Foliar herbivory modifies arbuscular mycorrhizal fungal colonization likely through altering root flavonoids

Abstract Insect herbivory may influence arbuscular mycorrhizal (AM) fungal colonisation by changing plant chemistry, and these effects can vary from negative to positive. Yet the underlying mechanisms are unclear. We investigated AM fungal colonization of Chinese tallow tree ( Triadica sebifera ) after exposing its seedlings to four different foliar‐feeding insects at two levels of herbivory. We then examined the potential role of induced carbon allocation and secondary metabolites, particularly flavonoids, in AM fungal colonization. Light herbivory (ca. 10% leaf area removed) promoted early colonization by AM fungi whereas heavy herbivory (ca. 60% leaf area removed) decreased it. Root flavonoids were increased (in the short‐term) under light herbivory but decreased under heavy herbivory. Further, herbivore‐induced changes to root flavonoids were positively correlated with those of AM fungal colonisation. Moreover, quercitrin, one of the root‐secreted flavonoids, could mediate these AM fungal responses. Surprisingly, however, AM fungal colonization was not correlated with herbivore‐induced carbon allocation in roots or root exudates. We show that AM fungal responses to insect herbivory are at least partially flavonoid‐based and depend on herbivory intensity. Our work highlights the importance of secondary metabolites rather than carbon allocation in interactions between above‐ground herbivory and below‐ground mycorrhiza. Read the free Plain Language Summary for this article on the Journal blog.