Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/18/2006
Publication Date: 10/1/2006
Citation: Blackburn, M.B., Farrar, R.R., Novak, N.G., Lawrence, S.D. 2006. Remarkable susceptibility of the diamondback moth (plutella xylostella l.) to ingestion of pir toxins from photorhabdus luminescens. Entomologia Experimentalis et Applicata 121:31-37. Interpretive Summary: Pir toxins are a novel class of insecticidal proteins recently described from the bacterial insect pathogen Photorhabdus luminescens. One of the toxin proteins has some similarities with proteins from the Colorado potato beetle, and also has some similarities with insecticidal toxins produced by another bacterium that is pathogenic to insects, Bacillus thuringiensis. Pir toxins have been shown to be lethal when fed to larvae of a pest of cabbages and related crops, the diamondback moth. However, whether the toxins are lethal to other agricultural pests has not been tested, and the manner in which the toxins kill diamondback moth larvae is not known. To answer these questions, we inserted genes for a Pir toxin into the normally non-pathogenic bacterium E. coli in order to test the toxin against other pests, and to determine how the toxin kills diamondback larvae. We found that the Pir toxin produced by E. coli with Pir genes inserted was orally active only against the diamondback moth. Four other pests, the tobacco budworm, tobacco hornworm, gypsy moth, and Colorado potato beetle, were at least 300-fold less susceptible to the toxin than diamondback moth larvae. We determined that the Pir toxin kills diamondback larvae by destroying that portion of the digestive system known as the midgut. This study suggests that Pir toxins may have limited value in insect control, due to their narrow activity spectrum. These results will be useful to scientists attempting to develop Pir toxins for insect control, or studying Pir toxin mode of action against the diamondback moth.
Technical Abstract: Genes encoding Pir toxins were cloned and sequenced from Photorhabdus luminescens strain Hm. Cultures of E. coli expressing the Pir A and B were highly toxic when fed to larvae of Plutella xylostella, as had been reported previously. However, the recombinant E.coli had no effect on the growth or mortality of larval Heliothis virescens, Manduca sexta, Lymantria dispar, or Leptinotarsa decemlineata. Based on these results, P. xylostella is at least 300-fold more susceptible to Pir toxins than other insect species tested. Because Pir B has sequence similarities with N-terminal portions of cry proteins from Bacillus thuringiensis, we also tested the recombinant E. coli against a strain of P. xylostella that is resistant to the Cry 1A toxin, but found no difference in mortality between resistant and susceptible strains. Histological examination of P. xylostella larvae fed recombinant E. coli revealed gross abnormalities of the midgut epithelium, with profound swelling and shedding of the apical membranes.