Abstract

Iron (Fe) is an essential micronutrient for plants. However, both iron deficiency and excess can severely inhibit plant growth and productivity. Arbuscular mycorrhizal (AM) fungi have been shown to improve plant mineral nutrition and stress tolerance, yet the integrated physiological and molecular mechanisms underlying AM-mediated iron homeostasis regulation, particularly in woody tree species, remain poorly understood. In this study, we investigated the effects of inoculating Eucalyptus grandis seedlings with the AM fungus Rhizophagus irregularis on their growth, photosynthetic performance, antioxidant defense, and transcriptional responses under varying Fe supply levels (5, 25, and 200 µM). Our results demonstrated that AM symbiosis significantly alleviated the growth inhibition induced by both low-Fe (5 µM) and high-Fe (200 µM) stress, enhanced photosynthetic capacity, as evidenced by increased net photosynthetic rate (Pn), stomatal conductance (Gs), and PSII photochemical efficiency; under low-Fe, Pn, Gs, and Fv/Fm increased by 33.7%, 42.4%, and 25.2%, respectively. AM symbiosis also significantly enhanced the activities of antioxidant enzymes (POD, SOD, and CAT) under high-iron stress, accompanied by reduced accumulation of reactive oxygen species and lipid peroxidation; specifically, POD, SOD, and CAT activities increased by 79.3%, 88.7%, and 87.0%, respectively. Transcriptomic analysis identified 44 MYB transcription factors that were differentially induced by AM symbiosis under iron stress, among which six genes (EgMYB-2, EgMYB-3, EgMYB315-1, EgMYB315-2, EgMYB61, and EgMYB306) were significantly upregulated by AM under both low- and high-iron conditions, as verified by qRT-PCR. Correlation analysis revealed strong positive associations between the expression of these EgMYB genes and antioxidant enzyme activities as well as photosynthetic parameters, suggesting their potential involvement in coordinating iron stress responses. Overall, our findings indicate that AM fungus enhances iron stress tolerance in E. grandis through a multilevel strategy that includes photosynthetic protection, antioxidant defense activation, and transcriptional reprogramming involving MYB transcription factors as potential regulators. This study provides integrated physiological-molecular analysis and novel insights into AM-mediated Fe homeostasis regulation in the woody tree species E. grandis.

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