Suillus bovinus sesquiterpenes stimulate root growth and ramification of host and non-host plants by coordinating plant auxin signaling pathways

Prior to physical contact, ectomycorrhizal (ECM) fungi can regulate plant root growth and ramification by emitting volatile organic compounds (VOCs). However, the underlying mechanisms of these VOC effects, as well as the key signaling molecules within the VOC blends, are largely unknown. Under sterile conditions, we studied the effects of the Suillus bovinus VOCs on the root growth of Pinus massoniana or Arabidopsis thaliana before physical contact. Exogenously added auxin inhibitors and auxin-related mutants were used to explore the role of auxin in the promotion of plant root development by S. bovinus VOCs. S. bovinus VOCs stimulated host P. massoniana and non-host A. thaliana lateral root formation (LRF). Although these effects were independent of the host, they exhibited a symbiotic fungal-specific feature. Sesquiterpenes (SQTs) were the main S. bovinus VOC component that promoted LRF in plants. Two SQTs, α-humulene and β-cedrene, utilized different auxin pathways to promote plant root growth but did not affect the formation of an ECM symbiotic relationship between P. massoniana and S. bovinus . These findings enhance our understanding of the role played by SQTs in the signal recognition mechanism during the ECM presymbiotic stage and their role in promoting plant growth.