Control of cell division orientation during vascular development in Arabidopsis thaliana

Growth of plants depends on cell division and elongation of the divided cells. After cell division, plant cells are fixed immediately within their tissue context. Thus, a carefully positioning and orientation of the division plane is crucial for plants to properly pattern and grow. Anticlinal cell divisions result in more cells within a cell file and lead to longitudinal growth; while periclinal and radial divisions (PRD) result in the addition of cell files during radial growth. So far, how a cell controls the position and orientation of its division plane remains poorly understood. In this thesis we focused on a complex of two proteins called TMO5 and LHW that are capable of shifting the division plane orientation towards PRD. Our main aim was to identify which genes are activated by the TMO5/LHW complex and characterize these to understand how plant cells control the positioning and orientation of their cell divisions to allow normal growth. After introducing the main concepts and background information in Chapter 2, we continue to show in Chapter 3 how we generated and characterized an inducible line of TMO5 and LHW (dGR). We compared this line to the existing TMO5-LHW misexpression and overexpression lines and concluded we created an excellent tool to study cell division orientation. Because TMO5 and LHW are transcription factors, we next studied the transcriptional changes upon induction. While it is commonly assumed that the plant hormone cytokinin is required to control these divisions, we could prove that indeed cytokinin response is triggered upon induction of TMO5/LHW. In Chapter 4 we used the transcriptional data to select putative targets, which were verified using a misexpression screen. We identified DOF2.1 as a factor capable of inducing PRD and continued to show how DOF2.1 functions downstream of TMO5-LHW and controls division plane orientation during vascular development. Analysis of loss-of-function mutants of DOF2.1 and the two closest family members revealed these Dof-type TF family members act in a redundant manner to control PRD. Because our misexpression screen only yielded one factor controlling PRD, we reanalyzed our transcriptional data in Chapter 5 using an inferred network analysis algorithm. We generated a gene regulatory network which indicated that a novel family of bHLHs acts as key transcriptional hubs downstream of TMO5-LHW. We could confirm that some of these factors are indeed TMO5-LHW regulated and are part of the downstream cytokinin response. Overexpression of these bHLH did not result in any obvious phenotype; however we showed that they interact with members of a different family of bHLH TFs. This suggests that they might require this interaction in order to carry out their function. Loss-of-function of single genes did not reveal a clear phenotype, indicating that there is a high amount of redundancy within the novel bHLH subfamily. Analysis of higher order mutants will be required to identify the function of these bHLH genes. In conclusion, the identification of DOF2.1 and its involvement in controlling PRD brings us one step closer to identify the key players that reorient and shift the division plane. However, the actual role of the novel bHLH subfamily acting downstream of TMO5-LHW is yet to be unraveled