Non‐selective cation channel activity of aquaporin AtPIP2;1 regulated by Ca²⁺ and pH

Abstract

The aquaporin AtPIP2;1 is an abundant plasma membrane intrinsic protein in Arabidopsis thaliana that is implicated in stomatal closure, and is highly expressed in plasma membranes of root epidermal cells. When expressed in Xenopus laevis oocytes, AtPIP2;1 increased water permeability and induced a non-selective cation conductance mainly associated with Na . A mutation in the water pore, G103W, prevented both the ionic conductance and water permeability of PIP2;1. Co-expression of AtPIP2;1 with AtPIP1;2 increased water permeability but abolished the ionic conductance. AtPIP2;2 (93% identical to AtPIP2;1) similarly increased water permeability but not ionic conductance. The ionic conductance was inhibited by the application of extracellular Ca and Cd , with Ca giving a biphasic dose-response with a prominent IC of 0.32 mм comparable with a previous report of Ca sensitivity of a non-selective cation channel (NSCC) in Arabidopsis root protoplasts. Low external pH also inhibited ionic conductance (IC pH 6.8). Xenopus oocytes and Saccharomyces cerevisiae expressing AtPIP2;1 accumulated more Na than controls. Establishing whether AtPIP2;1 has dual ion and water permeability in planta will be important in understanding the roles of this aquaporin and if AtPIP2;1 is a candidate for a previously reported NSCC responsible for Ca and pH sensitive Na entry into roots.

Roles of Aquaporins in Setaria viridis Stem Development and Sugar Storage

“…We hypothesize that in stem tissues SvPIP2;1 is involved in cell growth and that SvNIP2;2 may facilitate water movement and potentially the flow of other solutes into the apoplasm to sustain solute transportation by bulk-flow, and possibly ‘recycle’ water used for solute delivery back to the xylem. It is expected that SvPIP2;1 could have additional roles, as other PIP water channels have been shown to also be permeable to CO 2 , hydrogen peroxide, urea, sodium and arsenic (Siefritz et al, 2001; Uehlein et al, 2003; Mosa et al, 2012; Bienert and Chaumont, 2014; Byrt et al, 2016b). SvNIP2;2 could have roles such as transporting neutral solutes to the apoplasm, as previous studies report silicic acid, urea, and boric acid permeability for other NIPS (Bienert et al, 2008; Ma et al, 2008; Ma and Yamaji, 2008; Li et al, 2009; Deshmukh et al, 2013).…”

NIP1;2 is a plasma membrane-localized transporter mediating aluminum uptake, translocation, and tolerance inArabidopsis

“…It has been widely accepted that members of the AQP family, including those from the NIP subfamily, transport noncharged molecules (25,26), although recent studies have suggested that some members of the AQP family could transport charged substrates (27,28). However, the major Al species at low pH (4.2) is the charged Al 3+ ion (29).…”

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes

“…Also, there is mounting evidence that aquaporins are also implicated in Na + uptake in plants (Byrt et al, 2017). The Na + absorbed by the root then moves to the xylem with the aid of other transporters and channels and is delivered to the shoot, especially the leaf blade, where its effects are most felt (Munns and Tester, 2008).…”

“…In animals, some aquaporins also mediate ion transport (Ikeda et al, 2002), which prompted investigation of possible similar roles in plants. It was not until recently that such a role was shown to be indeed present in plants, at least in Arabidopsis (Byrt et al, 2017) where the phenomenon was evaluated. In this study, electrophysiological studies using Xenopus oocytes and yeast complementation tests showed that Arabidopsis thaliana plasma membrane intrinsic protein AtPIP2;1 has strong Na + conductance, although it was not clear from the study whether it is Na + -selective or not.…”