FK506-binding proteins OsFKBP57 and OsFKBP73 confer salt tolerance in rice by activating the SOS pathway while...

Soil salinization poses a serious threat to rice productivity and grain quality. Although moderate increases in grain protein content can improve nutritional value, excessive protein accumulation under salt stress compromises rice quality and reduces consumer acceptance. However, the molecular mechanisms underlying salt-stress-induced changes in grain quality during rice production remain poorly understood. Here, we identified two FK506-binding proteins, OsFKBP57 and OsFKBP73, which regulate both grain protein content and salt tolerance in rice. Mechanistically, OsFKBP57 and OsFKBP73 interact with OsSOS2, enhancing its physical interaction with OsSOS3 and stabilizing the OsSOS2 protein, thereby reducing Na⁺ accumulation in rice and enhancing salt tolerance. In parallel, OsFKBP57 and OsFKBP73 interact with the branched-chain amino acid transaminase OsBCAT1, inhibiting its enzymatic activity and modulating amino acid levels in grains under salt stress. Knockout of OsFKBP57 and OsFKBP73 increases glutelin accumulation in grains and compromises rice quality under saline conditions. Together, these actions directly mediate total grain protein content under salt stress. Our study thus identifies OsFKBP57 and OsFKBP73 as central regulators that coordinate salt tolerance and grain quality in rice.