Plant sterols and host plant suitability for a phloem-feeding insect

Abstract

1. Arthropods, including insects, are unique among animals in that they cannot synthesize sterols, including cholesterol, de novo. Some phytophagous insects (e.g. caterpillars, grasshoppers) generate tissue cholesterol by metabolizing plant phytosterols. Currently, little is known about sterols in plant phloem sap, and their significance to the nutritional physiology and ecology of phloem-feeding insects. 2. The sterol profiles of leaves from two plant species, Chinese cabbage and tobacco, were dominated by the phytosterols sitosterol and stigmasterol, respectively. In contrast, the principle sterol in the phloem sap of both plants was cholesterol, which is traditionally considered an animal sterol. Cholesterol was also the most abundant sterol in the carcass and honeydew of Myzus persicae aphids feeding on these plants. 3. The effect of sterol structure on M. persicae was investigated using modified tobacco plants that contained high levels of atypical steroids, specifically ketone-steroids. Aphids reared on the modified tobacco plants had a high atypical steroid content, severely reduced reproduction and high mortality. 4. Our data indicate that the total sterol composition of plants is not necessarily representative of the sterol profile available to phloem-feeding insects, and that the sterol utilization patterns of phloem-feeding insects may differ from chewing insect herbivores utilizing the same plant. Atypical steroids are naturally at insufficient concentrations for significant deleterious effects on insect herbivores, and possible reasons why plants apparently do not use them as defensive compounds are considered.