Investigation of the steps involved in the difference of susceptibility of Ephestia kuehniella and Spodoptera littoralis to the Bacillus thuringiensis Vip3Aa16 toxin

BUPM95 is a Bacillusthuringiensis subsp. kurstaki strain producing the Vip3Aa16 toxin with an interesting insecticidal activity against the Lepidopteran larvae Ephestia kuehniella. Study of different steps in the mode of action of this Vegetative Insecticidal Protein on the Mediterranean flour moth (E. kuehniella) was carried out in the aim to investigate the origin of the higher susceptibility of this insect to Vip3Aa16 toxin compared to that of the Egyptian cotton leaf worm Spodoptera littoralis. Using E. kuehniella gut juice, protoxin proteolysis generated a major band corresponding to the active toxin and another band of about 22 kDa, whereas the activation of Vip3Aa16 by S. littoralis gut juice proteases generated less amount of the 62 kDa active form and three other proteolysis products. As demonstrated by zymogram analysis, the difference in proteolysis products was due to the variability of proteases in the two gut juices larvae. The study of the interaction of E. kuehniella BBMV with biotinylated Vip3Aa16 showed that this toxin bound to a putative receptor of 65 kDa compared to the 55 and 100 kDa receptors recognized in S. littoralis BBMV. The histopathological observations demonstrated similar damage caused by the toxin in the two larvae midguts. These results demonstrate that the step of activation, mainly, is at the origin of the difference of susceptibility of these two larvae towards B. thuringiensis Vip3Aa16 toxin.

Graphical abstractVip3Aa16 toxin of Bacillus thuringiensis is active against the Mediterranean flour moth Ephestia kuehniella with an LC50 of about 36 ng/cm2. The high sensitivity of E. kuehniella to Vip3Aa16 toxin, compared to that of Spodoptera littoralis, was due to the great yield of the active form (62 kDa) obtained after protoxin activation by midgut juice proteases and the difference in the number and molecular weights of recognized receptors by Vip3Aa16 in the two larvae midgut.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Bacillusthuringiensis Vip3Aa16 toxin is very active against Ephestia kuehniella with an LC50 of about 36 ng/cm2. ► Extensive damage was detected in the midgut cells of toxin-treated larvae. ► The high sensitivity of E. kuehniella to Vip3Aa16 was due to the great yield of the toxin active form (62 kDa). ► After activation, the Vip3Aa16 toxin recognizes midgut putative receptor of about 65 kDa. ► Recognized receptors by Vip3Aa16 in larvae midgut could contribute to the variability in toxin efficiency towards its hosts.