Chloroplast protein homeostasis is coupled with retrograde signaling

Singlet oxygen (¹O₂) is a potent oxidizing agent, principally generated by photosystem II (PSII) as a byproduct of photosynthesis. Hence, ¹O₂ damages PSII, especially the PSII reaction center (RC) proteins, promoting a process called PSII repair cycle. The hetero-hexameric FtsH protease, located in the thylakoid membrane, is essential in degrading these damaged PSII RC proteins, which defines the first step of the PSII repair. The loss of the central subunit of the FtsH protease, FtsH2 (VAR2), weakens the PSII repair, thereby impairing PSII proteostasis. A recent study demonstrated that the impaired proteostasis (or accumulation of damaged proteins) in the chloroplasts of the var2 mutant induces an unfolded/misfolded protein response (UPR)-like response, more appropriately referred to as a damaged protein response (DPR), as evident in the accumulation of proteins related to the protein quality control (PQC). Comparison of data from chloroplast proteomics data with RNA sequencing in the context of the UPR-like response suggests a plausible activation of retrograde signaling in the var2 mutant. Either through the enhanced level of ¹O₂ or by impairing the substrate-unfolding activity of FtsH2, the reinforced defect appears to induce stress-related genes via the stress hormone salicylic acid (SA). This finding suggests that impaired chloroplast proteostasis (specifically for PSII proteins) may activate the chloroplast-established isochorismate pathway to produce SA. If this assumption is correct, then SA serves as a retrograde signaling molecule. In this review, we will discuss the impact of chloroplast proteostasis on chloroplasts-to-nucleus communication.