Plant accession and insect infestation, rather than silicon supplementation, shape defence strategies of <i>Arabidopsis halleri</i> towards a leaf beetle

Little is known about the functional role of silicon (Si) in metal-hyperaccumulating plant species, such as Arabidopsis halleri. We investigated the responses of A. halleri from two accessions, Bestwig (Best) and Langelsheim (Lan), to Si supplementation and insect infestation in two controlled full-factorial experiments. Plants were grown in soil either unsupplemented (-Si) or supplemented (+Si) with Si. Some of these plants were kept either uninfested or infested by larvae of the leaf beetle Phaedon cochleariae. Shoot chemical and mechanical traits and plant resistance against the larvae were quantified. Detached leaves from the remaining plants were used to examine whether trichome density and leaf area consumed by larvae were influenced by the accession and/or Si. We found that Si supplementation, but not insect infestation or their interaction, led to twice as high concentrations of shoot Si in +Si in comparison to -Si plants. Insect relative growth rate was not impacted by Si, but by accession, namely lower when larvae fed on Lan than on Best plants. Likewise, leaf area consumed by larvae was consistently lower in the former accession. The density of trichomes was twice as high in plants of the Lan than the Best accession. Uninfested +Si plants contained the highest C/N in both accessions. The composition of glucosinolates differed between accessions, with some glucosinolates being induced by Si, insect infestation or both in the Best plants only. Our findings highlight distinct (induced) defence strategies within A. halleri plants, which may indicate different local adaptations of the source populations.