Effects of Soil Salinity on the Expression of Bt Toxin (Cry1Ac) and the Control Efficiency of Helicoverpa armigera in Field-Grown Transgenic Bt Cotton

Abstract

An increasing area of transgenic Bacillus thuringiensis (Bt) cotton is being planted in saline-alkaline soil in China. The Bt protein level in transgenic cotton plants and its control efficiency can be affected by abiotic stress, including high temperature, water deficiency and other factors. However, how soil salinity affects the expression of Bt protein, thus influencing the control efficiency of Bt cotton against the cotton bollworm (CBW) Helicoverpa armigera (Hübner) in the field, is poorly understood. Our objective in the present study was to investigate the effects of soil salinity on the expression of Bt toxin (Cry1Ac) and the control efficiency of Helicoverpa armigera in field-grown transgenic Bt cotton using three natural saline levels (1.15 dS m-1 [low soil-salinity], 6.00 dS m-1 [medium soil-salinity] and 11.46 dS m-1 [high soil-salinity]). We found that the Bt protein content in the transgenic Bt cotton leaves and the insecticidal activity of Bt cotton against CBW decreased with the increasing soil salinity in laboratory experiments during the growing season. The Bt protein content of Bt cotton leaves in the laboratory were negatively correlated with the salinity level. The CBW populations were highest on the Bt cotton grown in medium-salinity soil instead of the high-salinity soil in field conditions. A possible mechanism may be that the relatively high-salinity soil changed the plant nutritional quality or other plant defensive traits. The results from this study may help to identify more appropriate practices to control CBW in Bt cotton fields with different soil salinity levels.

Fig 1. Measurement of the soil salt content in fields of Bt cotton and non-Bt cotton in 2013 and 2014.

SS, seedling stage; BS, budding stage; FBS, flowering and bolling stage. Lowercase letters above the bars indicate significant differences among the salinity treatments (p < 0.05).

Fig 2. Effect of soil salinity on the number of CBW eggs on Bt cotton plants in 2013 and 2014.

2nd, period of second-generation CBW occurrence; 3rd, period of third-generation CBW occurrence; 4th, period of fourth-generation CBW occurrence. Lowercase letters above the bars indicate significant differences among the salinity treatments (p < 0.05).

Fig 3. Effect of soil salinity on the number of CBW larvae on Bt cotton plants in 2013 and 2014.

2nd, period of second-generation CBW occurrence; 3rd, period of third-generation CBW occurrence; 4th, period of fourth-generation CBW occurrence. Lowercase letters above the bars indicate significant differences among the salinity treatments (p < 0.05).

Fig 4. Corrected mortality of larvae on the leaves of Bt cotton plants at three developmental stages.

SS, seedling stage; BS, budding stage; FBS, flowering and bolling stage. Lowercase letters above the bars indicate significant differences among the salinity treatments (p < 0.05).

Fig 5. Bt protein content in Bt cotton leaves at three developmental stages.

SS, seedling stage; BS, budding stage; FBS, flowering and bolling stage. Lowercase letters above the bars indicate significant differences among the salinity treatments (p < 0.05).