Location: Molecular Plant Pathology Laboratory2014 Annual Report
Objective 1: Identify changes in host gene expression and small RNA-mediated regulation associated with viroid and bacterial infection and disease development as potential targets for disease management. Objective 2: Identify key metabolites that are involved in the early stages of pathogenesis and may have global effects on disease resistance through either their bioactive nature or effect on redox-status. Sub-objective 2.A. Identify and quantify secondary metabolites induced upon infection of tomato with either P. syringae or potato spindle tuber viroid (PSTVd). Sub-objective 2.B. Determine bioactivity of secondary metabolites induced upon infection of tomato with either P. syringae or PSTVd. Sub-objective 2.C. Determine the effect on redox status of secondary metabolites induced upon infection of tomato with either P. syringae or PSTVd. Objective 3: Identify the molecular signals and pathways used by viroids to move through the cytoplasm, enter the nucleus or chloroplast of the host cell, and begin replication. Sub-objective 3.A. Determine the role of host protein 4/1 (and other proteins interacting with 4/1) in the intra- and intercellular movement of PSTVd. Sub-objective 3.B. Use sequence motifs derived from Eggplant latent viroid (ELVd) to redirect mRNAs encoding enzymes involved in terpenoid biosynthesis into the chloroplast.
This project seeks to elucidate specific signaling mechanisms that are involved in plant disease resistance. Plants respond to biotic stress by the integration of responses located in two different cellular compartments, the symplast and the apoplast. Improvement of existing resistance to plant disease requires a more comprehensive understanding of key apoplastic and symplastic responses to pathogen invasion and how they are integrated. In the apoplast, we will examine the complex interplay between secondary metabolites and redox signaling that controls early events in bacterial and viroid pathogenesis as well as other responses triggered by long distance signaling. In the symplast, we will examine the molecular interactions between viroids and their hosts to determine how these small RNA molecules replicate and move between organelles within a single cell or over long distances between cells. The long-term objective of this multidisciplinary project is to understand the role of plant-pathogen signaling in disease resistance in sufficient detail that novel strategies can be developed to render plants resistant/immune to pathogen infection. In the intermediate term, we will test the ability of certain viroid-derived targeting signals to redirect mRNAs to the chloroplast, thereby adding novel biosynthetic capabilities to chloroplast metabolism with the need for chloroplast genome transformation.
The early events that take place when a bacterial pathogen first comes in contact with its host have been examined. This work continues to support the finding that certain pathogens induce the production of secondary metabolites by the plant. These metabolites can have a bioactive affect on the host/pathogen interaction. The metabolites and the pathogens they interact with vary with the plant species and may influence resistance and susceptibility. Recent work at ARS has demonstrated for the first time that these metabolites can accumulate at the site of pathogen ingress and be nearly instantly converted to toxic intermediates that actively stop pathogen development. One leaf metabolite has been identified that appears responsible for the major share of this response.
1. Antimicrobial resistance continues to be a high priority issue in agriculture and the routine use of antibiotics against bacterial plant pathogens is likely to be eliminated as it has in animals. ARS researchers have discovered a new defense mechanism that plants appear to use against bacteria. The mechanism relies on secondary metabolites that are produced by the plant upon induction by the pathogen. These extracellular metabolites are converted to highly toxic intermediates by an ‘oxidative burst’ that occurs in resistant interactions. Not all bacteria are affected by this event, but those that are become unable to multiply. Further examination has shown that other secondary metabolites are also active and affect different bacterial species. These data and findings will be used by researchers in developing “antibiotic-free” bacterial-resistant plants in the future.
Averyanov, A.A., Zakharenkova, T.S., Lapikova, V.P., Pasechnik, T.D., Gaivoronskaya, L.M., Baker, C.J. 2013. Exogenous superoxide dismutase may lose its antidotal ability on rice leaves. Russian Journal of Plant Physiology. 60:270-278.
Baker, C.J., Mock, N.M., Whitaker, B.D., Hammond, R., Roberts, D.P., Nemchinov, L.G., Aver'Yanov, A.A. 2014. Characterization of apoplast phenolics: Invitro oxidation of acetosyringone results in a rapid prolonged increase in the redox potential. Physiological and Molecular Plant Pathology. 86:57-63.