Proline metabolic reprogramming in mycorrhizal plants: a unified framework for stress adaptation

Proline serves as a key osmoprotectant and signaling molecule in plants under abiotic stress. Arbuscular mycorrhizal fungi (AMF), which form symbiotic relationships with most land plants, can enhance stress tolerance by modulating proline metabolism in a stress-dependent manner. This review comprehensively synthesizes current knowledge on how these fungi regulate plant proline metabolism across various abiotic stresses. AMF fine-tune the activities and gene expression of proline biosynthetic and catabolic enzymes. Under salt, cold, and aluminum stress, these fungi generally promote proline accumulation to reinforce osmotic adjustment and antioxidant defense. Under drought conditions, however, they may either reduce proline levels when water status improves or increase proline when additional protection is required. The response suggests a working framework: AMF-mediated proline enhancement correlates with reinforced stress defense, whereas proline reduction may signal effective stress mitigation in certain contexts. However, this pattern is not a universal rule, because proline concentration is shaped by stress intensity, tissue type, developmental stage, nitrogen status, biosynthesis and catabolism rates, and recovery status. Signaling molecules, including nitric oxide, abscisic acid, and mitogen-activated protein kinase cascades, integrate fungal signals with host stress responses. Nitrogen availability strongly influences both fungal colonization and proline regulation. Co-inoculation with beneficial microbes can further modulate proline metabolism. The review also identifies critical knowledge gaps, such as unknown fungal signals, functional specialization of Δ 1 -pyrroline-5-carboxylate synthetase isoforms, and interactions with other osmolytes, and highlights future directions including multi-omics approaches, gene editing, and long-term field trials. Understanding this mycorrhizal fungus–proline nexus offers practical strategies for sustainable, stress-resilient agriculture, and we explicitly present the proline-based interpretation as a testable framework rather than an established rule.