A geminivirus satellite-encoded βC1 protein disrupts pipecolic acid-mediated immunity by targeting OSB1 in chloroplasts

Abstract Geminiviruses employ sophisticated immune evasion strategies to incite devastating diseases, posing a significant threat to global agricultural security. A central tactic involves the subversion of chloroplast-mediated immunity via viral effectors. Here, we elucidate how the pathogenicity determinant βC1 from ageratum yellow vein China betasatellite (AYVCNB) hijacks this defense system by targeting the chloroplast-localized protein organellar single-stranded DNA-binding protein 1 (OSB1). Functionally, OSB1 acts as a positive regulator of immunity by stabilizing AGD2‐LIKE DEFENSE RESPONSE PROTEIN 1 (ALD1) and promoting pipecolic acid biosynthesis. We demonstrate that AYVCNB-encoded βC1 physically interacts with OSB1, triggering its ubiquitin-proteasome-dependent degradation and consequently abrogating the OSB1-ALD1 defense module. Parallel to this finding, our recent work revealed that βC1 from tomato yellow leaf curl China betasatellite re-localizes OSB1 from the chloroplast to the cytoplasm and promotes its degradation. These results collectively demonstrate that targeting the OSB1-ALD1 defense module represents a widespread and conserved geminivirus mechanism to subvert chloroplast immunity.