Symbionts as hidden drivers: reshaping the fight between plants and insects

While insect symbionts are known for their roles in nutrient supplementation and toxin detoxification, their potential roles in modulating plant immunity remain incompletely understood. Here, we synthesize how symbionts shape plant defenses against insects across three levels - physical barriers, immune signaling, and defensive chemistry. These effects arise through both host-mediated modifications - such as altered salivary composition - and direct microbial inputs, including symbiont-derived molecules and effectors. Symbionts can either inhibit or stimulate plant defenses, with outcomes that are highly context-dependent and shaped by host genotype, symbiont diversity, and the plant's background. Notably, the molecular patterns that plants perceive from symbionts remain largely unidentified, representing a research gap in the field. We argue that integrating multi-omics approaches with functional genetics and mechanistic validation will be essential to advance a predictive framework for plant-insect-symbiont interactions.