Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/25/2012
Publication Date: 5/15/2012
Citation: Wallis, C.M., Chen, J. 2012. Zebra chip symptoms are associated with increased phenolic, pathogenesis-related protein, and amino acid levels. In: Workneh F. Rashed A. Rush CM (eds). Proceedings of the 11th annual SCRI zebra chip reporting session. Fredric Printing, Aurora, NY. p.159-162. Interpretive Summary: Zebra chip disease (ZC) is an emerging problem for potato growers throughout the United States and elsewhere. Symptoms of ZC include browning of cut potatoes slices that, upon frying, are unmarketable. This research examines whether changes in host physiology result in ZC symptom formation. Phenolic compounds, amino acids, and host defense-associated enzymes (such as polyphenol oxidases) were found present in greater amounts in ZC-symptomatic tubers than in those not exhibiting symptoms. Levels of these compounds were significantly correlated with symptom severity. The putative causal agent of ZC, 'Candidatus Liberibacter solanacearum', was found present within all ZC symptomatic tubers. Although titers of this bacteria were not correlated with ZC symptoms or changes in host chemistry, it is likely that this putative pathogen triggered defense responses throughout infected tubers. As part of the tuber defense response, increases in phenolic, amino acid, and defense enzyme levels occurred, resulting in ZC symptom formation.
Technical Abstract: Zebra chip disease (ZC) is an emerging problem for the potato industry as it causes undesirable symptoms such as increased browning of freshly-cut tubers and brown-striping of fried tuber slices. ZC is putatively caused by infection of the bacterium ‘Candidatus Liberibacter solanacearum’ (CLso). This project examined association of pathogen-induced changes in host physiology, especially in phenolic, amino acid, and defense-associated protein levels, with symptoms. To this end, potato tubers were obtained from a field where ZC was present, visually assessed for ZC disease, and mashed for chemical analysis. High performance-liquid chromatography of methanol extractions of mashed tuber tissue found that tubers with more severe ZC symptoms possessed greater levels of phenolic compounds. A gas chromatography-based amino acid extraction/ derivativization determined that tubers with more severe ZC symptoms also possessed greater levels of five individual amino acids. Finally, colorimetric assays determined that symptomatic tubers possessed greater levels of peroxidase, polyphenol oxidase, chitinase, and ß-1,3-glucanase. These results are consistent with pathogen infection of a plant. Observed increases in phenolics, amino acids, and defense-associated enzymes were significantly correlated with symptom severity ratings, but uncorrelated with bacterial titers. These results verify the hypothesis that increased phenolic and polyphenol oxidase levels result in increased browning of freshly cut tubers, and that increased amino acid levels result in increased browning when tuber slices are fried.