Ubiquitin-mediated stabilization of SlPsbS regulates low night temperature tolerance in tomatoes

Non-photochemical quenching (NPQ) plays a fundamental role in safely operating photosynthesis under low night temperatures (LNT). PsbS protein is essential for the rapid induction of NPQ, and its stability is often affected by adverse environmental conditions. However, the regulatory mechanism for the stability of PsbS or chloroplast proteins remains to be fully characterized. We showed that LNT decreased NPQ levels and SlPsbS protein abundance in tomato leaves. COP9 signalosome subunit 5A (SlCSN5A) facilitated SlPsbS ubiquitination and degradation in the cytosol. Further, tomato chloroplast vesiculation (SlCV) was activated by LNT. Under LNT, SlCV targeted the chloroplasts and induced the formation of CV-containing vesicles (CCVs) containing SlPsbS, which were exported from the chloroplasts. Subsequently, SlCV and SlPsbS contact SlCSN5A in the cytosol and are ubiquitinated and degraded. Genetic evidence demonstrated that overexpression of SlCV aggravated SlPsbS protein degradation, whereas silencing of SlCSN5 and SlCV delayed LNT-induced NPQ reduction and SlPsbS protein turnover. This study provides evidence that CSN5A is associated with chloroplast proteins, and reveals a ubiquitin-dependent degradation pathway of chloroplast proteins co-mediated by CV and CSN5A, thereby providing new insights into the regulation of chloroplast protein stability under stress conditions. ### Competing Interest Statement The authors have declared no competing interest.