Root symbionts:Powerful drivers of plant above- and belowground indirect defences

Soil microbial mutualists of plants, including mycorrhizal fungi, non-mycorrhizal fungi and plant growth promoting rhizobacteria, have been typically characterized for increasing nutrient acquisition and plant growth. More recently, soil microbes have also been shown to increase direct plant defence against above- and belowground herbivores. Plants, however, do not only rely on direct defences when attacked, but they can also recruit pest antagonists such as predators and parasitoids, both above and belowground, mainly via the release of volatile organic compounds (i.e. indirect defences). In this review, we illustrate the main features and effects of soil microbial mutualists of plants on plant indirect defences and discuss possible applications within the framework of sustainable crop protection against root- and shoot-feeding arthropod pests. We indicate the main knowledge gaps and the future challenges to be addressed in the study and application of these multifaceted interactions. This article is protected by copyright. All rights reserved.