Abstract
Vip3A proteins are important for the control of spodopteran pests in crops, including Spodoptera frugiperda (fall armyworm). Native Vip3Ab1 controls S. frugiperda, but it is ineffective against S. eridania (southern armyworm), a major pest of soybean in South America. Recently, a Vip3Ab1 chimera with a modified C-terminus was described, Vip3Ab1-740, which has increased potency against S. eridania while maintaining activity against S. frugiperda. As S. frugiperda and S. eridania are differentially susceptible to Vip3Ab1, experiments were conducted to identify and understand the mechanism by which this expanded potency is conferred. The role of protein stability, processing, and in vivo effects of Vip3Ab1 and Vip3Ab1-740 in both of these species was investigated. Biochemical characterization of the midgut fluids of these two species indicated no obvious differences in the composition and activity of digestive enzymes, which protease inhibitor studies indicated were likely serine proteases. Histological examination demonstrated that both proteins cause midgut disruption in S. frugiperda, while only Vip3Ab1-740 affects S. eridania. Immunolocalization indicated that both proteins were present in the midgut of S. frugiperda, but only Vip3Ab1-740 was detected in the midgut of S. eridania. We conclude that the gain of toxicity of Vip3Ab1-740 to S. eridania is due to an increase in protein stability in the midgut, which was conferred by C-terminal modification.
Highlights
Vegetative insecticidal proteins (Vip) are produced by Bacillus thuringiensis (Bt) as soluble proteins during the vegetative phase of their life cycle, and are valued for their broad-spectrum activity against lepidopteran pests [1,2]
Vip3A genes encode insecticidal proteins with a mode of action that is different than Cry proteins and are currently used to provide protection from a broad spectrum of lepidopteran crop pests [14]
We have shown that modification of the C-terminus of Vip3Ab1 can confer lethal activity towards S. eridania, a major threat to South American soybean crops that is not controlled by native Vip3Ab1 [20,21]
Summary
Vegetative insecticidal proteins (Vip) are produced by Bacillus thuringiensis (Bt) as soluble proteins during the vegetative phase of their life cycle, and are valued for their broad-spectrum activity against lepidopteran pests [1,2]. Vip3A proteins are generally accepted to be pore-forming proteins bearing some similarity to the more described Cry family of insecticidal proteins. Vip3A proteins associate as tetramers requiring proteolytic processing prior to insecticidal pore-forming activity [3,4,5]. In vitro studies demonstrate that Vip3A proteins do not compete with Cry proteins for binding sites on brush border membrane vesicles (BBMV) and in vivo studies have shown Vip3A proteins maintain potent insecticidal activity against Cry-resistant insects [6,7,8,9,10,11]. Vip3A is especially important for control of S. frugiperda (fall armyworm, FAW), which has documented resistance to first generation transgenic crops containing the Bt crystal proteins, Cry1Fa and Cry1Ab [12,13,14,15,16,17]. Vip3A has differing levels of toxicity to various spodopteran insects; for example, native Vip3Ab1 has potent lethal activity on S. frugiperda, but has very little effect on S. eridania (southern armyworm, SAW)
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