The privet tree (Ligustrum obtusifolium) defends with oleuropein, a phenolic secoiridoid glycoside. When leaves are damaged, oleuropein is activated by foliar b-glucosidase and show a strong protein denaturing activity that makes foliar protein innutritive to herbivores by decreasing the amount of lysine, an essential amino acid. High concentrations of glycine (0.4-0.8%) and some other amino acids were found from the digestive juices of several privet-specialist Lepidoptera larvae. In one of such species, Brahmaea wallichii (Brahmaeidae), glycine was secreted in midgut lumen in an active and selective manner. In vitro experiments showed that glycine inhibits the denaturing activity, suggesting that privet-specialists have adapted by secreting glycine in their digestive juices as a neutralizer. However, direct in vivo evidence was lacking.
Here we show in vivo evidence that comes from a novel in vivo bioassay method using larvae of the Eri silkmoth, Samia ricini (Saturuniidae). The Eri silkworm is a suitable insect for bioassaying plant defenses and insect adaptations due to its feature to eat many plants whether or not the plant are toxic, and then to show a symptom corresponding to each plant. Though the privet tree is not their host plant, the larvae ate the fresh leaves well, but the growth rate was very low. When the leaves were steamed in order to inactivate foliar enzymes (oleuropein is heat-stable), the larvae ate the steamed leaves well, and the growth rate was much higher, indicating that enzymatic activation is crucial for the plant defense. Then, we put glycine on the surface of fresh privet leaves by dipping them into 1% glycine solution and drying the surface. The larvae ate the leaves well and grew rapidly. This result shows that glycine really works in vivo as a counter-adaptation against the chemical defense of the privet tree.
Species 1: Lepidoptera Saturnidae Samia ricini (Eri silkworm)
Species 2: Lepidoptera Brahmaeidae Brahmaea wallichii (Ligustrum moth, owl moth)
Species 3: Oleaceae Ligustrum obtusifolium (privet, privet)
Keywords: Plant-Herbivore Interactions, glycine
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA