Vascular tissue development in Arabidopsis thaliana

Communication between cells and tissues is a prerequisite for defining the position of a single cell within the context of the whole organism and to establish its particular identity. Positional information is then translated into specific gene expression profiles by the interaction of proteins with DNA. Such interaction includes the regulation of chromatin conformation to establish and maintain a specific repertoire of active and inactive genes in a particular cell type. In plants, the vascular tissue is the main system mediating transport over long distances and also determines the mechanical properties of the plant body. Due to the highly diverse cell repertoire and the close interaction with surrounding tissues, plant vascular tissue represents an attractive target for analysing cell specification and patterning in complex organisms. Remarkably, despite their interesting diversity and their importance for plant development and physiology, knowledge of the molecular processes which control the establishment and maintenance of vascular cell types and their physiological properties is still very fragmentary. In this proposal we, therefore, suggest isolating and characterising new developmental regulators involved in the determination of vascular cell identity and addressing the contribution of chromatin regulation to the tissue-specific activity of two characterised regulators. This will be achieved by performing a mutagenesis screen and by analysing tissue-specific chromatin properties. The outcomes of the proposed experiments will result in a more detailed view of the whole body organisation of plants by addressing the question of how their essential transport system is established and how its specific pattern is generated. The elucidation of such molecular mechanisms will also have implications for the understanding of developmental processes in other organisms, in particular, due to our combination of a more classical developmental question with emerging concepts of epigenetic gene regulation. Beyond scientific aspects, the understanding of the molecular control of vascular development also has implications for various aspects of crop production as it will facilitate the modulation of vascular properties by genetic and transgenic approaches.

The vascular system in plants is the main long-distance transport system responsible for the distribution of nutrients, water, and signaling molecules, and also provides mechanical stability to the plant body. A continuous network of vascular bundles, consisting of xylem and phloem, pervades every organ of the plant. The xylem part of the vasculature is responsible for the transport of water and nutrients, while the phloem transports sugars, proteins, RNA, and other signaling molecules. In this project we analyzed the function of APL, a gene described to be essential for the establishment of phloem identity during all stages of plant development. Importantly, after completing the project, we suspect that the role of APL is broader than originally anticipated. This conclusion was based on the analysis of a novel apl mutant allele (apl-2) which appears to carry a less active APL gene than the mutants described to date. apl-2 mutants show strong defects during embryogenesis suggesting that APL fulfils essential roles during very early stages of plant development and not only during phloem differentiation after germination. This discovery allows novel functions of the gene to be dissected and will increase our understanding of fundamental processes in plant development. Interactions between different tissue types are essential for the establishment of the body organization in plants. The findings obtained during this project provide novel perspectives to look at this process and thus, contribute significantly to our understanding of the basic processes regulating plant development.