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.