STN7/STN8-Mediated Thylakoid Protein Phosphorylation: From Redox Signaling to Photosynthetic Acclimation Under Abiotic Stress

Background/Objectives: Reversible thylakoid protein phosphorylation regulates photosynthetic acclimation by coordinating light harvesting, electron transport, and photoprotection. This review examines STN7/STN8 kinases, their counteracting phosphatases, and their established and emerging substrates in response to abiotic stress. Methods: This narrative review synthesizes literature retrieved from Web of Science, Scopus, PubMed, and Google Scholar, focusing on kinase–phosphatase regulation, substrate specificity, phosphoproteomics, and photosynthetic acclimation. Results: STN7 and STN8 regulate complementary but partially overlapping phosphorylation networks. STN7 mediates light-harvesting complex II (LHCII) phosphorylation, state transitions, and excitation-energy redistribution, whereas STN8 phosphorylates photosystem II (PSII) core proteins involved in PSII repair and recovery. Their regulation is linked to the photosynthetic electron transport chain; the redox state of plastoquinone (PQ) promotes STN7 activation via cytochrome b6f, while the upstream regulation of STN8 remains unclear. Abiotic stresses disrupt electron transport and PQ redox homeostasis in stress- and time-dependent ways that may shape phosphorylation responses. Beyond canonical substrates, phosphoproteomic and genetic studies extend the STN7/STN8-associated network to calcium signaling, electron partitioning, thylakoid architecture, metabolism, and chloroplast gene regulation. However, the kinase dependence and physiological significance of many emerging targets remain unresolved and may differ among species and lineages. Conclusions: STN7/STN8-mediated thylakoid protein phosphorylation links chloroplast redox signaling with photosynthetic acclimation under abiotic stress. Future studies combining direct substrate validation, kinase–phosphatase genetics, redox measurements, and comparative analyses across photosynthetic lineages are needed to distinguish conserved mechanisms from stress- and species-dependent responses.