An endoplasmic reticulum-localized cytochrome b₅ regulates high-affinity K⁺ transport in response to salt stress in...

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 b₅ (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 b₅ and highlighted the coordinated action of two proteins to perceive Na⁺ in response to salt stress.