Spatiotemporal modelling of hormonal crosstalk in the Arabidopsis root.

Abstract

Plant development, growth and response to varying environmental conditions, involves a complex network of overlapping interactions between plant signalling hormones and gene expression, known as ‘CROSSTALK’, which controls cell proliferation, elongation and differentiation. Hormone response, concentrations and gene expression levels vary through the root tip and display patterning, which ultimately drives development; however, little is known about how this is established. Models have been constructed to explain patterning, including a ‘physical’ auxin flux model in a simple rectangular 2-D multicellular Arabidopsis root which excludes crosstalk (Grieneisen et al., 2007), and a single cell ‘biological crosstalk’ model of multiple hormone and protein interactions in WT and mutants (Liu et al., 2010; Liu et al., 2013). The project goal was to combine these approaches by embedding the single cell biological crosstalk relationships into a 2-D multicellular root structure to reproduce experimentally observed hormone and gene expression patterning. An initial model was constructed and parameter values calibrated to meet fit criteria and produce a WT parameter set. The model proved robust to parameter variation, indicating that results did not rely on unique parameter value selections. Model results were compared to experimental data to test predictive capability and matched experimentally observed patterning and concentration trends for most species and mutants. A more realistic digital root map was then developed with additional auxin carriers to allow improved comparison between model and experimental images at a cell-scale level. The roles of auxin influx and efflux carriers in regulating auxin patterning were investigated by developing a ‘Recovery Principle’, where pattern perturbations due to changes in one carrier set could be recovered by adjustments to the other carrier set. Finally, using additional experimental data from the literature, the crosstalk network was revised to produce more representative cytokinin patterning. The model provides an explanation of crosstalk control of gene expression and patterning, and forms a foundation for future expansion of hormonal crosstalk and gene expression modelling in the Arabidopsis root. In summary, this project has developed predictive models to further explore hormone and gene expression levels and spatiotemporal pattern formation in the Arabidopsis thaliana root tip.