Cell membrane water channels with built in ion channels

Byrt CS1,2, Qiu J1,2, McGaughey S1,2, Groszmann M3,4, Bose J1,2 and Tyerman SD1,2

  1. The University of Adelaide.
  2. ARC Centre of Excellence in Plant Energy Biology.
  3. Australian National University.
  4. ARC Centre of Excellence for Translational Photosynthesis.

Cell function is dependent on maintenance of water and ionic homeostasis. Aquaporins are regulated by cells to achieve water homeostasis, but in addition they may also be required for ion homeostasis. Cell water permeability is determined by the water conductance and density of aquaporins present in the plasma membrane. Plants express in the order of 30 to 70 different types of aquaporins, depending on the plant species, and these include a group called PIPs that are particularly abundant in plasma membranes. PIPs generally form tetramers with each monomer capable of allowing the passage of water. There are subsets of PIP tetramers that allow passage of other solutes, and we have identified PIPs that can change between functioning as water channels and non-selective cation channels (NSCCs) when expressed in heterologous systems. In plants there are likely to be multiple types of NSCCs and previous studies have revealed that NSCCs, for which the molecular candidates are so far unidentified, can provide a pathway for nutrient transport, and also for sodium transport under salinity stress. PIPs functioning as ion channels can allow passage of sodium and potassium, and they share similar properties with previously reported NSCCs. For example, NSCC and PIP ionic conductance are sensitive to calcium, pH and cyclic nucleotides. We are testing whether PIPs can account for any of the previously observed NSCC functions in plants by studying ion transport traits in mutant and transgenic lines of Arabidopsis where the PIPs of interest are either knocked out, overexpressed or mutated to change their ion channel function. Testing whether PIPs are implicated in maintaining water and ionic homeostasis in plants is an important step towards resolving the roles of PIPs in plant tolerance to dry, saline and nutrient deficient environments.