BR signaling mitigates Cu toxicity in rice by integrating ROS homeostasis with Copper Translocation.

Excessive copper (Cu) accumulation in soils triggers phytotoxicity and severely impairs crop growth and yield. Cu absorbed by plant roots is distributed to various tissues through root-to-shoot translocation. Although phytohormones have been widely documented to mediate plant responses to heavy metal stress, the role of brassinosteroids (BRs) in regulating Cu tolerance in rice remains unclear. Here, we show that Cu stress upregulates BR biosynthesis genes in rice, exogenous BR reduces ROS accumulation and lignin deposition under Cu stress. Phenotypic and physiological analyses of a series of BR-related genetic materials revealed that enhanced BR signaling, as in OsBZR1-overexpressing lines, improved shoot and root growth under Cu stress and attenuated ROS accumulation through enhanced activity of ROS-scavenging enzymes. Interestingly, BR promoted root Cu uptake and root-to-shoot translocation under normal conditions, but under Cu stress, however, plants with enhanced BR signaling exhibited reduced Cu accumulation and uptake capacity in roots, thereby alleviating growth inhibition, OsBZR1 serves as a key downstream hub mediating BR-dependent copper tolerance. Furthermore, BR modulated root Cu uptake and root-to-shoot translocation under Cu stress through transcriptional regulation of the Cu transport-related genes OsCOPT6, OsNPF6.5, and OsYSL16. In summary, our findings reveal that BR signaling mitigates Cu toxicity by integrating ROS homeostasis with transcriptional control of Cu transporters, leading to improved growth under stress, highlighting functional roles of multiple BR signaling components in mediating rice responses to Cu stress.