Summary: We examined how the removal of soil biota affects plant–soil feedback (PSF) and defense chemistry of Jacobaea vulgaris, an outbreak plant species in Europe containing the defense compounds pyrrolizidine alkaloids (PAs). Macrofauna and mesofauna, as well as fungi and bacteria, were removed size selectively from unplanted soil or soil planted with J. vulgaris exposed or not to above‐ or belowground insect herbivores. Wet‐sieved fractions, using 1000‐, 20‐, 5‐ and 0.2‐μm mesh sizes, were added to sterilized soil and new plants were grown. Sieving treatments were verified by molecular analysis of the inocula. In the feedback phase, plant biomass was lowest in soils with 1000‐ and 20‐μm inocula, and soils conditioned with plants gave more negative feedback than without plants. Remarkably, part of this negative PSF effect remained present in the 0.2‐μm inoculum where no bacteria were present. PA concentration and composition of plants with 1000‐ or 20‐μm inocula differed from those with 5‐ or 0.2‐μm inocula, but only if soils had been conditioned by undamaged plants or plants damaged by aboveground herbivores. These effects correlated with leaf hyperspectral reflectance. We conclude that size‐selective removal of soil biota altered PSFs, but that these PSFs were also influenced by herbivory during the conditioning phase.

Plants often are exposed to antagonistic and symbiotic organisms both aboveground and belowground. Interactions between above- and belowground organisms may occur either simultaneously or sequentially, and jointly can determine plant responses to future enemies. However, little is known about time-dependency of such aboveground-belowground interactions. We examined how the timing of a 24 h period of aboveground herbivory by Spodoptera exigua (1–8 d prior to later arriving conspecifics) influenced the response of Plantago lanceolata and the performance of later arriving conspecifics. We also examined whether these induced responses were modulated by the arbuscular mycorrhizal fungus (AMF) Funneliformis mosseae. The amount of leaf area consumed by later arriving herbivores decreased with time after induction by early herbivores. Mycorrhizal infection reduced the relative growth rate (RGR) of later arriving herbivores, associated with a reduction in efficiency of conversion of ingested food rather than a reduction in relative consumption rates. In non-mycorrhizal plants, leaf concentrations of the defense compound catalpol showed a linear two-fold increase during the eight days following early herbivory. By contrast, mycorrhizal plants already had elevated levels of leaf catalpol prior to their exposure to early herbivory and did not show any further increase following herbivory. These results indicate that AMF resulted in a systemic induction, rather than priming of these defenses. AMF infection significantly reduced shoot biomass of Plantago lanceolata. We conclude that plant responses to future herbivores are not only influenced by exposure to prior aboveground and belowground organisms, but also by when these prior organisms arrive and interact.

Summary: Soil‐dwelling entomopathogenic nematodes ( EPN s) kill arthropod hosts by injecting their symbiotic bacteria into the host hemolymph and feed on the bacteria and the tissue of the dying host for several generations cycles until the arthropod cadaver is completely depleted. The EPN –bacteria–arthropod cadaver complex represents a rich energy source for the surrounding opportunistic soil fungal biota and other competitors. We hypothesized that EPN s need to protect their food source until depletion and that the EPN symbiotic bacteria produce volatile and non‐volatile exudations that deter different soil fungal groups in the soil. We isolated the symbiotic bacteria species ( Alcaligenes faecalis ) from the EPN Oscheius spp. and ran infectivity bioassays against entomopathogenic fungi ( EPF ) as well as against plant pathogenic fungi ( PPF ). We found that both volatile and non‐volatile symbiotic bacterial exudations had negative effects on both EPF and PPF . Such deterrent function on functionally different fungal strains suggests a common mode of action of A. faecalis bacterial exudates, which has the potential to influence the structure of soil microbial communities, and could be integrated into pest management programs for increasing crop protection against fungal pathogens.

Plants use different types of responses such as tolerance and induced defense to mitigate the effects of herbivores. The direction and magnitude of both these plant responses can vary with plant age. However, most studies have focused on aboveground herbivory, whereas important feeding occurs belowground. Here, we tested the hypothesis that plant tolerance and defense following shoot damage or root herbivory depends on plant age. In order to test our hypothesis, we exposed the perennial grass species Holcus lanatus to defoliation and root nematode inoculation at three growth stages (young, intermediate and old plants), and examined responses of plant traits related to tolerance (regrowth following defoliation) and defense (leaf and root nitrogen and phenolics). Defoliation overall reduced plant shoot and root biomass as well as foliar concentrations of phenolics regardless of plant age at defoliation. In contrast, defoliation increased foliar N concentrations, but only when defoliation occurred at intermediate and old plant age. Inoculation with root-feeding nematodes reduced root N concentrations after a prolonged period of growth, but only when nematodes had been inoculated when plants were young. The relative shoot regrowth rate of plants increased immediately after defoliation but this was independent of the plant age at which defoliation occurred, i.e. was not stronger in plants that were defoliated at a more advanced age, as hypothesized. Similarly, relative root growth rates increased shortly after defoliation, but this was only observed for plants defoliated when they were young. We conclude that plant responses to aboveground and belowground herbivory in traits related to both defense and tolerance are affected by plant age, but do not generally change with plant age.