Figure 7. A proposed signaling pathway controlling SOS1 mRNA stability and SOS1-conferred paraquat sensitivity.

(a) Hypothetical model showing the early signaling events and downstream signal transduction regulating the stability of SOS1 mRNA. Under Na⁺ stress, operation of SOS1 would cause extracellular pH elevation, which could be required for activation and/or maintenance of the plasma membrane-bound NADPH oxidase activity that produces extracellular reactive oxygen species (ROS). Extracellular ROS could serve as signaling molecules to trigger gene regulation. The stability of SOS1 mRNA could be increased through a positive feedback regulation upon salt stress.

(b) Redox cycling of paraquat (PQ) coupling with the plasma membrane NADPH oxidases. The plasma membrane-bound NADPH oxidase could be an enzymatic source of electrons for the formation of free radical paraquat (PQ⁺) from normal divalent cation paraquat (PQ²⁺). Rapid reoxidation of PQ⁺ causes production of ROS through transfer of electrons to molecular oxygen. The PQ redox cycling, which causes enormous extracellular ROS production deleterious to plant cells, could be dependent on SOS1 activity that would be required for activation and/or maintenance of the NADPH oxidase activity. See the Discussion in the text for a detailed explanation.