More than just the sweet stuff: Parasitic dodder vines imbibe host plant secondary metabolites that are toxic to insect herbivores

Parasitic plants and insect herbivores both feed on plants to acquire nutrition and other resources. Some herbivores also obtain toxins from host plants and sequester them for protection against their enemies. While this defense mechanism is well-studied in insects, remarkably little research has explored analogous host-derived defenses in parasitic plants. Nevertheless, a few studies indicate that the resistance of parasitic plants to insect enemies can depend on the identity of the parasite’s host. For example, aphids feeding on the parasitic vine dodder (genus Cuscuta) generally perform well when dodder is grown on tomato, but poorly when it is grown on turnip or onion—two plants bearing anti-insect toxins that are accessible to dodder via phloem. However, cabbage aphids (Brevicoryne brassicae), brassica specialists that are well adapted to the glucosinolate defense compounds produced by these plants, performed optimally on turnip-hosted dodder. In light of these findings we hypothesize that glucosinolate toxins from infested turnips translocate into dodders and confer resistance to aphids. This talk will report initial results from chemical and biological tests of this hypothesis, including the key finding that glucosinolates move readily into dodders from their Brassicaceous host plants. Our methods include HPLC analysis of glucosinolates and caged-aphid performance assays. This work has significant implications for the ecology of parasitic plants and their insect herbivores, as well as for the management of some of the world’s most devastating agricultural weeds.