Histopathological Effects of a Protein Complex Purified from Xenorhabdus nematophila on the Midgut of Helicoverpa armigera Larvae

Entomopathogenic nematodes in the field carry symbiotic bacteria (Xenorhabdus or Photorhabdus) into the host insect through a natural orifice or the body wall and then release the symbiotic bacteria into the blood cavity of the insect. The symbiotic bacteria multiply and release a variety of active substances, including insecticidal proteins, which kill the host insect rapidly. PirAB toxin was initially found in the Photorhabdus luminescens TT01 strain and is a demonstrated binary toxin with high insecticidal activity. We examined the bioactivity of PirAB toxin on Helicoverpa armigera. First, the pirAB toxin gene of Xenorhabdus nematophila HB310 was co-expressed in a prokaryotic expression system. The purified recombinant protein showed high hemocoel insecticidal activity against the 4th instar larvae of H. armigera (LD₅₀=2.003μg/larva). We then quantified the influence of PirAB against the related enzymes (Acetylcholinesterase, Carboxylesterase, Polyphenol oxidase, Tyrosinase, and Peroxidase). After injection of the recombinant PirAB protein with LD₅₀, H. armigera larva gradually turned dark brown and moribund during the process. The acetylcholine esterase activity did not vary between treated and control hosts. The activity of carboxyl esterase was activated first, then inhibited and reactivated again, and remained a stable level after injection for 36 h. The activity of polyphenol oxidase also was inhibited and then maintained a stable level after injection for 24 h, which was lower than that of control. Tyrosinase activity remained largely at an inhibitory state which was lower than that of the control group, and decreased sharply and increased again after injection for 12 to 36 h. The overall trend of peroxidase activity was not significant and continued to be lower than that of control. These experimental results provide the research basis for revealing the host insect’s immune suppression to PirAB toxin.