FtsH2-Dependent Proteolysis of EXECUTER1 Is Essential in Mediating Singlet Oxygen-Triggered Retrograde Signaling in Arabidopsis thaliana

Photosystem II reaction center (PSII RC) and light-harvesting complex inevitably generate highly reactive singlet oxygen ((1)O(2)) that can impose photo-oxidative damage, especially when the rate of generation exceeds the rate of detoxification. Besides being toxic, (1)O(2) has also been ascribed to trigger retrograde signaling, which leads to nuclear gene expression changes. Two distinctive molecular components appear to regulate (1)O(2) signaling: a volatile signaling molecule β-cyclocitral (β-CC) generated upon oxidation of β-carotene by (1)O(2) in PSII RC assembled in grana core, and a thylakoid membrane-bound FtsH2 metalloprotease that promotes (1)O(2)-triggered signaling through the proteolysis of EXECUTER1 (EX1) proteins associated with PSII in grana margin. The role of FtsH2 protease in (1)O(2) signaling was established recently in the conditional fluorescent (flu) mutant of Arabidopsis thaliana that generates (1)O(2) upon dark-to-light shift. The flu mutant lacking functional FtsH2 significantly impairs (1)O(2)-triggered and EX1-mediated cell death. In the present study, the role of FtsH2 in the induction of (1)O(2) signaling was further clarified by analyzing the FtsH2-dependent nuclear gene expression changes in the flu mutant. Genome-wide transcriptome analysis showed that the inactivation of FtsH2 repressed the majority (85%) of the EX1-dependent (1)O(2)-responsive genes (SORGs), providing direct connection between FtsH2-mediated EX1 degradation and (1)O(2)-triggered gene expression changes. Furthermore, the overlap between β-CC-induced genes and EX1-FtsH2-dependent genes was very limited, further supporting the coexistence of two distinctive (1)O(2) signaling pathways.