A Survey of Barley PIP Aquaporin Ionic Conductance Reveals Ca2+-Sensitive HvPIP2;8 Na+ and K+ Conductance

Abstract

Some plasma membrane intrinsic protein (PIP) aquaporins can facilitate ion transport. Here we report that one of the 12 barley PIPs (PIP1 and PIP2) tested, HvPIP2;8, facilitated cation transport when expressed in Xenopus laevis oocytes. HvPIP2;8-associated ion currents were detected with Na⁺ and K⁺, but not Cs⁺, Rb⁺, or Li⁺, and was inhibited by Ba²⁺, Ca²⁺, and Cd²⁺ and to a lesser extent Mg²⁺, which also interacted with Ca²⁺. Currents were reduced in the presence of K⁺, Cs⁺, Rb⁺, or Li⁺ relative to Na⁺ alone. Five HvPIP1 isoforms co-expressed with HvPIP2;8 inhibited the ion conductance relative to HvPIP2;8 alone but HvPIP1;3 and HvPIP1;4 with HvPIP2;8 maintained the ion conductance at a lower level. HvPIP2;8 water permeability was similar to that of a C-terminal phosphorylation mimic mutant HvPIP2;8 S285D, but HvPIP2;8 S285D showed a negative linear correlation between water permeability and ion conductance that was modified by a kinase inhibitor treatment. HvPIP2;8 transcript abundance increased in barley shoot tissues following salt treatments in a salt-tolerant cultivar Haruna-Nijo, but not in salt-sensitive I743. There is potential for HvPIP2;8 to be involved in barley salt-stress responses, and HvPIP2;8 could facilitate both water and Na⁺/K⁺ transport activity, depending on the phosphorylation status.

Keywords: aquaporins; barley; ion transport; oocytes; plasma membrane intrinsic proteins (PIPs).