AUTHOR=Foster Kylie J. , Miklavcic Stanley J. 

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

JOURNAL=Frontiers in Plant Science

VOLUME=Volume 8 - 2017

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01326

DOI=10.3389/fpls.2017.01326

ISSN=1664-462X

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 of this model 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. We have
applied this model 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 effect 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 of the Casparian strip cannot be replaced by the action of membrane
transporters. The simulations also demonstrated that the positive effect
of the Casparian strip, in terms of controlled Na$^+$ uptake, was somewhat
offset by its contribution to the osmotic stress component: a more
effective Casparian strip 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 function of the suberin lamellae which has not
previously been considered. Specifically, the lamellae reduced plant
energy requirements by acting as a physical barrier to prevent the passive
leakage of Na$^+$ into the endodermal cells. The model results suggest
that more and particular experimental attention should be paid to the
character of the Casparian strip present when assessing the salt tolerance
abilities of different plant varieties and species. Indeed, the Casparian
strip appears to be a more promising target for plant breeding