Abstract: Chloroplasts are central to plant immunity, with the chloroplast-localized protein AGD2-LIKE DEFENSE RESPONSE PROTEIN 1 (ALD1) playing a critical role in producing pipecolic acid (Pip), a key immune signal. However, the regulation of ALD1 and how pathogens evade ALD1-mediated defenses remain poorly understood. Using the geminivirus tomato yellow leaf curl China virus and its associated betasatellite (TYLCCNV/TYLCCNB) as a model, we uncovered a defense mechanism involving organellar single-stranded DNA-binding protein 1 (OSB1), which stabilizes ALD1 and promotes Pip biosynthesis to strengthen immunity. Crucially, the viral βC1 effector encoded by TYLCCNB disrupts this pathway by binding OSB1 and sequestering it away from chloroplasts, thereby blocking OSB1–ALD1 interaction, destabilizing ALD1, and suppressing Pip-dependent defenses. Strikingly, βC1 mutants defective in OSB1 binding fail to interfere with the OSB1–ALD1 stability, and TYLCCNV infections carrying these mutants induce attenuated symptoms in Nicotiana benthamiana. Our study not only reveals how ALD1–OSB1 cooperates in chloroplast immunity but also demonstrates how geminiviruses, as a tractable model, can dissect pathogen counter-defense strategies.

Key words: ALD1, βC1, betasatellite, chloroplast immunity, geminivirus, pipecolic acid