Juglone is a pivotal allelopathic and defensive secondary metabolite in walnuts ( Juglans regia ), yet the regulation mechanisms govering its biosynthesis, particularly during mycorrhization, remain poorly understood. This study investigated the effects of five arbuscular mycorrhizal fungi (AMF) species, including Paraglomus occultum , Acaulospora scrobiculata , Rhizophagus intraradices , Funneliformis mosseae , and Diversispora spurca on walnuts. The changes in plant growth, root morphology, β-glucosidase activity, juglone accumulation, and the expression of β-glucosidase gene family ( JrBGLUs ) were studied. AMF colonized walnut roots, with D . spurca exhibiting the highest colonization rate and extraradical hyphal length. The responses of plant growth, root morphology, and leaf gas exchange to AMF inoculation varied, with D . spurca exerting greatest improvements. AMF inoculations significantly boosted juglone accumulation in roots (23−49%) and soil (90−148%), alongside a significant rise in β-glucosidase specific activity in roots (39−124%) and soil (27−54%). Notably, the increase in soil juglone was 2−3 times more than that in roots, whereas the induction of β-glucosidase activity was 2.3-4.6-fold higher in roots than in soil, with D . spurca triggering the most pronounced differences. A total of 18 JrBGLUs were identified, and their expression was differentially modulated by AMF colonization. Among these, 50−83% of JrBGLU members were consistently upregulated by AMF inoculation, with JrBGLU2 ‒ 6 / 9 / 17 / 18 up-regulated by all AMF species. D . spurca caused the greatest number of JrBGLU genes to be upregulated, while F . mosseae eliciting the strongest upregulation of their expression levels. Significantly positive correlations were observed among core JrBGLU genes ( JrBGLU2-4/6/7/9/12 ‒ 14/16/18 ), mycorrhiza, juglone, and β-glucosidase. This study provides the first evidence linking AMF-mediated activation of JrBGLU genes to promote juglone accumulation in walnuts. The highly effective D . spurca −walnut combination achieves a dual benefit: promoting plant productivity while simultaneously elevating juglone. These findings emphasize the necessity of context-specific application of AMF in walnut cultivation, considering both growth enhancement and potential allelopathic effects. • AMF colonized walnut roots, with D. spurca showing the highest colonization and extraradical hyphal length. • Plant growth, root morphology, and gas exchange responses to AMF varied, with D. spurca showing greatest improvements. • AMF inoculations boosted juglone in roots and soil, paralleled by enhanced β-glucosidase activity in roots and soil. • A total of 18 JrBGLUs was identified, with AMF colonization differentially modulating their expression patterns. • A positive correlation observed among core JrBGLU genes, mycorrhizas, juglone, and β-glucosidase.
Liu et al. (Sun,) studied this question.