Binding and toxicity of plant lectins to insects
The toxicity of plant lectins to insects after oral ingestion was investigated by incorporating the proteins into insect diets. Bioassays carried out using an artificial diet system demonstrated that kidney bean (Phaseolus vulgaris) lectin (PHA) caused a significant decrease in survival of larvae of the tomato moth, Lacanobia oleracea. Jackfruit (Artocarpus integrifolia) lectin (jacalin) and black mulberry (Morus nigra) lectin both caused a significant decrease in growth of the peach potato aphid (Myzus persicae) when compared to controls in an artificial diet based bioassay. Interactions of lectins with insect gut tissues in vivo were studied by immunolocalisation. Binding of the snowdrop lectin (Galanthus nivalis agglutinin;GNA) and jack bean (Canavalia ensifomiis) lectin (Concanavalin A; Con-A) to the digestive tract of L oleracea larvae was observed and localised at the electron microscope level after oral ingestion of the proteins. GNA was also observed to bind to the midgut of the two-spot ladybird Adalia bipunctata. No disruption of the brush border membrane of either L oleracea or A. bipunctata was observed. Binding of GNA to the peritrophic membrane of L. oleracea was observed by fluorescence microscopy. Histological evidence of lectin binding to insect guts in vivo was corroborated by in vitro studies, which showed that the lectins GNA and Con- A bind to sections of the digestive tract of L. oleracea larvae. Binding of Con-A to proteins from brush border membranes, solubilised brush border membranes and peritrophic membranes was also observed. The use of confocal microscopy showed that GNA bound to the midgut and haemocytes of the peach potato aphid Myzus persicae, both when incubated with isolated tissues and cells and when fed orally to live insects, providing evidence for transport of GNA across the gut wall. Larvae of L.oleracea fed the lectins GNA and PHA showed a significant increase in polyphenoloxidase levels within the haemolymph, suggesting that the lectins were causing systemic responses in the insects. A partial sequence for leucine aminopeptidase a potential receptor for lectin binding was obtained from a cDNA library constructed from the midgut of the tomato moth larvae.