Inhibition of baculoviral disease by plant-mediated peroxidase activity and free radical generation

The susceptibility of noctuid larvae to baculoviral infection is markedly affected by phytochemicals ingested during the acquisition of viral inoculum on foliage. We hypothesized that a major process causing phytochemical inhibition of viral disease is phenolic oxidation by phenolases, particularly peroxidase (POD), which subsequently generates free radicals. To test this hypothesis, we manipulated the chemical interactions in foliage of cotton, tomato, and lettuce by application of antioxidants, prooxidants, enriched extracts of phenolases, and/or phenolic substrates. Larvae of Heliothis virescens or Helicoverpa tea that received viral inoculum on treated foliage were less likely to die from viral infection the higher the POD activity of this foliage. Furthermore, the higher the POD activity, the more free radicals were generated in crushed foliage, and the more free radicals generated, the lower the incidence of viral disease. We present a series of reactions hypothesized to lead to inhibition of viral disease by free radicals, the generation of which is mediated, at least in part, by POD. Phenolic redox cycling catalyzed by POD involving clastogenesis (generation of H2O2) appeared to be a critical driver of phytochemical reactions leading to free radical generation and inhibition of baculoviral disease in their noctuid hosts. We also report application of an assay for the detection of free radicals by using methemoglobin as a new modification of this method for detecting radicals in plant foliage in the immediate aftermath of an oxidative burst.