Fungus–plant interaction influences plant-feeding insects

Abstract

Fungal infection of plants alters plant metabolism and therefore their chemistry, by either increasing levels of defence compounds or decreasing levels of nutrients. Such alterations in plants, in turn, influence the performance of the insects that feed on them. Pathogenic fungi live on plants as either biotrophs or necrotrophs. Both bio- and necrotrophic fungi alter the physiologies of plants in distinctly different pathways. This article explores the patterns of relationships evident between insects and plants, the latter infected by biotrophic and necrotrophic fungi, using established examples from the vertically and horizontally transmitted biotrophic, endophytic fungi. A curious pattern evident in such interactions refers to the biotrophic, endophytic fungi that live in galls induced by the Cecidomyiidae (Diptera). The insects associated with necrotrophic fungi stand distinct from the patterns of interactions evident among insects, plants, and fungi. An understanding of the ecology of three-way interactions involving distantly related organisms, viz., insects, fungi, and plants, will bear long-term consequences in the better management of annual crop and perennial forest trees. A majority of previously published papers in the context of insect–plant–fungus interactions liberally use the terms ‘mutualism’ and ‘symbiosis’, implicating the various benefits conferred on one or more of the participants. In the examples referred to, and the interacting contexts analyzed in this paper, what emerges is that the fungus is parasitically associated with the plant. In the eventuality of an insect interacting with the infected (= parasitized) plant, the insect gains (e.g., gall-inducing Cecidomyiidae) or in occasional instances the plant gains (e.g., Epichloë infections of Poaceae). A unifying model for insect–plant–fungus interacting systems is not readily apparent. The only possible explanation is the independent origins of insect behaviour in either preferring or rejecting the fungus-infected plants. This is possibly driven by specific environmental conditions, in which a specific fungus–plant–insect system would be operating.