FK506-binding proteins, OsFKBP57 and OsFKBP73, confer salt tolerance through SOS pathway and maintain grain protein content in rice

Although moderate increases in grain protein content enhance nutritional value, excessive protein accumulation under salt stress degrades rice quality and reduces consumer acceptability. However, the molecular mechanisms underlying salt stress-induced changes of grain quality remain a long-standing issue in rice production.

Here, we identified two FK506-binding proteins OsFKBP57 and OsFKBP73, which regulate grain protein content and salt tolerance in rice. Mechanistically, OsFKBP57 and OsFKBP73 interact with OsSOS2, enhancing its physical interaction with OsSOS3 and stabilizing OsSOS2 protein; this reduces Na⁺ accumulation in rice and enhances salt tolerance. Concurrently, OsFKBP57 and OsFKBP73 interact with the branched-chain amino acid transaminase OsBCAT1, inhibiting its enzymatic activity and regulating the amino acid levels in grains under salt stress.