Purification And Insecticidal Mechanism Of Insecticidal Proteins From Xenorhabdus Nematophila

Xenorhabdus is harbored as symbionts in the intestine of the infective juvenile stage of nematodes belonging to the family Steinernematidae. The nematodes carried the bacteria into the hemolymph of the host insects and released them. Then the bacteria rapidly killed the pest larva. Beside of this natural condition, in most cases the bacteria alone is also highly virulent to some kinds of pests.In this study, two oral insecticidal proteins( toxin Ⅰ and toxin Ⅱ) and two injectable insecticidal proteins( toxin Ⅲ and toxin Ⅶ) were purified from the intracellular protein of X. nematophila HB310 by native-PAGE. Bioassay results indicated that the oral insecticidal activity of toxin II was higher than that of toxin I to Helicoverpa armigera neonates.The histopathology of toxin II to H. armigera larvae was studied by dissecting and olefin slice of midgut. The symptoms showed that the histopathology of toxin II on H. armigera midgut was similar to that of other novel midgut-active toxins, such as the δ -endotoxins from Bacillus thuringiensis or Tca from Photorhabdus luminescens W-14. The midgut tissues of H. armigera began to change after taken the toxin II orally in six hours. The epithelium was decomposed gradually. The cells were loose or disordered at last. The peritrophic membrane disappeared after 12 hours , but appeared again after 72 hours following transient or sublethal exposure to the toxin. The symptoms showed that the histopathology of the Plutulla xylostella and Pieris rapae midguts was similar to that of H. armigera but more serious.Two injectable insecticidal proteins( toxin Ⅲ and toxin Ⅶ) have different effects on Galleria mellonella fifth-intar larvae. Toxin Ⅲ destroyed the insect hemocytes into fragmenry, and the colour of the hemocoel became brown. Toxin Ⅶ could not destroy the hemocytes and the hemocoel colour did not change. Perhaps toxin Ⅶ affected other tissues of the G mellonella.