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 F₃ 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 F₃ 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.

植物已经进化出多种防御能力，以减少对食草动物和病原体的损失。然而，这些好处可能会因拮抗剂的抗性进化、拮抗动物群的变化、与环境相关的“防御成本”以及拮抗剂之间的相互作用而改变。在Barbarea vulgaris (Brassicaceae) 中，所谓的“G 型”产生三萜皂苷，可以阻止重要的专业昆虫食草动物，而“P 型”产生其他皂苷并且不抗虫。相比之下，P 型植物主要对生物营养病原体Albugo sp.具有抗性，导致白色水疱锈病，而大多数 G 型植物易感。在 F ₃的现场试验中在 G 和 P 植物之间的杂种中，我们测试了这两种抗性是否在功能上耦合，导致较少的疾病和食草性以及更好的植物性能，以及昆虫食草动物和病原体是否相互作用对植物性能的影响。的白锈病和昆虫抗性在F之间连续改变₃植物和映射到不同的连锁群，表示独立的机制和演化。具有高Albugo抗性的植物比更易感的植物产生更多的生物量并且存活得更好。阿尔布戈在表面消毒的绿色长角果中检测到 DNA，表明系统性感染，有时无症状感染。具有高抗虫性的植物受到食草动物的伤害略小，但生长或存活率并不比更易感的植物好。Albugo和昆虫食草动物之间的相互作用不影响植物性能。与明显有利于植物的Albugo抗性相反，我们的结果表明，尽管在对照实验中具有威慑作用，但基于皂苷的昆虫抗性在给定条件下确实带来了任何好处。