Figure 7.

A working model depicting the function of the two chloroplast proteins in plant drought resistance. Drought stress promotes the biosynthesis of ABA and activates the ABA signaling pathway. In addition to phosphorylating the plasma membrane-bound NADPH oxidase RbohF and promoting the production of H₂O₂, OST1 can also interact and phosphorylate the chloroplast protein PPD5, which negatively regulates ROS and ABA accumulation under drought stress conditions. This negative regulation may involve a competition for OST1 between PPD5 and other OST1 substrate proteins. Another chloroplast protein, THF1, modulates H₂O₂ accumulation and plant drought resistance in an OST1-independent manner.