Regulating Root Aquaporin Function in Response to Changes in Salinity

Abstract

Rapid changes in soil salinity present plants with an osmotic stress. If the concentration of salt in the soil water is too high then water may flow from the plant roots back into the soil. As part of an adaptive response, the regulation of proteins associated with water transport, such as aquaporins, is altered. The change in regulation of aquaporins in roots in response to salt treatments occurs at transcriptional and post‐translational levels. Cell specific changes in aquaporin transcript levels, protein abundance, subcellular localisation, protein phosphorylation, and protein:protein interactions have been observed. It has recently been revealed that a subset of plant aquaporins can function both as water and as ion channels. Both Arabidopsis PIP2;1 and PIP2;2 function as water and ion channels in heterologous systems. AtPIP2;1 and AtPIP2;2 ionic conductance was carried by sodium (Na + ), indicating that a subset of plant PIP2s could have a role in Na + transport in plants. We show quantitatively how AtPIP2;1 may account for the calcium (Ca 2+ ) and pH sensitive Arabidopsis root non‐selective cation channel. Salt treatments are known to result in a change in AtPIP2;1 phosphorylation, and in Arabidopsis roots this is associated with internalisation of AtPIP2;1 into pre‐vacuolar compartments. Here we review how the regulation of aquaporins, in particular PIP2s, changes in root tissues in response to salt treatments.

Energy costs of salt tolerance in crop plants

“…5a) is likely achieved through NSCC (Tyerman et al, 1997;Davenport & Tester, 2000;Demidchik & Tester, 2002). Sub-sets of plasma membrane water channels such as the Arabidopsis PIP2;1 and PIP2;2 aquaporins, which are abundant in root epidermal cell plasma membranes and are permeable to Na + , have been proposed as candidates for NSCCs (Byrt et al, 2017;Kourghi et al, 2017); these aquaporins qualitatively and semi-quantitatively match the NSCC properties as measured in isolated root protoplasts by patch-clamp (McGaughey et al, 2018).…”

“…This may be genotype-and Ca 2+dependent (Wu et al, 2015). The vi-NSCCs could allow passive efflux if the electrochemical gradients are appropriate (McGaughey et al, 2018). Future progress in development of crop cultivars with improved Na + transport traits will benefit from resolving the molecular identity of key Na + transporters.…”

“…It should be noted that the osmotic and nonosmotic water uptake mechanisms proposed by Wegner (2017) are mutually exclusive, at least at the cellular level, because water fluxes across the plasma membrane are facilitated by a high activity of aquaporins, which would short-circuit nonosmotic water uptake, rendering it unfeasible from an energetic point of view (Wegner, 2015(Wegner, , 2017Fricke, 2017). These considerations may help to understand the complex regulation of aquaporin activity under salt stress (McGaughey et al, 2018).…”

Phosphorylation influences water and ion channel function of AtPIP2;1

“…This activity was more evident at low external calcium and high pH (Byrt et al, 2017; Kourghi et al, 2017). The ability of some plant aquaporins to facilitate Na + transport has implications in relation to plant salinity stress responses and tolerance to osmotic stress (McGaughey, Qiu, Tyerman, & Byrt, 2018)…”

“…It is therefore conceivable that phosphorylation could augment the ability of PIPs to facilitate water and Na + transport in isoforms with this dual function. In which case, changes in phosphorylation states would not only regulate PIP protein trafficking and localization in response to salt and osmotic stresses but also water and Na + transport capacity (Byrt et al, 2017; McGaughey et al, 2018).…”