Singlet oxygen- and EXECUTER1-mediated signaling is initiated in grana margins and depends on the protease FtsH2

Abstract

Formation of singlet oxygen (1 O2 ) has been implicated with damaging photosystem II (PSII) that needs to undergo continuous repair to maintain photosynthetic electron transport. In addition to its damaging effect,1 O2 has also been shown to act as a signal that triggers stress acclimation and an enhanced stress resistance. A signaling role of1 O2 was first documented in the fluorescent (flu) mutant of Arabidopsis. It strictly depends on the chloroplast protein EXECUTER1 (EX1) and happens under nonphotoinhibitory light conditions. Under severe light stress, signaling is initiated independently of EX1 by 1O2 that is thought to be generated at the acceptor side of active PSII within the core of grana stacks. The results of the present study suggest a second source of1 O2 formation in grana margins close to the site of chlorophyll synthesis where EX1 is localized and the disassembly of damaged and reassembly of active PSII take place. The initiation of 1O2 signaling in grana margins depends on EX1 and the ATP-dependent zinc metalloprotease FtsH. As FtsH cleaves also the D1 protein during the disassembly of damaged PSII, EX1-and 1O2 -mediated signaling seems to be not only spatially but also functionally associated with the repair of PSII.