Abstract: Inter-organellar communication has emerged as a critical factor in maintaining cellular homeostasis under stress conditions. Chloroplasts function not only as central organelles for energy production but are also increasingly recognized as stress sensors and signal integrators. SIGMA FACTOR-BINDING PROTEIN 1 (SIB1), encoded by a nuclear gene, has been identified as a positive regulator of plant immunity, with localization in both chloroplasts and the nucleus. In this study, we identified Arabidopsis SEC23A, a COPII complex component known to mediate membrane trafficking between the endoplasmic reticulum (ER) and the Golgi apparatus, as a novel interactor of SIB1. Our findings reveal that under normal conditions, both SIB1 and SEC23A localize to the ER, while SIB1 is also localized in the nucleus. Upon ER stress and/or treatment with the immunity inducer salicylate, both proteins are relocated from the ER to the chloroplasts. Notably, SEC23A, similar to SIB1, also functions as a positive regulator of disease resistance. In response to pathogen infection, SIB1 and SEC23A downregulate expression of chloroplast- and nucleus-encoded genes associated with photosynthesis while enhancing expression of defense-related genes. Together, our findings reveal a previously uncharacterized pathway of ER–chloroplast communication mediated by SIB1 and SEC23A during plant stress responses and immunity, providing novel insights into the intricate regulatory networks that govern inter-organellar communication under stress in plants.

Key words: Arabidopsis, chloroplast, ER stress, inter-organelle communication, plant immunity, SEC23A, SIB1