Pyramiding the Delta-endotoxin from Bacillus thuringiensis with plant resistance traits in cotton

Abstract

Insects are likely to develop resistance to Bacillus thuringiensis spp. kurstaki CrylA insecticidal proteins expressed in cotton (Gossypium hirsutum L.) unless strategies are implemented to counter pest adaptation. Three studies were undertaken to evaluate the potential for pyramiding the CrylA protein with other plant insect-resistance traits to increase insect resistance and improve the durability of cotton expressing a crylA gene. First, the expression of two foreign crylA genes was characterized in insect-resistant cotton lines derived in three backgrounds. Next, the effects of pyramiding the CrylA(b) protein with the high-terpenoid plant insect-resistance trait on tobacco budworm [Heliothis virescens (F.)] growth, survival, and feeding behavior were examined in field and laboratory no-choice and free-choice experiments. Finally, the effects of pyramiding the CrylA(b) protein with other antibiotic or antixenotic plant insect-resistance traits on cotton injury and yield were examined in free-choice field experiments in two environments. CrylA gene expression was variable and influenced by genetic and environmental factors. Variation from site of insertion and cotton background effects may provide significant opportunity for increasing crylA gene expression using traditional breeding techniques. Plant-to-plant variation from epistatic and/or somaclonal effects caused CrylA protein concentration to behave as a quantitative trait. The strong influence of environmental factors on crylA gene expression shows the importance of controlling environmental variation in experiments designed to evaluate crylA expression or plant insect-resistance. Pyramiding the CrylA(b) protein with the high-terpenoid plant insect-resistance trait injury compared to using the CrylA(b) protein alone. Pyramiding the CrylA(b) protein with other antibiotic or antixenotic plant insect-resistance traits effectively reduced budworm-bollworm injury to flower buds and bolls compared to using the CrylA(b) protein alone. The glabrous, nectariless, and high-terpenoid traits were useful pyramiding characters. These results demonstrate that gene pyramiding can be an effective strategy for improving the insect resistance and durability of cotton producing the CrylA(b) protein.