Abstract
We assessed the effectiveness of a biofortified maize line (4BtxHC) which accumulates high levels of antioxidant carotenoids that also expressed the insecticidal Cry1Ac Bacillus thuringiensis (Bt) gene against the European corn borer Ostrinia nubilalis. This line had been previously engineered to accumulate carotenoids specifically in the seed endosperm, whereas the Bt gene was expressed constitutively. The concentrations of Bt toxin (Cry 1Ac) in the leaves of the 4Bt and 4BtxHC lines were not significantly different at 47±6 μg/g of fresh weight (FW); neither were they in the kernels of both lines (35±3 μg/g FW). The kernels and leaves were toxic to the larvae of O. nubilalis. However, the insecticidal activity was substantially lower (ca. 20%) than that of lines that expressed only Bt in spite that the two lines showed a quantity of toxin not significantly different in kernels or in leaves. Although the reduced effectiveness of Cry1Ac in kernels may not be entirely surprising, the observation of the same phenomenon in vegetative tissues was unexpected. When semi-artificial diets containing kernels from 4Bt supplemented with different levels of β-carotene were used in insect bioassays, the β-carotene moderated the effectiveness of the Bt similarly to the plant material with carotenoid enrichment. To elucidate the biochemical basis of the reduced effectiveness of Bt toxin in the carotenoid-enriched plants, we measured the activity of three enzymes known to be implicated in the detoxification defence, namely, catalase, superoxide dismutase and glutathione S-transferase. Whereas Cry1Ac expression significantly increased SOD and CAT enzymatic activity in the absence of carotenoids, carotenoids, either in 4BtxHC or in artificial diets enriched with β-carotene, significantly lowered CAT activity. Carotenoids can therefore moderate the susceptibility of the maize borer O. nubilalis to Cry1Ac, and we hypothesize that their role as antioxidants could explain this phenomenon via their scavenging of reactive oxygen species produced during Cry1Ac detoxification in the larvae. The involvement of this mechanism in the decreased mortality caused by Cry1Ac when carotenoids are present in the diet is discussed.
Highlights
Bt crops engineered to express the insecticidal Cry protein(s) of the entomopathogenic bacterium Bacillus thuringiensis (Bt) have been grown commercially since 1996
Β-Carotene content in kernel-based diets β-Carotene was not present in the M37W and 4Bt diets; in the 4Bt+60 diet, β-carotene accumulated at 15.8 ± 0.16 μg/g dry weight (DW), whereas the high-carotenoid content trait (HC), 4BtxHC and 4Bt+0.6 diets had similar β-carotene contents (1.09 ±0.06 μg/g DW). β-Carotene was present at similar levels in the M37W+6 and 4Bt+6 diets (3.02 ± 0.17 μg/g DW)
Through our transgenic 4Bt plants, we have shown that maize plants expressing cry1Ac, with Cry1Ac toxin at a concentration of 50 μg/g fresh weight (FW), are as effective as maize plants expressing cry1Ab in controlling young larvae of O. nubilalis
Summary
Bt crops engineered to express the insecticidal Cry protein(s) of the entomopathogenic bacterium Bacillus thuringiensis (Bt) have been grown commercially since 1996. Whereas most commercialized biotech crops exhibit enhanced agronomic traits such as insect resistance and/or herbicide tolerance, many new biofortified crops with enhanced nutritional traits are just under development or nearing commercialization. Metabolic engineering is used to produce one or more nutritionally important molecules such as vitamins, antioxidants or health-promoting fatty acids. Little is known of the effects of biofortified crops on herbivorous insects, and few studies have been devoted to known how carotenoid enhancement in host plants may affect herbivore survival, development, and behaviour. The oxidative cleavage of carotenoid substrates produces volatile apocarotenoids, which are important compounds in herbivore-plant communication, since they can be either strongly attractive or repellent to insects [5]. It is important to investigate how plants with enhanced nutritional traits such as increased carotenoids behave in terms of susceptibility to feeding, as herbivores may change their susceptibility to pest control agents on this basis
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