Abstract
This study was conducted to investigate the effects of soil water deficit on insecticidal protein expression in boll shells of cotton transgenic for a Bt gene. In 2014, Bt cotton cultivars Sikang 1 (a conventional cultivar) and Sikang 3 (a hybrid cultivar) were planted in pots and five soil water content treatments were imposed at peak boll stage: 15% (G1), 35% (G2), 40% (G3), 60% (G4), and 75% field capacity (CK), respectively. Four treatments (G2, G3, G4, and CK) were repeated in 2015 in the field. Results showed that the insecticidal protein content of boll shells decreased with increasing water deficit. Compared with CK, boll shell insecticidal protein content decreased significantly when soil water content was below 60% of maximum water holding capacity for Sikang 1 and Sikang 3. However, increased Bt gene expression was observed when boll shell insecticidal protein content was significantly reduced. Activity assays of key enzymes in nitrogen metabolism showed that boll shell protease and peptidase increased but nitrogen reductase and glutamic-pyruvic transaminase (GPT) decreased. Insecticidal protein content exhibited significant positive correlation with nitrogen reductase and GPT activities; and significant negative correlation with protease and peptidase activities. These findings suggest that the decrease of insecticidal protein content associated with increasing water deficit was a net result of decreased synthesis and increased decomposition.
Highlights
Bollgard II R varieties containing the additional δ-endotoxin of Bacillus thuringiensis, Cry2A (b), have been employed in both Australia and the United States (Morse, 2016), the single toxin Bt-cotton (Bollgard I) has dominated domestic production in Asia and Africa (Huang et al, 2010; Clive, 2012)
Soil water deficit significantly affected the contents of insecticidal protein in the boll shells of two Bt cotton varieties (Figure 1)
Previous studies have shown that soil water deficit reduced the insecticidal protein content of Bt cotton
Summary
Bollgard II R varieties containing the additional δ-endotoxin of Bacillus thuringiensis, Cry2A (b), have been employed in both Australia and the United States (Morse, 2016), the single toxin Bt-cotton (Bollgard I) has dominated domestic production in Asia and Africa (Huang et al, 2010; Clive, 2012). Soil water deficit significantly affects insecticidal protein expression in the leaves of Bt cotton (Rochester, 2006; Parimala and Muthuchelian, 2010). Increased damage to Bt cotton by cotton worm in Shandong and Hebei provinces of China in 2005 and 2006 may have been due to lack of rain and a resulting soil water deficit from June to July (Liu et al, 2008). Carter et al (1997) and Benedict et al (1996) found that lack of rain resulted in soil water deficit and associated water stress reduced the content of total soluble protein and insecticidal protein in June and July. Drought stress could lead to DNA degradation in cotton seedling tissues, producing many residual DNA fragments that could inhibit the synthesis of functional proteins and structural proteins (Yang et al, 2016). In most of the world, drought is an important problem during the cotton whole growing period (Li et al, 2010)
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