Role of Chloroplast–Nucleus Communication in Plant Stress Response: An Integrated Review of Retrograde Signaling Pathways

Abstract

ABSTRACT Plants are continuously exposed to environmental stresses such as drought, salinity, temperature fluctuations, excessive light and pathogen attack. These stresses disturb cellular homeostasis and photosynthetic activity and require coordinated responses between different cellular compartments. Chloroplasts are not only the principal sites of photosynthesis but also important sensors of environmental changes. Information generated within chloroplasts is transmitted to the nucleus through chloroplast-to-nucleus or retrograde signaling pathways. The present review summarizes the role of major chloroplast-derived signals in plant stress responses, based exclusively on the literature analyzed in the source dissertation. Particular attention is given to reactive oxygen species (ROS), redox signals, tetrapyrrole intermediates, methylerythritol cyclodiphosphate (MEcPP), 3′-phosphoadenosine 5′-phosphate (PAP), and the GENOMES UNCOUPLED (GUN) pathway. The reviewed studies indicate that ROS represent important early stress signals, while PAP and MEcPP function as metabolic messengers under specific stress conditions. Tetrapyrrole-associated signaling contributes to the regulation of photosynthesis-associated nuclear genes. GUN1 and associated nuclear regulatory factors participate in coordinating chloroplast status with nuclear gene expression. Retrograde signaling was associated with drought, salinity, cold, heat, high-light and pathogen-related responses. Interaction with abscisic acid, salicylic acid and jasmonic acid provides an additional regulatory layer. The review indicates that chloroplast–nucleus communication is an integrated signaling network that contributes to stress adaptation, cellular protection and plant survival.