Bt protein expression in the transgenic insect-resistant cotton in China
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According to statistics of International Service for the Acquisition of Agri-biotech Applications (ISAAA), 18 million farmers in 28 countries planted more than 181 million hectares of genetically modified (GM) crops in 2014, at an annual growth rate of between 3 %–4 %, with 6.3 million ha more than 2013 . Among these counties, China ranks the sixth with 3.9 million ha. Globally, 27 kinds of transgenic crops have been approved. The four main food groups for which GM varieties are cultivated and the proportion of each worldwide that consists of genetically modified strains are soybean, cotton, maize and rapeseed . Among these crops, transgenic cotton has been one of the most rapidly adopted GM crops in the world, which expressed insecticidal proteins from Bacillus thuringiensis (Bt). Especially, more than 90 % of cotton were transgenic, containing Cry gene(s) such as Cry1Ac, Cry1Ac + Cry2Ab or Cry1Ac + Cry1AF. Extensive planting of Bt cotton efficiently controlled target pests either in the USA or in China and is highly beneficial to the environment by reducing chemical insecticide sprays and preserving population of beneficial arthropods. In recent years, Cry genes were transformed into maize , rice , and other species.
It is very important that the Bt toxin protein be sustainably expressed in adequate quantities in plant parts at the requisite time of the whole season to afford protection against target insect pests. We screened Bt protein expression in different tissues at whole growth stage of 32 transgenic pest-resistant varieties using enzyme-linked immunosorbent assay (ELISA) (Fig. S1). Results showed that Cry1Ac protein underwent rapid degradation in the early stage, followed by a slow decline in the later stage. Among the four tissues, the contents of Cry1Ac protein in leaves were the highest, followed by the buds, and the bolls were the lowest. The expression of Cry1Ac in leaves and buds gradually declined with cotton growth, while those in flowers and bolls were relatively constant. It indicated that the levels of Bt protein in cotton tissues fluctuate during the whole growing season [5, 6] Some studies concluded that the overexpression of bt gene at earlier stages led to gene regulation at the post-transcription level and contributed to the consequent gene silencing [7, 8]. Moreover, Xia et al. reported lower expression level of bt gene at late stage correlated with changes in the methylation state of the 35S promoter region. Although the amount of the insecticidal protein in Bt-cotton tissues was considerable reduced at later stage, the toxin level did not fall below the critical level, and still maintains relative high efficacy against insect pest . In this study, the efficacy against insect pest of these varieties also maintained high level, indicated by less Helicoverpa armigera survived after fed by leaves, buds, flowers and young bolls of experimental varieties (unpublished data).
Most of varieties had similar Bt protein levels, with only a few showed higher expression. In recent years, some farmers found that the cotton transferred bt could not well resisted the cotton bollworm, Heliothis armigera. In some districts of Shandong, insects overflow was found even in the fields of transgenic cotton. As the Bt levels did not decline rapidly with time (Fig. S4), the possible reasons that account for the above phenomenon may include the increasing resistance of target pests [10, 11], purity of seed , and environmental factors .
In China, nearly 90 % of the transgenic pest-resistant cotton were bred by crossing between conventional varieties, which caused relatively low genetic diversity, narrow hereditary basis and high similarities in genetic background. To breed new pest-resistant cotton, a more promising method is to transfer bt gene into non-transgenic variety using genetic engineering technique, rather than crossing conventional varieties and authorized bt-transgenic varieties. Meanwhile, understanding the underlying mechanisms that regulate the bt gene expression may also contribute to the successful breeding of pest-resistant cotton in China.
Conflict of interest
The authors declare that they have no conflict of interest.
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