Photodamage of the PSII reaction center (RC) is an inevitable process in an oxygen-rich environment. The damaged PSII RC proteins (Dam-PSII) undergo degradation via the thylakoid membrane-bound FtsH metalloprotease, followed by posttranslational assembly of PSII. While the effect of Dam-PSII on gene regulation is described for cyanobacteria, its role in land plants is largely unknown. In this study, we reveal an intriguing retrograde signaling pathway by using the Arabidopsis (Arabidopsis thaliana) yellow variegated2-9 mutant, which expresses a mutated FtsH2 (FtsH2 G267D) metalloprotease, specifically impairing its substrate-unfolding activity. This lesion leads to the perturbation of PSII protein homeostasis (proteostasis) and the accumulation of Dam-PSII. Subsequently, this results in an up-regulation of salicylic acid (SA)-responsive genes, which is abrogated by inactivation of either an SA transporter in the chloroplast envelope membrane or extraplastidic SA signaling components as well as by removal of SA. These results suggest that the stress hormone SA, which is mainly synthesized via the chloroplast isochorismate pathway in response to the impaired PSII proteostasis, mediates the retrograde signaling. These findings reinforce the emerging view of chloroplast function toward plant stress responses and suggest SA as a potential plastid factor mediating retrograde signaling. Chloroplast-generated reactive oxygen species (ROS) damage primarily the photosynthetic apparatus, thereby deleteriously affecting plant development. Among ROS, singlet oxygen (1 O 2), mainly generated by PSII, has been considered as a prime cause of PSII damage because of its proximity to PSII (Triantaphylidès et al., 2008; Kato and Sakamoto, 2009). Since 1 O 2 is a known by-product of photosynthesis, PSII reaction center (PSII RC) proteins undergo constant damage in a light-dependent manner (Ohad et al., 1984; Mishra et al., 1994). The damaged PSII (Dam-PSII) becomes repaired via a series of steps: (1) D1 oxidation and subsequent phosphorylation; (2) migration of the Dam-PSII from the grana core (appressed membranes) to the grana margin (nonappressed membranes); (3) dephosphorylation of D1; (4) degradation of the damaged D1 protein by the membrane-bound FtsH metalloprotease; (5) de novo synthesis of D1; and (6) reassembly and subsequent remigration to the grana core (Yamamoto et al., 2013; Yoshioka-Nishimura and Yamamoto, 2014). Accordingly, the impaired PSII repair in Arabidopsis (Arabidopsis thaliana) mutants lacking the membrane-bound FtsH protease leads to the failure of acclimation to a sublethal intensity of

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