Bacillus thuringiensis δ-Endotoxin Proteins Show a Correlation in Toxicity and Short Circuit Current Inhibition Against Helicoverpa zea

Abstract

Pesticidal activity of Bacillus thuringiensis delta-endotoxins, Cry1Aa, Cry1Ab, Cry1Ac, and Cry2A, was determined by using the force-feeding bioassay method to 4(th) instar larvae of Helicoverpa zea. H. zea was susceptible to Bt toxins in the order Cry1Ac > Cry1Ab > Cry1Aa > Cry2A with 63.60, 89.04, 159.65, and 375.78 ng/larvae respectively. The abilities of selected Bacillus thuringiensis toxins to inhibit short circuit current (I(SC)) in midgut epithelia of H. zea were also investigated by voltage clamp assay. The voltage-clamp studies were conducted on isolated midguts, measuring the inhibition of short circuit current (I(SC)) by activated toxin. A Cry1Aa toxin dilution of 33.3 and 500 ng/ml resulted in inhibition of I(SC) of -2.29 microA/min (lag time 15 min) and -4.48 microA/min (lag time, 2 min) respectively. The Cry1Ab dilution of 25 ng/ml inhibited I(SC) to -1.39 microA/min, a lag time of 14 min, and 333.3 ng/ml dilution resulted in decay of I(SC) -2.49 microA/min, lag time 1 min respectively. The Cry1Ac lower dilution 16.7 ng/ml inhibited I(SC) to -1.39 microA/min, lag time 4 min, and a high dilution 333.3 ng/ml decay I(SC) to -2.44 microA/min, lag time 1 min. The inhibition of I(SC) (-1.10 microA/min, lag time 25) at lower dilution (33.3 ng/ml) and high dilution (500 ng/ml), decay (-2.38 microA/min, lag time 5 min), showed a correlation between toxin concentration and inhibitory response with Cry2A toxin. The lag time decreased with increasing concentration of toxin applied, which is additional evidence of dose response besides direct correlation of toxicity assays and I(SC).

A Novel Secreted Protein Toxin from the Insect Pathogenic Bacterium Xenorhabdus nematophila

“…1). The measured molecular weight (41,485) was within instrumental error (0.05%) of the calculated molecular weight (41,502). Both the whole protein and tryptic digest spectra suggested that the N-terminal methionine residue of the recombinant protein had not been processed.…”

“…3B). The dose of A24tox required to kill insects (30 -40 ng/g of larvae) is of the same order of magnitude as that of other insecticidal toxins, such as the Tc toxins from Photorhabdus against M. sexta larvae (18), and the insecticidal crystal proteins of B. thuringiensis against Helicoverpa zea (41) and Leptinotarsa decemlineata (42). Injection or ingestion of the Tca complex of P. luminescens strain W14 by M. sexta larvae causes damage to the midgut cells, resulting in a shedding of the midgut epithelium into the gut lumen followed by lysis of the epithelium (43).…”

Expression and inheritance of chimeric cry2AX1 gene in transgenic cotton plants generated through somatic embryogenesis

“…Basri (2005) also observed instability in expression of the bar gene in T 1 , T 2 , and T 3 transgenic wheat plants. Karim et al (2000) studied pesticidal activity of Bt endotoxins Cry1Aa, Cry1Ab, Cry1Ac, and Cry2A against fourth instar larvae of Helicoverpa zea and found that H. zea was susceptible to Bt toxins in the order Cry1Ac, Cry1Ab, Cry1Aa, and Cry2A with 63.60, 89.04, 159.65, and 375.78 ng/larvae, respectively. A high concentration of Cry2A protein was needed to achieve complete insect mortality when compared to Cry1A proteins.…”

Genetic engineering of cotton with a novel cry2AX1 gene to impart insect resistance against Helicoverpa armigera

“…The Cry2Aa protein is known to be toxic to prominent lepidopteran insect pests like rice stem borers and brinjal fruit and shoot borer (Maqbool et al 1998, Rao et al 1999. Pesticidal activity of B. thuringiensis δ-endotoxins, Cry1Aa, Cry1Ab, Cry1Ac, and Cry2A against Helicoverpa zea was determined by Karim et al (2000). H. zea was susceptible to Bt toxins in the order Cry1Ac,Cry1Ab,Cry1Aa,and Cry2A with 63.60,89.04,159.65,and 375.78 ng/larvae, respectively.…”