A Novel Insecticidal Toxin from Photorhabdus luminescens , Toxin Complex a (Tca), and Its Histopathological Effects on the Midgut of Manduca sexta

Abstract

ABSTRACT Photorhabdus luminescens is a bacterium which is mutualistic with entomophagous nematodes and which secretes high-molecular-weight toxin complexes following its release into the insect hemocoel upon nematode invasion. Thus, unlike other protein toxins from Bacillus thuringiensis (δ-endotoxins and Vip’s), P. luminescens toxin (Pht) normally acts from within the insect hemocoel. Unexpectedly, therefore, the toxin complex has both oral and injectable activities against a wide range of insects. We have recently fractionated the protein toxin and shown it to consist of several native complexes, the most abundant of which we have termed Toxin complex a (Tca). This complex is highly active against the lepidopteran Manduca sexta . In view of the difference in the normal mode of delivery of P. luminescens toxin and the apparent communality in the histopathological effects of other gut-active toxins from B. thuringiensis , as well as cholesterol oxidase, we were interested in investigating the effects of purified Tca protein on larvae of M. sexta . Here we report that the histopathology of the M. sexta midgut is similar to that for other novel midgut-active toxins. Following oral ingestion of Tca by M. sexta , we observed an acceleration in the blebbing of the midgut epithelium into the gut lumen and eventual lysis of the epithelium. The midgut shows a similar histopathology following injection of Tca into the insect hemocoel. These results not only show that Tca is a highly active oral insecticide but also confirm the similar histopathologies of a range of very different gut-active toxins, despite presumed differences in modes of action and/or delivery. The implications for the mode of action of Tca are discussed.