A Comprehensive Biophysical Model of Ion and Water Transport in Plant Roots. I. Clarifying the Roles of Endodermal Barriers in the Salt Stress Response.

Kylie J Foster,Stanley J Miklavcic

doi:10.3389/fpls.2017.01326

Abstract

In this paper, we present a detailed and comprehensive mathematical model of active and passive ion and water transport in plant roots. Two key features are the explicit consideration of the separate, but interconnected, apoplastic, and symplastic transport pathways for ions and water, and the inclusion of both active and passive ion transport mechanisms. The model is used to investigate the respective roles of the endodermal Casparian strip and suberin lamellae in the salt stress response of plant roots. While it is thought that these barriers influence different transport pathways, it has proven difficult to distinguish their separate functions experimentally. In particular, the specific role of the suberin lamellae has been unclear. A key finding based on our simulations was that the Casparian strip is essential in preventing excessive uptake of Na+ into the plant via apoplastic bypass, with a barrier efficiency that is reflected by a sharp gradient in the steady-state radial distribution of apoplastic Na+ across the barrier. Even more significantly, this function cannot be replaced by the action of membrane transporters. The simulations also demonstrated that the positive effect of the Casparian strip of controlling Na+ uptake, was somewhat offset by its contribution to the osmotic stress component: a more effective barrier increased the detrimental osmotic stress effect. In contrast, the suberin lamellae were found to play a relatively minor, even non-essential, role in the overall response to salt stress, with the presence of the suberin lamellae resulting in only a slight reduction in Na+ uptake. However, perhaps more significantly, the simulations identified a possible role of suberin lamellae in reducing plant energy requirements by acting as a physical barrier to preventing the passive leakage of Na+ into endodermal cells. The model results suggest that more and particular experimental attention should be paid to the properties of the Casparian strip when assessing the salt tolerance of different plant varieties and species. Indeed, the Casparian strip appears to be a more promising target for plant breeding and plant genetic engineering efforts than the suberin lamellae for the goal of improving salt tolerance.

Highlights

In this paper, we present and implement a detailed theoretical model of the uptake and transport of ions and water through a plant root that captures a large set of the anatomical and biophysical features that have been identified through experiments to play an important role in the salt stress response of plants
In solving the governing system of equations presented in Section 3, an enormous wealth of information becomes available for scrutiny
A functional Casparian strip (CS) is essential to restrict the amount of Na+ being transported from the root to the shoot

Summary

Introduction

We present and implement a detailed theoretical model of the uptake and transport of ions and water through a plant root that captures a large set of the anatomical and biophysical features that have been identified through experiments to play an important role in the salt stress response of plants. The two key barriers, namely the Casparian strip (CS) and suberin lamellae (SL), develop in the endodermis, and in some plant species in the hypodermis (Enstone et al, 2002; Geldner, 2013) These barriers are understood to restrict the transport of toxic ions (and water) into root vascular tissues, which in turn limits the amount that enters leaves. These barriers play potentially significant roles in the salt stress response (Enstone et al, 2002)

Results

Discussion

Conclusion