Asymmetric exposure to MoS2 nanosheets triggers redox-mediated hormesis effect in plant roots: Insights from in situ chip tracking

Abstract

The rhizosphere is microscopically heterogeneous, yet the impacts of uneven distribution of nanomaterials on plant roots remain unclear. Here, using Arabidopsis thaliana and MoS₂ nanosheets as the model plant and nanomaterial, we employed a microfluidic root chip housed in a Petri dish with moist dust-free paper to investigate root responses to spatially asymmetric exposure. We compared symmetric exposure (half dose delivered to each root side; a homogeneous scenario) with asymmetric exposure (full dose to one side; a heterogeneous scenario) of MoS₂ nanosheets over a 5-day period. Results showed that this spatial distinction alone is sufficient to elicit divergent plant growth and physiological responses in seedling roots. Real-time imaging revealed that symmetric exposure induced a classic dose-dependent toxicity from excess MoS₂ nanosheet-induced stress oxidative damage, whereas asymmetric exposure elicited a hormesis effect on root growth, with asymmetric exposure of 50 mg L⁻¹ MoS₂ nanosheets for 64 h increasing root length by 18% compared to symmetric exposure at 25 mg L⁻¹. Mechanistic investigations, integrating dynamic monitoring and inhibition experiments using fluorescence probes and an auxin reporter, showed that localized stimulation promoted root growth via Fe²⁺-dependent reactive oxygen species (ROS). The data signaled that asymmetric exposure increased Fe²⁺ and ROS levels by 1.7-fold and 2.0-fold relative to symmetric exposure, leading to auxin accumulation in root tips. Our findings demonstrate that spatial heterogeneity is a critical consideration for evaluating and designing the application of redox-active nanomaterials.