Endophyte-Mediated Aphid Resistance in Wheat via Chemical and Structural Defenses under Nonsterilized Soil Condition

Endophyte-based coatings enhance plant resistance and offer a sustainable alternative to chemical insecticides; however, their efficacy under nonsterilized soil conditions remains unclear. Here, we examined how nonsterilized soil conditions influence aphid resistance in wheat conferred by the endophytic fungus Beauveria bassiana strain YC. YC inoculation reduced Rhopalosiphum padi populations in both sterilized and nonsterilized soils, with stronger resistance observed under nonsterilized conditions, as evidenced by reduced aphid performance and altered feeding behavior. These effects were accompanied by shifts in the rhizosphere bacterial and fungal communities. Integrated transcriptomic and metabolomic analyses revealed coordinated activation of plant defense responses, particularly phenylpropanoid metabolism, including enhanced flavonoid accumulation and the upregulation of lignin and cell wall biosynthesis-related genes (e.g., PAL, 4CL, CCR). Overall, nonsterilized soil conditions modulated the magnitude of endophyte-mediated resistance, suggesting that soil environments should be considered when evaluating and applying endophyte-based strategies for enhancing crop resistance against insect herbivores.