Penetration through the peritrophic matrix is a key to lectin toxicity against Tribolium castaneum

In the last decades lectins have received a lot of attention as potential tools in pest control. Despite substantial progress in the field not all the factors determining insecticidal potency and selectivity of these proteins have been described. Recently, three lectins, RSA (Rhizoctonia solani agglutinin), SNA-I and SNA-II (Sambucus nigra agglutinin I and II) have been shown to be toxic to aphids and caterpillars. In this project we investigated if these lectins are also toxic against larvae and a cell line of the red flour beetle, Tribolium castaneum, a model organism and important pest of stored products. Furthermore, we analyzed the stability of the lectins in the larval gut and used confocal microscopy to compare their efficiency in passing through the peritrophic matrix (PM). We observed that all three lectins were toxic against the T. castaneum cell line and their effectiveness in vitro was in decreasing order SNA-II>SNA-I>RSA with the respective EC50 being 0.1, 0.5 and 3.6 μg/ml. Larvae feeding for 16 day on diets containing 2% RSA, 2% SNA-II and 2% SNA-I weighed 0.14 ± 0.07 mg, 0.67 ± 0.44 mg and 1.89 ± 0.38 mg, corresponding to approximately 7%, 36% and 80% of control larvae, respectively. As a consequence, RSA increased the time to adult emergence by over 3-fold, SNA-II by 1.9-fold and SNA-I by 1.2-fold. RSA and SNA-II were stable in the larval gut, while SNA-I was digested and excreted with the feces. Finally, confocal microscopy confirmed that RSA passed through the PM more efficiently than SNA-II. In conclusion, our data suggest that the lectin ability to pass through the PM, governed by molecule dimensions, charge and size of PM pores, is one of the features that determine the toxicity of these insecticidal proteins.