Mutualist-Provided Resources Increase Susceptibility to Parasites

Background and AimsO_LIMutualistic microbes often increase their hosts susceptibility to parasite infection, but the mechanism underlying this pattern remains unknown. C_LIO_LIWe tested two competing hypotheses to identify the cause of this phenomenon. C_LIO_LIFirst, mutualist-provided resources could attract antagonists by making hosts more resource-rich. C_LIO_LISecond, mutualism establishment itself might increase host vulnerability to antagonists by enabling antagonists to exploit host defenses or immune systems, which are often downregulated during mutualism establishment. C_LI MethodsO_LITo test which mechanism underlies increased parasite susceptibility, we experimentally decoupled mutualism establishment and mutualist-provided resources in the legume-rhizobia mutualism. C_LIO_LIWe measured parasite load on Medicago truncatula plants infected with root-knot nematodes in a full-factorial design, in which we independently manipulated rhizobia inoculation (mutualism establishment) and nitrogen availability (mutualist-provided resources). C_LI Key ResultsO_LIIncreased nitrogen availability increased parasite infection. However, its effect was non-linear and was not explained by nitrogen assimilation into plant tissues, indicating that this effect is not driven by parasite attraction to resource-rich hosts. C_LIO_LIWe found little effect of mutualism establishment on susceptibility, as neither rhizobia inoculation nor nodulation without nitrogen fixation significantly increased parasite infection. C_LI ConclusionsO_LIOur results suggest that mutualist-provided resources are an important driver of indirect ecological costs of mutualism, although the mechanism linking mutualist-provided resources and susceptibility to infection remains unknown. C_LI