Insect Egg Deposition Induces Indirect Defense and Epicuticular Wax Changes in Arabidopsis thaliana

Egg deposition by the Large Cabbage White butterfly Pieris brassicae on Brussels sprouts plants induces indirect defense by changing the leaf surface, which arrests the egg parasitoid Trichogramma brassicae. Previous studies revealed that this indirect defense response is elicited by benzyl cyanide (BC), which is present in the female accessory reproductive gland (ARG) secretion and is released to the leaf during egg deposition. Here, we aimed (1) to elucidate whether P. brassicae eggs induce parasitoid-arresting leaf surface changes in another Brassicacean plant, i.e., Arabidopsis thaliana, and, if so, (2) to chemically characterize the egg-induced leaf surface changes. Egg deposition by P. brassicae on A. thaliana leaves had similar effects to egg deposition on Brussels sprouts with respect to the following: (a) Egg deposition induced leaf surface changes that arrested T. brassicae egg parasitoids. (b) Application of ARG secretion of mated female butterflies or of BC to leaves had the same inductive effects as egg deposition. Based on these results, we conducted GC-MS analysis of leaf surface compounds from egg- or ARG-induced A. thaliana leaves. We found significant quantitative differences in epicuticular waxes compared to control leaves. A discriminant analysis separated surface extracts of egg-laden, ARG-treated, untreated control and Ringer solution-treated control leaves according to their quantitative chemical composition. Quantities of the fatty acid tetratriacontanoic acid (C34) were significantly higher in extracts of leaf surfaces arresting the parasitoids (egg-laden or ARG-treated) than in respective controls. In contrast, the level of tetracosanoic acid (C24) was lower in extracts of egg-laden leaves compared to controls. Our study shows that insect egg deposition on a plant can significantly affect the quantitative leaf epicuticular wax composition. The ecological relevance of this finding is discussed with respect to its impact on the behavior of egg parasitoids.