Chloroplast-generated ROS dominate NaCl-induced K+ efflux in wheat leaf mesophyll

Mesophyll K⁺ retention ability has been recently reported as an important component of salinity stress tolerance in wheat. In order to investigate the role of ROS in regulating NaCl^-induced K⁺ efflux in wheat leaf mesophyll, a series of pharmacological experiments was conducted using MV (methyl viologen, superoxide radical inducer), DPI (an inhibitor of NADPH oxidase), H₂O₂ (to mimic apoplastic ROS), and EGCG ((−)-Epigallocatechin gallate, ROS scavenger). Mesophyll pre-treatment with 10 μM MV resulted in a significantly higher NaCl^-induced K⁺ efflux in leaf mesophyll, while 50 μM EGCG pre-treatment alleviated K⁺ leakage under salt stress. No significant change in NaCl^-induced K⁺ efflux in leaf mesophyll was found in specimens pre-treated by H₂O₂ and DPI, compared with the control. The highest NaCl^-induced H⁺ efflux in leaf mesophyll was also found in samples pre-treated with MV, suggesting a futile cycle between increased H⁺-ATPase activity and ROS-induced K⁺ leak. Overall, it is suggested that, under saline stress, K⁺ efflux from wheat mesophyll is mediated predominantly by non-selective cation channels (NSCC) regulated by ROS produced in chloroplasts, at least in bread wheat.