Plant-mediated interactions between above- and below-ground communities at multiple trophic levels

P>1. Above-ground (AG) and below-ground (BG) biota show little overlap and are usually investigated independently. However, both communities are connected by a group of organisms that intrinsically belong to both worlds: plants. The responses of plants to environmental stresses often move from the aerial compartments to the roots and vice versa and thereby enable interactions among species that never come into direct physical contact. 2. Plant-mediated AG-BG interactions can have positive or negative effects on organisms from all phyla. For example, damage inflicted on roots or root symbioses with mycorrhizal fungi and rhizobia can affect the interactions between the aerial compartment and herbivores, pollinators, predators and parasitoids, whereas resistance expression in leaves can affect root herbivores as well as mutualistic or parasitic soil microorganisms. Given the tremendous diversity in the interacting organisms and also in the outcomes of the interactions, we are only just beginning to identify general patterns in the net effects of plant-mediated interactions between AG and BG communities. 3. Synthesis. The papers in this Special Feature reveal an important problem that remains to be solved: for most AG-BG interactions, the driving evolutionary forces remain to be identified. Because several plant responses in the context of AG-BG interactions ultimately result in negative effects on the plant itself, it appears obvious that in some cases the interacting partners manipulate the plant for their own benefit. In the future we will need to combine knowledge on the underlying physiological mechanisms with data on fitness effects. 'Negative' results, which demonstrate that the other compartment does NOT respond to a certain stress, or which demonstrate no or negative fitness effects on the involved species, are as important as reports on successful systemic resistance responses. Integrating knowledge on the physiological and genetic mechanisms with data on the fitness effects on all interacting partners will help to determine whether a certain AG-BG interaction is adaptive from the perspective of the plant, or the interacting organism, or both.