Research

Research in our laboratory centers on the developmental genetics of plant reproduction. We are interested in both sexual reproduction and apomixis, a form of asexual reproduction through seeds. We use genetic and molecular approaches to identify genes controlling the underlying developmental processes using Arabidopsis thaliana and maize as model systems. Our studies have shown that both genetic and epigenetic mechanisms play a key role in plant reproduction. The long term goal of our research is to elucidate the role of positional information, cell lineage, cell-cell communication and epigenetic regulation of gene expression in plant morphogenesis and cellular differentiation.

Sexual Reproduction
Maternal Effects
Epigenetic Mechanisms
Apomixis

The formation of the female gametes is a key step in plant reproduction. We use genetic and molecular approaches to identify genes controlling this process. Megasporogenesis leads to the formation of a functional megaspore within the ovule, the precursor of the seed. This haploid megaspore develops into a multicellular embryo sac or female gametophyte. Although the seven cells of the embryo sac are of clonal origin, they develop along four alternative developmental pathways. The molecular and genetic basis controlling this process is unknown. The highly polar nature of the embryo sac, its small number of distinct cell types and its closely coordinated development with surrounding tissues of the ovule make it an ideal system to study fundamental aspects of plant development, such as the role of positional information, cell lineage and cell-cell communication in plant morphogenesis and cellular differentiation. To identify genes involved in megasporogenesis and megagametogenesis we use enhancer detection, an approach that allows the identification of genes based on their pattern of expression, and classical genetic screens. We focus on mutants affecting megasporogenesis, cell specification in the embryo sac, or the process of double fertilization.

References:
Grossniklaus U, Schneitz K. (1998) Genetic and molecular control of ovule development and megagametogenesis. Seminars in Cell & Devl. Biol., 9:227-238.

Huck N, Moore JM, Federer M, Grossniklaus U. (2003) The female gametophytic control of pollen tube reception is disrupted in the Arabidopsis mutant feronia. Development 130:2149-59.

One of our mutants, medea, is of particular interest because it displays a gametophytic maternal effect on seed development. If the seed is derived from a mutant medea gametophyte, the seeds abort irrespective of the paternal contribution and dosage. In plants, maternal effects were not thought to play a crucial role for development becasue embryos can be formed from any cell under appropriate conditions in culture. We have shown, however, that early embryogenesis is largely under maternal control during sexual reproduction, as most paternally inherited genes are not active for some time after fertilization. We are investigating the foundation of this phenomenon using molecular and (cyto)genetic tools.

References:
Grossniklaus U, Vielle-Calzada, J-P, Hoeppner MA, Gagliano WB. (1998) Maternal control of embryogenesis by MEDEA, a Polycomb-group gene in Arabidopsis. Science, 280: 446-450.

Vielle-Calzada J-P, Baskar R, Grossniklaus U. (2000) Delayed activation of the paternal genome during seed development. Nature 404: 91-94.

Our interest in epigenetics originated in our studies on MEDEA, which is connected to epigenetic processes at two levels: genomic imprinting and Polycomb repression. We could show that the medea maternal effect is caused by genomic imprinting, that is only the maternally inherited allele is active after fertilization. Our current efforts focus on the elucidation of the genetic control of genomic imprinting. The medea mutant provides a unique entry point for identifying genes involved in the establishment and maintenance of genomic imprinting through second site modifier screens. Furthermore, MEDEA shares structural and functional similarities with Polycomb group proteins, which are involved in the epigenetic control of gene expression and keep target genes in a silent state over many cell divisions, thus providing a molecular basis for a cellular memory mechanism. We have identified target genes of MEDEA and are elucidating its biochemical function. MEDEA is part of a multiprotein complex, which is very similar to the E(z)-ESC complex of Drosophila.

References:
Vielle-Calzada J-P, Thomas J, Spillane C, Coluccio A, Hoeppner MA, Grossniklaus U. (1999) Maintenance of genomic imprinting at the Arabidopsis medea locs requires zygotic DDM1 activity. Genes & Dev, 13:2971-2982.

Köhler C, Hennig L, Spillane C, Pien S, Gruissem W, Grossniklaus U. (2003) The Polycomb group protein MEDEA regulates seed development by controlling expression of the MADS-box gene PHERES1. Genes & Dev. 17:1540-53.

A better understanding of the molecular mechanisms controlling plant reproduction will not only provide important insights into fundamental aspects of plant development but also provide tools for the manipulation of the reproductive system. We are particularly interested in the engineering of apomixis, a form of asexual reproduction through seeds. Over the last few years we performed genetic screens in maize that should allow the isolation of mutants displaying apomictic traits. We are also performing sceens in Arabidopsis with the goal of identifying elements of apomixis and use natural apomicts in comparative studies. The introduction of apomixis into crops will have revolutionary implications for plant breeding and agriculture, whose social and economic benefits promise to exceed those of the green revolution if fair access to the technology can be guaranteed.

References:
Koltunow AM, Grossniklaus U. (2003) Apomixis: a developmental perspective. Ann. Rev. Plant Devl. 54:547-74.

Spillane C, Curtis MD, Grossniklaus U. (2004) Apomixis technology development: virgin births in farmer’s fields? Nature Biotechnology 22:687-691.