Control of flowering time

Work in our group focuses on the transition to flowering in Arabidopsis thaliana. To ensure reproductive success, plants have developed an elaborate genetic network that integrates endogenous and environmental factors to correctly time the onset of flowering. One of the most important factors that control this developmental switch is day length, or photoperiod. Based on grafting experiments it has been proposed that photoperiod is perceived in leaves where it leads to the induction of a flower-forming substance called ‘florigen’. The florigen is than transmitted to the shoot apex where it induces the transition to flowering. The molecular nature of florigen has eluded characterization for the past 70 years, but recent data suggested that the FLOWERING LOCUS T (FT) protein can be transmitted from the leaves to the apex. Whether the transport of FT protein is absolutely necessary to induce flowering and the role of the FT mRNA are still not entirely clear. 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 have identified a negative regulator of the floral transition, SCHLAFMÜTZE (SMZ), which acts in a long-day-specific manner. The molecular mechanism by which SMZ is preventing flowering is investigated with gain- and loss-of-function approaches. SMZ is a member of a clade of six APETALA2 (AP2)-like transcription factors, all of which are targeted by miRNA172. The function of miR172 in regulating flowering time is investigated in collaboration with the group of Detlef Weigel. We could recently show that SMZ represses the induction of FT in leaves. The identification of SMZ as a negative regulator of FT provided us with the means to investigate the role of the FT mRNA as a florigen. We have applied misexpression of SMZ to locally repress induction of FT in various parts of the plant, especially in the leaves and at the apex. Ours results indicate that transcription of FT at the apex is not required for the induction of flowering. This notion is supported by our finding that targeted an artificial microRNA against the FT transcript delays flowering only when targeted to leaves, but not to the apex (Mathieu et al., 2007). Together, these observations suggest that it is the FT protein, rather than the FT mRNA that needs to be exported from the leaves to induce flowering.

To be able to discriminate between mRNA and protein movement we have devised an innovative way to trap the protein in the cells where it is expressed. This is achieved by fusing FT (~19 kD) to three copies of GFP (~81 kD). The FT:3xGFP fusion protein is fully functional, as shown by expressing it at the apex, where FT binds to the FD transcription factor. Expression of FT:3xGFP in leaves does not induce flowering, indicating that FT is not inducing an unknown mobile signal in the leaves. More recently we have developed an in vivo protease cleavage assay that enables us to release mature FT protein from the large FT:3xGFP precursor. To that end we have introduced a TEV protease cleavage site between FT and the 3xGFP protein (FT:TEVrs:3xGFP). Lines transformed with this construct under the control of the phloem-specific SUC2 promoter do not flower early. Similar, flowering time is not affected in lines expressing the TEV protease under the control of either the SUC2 or the ubiquitous 35S promoter. However, when we crossed 35S::TEV lines with SUC2::FT:TEVrs:3xGFP plants, the progeny was extremely early flowering, similar to what has been observed in SUC2::FT lines. This suggests that mature FT protein is released from the larger precursor, and that it is FT protein translocation rather than mRNA movement that is crucial for the induction of flowering (Mathieu et al., 2007). We are now further investigating this possibility in collaboration with Ove Nilsson and our former colleague Phil Wigge.

Apart from our work on bona-fide flowering time genes, we are also studying by reverse genetics a family of four related genes, now called FANTASTIC FOUR (FAF). The FAF genes were first identified in a microarray experiment aimed at identifying novel floral regulators. They encode plant-specific, single-exon genes without domains of known function. Two, FAF2 and FAF4, are expressed in the organizing centre of the shoot meristem. In addition, all four genes are expressed in the vasculature. In agreement with a role for the FAF genes in the regulation of meristems, overexpression of FAFs resulted in an arrest of both the shoot and the root meristem. Additional results indicate that the FAF genes have the potential to repress the homeo-domain transcription factor WUSCHEL (WUS) in the organizing centre of the shoot meristem, and that they are themselves under repression by CLAVATA3 (CLV3). Thus it appears that the FAF genes can modulate the WUS-CLV feedback loop at the shoot apex. That overexpression of FAF genes also affects the root meristem indicates that they regulate a general meristem maintenance pathway. This notion is supported by the fact that the arrest of the root meristem can be rescued by exogenous sucrose. An additional connection between the FAF proteins and sugar metabolism comes from the observation that the FAF proteins interact in yeast with TREHALOSE-6-PHOSPHATE SYNTHASE 1 (TPS1), a key enzyme in trehalose synthesis. It is interesting to note that trehalose has been implicated with the control of meristem development before. Our finding that a number of TPP genes, which encode enzymes that convert trehalose-6-phosphate to trehalose, are strongly upregulated in FAF overexpressers, provides additional support for an intimate link between the FAF genes and trehalose metabolism. Interestingly, it was recently ben shown that an Arabidopsis tps1 mutant remained vegetative and failed to induce flowers throughout life (van Dijken et al., 2004). We are planning to investigate the connection between induction of flowering and trehalose in more detail in the future.

Personnel

Dr. Markus Schmid

Group Leader

Anusha Srikanth

Ph.D. Student

Frank Küttner

Technician

Janina Vogt

Technican

Jathish Ponnu

Ph.D. Student

Johannes Mathieu

Ph.D. Student

Levi Yant

Ph.D. Student

Vinicius Costa Galvão

Ph.D. Student

Former lab members

Tanja Weinand

Diploma Student

Sarah Fehr

Diploma Student

Florian Aldehoff

Diploma Student

Vanessa Wahl

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

Key publications

• 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., Lohmann, J.U., and Weigel D. (2005), Integration of spatial and temporal information during floral induction in Arabidopsis. Science 309, 1056-1059.
  

• Schmid, M., Davison, T.S., Henz, S.R., Pape, U.J., Demar, M., Vingron, M., Schölkopf, B., Weigel, D., Lohmann, J.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.