Fig. 9.

SAL1 as an oxidative stress sensor in the chloroplast for regulation of PAP-mediated retrograde signaling. Oxidative stresses (drought and HL) invoke physiological and biochemical changes in chloroplasts, such as ROS accumulation and altered redox poise. These are sensed by SAL1 via redox-mediated inhibition of SAL1 activity, enabling PAP accumulation. Stress responses, including transcription of stress-responsive genes, are activated. ROS sensors may also control other HL retrograde signals, such as β-cyclocitral or MEcPP, or use stromules; their identity and interaction with SAL1-PAP remain to be determined (gray arrows).