|Williams, William - Paul|
Submitted to: PLoS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2008
Publication Date: 3/12/2008
Citation: Mohan, S., Ma, P., Williams, W.P., Luthe, D.L. 2008. A naturally occurring plant cysteine protease possesses remarkable toxicity against insect pests and synergizes Bacillus thuringiensis. PLoS One. Available: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0001786.
Interpretive Summary: Fall armyworm and southwestern corn borer are major insect pests of corn in the South. USDA-ARS scientists at Mississippi State University identified, developed, and released corn germplasm with resistance to these insects. A gene encoding a defensive protein associated with resistance was discovered in this germplasm. The protein, a cysteine proteinase, accumulates at the feeding site. When insects ingest the protein, the peritrophic membrane in the midgut is degraded. Laboratory tests using the purified protein indicated the toxicity to corn earworm, tobacco budworm, fall armyworm, and southwestern corn borer is similar to that of the Bacillus thuringiensis toxin in Bt-CryIIA. In combination these toxins could improve insect control and reduce the chances of a buildup of resistance to Bt in insect populations.
Technical Abstract: When caterpillars feed on maize (Zea maize L.) lines with native resistance to several Lepidopteran pests, a defensive systeine protease, Mir1-CP, rapidly accumulates at the wound site. Mir1-CP has been shown to inhibit caterpillar growth in vivo by attacking and permeabilizing the insect’s peritrophic matric (PM), a structure that surrounds the food bolus, assists in digestion and protects the midgut from microbes and toxins. PM permeabilization weakens the caterpillar defenses by facilitating the movement of other insecticidal proteins in the diet to the midgut mocrovilli and thereby enhancing their toxicity. To directly determine the toxicity of Mir1-CP, the purified recombinant enzyme was directly tested against four economically significant Lepidopteran pests in bioassays. Mir1-CP LC50 values were 1.8, 3.6, 0.6, and 8.0 ppm for corn earworm, tobacco budworm, fall armyworm and southwestern corn borer, respectively. These values were the same order of magnitude as those determined for the Bacillus thuringiensis toxin Bt-CryIIA. In addition to being directly toxic to the larvae, 60 ppb Mir1-CP synergized sublethal concentrations of Bt-CryIIA in all four species. Permeabilization of the PM by Mir1-CP probably provides ready access to Bt-binding sites on the midgut microvilli and increases its activity. Consequently, Mir1-CP could be used for controlling caterpillar pests in maize using non-transgenic approaches and potentially could be used in other crops either singly or in combination with Bt-toxins.