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 O2 ) that can impose photo-oxidative damage, especially when the rate of generation exceeds the rate of detoxification. Besides being toxic, 1 O2 has also been ascribed to trigger retrograde signaling, which leads to nuclear gene expression changes. Two distinctive molecular components appear to regulate 1 O2 signaling: a volatile signaling molecule β-cyclocitral (β-CC) generated upon oxidation of β-carotene by 1 O2 in PSII RC assembled in grana core, and a thylakoid membrane-bound FtsH2 metalloprotease that promotes 1 O2 -triggered signaling through the proteolysis of EXECUTER1 (EX1) proteins associated with PSII in grana margin. The role of FtsH2 protease in 1 O2 signaling was established recently in the conditional fluorescent ( flu ) mutant of Arabidopsis thaliana that generates 1 O2 upon dark-to-light shift. The flu mutant lacking functional FtsH2 significantly impairs 1 O2 -triggered and EX1-mediated cell death. In the present study, the role of FtsH2 in the induction of 1 O2 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 O2 -responsive genes (SORGs), providing direct connection between FtsH2-mediated EX1 degradation and 1 O2 -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 O2 signaling pathways.