Identification of STN7-associated leaf senescence networks in thylakoid membranes of Arabidopsis

Photosynthetic protein complexes in thylakoid membranes disassemble during leaf senescence, leading to a decline in photosynthetic efficiency. However, the mechanisms by which these protein complexes in chloroplast thylakoid membranes modulate leaf senescence are poorly understood. This study investigated the roles of <i>Arabidopsis thaliana</i> STN7 and STN8 kinases during leaf senescence using proteomic tools, characterizing STN7 and STN8 overexpressors and loss-of-function mutants. During leaf senescence, proteins involved in photosynthesis-associated processes decreased, while those related to lipid metabolism and jasmonic acid-mediated stress responses increased. The abundance and phosphorylation of the thylakoid membrane proteome were regulated by STN7, but not by STN8. Thirty-four STN7 target proteins overlapped with senescence-associated proteins in the wild type, most of which are related to the photosynthetic protein apparatus. Three out of nine mutants defective in STN7 downstream targets (<i>lfnr2</i>, <i>tsp9</i>, and <i>riq2</i>) exhibited accelerated leaf senescence based on chlorophyll a/b ratio measurements and photochemical analyses. Photosynthetic complex formation was normal in all STN7 target protein loss-of-function mutants, indicating the absence of feedback regulation upstream of STN7 during photosynthetic complex assembly. Overall, the data reveal that STN7 is a major kinase component in signaling cascades that link photosynthetic complex formation and leaf senescence.