Microbial volatile organic compounds reshape plant hormonal networks and root herbivore defense

Root-feeding herbivores impose substantial constraints on plant performance, yet the chemical signals coordinating belowground defence remain poorly resolved. Microbial volatile organic compounds (mVOCs)—highly diffusible metabolites produced by rhizosphere and endophytic microbes—have emerged as pivotal regulators of plant immunity and development. Recent evidence shows that specific mVOCs modulate jasmonic acid, salicylic acid, ethylene, auxin, and ROS-associated pathways, thereby reprogramming root architecture and priming defence responses during herbivore attack. Despite these advances, major mechanistic gaps persist, including how plants perceive mVOCs, how soil physicochemical conditions shape their diffusion and bioactivity, and how mVOCs integrate with plant-derived volatiles and metabolites to coordinate systemic signalling. Moreover, the roles of mVOCs in mediating multitrophic interactions—particularly their influence on root herbivore behaviour, microbial recruitment, and defence hormone crosstalk—remain largely unexplored. This review synthesizes current advances in mVOC biology and proposes conceptual frameworks linking microbial volatilomes to plant hormonal networks and belowground herbivore defence. A deeper understanding of these hidden chemical dialogues will inform strategies for enhancing crop resilience and developing sustainable root pest management. • mVOCs regulate plant growth and defense through complex hormonal networks. • Interactions between mVOCs and auxin, JA, SA, and ethylene mediate root responses. • mVOCs influence plant adaptation to belowground herbivory and stress. • Understanding mVOC–hormone–herbivore networks provides novel strategies for sustainable crop protection.