1. Plants possess numerous mechanisms to control infections by deleterious organisms. Unspecific resistance mechanisms may, however, also exert ecological costs when they have a negative effect on beneficial plant–microbe interactions. Such negative effects may even cross the border between a plant’s aerial parts and its roots and then affect very central functions such as nutrient uptake and root resistance to micro-organisms. Whereas an impaired nodulation indeed appears common after resistance expression in the leaves, contradictory results have been published for the case of arbuscular mycorrhizal (AM) fungi.
2. We analysed the effect of induction of resistance mechanisms in foliar tissues on AM colonization in soybean plants, using acibenzolar-S-methyl (ASM) as chemical elicitor. By determining different physiological and biochemical parameters, we assessed whether the effects are related to the activation of the plant defence mechanisms or rather to the re-allocation of primary metabolites.
3. Colonization with AM fungi transiently decreased after pathogen resistance mechanisms were elicited in the aerial parts of the plant. The induction with ASM led to a significant, yet moderate, defence response in the roots, which was modulated in mycorrhizal plants. No allocation or fitness costs associated with the induction of resistance were detected in this study.
4. Synthesis. Our study confirms a transient negative impact of the elicitation of foliar defences on root–AM interactions. The results show that induced resistance to foliar pathogens can (i) move from the above-ground to the below-ground compartment and (ii) affect mutualistic micro-organisms as well as plant pathogens. We also conclude that (iii) the negative effect is likely linked to changes in the defence status of the plant rather than to changes in resource allocation patterns and (iv) the AM association can modulate the activation of the plant defence mechanisms and overcome such effects.