Formation of singlet oxygen ( 1 O 2 ) 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 O 2 has also been shown to act as a signal that triggers stress acclimation and an enhanced stress resistance. A signaling role of 1 O 2 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 1 O 2 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 of 1 O 2 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 1 O 2 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 1 O 2 -mediated signaling seems to be not only spatially but also functionally associated with the repair of PSII.

Chloroplasts are crucial players in the activation of defensive hormonal responses during plant-pathogen interactions. Here, we show that a plant virus-encoded protein re-localizes from the plasma membrane to chloroplasts upon activation of plant defense, interfering with the chloroplast-dependent anti-viral salicylic acid (SA) biosynthesis. Strikingly, we have found that plant pathogens from different kingdoms seem to have convergently evolved to target chloroplasts and impair SA-dependent defenses following an association with membranes, which relies on the co-existence of two subcellular targeting signals, an N-myristoylation site and a chloroplast transit peptide. This pattern is also present in plant proteins, at least one of which conversely activates SA defenses from the chloroplast. Taken together, our results suggest that a pathway linking plasma membrane to chloroplasts and activating defense exists in plants and that such pathway has been co-opted by plant pathogens during host-pathogen co-evolution to promote virulence through suppression of SA responses.

Environmental information perceived by chloroplasts can be translated into retrograde signals that alter the expression of nuclear genes. Singlet oxygen ( 1 O 2 ) generated by photosystem II (PSII) can cause photo-oxidative damage of PSII but has also been implicated in retrograde signaling. We previously reported that a nuclear-encoded chloroplast FtsH2 metalloprotease coordinates 1 O 2 -triggered retrograde signaling by promoting the degradation of the EXECUTER1 (EX1) protein, a putative 1 O 2 sensor. Here, we show that a 1 O 2 -mediated oxidative post-translational modification of EX1 is essential for initiating 1 O 2 -derived signaling. Specifically, the Trp643 residue in DUF3506 domain of EX1 is prone to oxidation by 1 O 2 . Both the substitution of Trp643 with 1 O 2 -insensitive amino acids and the deletion of the DUF3506 domain abolish the EX1-mediated 1 O 2 signaling. We thus provide mechanistic insight into how EX1 senses 1 O 2 via Trp643 located in the DUF3506 domain.