Investigating the impact of cell wall dynamics on secondary growth regulation

The morphology of plants can be highly adaptive due to the plasticity governed by postembryonic development. In plants, stem cell populations, called meristems are maintained through the entire life and enable lifelong development. Longitudinal growth is realized by the meristematic activity in the apices of the shoot and root, while lateral growth is enabled by the activity of the vascular cambium that is able to produce specialized tissues in a bifacial manner. Initially, the fascicular cambium is active solely in vascular bundles. At a later stage of development, the fascicular cambium extends and cells in the region between vascular bundles – the so-called interfascicular region – start to divide, cumulating in the post-embryonic establishment of the vascular cambium. Its formation is potentially based on de-differentiation of differentiated cells. However, how exactly this process is initiated is still poorly understood. Here, I used the process of vascular cambium formation, as a unique model to study cell fate change. To control the formation of vascular cambium, I used a genetic tool that allowed me to induce auxin biosynthesis in a cell type-specific manner. Strikingly, induction of auxin biosynthesis in fully differentiated starch sheath cells provoked a cell fate change and initiated the formation of interfascicular cambium. Based on detailed analysis of interfascicular cambium formation, nuclear auxin signaling is shown to be critical for this process. Furthermore, I investigated the role of cell wall during vascular cambium formation, as plant cells are immobilized by cell walls – a rigid type of extracellular matrix. The thereby fixed position of the cell can contain important information for cell fate determination. Interestingly, extensive cell wall remodeling takes place during cambium formation, as revealed by immunohistochemical quantification of extensin abundance and pectin modifications. Genome-wide transcriptional profiling upon auxin-induced interfascicular cambium formation further supports this result, as genes related to cell wall remodeling were disproportionately represented among differentially expressed genes. Finally, I demonstrated the importance of cell wall remodeling during interfascicular cambium formation by blocking auxin induced interfascicular cambium formation via inhibiting cell wall remodeling. The reported change in cell wall composition during vascular cambium formation may cause a change in the elasticity of the cell wall, which might in turn be a prerequisite for lateral growth. To quantitatively measure the elasticity of cell walls, I established Brillouin microscopy in our lab and investigated cell wall properties depending on their orientation and localization. I was able to show that Brillouin microscopy is a suitable method to further investigate cell wall mechanics and its impact in the field of plant science