Glycine addition improves feeding performance of non-specialist herbivores on the privet, Ligustrum obtusifolium: In vivo evidence for the physiological impacts of anti-nutritive plant defense with iridoid and insect adaptation with glycine

The leaves of the privet tree, Ligustrum obtusifolium (Oleaceae), retain a strong lysine-decreasing activity caused by enzymatically-activated oleuropein, an iridoid glycoside. Protein treated with this activity become innutritive to insects because of the loss of lysine. We previously found that several privet specialist caterpillars secrete high concentrations of glycine in their digestive juices. Previous in vitro experiments showed that glycine inhibits lysine-decreasing activity of oleuropein. Therefore, we hypothesized that the lysine-decreasing activity acts to defend privet tree against herbivores and that glycine secretion by insects is an adaptive trait to counter the privet defense. In the present study, we aimed to examine whether these assumptions hold true under physiological conditions, and performed in vivo bioassays and physiological analyses using the Eri silkworm, Samia ricini (Saturniidae), a non-privet specialist. Significant decreases in larval growth and lysine concentration in the midgut lumen were observed when larvae were fed intact privet leaves compared to when they were fed heat inactivated privet leaves. These decreases were inhibited when larvae were fed intact privet leaves together with glycine, indicating that the privet defense with oleuropein and the specialist adaptation with glycine do function under physiological conditions. This study thus provides a rare view into the detailed physiological impacts of anti-nutritive plant defense and insect physiological adaptation in vivo.