Iron Nanostructure Primes Arbuscular Mycorrhizal Fungi Symbiosis Tightly Connecting Maize Leaf Photosynthesis via a Nanofilm Effect

It is crucial to clarify how the iron nanostructure activates plant growth, particularly in combination with arbuscular mycorrhizal fungi (AMF). We first identified 1.0 g·kg–1 of nanoscale zerovalent iron (nZVI) as appropriate dosage to maximize maize growth by 12.7–19.7% in non-AMF and 18.9–26.4% in AMF, respectively. Yet, excessive nZVI at 2.0 g·kg–1 exerted inhibitory effects while FeSO4 showed slight effects (p > 0.05). Under an appropriate dose, a nano core–shell structure was formed and the transfer and diffusion of electrons between PS II and PS I were facilitated, significantly promoting the reduction of ferricyanide and NADP (p < 0.05). SEM images showed that excessive nZVI particles can form stacked layers on the surface of roots and hyphae, inhibiting water and nutrient uptake. TEM observations showed that excessive nanoparticles can penetrate into root cortical cells, disrupt cellular homeostasis, and substantially elevate Fe content in roots (p < 0.05). This exacerbated membrane lipid peroxidation and osmotic regulation, accordingly restricting photosynthetic capacity and AMF colonization. Yet, appropriate nZVI can be adhered to a mycelium surface, forming a uniform nanofilm structure. The strength of the mycelium network was evidently enhanced, under an increased root colonization rate and an extramatrical hyphal length (p < 0.05). Enhanced mycorrhizal infection was tightly associated with higher gas exchange and Rubisco and Rubisco enzyme activities. This enabled more photosynthetic carbon to input into AMF symbiont. There existed a positive feedback loop connecting downward transfer of photosynthate and upward transport of water/nutrients. FeSO4 only slightly affected mycorrhizal development. Thus, it was the Fe nanostructure but not its inorganic salt state that primed AMF symbionts for better growth.