An endoplasmic reticulum-localized cytochrome regulates high-affinity K transport in response to salt stress in rice.

Potassium (K) is an essential element for growth and development in both animals and plants, while high levels of environmental sodium (Na) represent a threat to most plants. The uptake of K from high-saline environments is an essential mechanism to maintain intracellular K/Na homeostasis, which can help reduce toxicity caused by Na accumulation, thereby improving the salt tolerance of plants. However, the mechanisms and regulation of K-uptake during salt stress remain poorly understood. In this study, we identified an endoplasmic reticulum-localized cytochrome (OsCYB5-2) that interacted with a high-affinity K transporter (OsHAK21) at the plasma membrane. The association of OsCYB5-2 with the OsHAK21 transporter caused an increase in transporter activity by enhancing the apparent affinity for K-binding but not Na-binding. Heme binding to OsCYB5-2 was essential for the regulation of OsHAK21. High salinity directly triggered the OsHAK21-OsCYB5-2 interaction, promoting OsHAK21-mediated K-uptake and restricting Na entry into cells; this maintained intracellular K/Na homeostasis in rice cells. Finally, overexpression of increased OsHAK21-mediated K transport and improved salt tolerance in rice seedlings. This study revealed a posttranslational regulatory mechanism for HAK transporter activity mediated by a cytochrome and highlighted the coordinated action of two proteins to perceive Na in response to salt stress.