Limited impact of elevated levels of polyphenol oxidase on tree-feeding caterpillars: assessing individual plant defenses with transgenic poplar

Abstract

Polyphenol oxidase (PPO) is commonly believed to function as an effective antiherbivore defense in plants. PPO is induced in plants following herbivory, and insect performance is often negatively correlated with PPO levels. However, induced defenses create numerous changes in plants, and very little work has been done to test the direct effects of PPO on insect herbivores separately from other changes. This study examined the impacts of high levels of PPO on the performance of two species of tree-feeding caterpillars (Lymantria dispar and Orgyia leucostigma) on poplar. Transgenic PPO-overexpressing poplar (Populus tremula x Populus alba) was used as a source of elevated-PPO leaves, thereby controlling for the multiple effects of induction. In addition, the impacts of treating poplar foliage with high levels of purified mushroom PPO were examined on the two caterpillar species. Contrary to expectation, in several cases increased PPO levels had no significant effect on insect consumption or growth rates. Although one of the mechanisms by which PPO is believed to impact herbivores is via increased oxidative stress, the ingestion of large amounts of PPO had little or no effect on semiquinone radical and oxidized protein levels in the gut contents of lymantriid caterpillars. PPO activity in caterpillars is likely limited by the low oxygen and high ascorbate levels commonly found in their gut contents. This study questions whether induced PPO functions as an effective post-ingestive defense against tree-feeding caterpillars, and indicates that controlled, mechanistic studies are needed in other plant-herbivore systems to test for a direct effect of PPO on insect performance.

Evaluating Ascorbate Oxidase as a Plant Defense Against Leaf-Chewing Insects Using Transgenic Poplar

“…The lack of effect of AO on ascorbate levels in the grasshopper M. sanguinipes suggests that AO activity is also limited in the guts of insects with acidic pH. As in previous work on polyphenol oxidase-overexpressing poplars (Barbehenn et al 2007), we conclude that low oxygen levels appear to limit the effectiveness of these oxygen-dependant enzymes as postingestive defenses against a variety of leaf-chewing insects.…”

The polyphenol oxidase gene family in poplar: phylogeny, differential expression and identification of a novel, vacuolar isoform

“…Similarly, forest tent caterpillar (Malacosoma disstria) larvae reared on transgenic hybrid aspen foliage with enhanced PPO activity also showed decreased growth rates, but only when larvae were hatched from old egg masses (Wang and Constabel 2004a). However, additional studies in the same PPO overexpressing hybrid aspen lines with lymantriid lepidopterans gypsy moth (Lymantria dispar) and white-marked tussock (Orgyia leucostigma) failed to show a clear correlation between increased levels of PPO and insect performance (Barbehenn et al 2007). Therefore, the eYcacy of PPO as a defensive protein may depend on the speciWc plant-insect interaction and other as yet unknown factors.…”

Characterization of rice polyphenol oxidase promoter in transgenic Arabidopsis thaliana

“…In some plant species PPO activity is strongly induced by insect attacks (Ruuhola et al, 2008). Using modified PPO expression in transgenic plants, a defensive role was established against insects (Barbehenn et al, 2007;Mahanil et al, 2008;Bhonwong et al, 2009) and pest infestations (Wang and Constabel, 2004;Richter et al, 2012;Chai et al, 2013). As anticipated, the downregulation or silencing of PPOs resulted in enhanced susceptibility to pathogens in transgenic tomato and potato lines (Thipyapong et al, 2004;Richter et al, 2012).…”