Chalmers Conferences, 9th European Conference on Mathematical and Theoretical Biology

Modeling of Zinc Uptake and Transport in Plant Roots
Juliane Claus, Andrés Chavarría-Krauser

Last modified: 2014-06-09


Zinc is an essential micronutrient in green plants, yet toxic at high concentrations. Only specialized hyperaccumulator plants can tolerate high zinc doses and are therefore of special interest for their potential application in phytoremediation and crop development. Zinc ions are taken up from the soil along with water and are transported radially towards the root's vascular bundle in two parallel pathways: cell wall (apoplast) and cytoplasm (symplast). Cross-membrane transport into and out of the cytoplasm is mediated by ZIP and HMA transporter proteins, respectively. The ZIP transporters responsible for zinc uptake are highly regulated to guarantee an optimal internal zinc concentration under varying external conditions.

A dynamical model based on ordinary differential equations is used to study the regulation of ZIP transporters. Simulations of different model versions suggest an activator-inhibitor model as the most likely mechanism, because it provides more robust zinc homeostasis than simpler models without inhibitor. High robustness of the steady state towards external zinc variations, however, leads to instability of the steady state and high-amplitude oscillations. These oscillations form stable periodic solutions and emerge from a supercritical Hopf bifurcation in certain critical values of the external zinc concentration. Buffering can dampen the oscillations and lead to stability of the steady state. Since experiments do not suggest oscillatory behavior in the cellular zinc concentration, these results indicate the existence of strong zinc buffers.

To study spatial aspects of the zinc distribution in root tissues, the ZIP regulatory model was coupled to a radial transport model. This model accounts for the structure of the root consisting of symplast and apoplast and includes effects of water flow, diffusion, and cross-membrane transport via transporters. It also incorporates the radial geometry and varying porosity of root tissues. We use existing biological data to estimate parameters and analyze the properties of the model in numerical simulations. Experimental results show a pattern of zinc accumulation close to the centre of the root, which disappears at high levels of the efflux transporter HMA. Using our model, we study the roles of ZIP regulation, HMA level and water flow velocity in the creation of this radial pattern. In the steady state, the model reproduces the zinc gradient found in experiments as well as its loss at increased levels of HMA. Surprisingly, water flow velocity is found to be also a key parameter for producing this gradient.

These results give insight into the uptake and transport of zinc in roots and suggest improved experimental measurements.