Symbiosis with Mycorrhizal Fungi Alters Sesquiterpene but not Monoterpene Profile in the South American Willow Salix humboldtiana.

The emission of volatile organic compounds (VOC) in plants can be influenced by abiotic factors such as light, temperature and moisture, as well as biotic factors like herbivory, oviposition, and pathogen damage. The influence of symbiotic microorganisms on VOC emission is less explored. Although it is widely known that arbuscular mycorrhizal (AM) fungi can significantly affect host plant metabolism, their role in VOC emission in trees remains under-investigated. Here, we examine the impact of AM fungi on VOC production in the South American willow tree, Salix humboldtiana. We assessed the effects of inoculation with AM fungi on plant growth and larval feeding by the willow sawfly Nematus oligospilus, as well as its impact on the plant's VOC emission profile. Willow plants inoculated with AM fungi exhibited increased leaf biomass and reduced damage incidence from willow sawfly larvae, supporting the role of mycorrhiza as a protective symbiosis. Notably, AM fungi-inoculated plants emitted 40% less total VOC compared to non-inoculated plants. Both groups emitted similar levels of monoterpenes; however, inoculated plants produced 30% fewer sesquiterpenes. Herbivory did not alter total VOC emission, but non-inoculated plants showed a reduction in (E)-β-ocimene, which was not observed in inoculated plants. The significant decline in sesquiterpene emission of inoculated willow saplings points out the importance of considering the symbiotic microorganisms in the study of plant defenses and insect-plant interactions.