Symbiotic soil microorganisms as players in aboveground plant–herbivore interactions – the role of rhizobia

Rhizobia play a key role for performance of leguminous plants and ecosystem productivity. However, no studies to date have investigated the importance of the rhizobial symbiosis for legume herbivore interactions. The additional nitrogen provided by the rhizobia improves the nutritional quality of plants, but may also be used for the synthesis of defence compounds. We performed greenhouse experiments with nodulating and non-nodulating, as well as cyanogenic and acyanogenic strains of Trifolium repens to study the effects of rhizobia Rhizobium leguminosarum on plant growth and the performance of the chewing herbivore Spodoptera littoralis and the phloem-sucking aphid Myzus persicae. We demonstrate that for nodulating strains of T. repens rhizobia increased plant growth and the performance of Spodoptera littoralis. However, this positive effect of rhizobia on the caterpillars did not occur in a cyanogenic clover strain. Reproduction of the phloem-sucking aphid Myzus persicae was inconsistently affected by rhizobia. Our study provides evidence that the additional nitrogen provided by the rhizobia may be used for the production of nitrogen-based defence compounds, thereby counteracting positive effects on the performance of chewing herbivores. The symbiosis with rhizobia is therefore an important driver of legume herbivore interactions.