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.
Highlights
Aquaporins are well known for their transport of water and other small neutral solutes [1]
Higher plants have five aquaporin subfamilies (plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), Nodulin 26-like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs), and X-intrinsic proteins (XIPs) [2], where the PIP group aquaporins consist of two separate groups, PIP1 and PIP2
We investigated whether the activity of the endogenous kinases in the X. laevis oocytes might influence the phosphorylation state of HvPIP2;8 or HvPIP2;8 S285D by applying a kinase inhibitor H7
Summary
Aquaporins are well known for their transport of water and other small neutral solutes [1]. PIPs can influence plant hydraulic conductivity [3], and some have been implicated in guard cell closure in response to ABA [4], in signaling in guard cells [5], and in CO2 sensing [6]. The regulation of PIPs in plants changes in response to salt treatments, and these changes might influence plant adaptation to salinity [7,8]. Targeting of PIPs to the plasma membrane and regulation of their internalization under salinity is dependent on the phosphorylation status of a serine (S283) in the carboxyl terminal domain [1,9]
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