A viral protein-driven ubiquitination switch fine-tunes chloroplast outer envelope protein homeostasis

Chloroplast integrity is essential for plant growth, development, and stress resilience; however, viral infection frequently induces chloroplast dysfunction. The mechanism by which viruses perturb and subsequently reprogram chloroplast homeostasis remains poorly understood. Here, we demonstrate that host pheophytinase (PPH) and the barley stripe mosaic virus (BSMV) γb protein engage in an antagonistic interaction to govern the stability of the chloroplast outer-envelope translocon protein Toc34 in Nicotiana benthamiana. Outer envelope-localized PPH recruits the E3 ubiquitin ligase PUB4 to Toc34, driving ubiquitin-dependent degradation of Toc34 and thereby compromising envelope integrity and conferring broad-spectrum antiviral defenses. Upon infection, the viral γb protein directly interacts with PPH and redirects PUB4-mediated ubiquitination toward PPH itself, accelerating PPH turnover and restoring Toc34 levels. This γb-mediated ubiquitination switch preserves envelope integrity and creates a cellular environment that favors viral replication. Our findings reveal a dynamic tripartite PPH-PUB4-γb regulatory circuit that fine-tunes chloroplast homeostasis during infection, illustrating how viruses exploit host ubiquitin machinery to reconcile organelle stability with pathogen fitness.