Arbuscular mycorrhizal network-mediated allelochemical transfer: a critical hypothesis of juglone-walnut case and its ecological implications

Juglone (5-hydroxy-1,4-naphthoquinone), a key allelochemical exuded by walnut trees (Juglans spp.), is widely recognized for its strong phytotoxic effects, which contribute significantly to the well-known “walnut toxicity” phenomenon in both walnut monocropping and intercropping systems. This review synthesizes current evidence and proposes that arbuscular mycorrhizal fungi (AMF) play a pivotal yet complex role in modulating multiple stages of juglone’s lifecycle. These fungi form extensive underground hyphal networks, known as arbuscular mycorrhizal networks (AMNs), which may interconnect neighboring plants. Evidence suggests that mycorrhizal symbiosis can enhance juglone biosynthesis in walnuts through physiological and molecular mechanisms. Furthermore, AMF are hypothesized to facilitate juglone immobilization and degradation. A critically important yet less established hypothesis is that AMNs could potentially act as conduits for the hyphal transport of juglone. The AMN-mediated transfer of juglone would be a key mechanism with profound ecological implications, potentially extending the compound’s influence beyond the root zone of walnut trees and impacting the structure of connected plant communities. Understanding the potential AMF-juglone interactions offers valuable insights for managing sustainable walnut-based agroforestry systems and optimizing intercropping systems by leveraging mycorrhizal functionality. These findings underscore the need to re-evaluate belowground chemical ecology in walnut-dominated ecosystems, where AMF may serve dual roles as both symbiotic facilitators and allelochemical vectors.