Genotype-dependent Regulation of Δ-endotoxin Reveals That Insect-resistant Cotton Contains Toxin Predominantly in Seeds

Engineering plant genomes for useful traits leads toward sustainable agriculture. Amongst useful traits, resistance against insects is developed by using cry proteins from Bacillus thuringiensis (Bt). The Bacillus thuringiensis is an endospore forming, aerobic and gram-positive bacterium found in soil having crystal (cry) proteins in the cytoplasm of its sporulating cells. The cry proteins are extremely toxic to some chewing and sucking insects belonging to orders Lepidoptera and dipteral (Soberon et al., 2016). Crossbred cotton varieties expressing BT toxins are being grown on more than 80% of the agricultural land in Pakistan, irrespective to formal approval by the statutory bodies. Cotton was engineered using cry1Ac gene to produce its own insecticide, limiting the insecticidal sprays to reduce burden on farmers’ budget. Unfortunately, farmers have to spray their fields to protect crop from insect attack, though number is almost half of the number of sprays on non-Bt-cotton varieties, because of variable accumulation of toxin in different genotypes throughout the growing season (Kranthi et al., 2005b; Olsen et al., 2005; Adamczyk et al., 2009; Bakhsh et al., 2010). As the crop growth and yield performance was much influenced by environmental factors, the abiotic factors could play important role in gene expression and thus performance of Bt cotton. The experimental evidences for the influence of abiotic factors on cry gene expression are meager compared to influence of biotic factors. Among these, effect of soil, nutrition, temperature, CO2 and various stresses are found to be important (Chen et al., 2005; Adamczyk and Sumerford, 2001). In addition to temperature stress, the age of the plant also plays important role in the expression of cry genes. Toxic accumulation levels vary with the age of plant as well (Kranthi et al., 2005a). Hence, it is assumed that the changed efficiency might be linked with high temperature. To make sure that resistance management designed for the use of Bt cotton successful, the estimation of the insecticidal toxic proteins expression by high temperature is required (Chen et al., 2005). Since cake is developed for animals from seeds therefore, it is imperative to quantify the toxin levels in seeds. Though CaMV 35S promoter directs constitutive expression of transgenes but varied expression levels may result from its interaction with environmental factors and physiological state Pak. J. Agri. Sci., Vol. 54(3), 627-634; 2017 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 DOI: DOI: 10.21162/PAKJAS/17.6382 http://www.pakjas.com.pk