Plant defenses: Chlorogenic acid and polyphenol oxidase enhance toxicity of Bacillus thuringiensis subsp. kurstaki to Heliothis zea

Two chemicals implicated in resistance of the tomato plant, chlorogenic acid and polyphenol oxidase, are known to form orthoquinones in damaged plant tissue. Orthoquinones have been reported to alkylate -NH₂ and -SH groups of proteins and amino acids, altering solubility, digestibility, and, for some pathogenic viruses, infectivity. Here we explore effects of quinone alkylation on toxicity of an important microbial insecticide, Bacillus thuringiensis subsp. kurstaki (BTk), to larval Heliothis zea. BTk incubated with these phytochemicals and fed to larvae was more toxic than untreated BTk. Similar but less dramatic results arose when BTk was incubated with polyphenol oxidase alone. Digestibility experiments suggest that alkylation enhanced the solubilization and/or proteolysis of crystal protein in vivo. Implications of our results for compatibility of BTk with host-plant resistance and biological control are discussed.