Conclusions
The data presented here reveal the potential for differential AtPIP2;1 CTD phosphorylation to be a key factor in the processes controlling water and ion channel transport functions, in addition to localisation trends. Mimicking a phosphorylated state of AtPIP2;1 S280 and S283 sites resulted in a transport function where the Na+ and K+ ion conductance reached a magnitude similar to that observed for other plant ion channels expressed in X. laevisoocytes; indicating that phosphorylation-dependent ion flux through AtPIP2;1 could hypothetically be significant in planta (McGaugheyet al., 2018). Plant aquaporins capable of facilitating ion transport are key candidates for the elusive non-selective cation channels responsible for a large proportion of Na+ and K+ flux across the PM (Demidchik and Tester, 2002; Rubio et al., 2010). Further testing is needed to explore the influence of phosphorylation at S280 and S283 on water and ion transport functions in planta , and to resolve whether the observations of AtPIP2;1 potential to switch between ion and water channel modes are applicable. Discovering the steps in the processes regulating the switching between ion and water channel functions, and resolving their relationship with osmotic stress tolerance mechanisms, holds great potential for generating discoveries that support the engineering of future strategies for improving plant productivity in dry and saline environments.