National Institute of Sericultural and Entomological Science, Tsukuba, Ibaraki 305-8634, Japan.
Our previous work1-3 suggested that the privet tree, Ligustrum
obtusifolium, defends itself with oleuropein, a phenolic secoiridoid glycoside. On leaf
damage, oleuropein is activated by foliar beta-glucosidase and polyphenol oxidase to show
a strong protein-denaturing activity that resembles that of glutaraldehyde, and makes
protein innutritive for herbivores by decreasing the lysine content. High concentrations of
glycine (0.4-0.8%) existed in the digestive juices of several privet-specialist Lepidoptera
larvae, and in vitro experiments showed that glycine inhibits the denaturing
activity, suggesting that privet-specialists have adapted themselves by secreting glycine
in their digestive juices as a neutralizer. However, direct in vivo evidence was
lacking. Here we show in vivo evidence by using larvae of Samia ricini
(Saturuniidae). Though the privet tree is not their host plant, the larvae ate the fresh leaves
very well, but the growth rate was very low. When steamed privet leaves whose enzymes
were inactivated were fed (oleuropein is heat-stable), the larvae grew much faster,
indicating that enzymatic activation is crucial for the plant defense. Next, we fed to the
larvae the fresh privet leaves whose surface was coated with glycine. The larvae grew very
rapidly as when the larvae were fed their natural host plant, the castor oil plant. This result
shows that glycine really works in vivo as a counter-adaptation against the
chemical defense of the privet tree.
1 Konno et al. Proc. Natl. Acad. Sci. USA, 96: 9159-9164 (1999).
2 Konno et al. J. Insect Physiol.43: 217-224 (1997).
3 Konno et al. J. Chem. Ecol. 24: 735-751 (1998).