Stem cell regulation and floral patterning in Arabidopsis

Jan U. Lohmann Group

Few scientific topics currently attract as much attention as stem cells. While public debate has focused on embryonic stem cells from animals and humans, stem cells are common to all multicellular organisms. In most animals, the only “true” totipotent stem cell is the fertilised egg and its immediate descendants. Many plant cells, in contrast, continue to be totipotent throughout the plant’s life, and plant stem cells are much more easily obtained and analyzed than those of animals.

Plant stem cells reside at the growing points of a plant, namely the root tip and the shoot apex and are embedded into specialised structures called meristems, which provide a local environment that regulates the homeostasis between proliferation and differentiation. After the transition from vegetative to reproductive development of the plant, floral meristems arise from the main shoot apical meristem and then develop into flowers. Stem cell regulation and floral patterning in Arabidopsis are closely linked, since the homeodomain protein WUSCHEL (WUS) regulates stem cell fate in both shoot and floral meristems. WUS interacts with the flower-specific transcription factor LEAFY (LFY) to activate transcription of the floral homeotic gene AGAMOUS (AG), which is essential for the correct patterning of flowers. WUS and AG in turn form a negative feedback loop in which AG represses WUS to terminate the maintenance of stem cells in flowers, bringing about the determinate character of flowers.

Because the proteins involved are all transcription factors, it follows that they exert their function by orchestrating the transcriptional response of cells. Therefore, current research in Jan Lohmann’s group is aimed at deciphering the global transcriptional network underlying the transition of stem cells from proliferation to differentiation, using flower development as a model. To this end, the Lohmann group studies the transcriptional response to stem-cell and differentiation promoting factors using expression profiling as well as transcription factor binding studies on a genome wide level. New genetic screens involving plants carrying an inducible form of WUS are used to uncover essential signal transduction components. Genetic screens in Arabidopsis and yeast are also employed to identify new regulators of the floral homeotic gene AG.

Recent results indicate that WUS modulates the expression of genes involved in the cellular response to the phytohormone cytokinin thereby integrating local transcriptional signals with global hormonal signals to regulate meristem behaviour. Furthermore, we identified flower-specific GARP and bZIP transcription factors, which bind to conserved regulatory elements in the AG enhancer to bring about differentaition of stem cells via activation of this gene.

The goal of these studies is to identify new stem-cell regulators and to understand the interactions between them. Ultimately, a functional map including mathematical models with predictive functions is envisioned. Comparing the mechanisms of stem cell homeostasis employed by plants and animals should allow for the identification of common as well as divergent features of stem cell control.

The Lohmann lab is currently accepting applications at the postdoctoral level from candidates with a strong background in systems biology, developmental biology or genetics.

Personnel

Stig U. Andersen: Postdoc
Jan U. Lohmann: Group leader
Annette Maier: Ph.D. student

Andrej Miotk
Diploma student

Sebastian Schultheiß: Ph.D. student
Christoph Schuster: Ph.D. student
Zhong Zhao: Postdoc

Collaborators

Dr. Joe Kieber: University of North Carolina, US

Dr. Markus Schmid: MPI for Developmental Biology
Dr. Thomas Schmülling: FU Berlin
Dr. Klaus Harter: ZMBP Tübingen

Key publications

Whole-genome analysis of the SHORT-ROOT developmental pathway in Arabidopsis.

Levesque, M. P., Vernoux, T., Busch, W., Cui, H., Wang, J. Y., Blilou, I., Hassan, H., Nakajima, K.,
Matsumoto, N., Lohmann, J. U., Scheres, B., and Benfey, P. N. (2006) PloS Biology 4, 739-752.

WUSCHEL controls meristem function by direct regulation of cytokinin inducible response regulators. (including Supporting Online Material)

Leibfried, A., To, J.P.C., Busch, W., Stehling, S., Kehle, A., Demar, M., Kieber, J.J., and Lohmann, J.U. (2005)
Nature 438, 1172-1175.

Integration of spatial and temporal information during floral induction in Arabidopsis. (including Supporting Online Material)

Wigge, P.A., Kim, M.C., Jaeger, K.E., Busch, W., Schmid, M., Lohmann, J.U., and Weigel D. (2005)
Science 309, 1056-1059.

A gene expression map of Arabidopsis thaliana development.

Schmid, M., Davison, T.S., Henz, S.R., Pape, U.J., Demar, M., Vingron, M., Scholkopf, B., Weigel, D., and Lohmann, J.U. (2005)
Nature Genetics 37, 501-506.