Aphid-induction of defence-related metabolites in Arabidopsis thaliana is dependent upon density, aphid species and duration of infestation

Plants display a wide range of chemical defence responses when challenged by sap feeding insects. In this study, we examined changes in leaf chemistry in Arabidopis thaliana when challenged by three species of aphid which were all able to grow and reproduce on Arabidopsis: a generalist with wide host range, Myzus persicae, and two brassica specialists Brevicoryne brassicae and Lipaphis pseudobrassicae. Most glucosinolates were reduced in concentration by aphid feeding, but Myzus persicae consistently increased the levels of 4-methoxy-indolyl-glucosinolate, which is a known feeding deterrent for M. persicae, whilst decreasing other indolyls, suggesting the plant is converting these compounds to the former. The foliar concentrations of jasmonic acid and salicyclic acid were increased by M. persicae but not by B. brasssicae and L. pseudobrassicae, whereas the phytoalexin camalexin and its precursor, the amino acid tryptophan, was induced after feeding by all three aphids. Many of the compounds induced by M. persicae (e.g., jasmonic acid; salicylic acid; camalexin; tryptophan; 4-methoxy-indolyl-glucosinolate) exhibited positive relationships with aphid density and the duration of feeding prior to harvest, indicating that they are responding to the overall level of herbivore challenge that has taken place. The study reinforces the need to consider components of the experimental system (e.g., insect density, insect species, duration of feeding prior to harvest) when making inter-study comparisons of the chemical responses of plants to aphid feeding.