Balancing Growth and Defense: miRNA-Mediated Regulation of Phosphorus Metabolism and Pathogen Resistance in Soybean Under Variable Light Conditions

This study reveals the molecular mechanisms by which soybeans dynamically balance phosphorus uptake and disease resistance through microRNAs. Under normal light conditions, during the early stages of SMV infection (3 dpi), miR397a promotes lignin synthesis by inhibiting laccase activity, thereby increasing disease resistance. In contrast, shading upregulated miR397a expression, weakening lignin-mediated defense. Phosphorus uptake is environmentally regulated: shading significantly increases the root phosphorus content, whereas viral infection only enhances early phosphorus absorption under normal light conditions. MiR399j negatively regulates the GmPHT1-4 gene, inhibiting phosphorus uptake under normal light at 3 dpi, with its expression decreasing by 10 dpi, leading to reduced phosphorus acquisition. Genetic analysis confirmed a positive correlation between defense activation and root phosphorus accumulation, indicating that under normal light, soybean plants prioritize coordinating early defense with phosphorus uptake, whereas shading promotes phosphorus accumulation to support growth. The hormone signaling network shows spatiotemporal-specific regulation, activating defense early under normal light and inhibiting related pathways under shading. This research elucidates the plasticity of resource allocation in soybeans under biotic and abiotic stresses, providing a theoretical basis for breeding resilient high-yield varieties. These findings are important for guiding the optimization of crop adaptability in intercropping systems.