Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/24/2013
Publication Date: 11/5/2013
Citation: Hijaz, F.M., Manthey, J.A., Folimonova, S.Y., Davis, C.L., Jones, S.E., Reyes-De-Corcuera, J.R. 2013. An HPLC-MS Characterization of the Changes in Sweet Orange Leaf Metabolite Profile following Infection by the Bacterial Pathogen Canditatus Liberibacter asiaticus . PLoS One. DOI: 10.1371/journal.pone.0079485. Interpretive Summary: The citrus industry is beset with a devastating disease (Huanglongbing or HLB). The time courses of the appearance of the HLB-causing bacterium throughout graft-inoculated Valencia and Hamlin orange seedlings were studied. Visual symptoms of the HLB disease did not appear before 20 weeks, but were notably symptomatic at 30 weeks post inoculation. During this transition notable changes occurred in the chemical profiles of the HLB affected leaves. The main classes of compounds involved in these transitions were identified.
Technical Abstract: Huanglonbing1 (HLB) presumably caused by Canditatus Liberibacter asiaticus (Clas) threatens the commercial U.S. citrus crop of an annual value of $3 billion. The earliest significant differences between the metabolomes of leaves from greenhouse-grown sweet orange trees infected with Clas and of healthy leaves were characterized by HPLC-MS concurrently with measurements of the levels of Clas bacteria and visual symptoms in planta. Twenty 8-month-old ‘Valencia’ and ‘Hamlin’ trees were grafted with budwood from PCR-positive HLB source trees. Five graft-inoculated trees of each variety and three control trees were sampled biweekly and analyzed by HPLC-MS and PCR. Thirteen weeks after inoculation, Clas was detected in newly growing flushes in 33% and 55% of the inoculated ‘Hamlin’ and ‘Valencia’ trees respectively. Inoculated trees remained visually asymptomatic in the first 20 weeks, but were symptomatic 30 weeks after grafting. No differences in the leaf metabolomes were detected in Clas-infected trees 24 weeks after inoculation. However, 28 weeks after inoculation, metabolomic differences between control leaves and those of Clas-infected trees were evident. Compounds involved in these differences were identified with authentic standards or structurally classified by their UV and mass spectra. Included among these compounds are flavonoid glycosides, polymethoxylated flavones, and hydroxycinnamates. Four structurally related hydroxycinnamate compounds increased more than 10-fold in the ‘Hamlin’ and ‘Valencia’ sweet orange leaves in response to Clas infection. Possible roles of these hydroxycinnamates as plant defense compounds against the Clas bacterium are discussed.