Singlet Oxygen- and Executer1-Mediated Signaling is Initiated in Grana Margins and Depends on the Protease Ftsh2

Significance Singlet oxygen ( 1 O 2 )- and EXECUTER1 (EX1)-dependent signaling triggers programmed cell death in seedlings and inhibits growth of mature plants of the fluorescent ( flu ) mutant of Arabidopsis . The EX1 protein has been located in chloroplasts to the grana margins close to where chlorophyll is synthesized and the disassembly of damaged photosystem II (PSII) and reassembly of active PSII take place. With the onset of 1 O 2 -mediated signaling there is a rapid decline of EX1 that depends on the ATP-dependent zinc metalloprotease FtsH. Generation of 1 O 2 without the decline of EX1 is not sufficient to activate 1 O 2 signaling. As FtsH cleaves also the D1 reaction center protein of damaged PSII, EX1-dependent signaling seems not only spatially but also functionally linked to the repair of PSII.