A western corn rootworm cadherin-like protein is not involved in the binding and toxicity of Cry34/35Ab1 and Cry3Aa Bacillus thuringiensis proteins

The western corn rootworm (WCR) Diabrotica virgifera virgifera LeConte is an important insect pest of corn.  Bacillus thuringiensis (Bt) insecticidal proteins Cry3Aa (as mCry3A)  and Cry34Ab1/Cry35Ab1 have been expressed in transgenic corn and are used to control the insect in the U.S. To date, there is limited information regarding the target sites for these proteins. In this study, a cadherin-like gene associated with the WCR larval midgut tissue was tested (GenBank accession # EF531715). Experiments were designed : (1) to examine the sensitivity of WCR to Cry34Ab1/35Ab1 and Cry3Aa after silencing the expression of the candidate receptor protein, cadherin, through RNA interference on overlay artificial diet bioassay, and (2) to determine whether Bt proteins bind to recombinant rootworm cadherin protein using ligand blots. The mortality and growth inhibition of WCR neonates exposed to cadherin dsRNA for 2 days followed by either Cry3Aa or Cry34Ab1/Cry35Ab1 for 4 days were comparable to the results observed in insect exposure to either Cry3Aa or Cry34Ab1/Cry35Ab1 treatment alone, even though quantitative real-time PCR confirmed the dsRNA knockdown of cadherin expression in the larvae. In addition, ligand blots of recombinant cadherin protein probed with Cry proteins did not reveal binding with the rootworm-active Cry proteins. These data indicate that the WCR cadherin is unlikely to be involved in the toxicity and binding of Cry34/35Ab1 and Cry3Aa.