Growing up and down the root: Applying multi scale modeling to primary and secondary root development

In this thesis we studied regulation of plant root growth by combining computer models with experiments. We studied both primary growth, which occurs at the root tip, and secondary growth, which occurs higher up the root. We contrasted data from the well-studied Arabidopsis thaliana model plant with a variety of computational models. By incorporating cell-level interactions into a multicellular computer model, we were able to study the tissue level effects that emerge from these interactions, as well as their feedback back onto the cell level, in great detail. In primary root growth we used this technique to determine how the size of the root tips proliferation domain is regulated. A pushing contest between the division stimulating PLETHORA and the differentiation inducing ARR1 and ARR12. The PLETHORAS and ARRs mutually repress each other’s production, creating two separated domains along the longitudinal axis of the root tip. Using the model we show how the onset of division initially favors PLETHORAs carried along by the dividing cells, while also diluting the PLETHORA further away from the PLETHORA producting tip. Initially, ARR12 is only able to slow down the expansion of the division zone, while later activation of the division repressing ARR1 forces a tie that dynamically stabilzes the division domain size. We further studied secondary growth, which determines the plants long distance transport capacity through the production of new phloem and xylem vascular cells. A thin layer of dividing cambial cells, which lies between these two cell populations, supplies both sides with new tissue. Combining a literature study with experiments, we determined a regulatory network that determines the cambial identity of the dividing cells, as well as the xylem and phloem identities of the differentiating cells. We further demonstrated how the binding of phloem derived TDIF peptides to their xylem localized PXY receptors robustly positioned the cambium under a variety of conditions. Then, we showed how different hormone concentrations and division speeds could regulate both absolute and relative xylem and phloem production by introducing growth to the model.