Theme 1: Developmental Biology of Plants

The research within this theme aims at elucidating the molecular and cellular bases of the processes that govern plant development. During the development of plants, the genetic information contained in the zygote is passed to successive generations of cells by cell division. During this process the cells are arranged in tissues and organs with different specific functions. In seed plants, the zygote develops first into an embryo that is stored in the seed, and only after seed germination the post-embryonic development of the plant body initiates. A key role in the development of plants is played by the primary meristems, which are laid down in the embryo. They produce new cells that differentiate into tissues and organs, depending on the position of the meristem in the plant. During the vegetative stage of the plant, and with the transition to flowering, the primary meristems and secondary meristems form organs such as roots, stems, leaves, and floral organs.

In addition to the genetic information carried by the meristematic cells, internal signal molecules such as growth regulators and external factors such as light also contribute to the embryonic and/or post-embryonic development of the plant.

Another property of plants, is that many of the differentiated somatic cells can reverse their development and regenerate into complete new plants. Investigations on this feature and on the mechanisms of the regeneration process are of great fundamental and practical importance in plant biology.

To date, a conspicuous amount of the research on plant development is focused on the identification of various genes and signals involved in the developmental processes. In the first place, unraveling the function of these genes will provide clues on regulatory events that are required for the formation of a particular plant organ. Secondly, these genes and their products can also be used as molecular markers to trace back the relations between cells, i.e. whether cells derive from the same cell lineage or not, when and where cells differentiate, and how cells communicate with each other. However, the biggest challenge remains to identify genes and signals that produce the positional information which ultimately determines pattern formation and cell differentiation, by which cells acquire their specific function and identity in the different tissues and organs.

The research on plant development is coordinated in the three following subthemes:

Subtheme 1a:

Perception & Transduction of Signals in Plant Development

Development and morphogenesis of plants are controlled by internal signals such as plant hormones and by external signals such as light, plant growth stimulating bacteria and other factors. The perception of these signals and their transduction play a role in almost every aspect of plant development. The research in this subtheme aims at the identification of signal molecules and the receptors involved in signal perception, and at the study of the pathways followed in signal transduction, that finally result in specific gene expression and morphogenesis. For example, the study on factors such as the plant hormones auxin, ethylene, cytokinins and gibberellins includes the work on embryogenesis, organogenesis, flower senescence, lateral root formation and the role of ethylene in plant reproduction. External factors such as light and growth regulators and microbial saccharides are investigated for example using the model system of anthocyanin biosynthesis in tomato seedlings, and the signal route of the ENOD factors and lectins in nodule and root hair development in leguminosae, respectively. The isolation and characterization of mutants disturbed in signal perception or transduction is an important part of this subtheme. The identification and function of receptors is undertaken with a genetic and a bio-chemical approach. The signal transduction pathways leading to morphogenesis are analyzed by biochemical, biophysical, physiological and cell biological methods.

The research encompasses studies on the cytoskeleton and its associated proteins as a line between signal transduction pathways and morphogenesis. During development certain specific cell types (e.g. xylem, phloem, root cap aleuron cells) are killed by a process of programmed cell death. Death signalling is also included as a topic of study.

Although most plant hormones have no counterpart in other eukaryotic systems, there is extensive similarity between components of signal transduction pathways in plants and other eukaryotes. These common components include histidine kinases, serine-threonine kinases, phosphatases, monomeric G-proteins, transcription activators and adenylyl cyclase. Where appropriate, model systems such as yeast and Dyctyostelium will be used to study signal transduction pathways in plants.

Subtheme 1b:

Embryogenesis and Organogenesis

The study of plant embryogenesis is complicated by the fact that the zygotic embryo is embedded in the nourishing endosperm and covered by the tissues of the ovule which later will become the seed coat. In general, this makes these embryos not easily accessible and not amenable for dissection of large quantities for biochemical studies.

Somatic embryogenesis, which involves the development of embryos from somatic cells of differentiated organs, provides an alternative experimental system for investigating the cellular and molecular bases of plant embryogenesis. Daucus, Brassica, Arabidopsis and rice (Oryza sativa) have become important model plants for the genetic analysis of somatic and zygotic embryogenesis, respectively, and several mutants affected in embryo development have been identified. A possible approach is to identify genes and products that have an effect on early somatic embryogenesis, and then analyze their expression pattern in zygotic embryos. This strategy will give clues on the first events of pattern formation and cell-cell communication in plant embryogenesis. The role which the classical phytohormones auxin and cytokinin play in embryogenesis will also be studied. In addition, similarities and differences between embryo-genesis in dicots and monocots will be investigated.

Organogenesis concerns the induction of a primordium that develops in a meristem for the differentiation of cells in the primordium. Usually the organ primordia arise at specific positions which are dependent on the developmental stage of the continuous growth process. The research on plant organ development also aims at understanding how the positional information controls the generation of an organ. A good example of this research is the determination of floral organ identity in the appropriate whorls in e.g. Petunia and Cucurbita flowers. Another example constitutes the formation of nodule primordia in front of xylem poles in roots of leguminosae. A particular aspect of organogenesis concerns plant regeneration from somatic cells and microspores and clonal propagation of plants, that results in starting material for fundamental studies and applications in horticulture. There is a similarity between plant and vertebrate organogenesis in that in both processes chitin oligosaccharides play a role. It is of importance to further investigate to which extent signal transduction of chitin oligosaccharides is similar in plants and animals.

Subtheme 1c:

Reproduction Biology

Sexual reproduction is a key process in the life cycle of the plant and of crucial importance for the diversity and the preservation of plant species. The research in this subtheme includes investigations on development of the male and the female gametophytes after meiosis (see subtheme 4b) within the flower, i.e. the pollen grain that develops in the anther, and the embryo-sac that develops in the ovule, respectively. For example, morphological, biochemical and molecular studies are addressed to unravel micro-sporogenesis in Nicotiana and Brassica, and mega-sporogenesis and megagametogenesis in Petunia and Nicotiana. At maturity pollen grains are transferred to the pistil, on/in which a series of interactions take place. Recognition or rejection of the pollen may take place already on the stigma in inter- and intra-specific crosses in some species, or in the style, in others. Self-incompatibility and unilateral incompatibility (or incongruity) are investigated in Solanum and in Nicotiana. The process of promotion of pollen tube growth by the pistil takes place only when the pollen or pollen tubes are accepted by the pistil, and fertilization can occur when the tube penetrates the embryo-sac in the ovule.

The interactions between pollen and pistil and pollen tube and ovule suggest an active mechanism of communication between different cell types. The signaling mechanisms that operate in these systems are studied in Gasteria, Lilium, Nicotiana and Petunia in this subtheme. The achievements in this area of research will provide the bases for biotechnological applications in the field of control of reproduction and seed production.