focus on key regulatory elements . Interestingly, the distal enhancer is located outside of the FT region enriched in the histone H3 modification H3K27me3, which is the chromatin mark indicative of repression by the Polcomb Group pathway. We have shown that the distal enhancer is not required for FT expression if these are defective in Polycomb-mediated repression. Such permissive enhancers that allow reprogramming of transcription of Polycomb Group-repressed genes have recently also been identified in mammals . A key question for the future is how the distal enhancer connects to the transcriptional start to regulate expression. A possible involves the formation of long range chromatin looping and we are currently working on experiments corroborating this model.
Flowering time has been and continues to be an important selected trait during crop domestication. The availability of plants with different responses to the environment made it possible to extend the distribution range of various crops such as rice, barley, wheat, cabbage and maize. Given the conserved role of FT-related genes as florigen, an intricate knowledge of their regulation will be helpful to select crop variants that are better adapted to changing climate conditions, which we expect as a result of the consequence of the observed global warming.
- Turck, F., F. Fornara, and G. Coupland, Regulation and identity of florigen: FLOWERING LOCUS T moves center stage. Annu Rev Plant Biol, 2008. 59: p. 573-94.
- Takada, S. and K. Goto, Terminal flower2, an Arabidopsis homolog of heterochromatin protein1, counteracts the activation of flowering locus T by constans in the vascular tissues of leaves to regulate flowering time. Plant Cell, 2003. 15(12): p. 2856-65
- Adrian, J., S. Torti, and F. Turck, From Decision to Commitment: The Molecular Memory of Flowering. Molecular Plant, 2009. 2: p. 628-642.
- Turck, F., F. Roudier, S. Farrona, M.L. Martin-Magniette, E. Guillaume, N. Buisine, S. Gagnot, R.A. Martienssen, G. Coupland, and V. Colot, Arabidopsis TFL2/LHP1 specifically associates with genes marked by trimethylation of histone H3 lysine 27. PLoS Genet, 2007. 3(6): p. e86.
- Wenkel, S., F. Turck, K. Singer, L. Gissot, J. Le Gourrierec, A. Samach, and G. Coupland, CONSTANS and the CCAAT box binding complex share a functionally important domain and interact to regulate flowering of Arabidopsis. Plant Cell, 2006. 18(11): p. 2971-84.
- Kumimoto, R.W., Y. Zhang, N. Siefers, and B.F. Holt, 3rd, NF-YC3, NF-YC4 and NF-YC9 are required for CONSTANS-mediated, photoperiod-dependent flowering in Arabidopsis thaliana. Plant J, 2010.
- Tiwari, S.B., Y. Shen, H.C. Chang, Y. Hou, A. Harris, S.F. Ma, M. McPartland, G.J. Hymus, L. Adam, C. Marion, A. Belachew, P.P. Repetti, T.L. Reuber, and O.J. Ratcliffe, The flowering time regulator CONSTANS is recruited to the FLOWERING LOCUS T promoter via a unique cis-element. New Phytol, 2010. 187(1): p. 57-66.
- Adrian, J., S. Farrona, J.J. Reimer, M.C. Albani, G. Coupland, and F. Turck, cis-Regulatory elements and chromatin state coordinately control temporal and spatial expression of FLOWERING LOCUS T in Arabidopsis. Plant Cell, 2010. 22(5): p. 1425-40.
- Franklin, K.A., S.H. Lee, D. Patel, S.V. Kumar, A.K. Spartz, C. Gu, S. Ye, P. Yu, G. Breen, J.D. Cohen, P.A. Wigge, and W.M. Gray, Phytochrome-interacting factor 4 (PIF4) regulates auxin biosynthesis at high temperature. Proc Natl Acad Sci U S A, 2011. 108(50): p. 20231-5.
- Lee, J.H., S.J. Yoo, S.H. Park, I. Hwang, J.S. Lee, and J.H. Ahn, Role of SVP in the control of flowering time by ambient temperature in Arabidopsis. Genes Dev, 2007. 21(4): p. 397-402.
- Farrona, S., G. Coupland, and F. Turck, The impact of chromatin regulation on the floral transition. Semin Cell Dev Biol, 2008. 19(6): p. 560-73.
- Farrona, S., F.L. Thorpe, J. Engelhorn, J. Adrian, X. Dong, L. Sarid-Krebs, J. Goodrich, and F. Turck, Tissue-specific expression of FLOWERING LOCUS T in Arabidopsis is maintained independently of polycomb group protein repression. Plant Cell, 2011. 23(9): p. 3204-14.
- Weigel, D. and R. Mott, The 1001 genomes project for Arabidopsis thaliana. Genome Biol, 2009. 10(5): p. 107.
- Taberlay, P.C., T.K. Kelly, C.C. Liu, J.S. You, D.D. De Carvalho, T.B. Miranda, X.J. Zhou, G. Liang, and P.A. Jones, Polycomb-repressed genes have permissive enhancers that initiate reprogramming. Cell, 2011. 147(6): p. 1283-94.