A Photorhabdus akhurstii toxin altered gut homeostasis prior conferring cytotoxicity in Spodoptera frugiperda, S. litura and Helicoverpa armigera.
The nematode-bacterium pair Heterorhabditis indica-Photorhabdus akhurstii is a model system to study symbiotic associations. Upon introduction into the host hemocoel by its nematode partner, the bacterium produces a number of toxins which kill the insect. Nevertheless, unlike Bt toxins majority of Photorhabdus toxins are not orally active which hinders their commercial exploitation as insect biological control agent. In the present study, an orally active 63 kDa protein, TcaB (derived from P. akhurstii strains IARI-SGHR2 and IARI-SGMS1), was investigated for its biological activity against agriculturally important insects including Spodoptera frugiperda, S. litura and Helicoverpa armigera. A force feeding-based oral administration of TcaB exhibited LD50 values of 92.25-105.5, 145.4-156.6 and 190.75-200.75 ng/g in S. frugiperda, S. litura and H. armigera, respectively, which was comparable with the injection LD50 values of 80.5-95.75, 138.6-150.4 and 186-197 ng/g. An oral delivery of 500 ng TcaB caused a continual disintegration of midgut architecture in S. frugiperda, S. litura and H. armigera over 12 to 48 h incubation period putatively leading to escape of TcaB into the hemocoel. Simultaneously, alike of intra-hemocoel injection effect, TcaB oral delivery caused cytotoxicity and immune-stimulatory effect in the hemocoel of all the test insects during 12 to 48 h after inoculation, supporting our hypothesis. The circulatory hemocyte numbers and cell viability was considerably reduced in the TcaB-inoculated larvae compared to control. In parallel, the hemolymph phenoloxidase activity was elevated by manifold in the toxin-treated insects. In silico docking analyses suggested that TcaB interacts with a number of S. frugiperda, S. litura and H. armigera receptor proteins in order to become a gut-active toxin. Current study describes the potential of orally-active Photorhabdus toxins as an alternative to Bt and fortifies the existing pest management tactics.