Control of flowering time

The induction of flowering is a central event in the life cycle of plants. When timed correctly, it helps ensure reproductive success, and therefore has adaptive value. Because of its importance, flowering is under the control of a complex genetic circuitry that integrates environmental and endogenous signals, such as photoperiod, temperature and hormonal status. Based on grafting experiments, it has been long proposed that photoperiod is perceived in leaves where it leads to the induction of a flower-forming substance, or ‘florigen’. The florigen is then thought to be transmitted to the shoot apex where it induces the transition to flowering. The molecular nature of florigen has eluded characterization for 70 years, but recent data suggested that the FLOWERING LOCUS T (FT) gene might play a central role in this process.

Work in our group aims to understand the precise mechanisms that govern flowering time, in particular in response to inductive photoperiod, in the best-understood model plant, Arabidopsis thaliana.

We have identified a negative regulator of the floral transition, SCHLAFMÜTZE (SMZ) that represses flowering in a long-day-specific manner. SMZ is a member of a clade of six APETALA2 (AP2)-like transcription factors, all of which are targeted by miRNA172 (Schmid et al., 2003). Using ChIP-chip, we could recently show that SMZ directly binds and represses FT expression in leaves. In addition, SMZ was found to directly regulate several other key flowering time and flower development genes, including its own family members, demonstrating a regulatory feedback loop (Mathieu et al., 2009). This work supports an emerging body of evidence, which envisions floral repressors of several transcription factor families playing a key role in the fine tuning of floral induction (Yant et al, in press).

The second focus of our work is on understanding the role of FT in transmitting the floral stimulus from the leaves to the shoot apex. Our data suggested that the FT protein can be transmitted from the leaves to the apex and is sufficient to induce flowering (Mathieu et al., 2007). At the apex, the FT protein finally interacts with a bZIP transcription factor FD to induce flower meristem identity genes such as APETALA1 (AP1) and FRUITFUL (FUL) (Wigge et al., 2005).  We are currently investigating FD and FT function by mapping the genome wide binding sites of this protein complex by ChIP-seq (ChIP coupled to massively parallel deep sequencing).

Apart from our work on the photoperiod pathway, we have recently begun to investigate the role of endogenous regulators of flowering time such as gibberellic acid and trehalose-6-phosphate. To better understand the transcription factor network that integrates the various flowering time signals, we employ next generation sequencing to systematically identify binding sites for several key regulators of flowering by ChIP-seq. Part of this work is being carried out in collaboration with several other European colleagues in the BLOOMNET consortium. These efforts are being complemented by collaboration with Karsten Borgwardt in our department, to create a network model of flowering time regulation based on empirical knowledge of in vivo, genome wide transcription factor binding.

Key publications

• Mathieu, J., Yant, L.J., Mürdter, F., Küttner, F., Schmid, M. (2009), Repression of flowering by the miR172 target SMZ. PLoS Biol., 7(7):e1000148.
• Mathieu, J., Warthmann, N., Küttner, F., Schmid, M. (2007), Export of FT protein from phloem companion cells is sufficient for floral induction in Arabidopsis. Current Biology 17, 1055-1060.
• Wigge, P.A., Kim, M.C., Jaeger, K.E., Busch, W., Schmid, M., Lomann, J.U., Weigel, D. (2005), Integration of spatial and temporal information during floral induction in Arabidopsis. Science 209, 1056-1059.
• Schmid, M., Davison, T.S., Henz, S.R., Pape, U.J., Demar, M., Vingron, M., Schölkopf, B., Weigel, D., Lohmann, L.U. (2005), A gene expression map of Arabidopsis thaliana development. Nature Genetics 37, 501-506. 
• Schmid, M., Uhlenhaut, N.H., Godard, F., Demar, M., Bressan, R., Weigel, D., Lohmann, J.U. (2003), Dissection of floral induction pathways using global expression analysis. Development 130, 6001-6012.

Personnel

Dr. Markus Schmid markus.schmid@tuebingen.mpg.de

Group Leader

Former lab members

Tanja Weinand

Diploma Student

Sarah Fehr

Diploma Student

Florian Aldehoff

Diploma Student

Vanessa Wahl

Ph.D. Student

Johannes Mathieu

Ph.D. Student

Collaborators

Dr. Ray Bressan

Purdue University, Indiana, US

Dr. James Carrington

Oregon State University, US

Dr. Jan Lohmann

MPI for Developmental Biology

Dr. Phil Wigge

John Innes Centre, Norwich, Uk