Two-way plant-mediated interactions between root-associated microbes and insects: from ecology to mechanisms

Abstract

Plants are members of complex communities and function as a link between above- and below-ground organisms. Associations between plants and soil-borne microbes commonly occur and have often been found beneficial for plant fitness. Root-associated microbes may trigger physiological changes in the host plant that influence interactions between plants and aboveground insects at several trophic levels. Aboveground, plants are under continuous attack by insect herbivores and mount multiple responses that also have systemic effects on belowground microbes. These belowground–aboveground plant-mediated interactions are mostly studied for interactions between single microbial species and single herbivore species, which are not representative of the actual situation in which plants interact with multiple soil-borne microbes and aboveground insects. In this review, we explore how colonization by either single non-pathogenic microbial species or a community of such microbes belowground affects plant growth and defense and how this affects the interactions of plants with aboveground insects at different trophic levels. Moreover, we review how plant responses to foliar herbivory by insects belonging to different feeding guilds affect interactions of plants with non-pathogenic soil-borne microbes. The role of phytohormones in coordinating plant growth, plant defenses against foliar herbivores while simultaneously establishing associations with non-pathogenic soil microbes is discussed.