Organelle Contact-Site Plasticity and Retrograde Stress Codes: Cellular Integration of Biotic–Abiotic Stress in Plants

Plants rarely encounter environmental constraints in isolation. Water deficit, temperature extremes, salinity, flooding, excessive irradiance, nutrient imbalance, herbivory, and microbial infection frequently overlap in time and space, creating composite cellular states that cannot be inferred by adding single-stress responses. This review develops an organelle-centred framework for such integration. We synthesize evidence that the plasma membrane and cell wall initiate partially shared Ca2+, reactive oxygen species (ROS), kinase, lipid, and electrical signals, whereas chloroplasts, mitochondria, peroxisomes, the endoplasmic reticulum (ER), and the vacuolar system transform these inputs into compartment-specific retrograde information. We distinguish experimentally supported plant contact-site functions from the hypothesis that contact-site remodeling encodes combined biotic–abiotic stress. We further examine how ABA-, SA-, JA-, and ET-centered hormonal circuits intersect with organellar signals, how the unfolded protein response and selective autophagy set stress thresholds, and how systemic signaling and chromatin-associated memory alter subsequent responses. Because most molecular memory evidence derives from single-stress acclimation or immune priming, persistent memory after combined stress remains an open question. An interface-to-memory model is proposed in which stress combinations are encoded through the timing, amplitude, localization, and persistence of signals, with implications for organelle-resolved experiments and field validation.