Location: Molecular Plant Pathology
2013 Annual Report
Objective 2: Expand and refine the current gene-based phytoplasma classification system.
Objective 3: Establish a framework toward DNA barcoding of plant pathogenic mollicutes, a system for multilocus genotyping, strain description, and eventual formal molecular taxonomy of spiroplasmas and phytoplasmas.
2. Gained insight into the effect of phytoplasma infection on host gibberellin (GA) homeostasis. Many agriculturally important crops are vulnerable to infection by phytoplasmas, which significantly alter the physiology of the plants, which often exhibit symptoms indicative of hormonal disorder. We found that phytoplasma infection in tomato reduces internal levels of gibberellic acid, a naturally-occurring plant hormone, and found that the reduced level was due to suppression of key genes responsible for the hormone’s synthesis. We discovered that phytoplasma infection triggered de-sensitization of the GA biosynthesis negative feedback regulation. External application of the hormone at early stages of phytoplasma infection compensated for the hormonal loss and reduced severity of disease. The findings offer clues for devising novel practical approaches to control these bacterial diseases. 3. Elucidated the role of gibberellic acid in plant defense against a bacterium, phytoplasma. Based on our observation that phytoplasma infection causes disruption of GA homeostasis in tomato and that can be partially reversed by exogenous application of GA, we hypothesized that GA plays a role in plant defense. By examining expression profiles of genes involved in GA signaling, we discovered that GA promotes production of other hormones that work together to enhance the plant’s defense system. Following GA application and inoculation of plants, there was coordinated down-regulation of the GA signaling and growth repressor gene (GAI) and up-regulation of genes involved in salicylic acid synthesis (ICS1), signaling (NIM1), and downstream defense responses (PRP-1). We further found that the differential gene regulation was correlated with increased activities of defense-related enzymes ß-1,3-glucanase (GLU) and chitinase (CHI). The findings present new opportunities for understanding plant defenses against bacteria, through studies of the GA signaling network. 4. New concepts and theories on the evolutionary emergence, lineage radiation, and pathogenicity of phytoplasmas. Discovered genes and genomic features that distinguish phytoplasmas from other cell-wall less bacteria. Knowledge from previous analyses opened only a small window on the uniqueness of the phytoplasma genome and its functional implications. Following our earlier discovery of unique genome architecture, we pinpointed phytoplasma-unique genes and genes that are essential to a minimal free-living bacteria but absent in phytoplasma genomes. We elucidated metabolic pathways that are undergoing either lineage-specific acquisition or loss, and provided molecular evidence to support our concept of raising the phytoplasma clade to a taxonomic level of family. We presented new concepts and theories on mechanisms giving rise to genetic diversity and the changing landscape of host range and pathogenicity, opening unexpected possibilities for devising novel disease control measures having significance for and beyond phytoplasmas. 5. Completed characterization and molecular differentiation of phytoplasmas causing with disease of blueberry in New Jersey. Recently, the recurrence of blueberry stunt disease became evident in many farms in New Jersey. The phytoplasmas causing the disease had not been previously characterized. A survey, throughout New Jersey, identified two distinct phytoplasmas, one previously unknown, in diseased blueberries in New Jersey. The information and molecular tools developed have facilitated studies on investigation of insect vectors and disease diagnosis and spread. This work will benefit students, research scientists, diagnostic laboratories and extension personnel engaged in disease management, and government agencies that implement quarantine regulations to prevent disease spread domestically and internationally.
6. Completed characterization and classification of phytoplasmas causing diseases of potato in Russia. Potato purple top diseases are widespread in major potato growing regions in the world and cause significant economic damage in potato production, but the identities of the causal pathogens have not been definitively determined. Following seven years of research in several potato growing regions in Russia, we found that the diseases were caused by at least five different phytoplasma species. The disease of potato crops in Russia is due to a complex of diverse phytoplasmas, likely spread by different insect vector species. The findings provide needed new understanding and molecular markers for quarantine agencies to prevent introduction of the exotic phytoplasmas into new geographic regions including the U.S., and should aid in the formulation of strategies to minimize potato crop losses wherever the diseases occur.
7. Web-based customer service and outreach. We reconfigured the interactive iPhyClassifier program, MPPL’s publically accessible online tool for phytoplasma classification and taxonomy. Now, the dynamically drawn images superimpose the in silico calculated RFLP patterns of both sequence heterogeneous rRNA operons. We also expanded the underlying database. We added new information concerning grapevine diseases, which have become of worldwide concern. The online functions, assembled under MPPL’s Phytoplasma Resource Center, are used by scientists, students, professors, quarantine agencies, and diagnostics companies to aid their identification and classification of phytoplasmas worldwide.
Davis, R.E., Zhao, Y., Dally, E.L., Lee, I., Jomantiene, R., Douglas, S.M. 2013. 'Candidatus Phytoplasmas pruni', a novel taxon associated with X-disease of stone fruits, Prunus spp.: multilocus characterization based on 16S rRNA, secY, and ribosomal protein genes. International Journal of Systematic and Evolutionary Microbiology. 63:766-776.
Lee, I., Bottner-Parker, K.D., Zhao, Y., Bertaccini, A., Davis, R.E. 2012. Differentiation and classification of phytoplasmas in the pigeon pea witches'-broom group (16SrIX): an update based on multiple gene sequence analysis. International Journal of Systematic and Evolutionary Microbiology. 62:2279-2285.
Zhao, Y., Wei, W., Lee, I., Shao, J.Y., Suo, X., Davis, R.E. 2012. The iPhyClassifier, an interactive online tool for phytoplasma classification and taxonomic assignment. Methods in Molecular Biology. 938:329-338.
Bagadia, P.G., Polashock, J.J., Bottner-Parker, K.D., Zhao, Y., Davis, R.E., Lee, I. 2013. Characterization and molecular differentiation of 16SrI-E and 16SrIX-E phytoplasmas associated with blueberry stunt disease in New Jersey. Molecular and Cellular Probes. 27:90-97.
Ding, Y., Wu, W., Wei, W., Davis, R.E., Lee, I., Hammond, R., Sheng, J., Shen, L., Jiang, Y., Zhao, Y. 2013. Potato purple top phytoplasma-induced disruption of gibberellin homeostasis in tomato plants. Annals of Applied Biology. 162:131-139.
Ding, Y., Wei, W., Wu, W., Davis, R.E., Jiang, Y., Lee, I., Hammond, R., Sheng, J., Shen, L., Zhao, Y. 2013. Role of gibberellic acid in tomato defense response to potato purple top phytoplasma infection. Annals of Applied Biology. 162(2)191-199.
Nejat, N., Vadamalai, G., Davis, R.E., Harrison, N.A., Sijam, K., Dickinson, M., Abdullah, S., Zhao, Y. 2012. Candidatus Phytoplasma malaysianum, a novel taxon associated with virescence and phyllody of Madagascar periwinkle (Catharanthus roseus). International Journal of Systematic and Evolutionary Microbiology. 63:540-548.
Martini, M., Lee, I. 2012. PCR and RFLP analyses based on the ribosomal protein operon. In: Dickinson, M., Hodgetts, J., editors. Springer Protocols: Methods in Molecular Biology. New York, NY: Humana Press. p. 173-188.
Duduk, B., Paltrinieri, S., Lee, I., Bertaccini, A. 2012. Nested PCR and RFLP analysis based on the 16S rRNA gene. In: Dickinson, M., Hodgetts, J., editors. Springer Protocols: Methods in Molecular Biology. New York, NY: Humana Press. p. 159-172.