Effects of a saponin-based insect resistance and a systemic pathogen resistance on field performance of the wild crucifer Barbarea vulgaris

Plants have evolved a variety of defences to reduce losses to herbivores and pathogens. The benefits of these may, however, be modified by resistance evolution in antagonists, changes in antagonist fauna, context-dependent costs of defence, and by interactions between antagonists. In Barbarea vulgaris (Brassicaceae), the so-called G-type produces triterpenoid saponins that deter important specialist insect herbivores, whereas the P-type produces other saponins and are not insect-resistant. In contrast, P-type plants are predominantly resistant to the biotroph pathogen Albugo sp., causing white blister rust, whilst most G-type plants are susceptible. In a field experiment with F-3 hybrids between G and P-plants, we tested whether the two resistances are functionally coupled, leads to less disease and herbivory and to better plant performance, and whether insect herbivores and the pathogen interact in their effects on plant performance. The Albugo and insect resistances varied continuously between the F-3 plants and mapped to different linkage groups, indicating independent mechanisms and evolution. Plants with high Albugo resistance produced more biomass and survived better than more susceptible plants. Albugo DNA was detected in surface-sterilized green siliques, indicating systemic and sometimes non-symptomatic infection. Plants with high insect resistance were slightly less damaged by herbivores, but did not grow or survive better than more susceptible plants. Interactions between Albugo and insect herbivores did not affect plant performance. In contrast to the Albugo resistance, which clearly benefited the plants, our results show that the saponin-based insect resistance did convey any benefit under the given conditions despite its deterrent effects in controlled experiments.

Automated summary

Plants have evolved various defenses against herbivores and pathogens, but the effectiveness of these defenses can be influenced by multiple factors. Research on Barbarea vulgaris revealed that different resistance mechanisms have independent evolution and effects, with Albugo resistance benefiting plants while insect resistance showed limited benefits under certain conditions.