Defining the link between epidermal patterning and auxin responsiveness in the Arabidopsis thaliana root.

The aim of this project was to define the link between developmental patterning and auxin response in the Arabidopsis thaliana root epidermis. The root epidermis was used as a model to understand the interaction of patterning mechanisms, such as those that define the hair and non-hair producing epidermal cells, and auxin. According to data published before this project began the epidermal patterning mechanism and the auxin response network are two independent aspects of plant growth. However the results detailed within this thesis have highlighted that this view is too simplistic, and they actually interact at multiple levels

Having established the presence of a repressive auxin response regime in the non-hair cells of the root epidermis, analysis of the gl2-1 mutant allowed us to further understand the functional significance of this. Results were consistent with ARF10 and/or ARF16 functioning to restrict root hair growth. In order to place the spatial control of auxin response in the current knowledge of the epidermal patterning mechanism, analysis of mutant and marker line crosses was carried out. These indicated that members of both pathways were involved in promoting and inhibiting the expression of components of the other pathway, thereby indicating the presence of multiple interactions, with both positive and negative feedback loops existing between the two. Specifically the non-hair promoting components WER and MYB23 were observed to promote the expression of the repressive ARF, ARF10. In turn ARF10 was observed to promote the expression of WER and GL2, but inhibit the expression of MYB23. Finally analysis of constructs that blocked the down-regulation of MYB23 by auxin highlighted the possibility that auxin mediated root hair elongation may function via the down-regulation of the epidermal patterning component MYB23, thus indicating a potentially novel role for an epidermal patterning component that has previously been considered to be somewhat insignificant.