Caterpillar-induced plant-soil feedback affects resistance in wild and cultivated cabbage

Abstract Background and aims Aboveground insect herbivory can change the plant rhizosphere and modulate the composition of the soil microbiome. However, it is unclear to what extent these changes in the rhizosphere affect plant resistance to above-ground herbivorous insects, and how these plant-soil feedback (PSF) mechanisms are shaped. Here, we investigated whether herbivore-induced changes in the rhizosphere increase resistance against caterpillars in cabbage, Brassica oleracea , and how intraspecific variation of the host plant, herbivory intensity, and soil type affect PSF outcomes. Methods PSF experiments with rhizosphere-soil transfer were performed for a wild and cultivated B. oleracea , with different densities of the caterpillar Mamestra brassicae , and different soil types. Results We found that caterpillar-induced soil conditioning affected the performance of M. brassicae feeding on the shoot, depending on both intraspecific variation of the host plant and the intensity and duration of herbivory. On wild cabbage, caterpillar-induced PSF positively affected plant resistance to M. brassicae , which needed more than two weeks to become detectable. In contrast, in cultivated cabbage, caterpillar-induced PSF had a neutral to negative effect on plant resistance and did not differ between soil types. The observed negative PSF effect was associated with downregulation of genes involved in jasmonic acid biosynthesis and downstream signalling. Conclusion Overall, we found that natural variation within one plant species can, depending on intensity and duration of herbivory, result in opposite PSF effects with consequences for jasmonic acid-mediated defences.