An endoplasmic reticulum-localized cytochrome b 5 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 b 5 (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 OsCYB5-2 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 b 5 and highlighted the coordinated action of two proteins to perceive Na + in response to salt stress.