Polyamines in citrus stress resilience and fruit quality: Metabolism, exogenous applications, and translational perspectives

Polyamines (PAs), primarily putrescine (Put), spermidine (Spd), and spermine (Spm), are low-molecular-weight organic amines involved in plant development, stress signaling, redox regulation, and fruit physiology. Their exogenous applications have been explored as a small molecule-based strategy to improve plant performance under adverse environmental conditions. Citrus is one of the world’s most economically important fruit crops, yet its productivity and fruit quality are increasingly threatened by drought, salinity, extreme temperature events, and heightened pathogen pressure. These stressors disrupt key physiological and metabolic pathways, ultimately reducing yield, tree longevity, and marketability. Despite the recognized importance of PAs in plant stress biology, research attention in citrus remains limited. While more than 1,000 peer-reviewed studies address PAs across plant systems, only 43 focus specifically on citrus, underscoring a substantial and timely research gap. Existing studies demonstrate that PAs act as context-dependent regulators of stress responses by interacting with hormonal and signaling networks, stabilizing membranes, and maintaining cellular homeostasis and redox balance. In citrus, exogenous PA applications have been shown to enhance root system architecture, improve nutrient acquisition, and increase symbiotic efficiency with arbuscular mycorrhizal fungi. PAs can also stimulate somatic embryogenesis by modulating key biosynthetic and regulatory genes. In reproductive and fruit-related contexts, PAs have been linked to flowering, pollen tube growth, fruit set, abscission, fruit quality, and postharvest performance. Under abiotic stress, applications of Put, Spd, and Spm can enhance antioxidant enzyme activity, regulate osmolyte accumulation, and maintain endogenous PA balance by interacting with ethylene and abscisic acid signaling. Postharvest treatments with Put and Spd have also been reported to preserve fruit firmness, reduce oxidative damage, and mitigate selected physiological disorders by stabilizing cellular membranes and improving antioxidant capacity. However, translating these promising findings from laboratory, greenhouse, and postharvest studies to consistent orchard-scale outcomes remains a major challenge. Future research should prioritize citrus-specific dose-response studies, scion-rootstock comparisons, and integrative multi-omic approaches to clarify PA-mediated mechanisms and define realistic application windows for sustainable citrus production.