Head(s) : Arp_SCHNITTGER 

Topic and strategy

How to control cell proliferation and coordinate it with cell differentiation and overall tissue and organ growth is one of the most vital challenges for every multicellular organism. In humans, a failure of this coordination result in severe maladies such as cancer.

In plants, this regulation is equally important and human life depends on the faithful control of plant growth. For instance plants build the source for many biomaterials and chemicals and in particular are now being explored as a source of renewable energies.

Cell proliferation control is especially crucial during the unique reproductive life phase of flowering plants that is decisive for reproductive success, genetic diversity and speciation in plants. Moreover, the product of fertilization, the plant seed, is the major food source for mankind and lifestock.

Our laboratory studies cell cycle control and plant reproduction by using a combination of biochemistry, cell biology, molecular biology, genetics, and genomics as well as proteomics approaches.

We compare for our work two evolutionary distant organisms: Arabidopsis thaliana, a dicotyledonous plant that is currently the best-studied and best-understood plant organism. In addition, we use Brachypodium distachyon, a grass species, which is closely related to the agronomically important species wheat and barley.

Key words

cell cycle, proliferation, endoreplication, gametophyte development, double fertilization, seed biology

Significant results

Our research team has made important discoveries that on the one hand have contribute to the elucidation of general cell cycle regulation and on the other have advanced our understanding of plant reproduction. In general, cell cycle regulators appear to be conserved in plants. And indeed, studying the plant cell cycle allowed us to reveal general features of cell cycle regulation. For instance, our laboratory has shown that cell cycle inhibitors can function in a non-cell-autonomous manner.

However, there are also profound differences between animal and plant cell cycle control and recent data from our group has demonstrated that – in contrast to animals - Cdc25-like phosphatases are not important in plants for entry into mitosis. Thus, the molecular mechanistics of at least some of the involved regulators appear to be conserved. The wiring, however, is likely to be fundamentally different and the work on plants allows us now to study the evolution of the molecular circuitry and their biological and biochemical constraints.

As an entry point to explore the plant cell cycle control in detail, we have followed a classical genetic approach isolating mutants for major cell cycle regulators in Arabidopsis. As expected, many of these mutants affect the early stages of the plant life cycle, i.e. the gametophytes (pollen and embryo sac) or the embryo. This has stimulated the work on plant reproduction and seed development.

A major step was the isolation and characterization of a mutant in the central cell cycle regulator in Arabidopsis, the Cdk1/Cdc2+/CDC28 homolog CDKA;1. Pollen of cdka;1 mutant plants develop only one instead of two gametes as found in wild type. The cdka;1 mutant represents a unique tool for to dissect of regulatory circuits during seed development. This has enabled us to unravel a previously unknown hierarchy during the unique double fertilization process, the hall mark of flowering plants. Moreover, we could identify a novel signal transduction cascade during seed development.

In subsequent experiments with could show that double fertilization is not required in sexually reproducing plants and that - like in gymnosperms (lower seed plants) – a single fertilization event is sufficient to generate a functional seed. This finding also had implications for the evolution of the double fertilization process and addressed an old developmental question raised in 1900 on the origin of the nutritive tissue (endosperm) in the seed of flowering plants.

Contracts

• ATIP jeune chercheur, 2007-2010
• ERC starting grant, 2008-2013
• SFERE, 2008-2012

Recent publications 

Jakoby, M.J., Falkenhan, D., Mader, M.T., Brininstool, G., Wischnitzki, E., Platz, N., Hudson, A., Hulskamp, M., Larkin, J., and Schnittger, A. (2008). Transcriptional profiling of Arabidopsis trichomes reveals that NOEK encodes the putative transcriptional regulator MYB106. Plant Phys, 148, 1583-602.

Ungru, A., Nowack, M.K., Reymond, M., Shirzadi, R., Kumar M., Biewers, S.,
Grini, P.E., and Schnittger, A. (2008). Natural variation in the degree of autonomous endosperm formation reveals independence and constraints of embryo growth during seed development in Arabidopsis thaliana. Genetics, 179, 829–841.

Nowack, M. K., Shirzadi, R., Dissmeyer, N., Dolf, A., Endl, E., Grini, P.E., and Schnittger, A. (2007). Bypassing of genomic imprinting allows seed development. Nature, 447, 312-5.

Dissmeyer, N., Nowack, M. K., Pusch, S., Stals, H., Inzé, D., and Schnittger, A. (2007). T-loop phosphorylation of Arabidopsis CDKA;1 is required for its function and can be partially substituted by an aspartate residue. Plant Cell, 19, 972-85.

Nowack, M.K., Grini, P.E., Jakoby, M.J., Lafos, M., Koncz, C., and Schnittger, A. (2006). A positive signal from the fertilization of the egg cell sets off endosperm proliferation in angiosperm embryogenesis. Nature Genetics, 38, 63-7.

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