Summary1. Plant-soil feedback (PSF) effects on plant performance strongly depend on the plant species that conditioned the soil. Recent studies have shown that PSF can change above-ground plant-insect interactions via soil-mediated changes in plant quality, but whether these effects depend on speciesspecific soil conditioning is unknown. We examined how PSF effects of several plant species influence above-ground plant-aphid interactions. 2. We grew ragwort (Jacobaea vulgaris) in field soil conditioned specifically by 10 plant species, belonging to three functional groups (grasses, forbs and legumes), in a multispecies mixture of the conditioned soils and in control (unconditioned) field soil. We measured plant biomass, concentrations of primary (amino acids) and secondary (pyrrolizidine alkaloids) metabolites in phloem exudates, and performance of the generalist aphid Brachycaudus cardui and the specialist Aphis jacobaeae. 3. We observed that plant species, via species-specific effects on soil fungal communities, exerted unique plant-soil effects on J. vulgaris biomass, amino acid concentrations in phloem exudates and aphid performance. The direction and magnitude of the species-specific PSF effects on aphid performance differed between both aphid species. PSF effects on soil fungal communities, plant biomass and A. jacobaeae performance also differed between grasses, forbs and legumes, with soil conditioning by forbs resulting in lowest plant biomass and aphid performance. 4. Synthesis. Our study provides novel evidence that PSF effects on above-ground plant-insect interactions are highly species specific. Our results add a new dimension to the rapidly developing research fields of PSF and above-below-ground interactions, and highlights that these fields are tightly linked.

Plant–soil feedback (PSF) effects on plant performance can be influenced by the availability of nutrients in the soil. Recent studies have shown that PSF effects can also change aboveground plant–insect interactions via soil‐mediated changes in plant quality, but whether this is influenced by soil nutrient availability is unknown. We examined how fertilisation influences PSF effects on aboveground plant‐aphid interactions in ragwort Jacobaea vulgaris. We grew J. vulgaris in soil conditioned by conspecific plants and in unconditioned soil at two levels of fertilisation and measured soil fungal communities, plant biomass, concentrations of primary (amino acids) and secondary (pyrrolizidine alkaloids; PAs) metabolites in phloem exudates, performance of the specialist aphid Aphis jacobaeae and sequestration of PAs by the aphid. We observed a strong interaction between soil conditioning and fertilisation on amino acid and PA concentrations in phloem exudates of J. vulgaris and on aphid performance, with opposite effects of soil conditioning at the two fertilisation levels. Plant biomass was reduced by soil conditioning and increased by fertilisation. Aphids contained high PA concentrations, converted N‐oxides into tertiary amines and preferentially sequestered certain PA compounds, but PA sequestration was not affected by any of the treatments. We conclude that effects of PSF and fertilisation on plant chemistry and aphid performance are interdependent. Our study highlights the need to consider the importance of abiotic soil conditions on the outcome of PSF effects on aboveground plant–insect interactions.

Canalization, an intrinsic robustness of development to external (environmental) or internal (genetic) perturbations, was first proposed over half a century ago. However, whether the robustness to environmental stress (environmental canalization [EC]) and to genetic variation (genetic canalization) are underpinned by the same molecular basis remains elusive. The recent discovery of the involvement of two endoplasmic reticulum (ER)‐associated DnaJ genes in developmental buffering, orthologues of which are conserved across Metazoa, indicates that the role of ER‐associated DnaJ genes might be conserved across the animal kingdom. To test this, we surveyed the ER‐associated DnaJ chaperones in the nematode Caenorhabditis elegans. We then quantified the phenotype, in the form of variance and mean of seam cell counts, from RNA interference knockdown of DnaJs under three different temperatures. We find that seven out of eight ER‐associated DnaJs are involved in either EC or microenvironmental canalization. Moreover, we also found two DnaJ genes not specifically associated with ER (DNAJC2/dnj‐11 and DNAJA2/dnj‐19) were involved in canalization. Protein expression pattern showed that these DnaJs are upregulated by heat stress, yet not all of them are expressed in the seam cells. Moreover, we found that most of the buffering DnaJs also control lifespan. We therefore concluded that a number of DnaJ chaperones, not limited to those associated with the ER, are involved in canalization as a part of the complex system that underlies development.