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Research Project: PHYSIOLOGICAL AND MOLECULAR SIGNALING IN VIROID AND BACTERIAL DISEASE

Location: Molecular Plant Pathology Laboratory

2017 Annual Report


Objectives
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.


Approach
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.


Progress Report
This is the final report for project 8042-22000-276-00D, “Physiological and Molecular Signaling in Viroid and Bacterial Disease." This project sought to elucidate specific signaling mechanisms that are involved in plant disease resistance. Plants respond to biotic stress by the integration of responses from two different cellular compartments, the symplast within the cell and the apoplast which surrounds it. During bacterial attack via leaves, the results of these responses, whether molecular or micro-environmental, are played out in the apoplast. We have developed several in planta techniques to elucidate how these two compartments, apoplast and symplast, work together to counter various bacterial interactions including saprophytic, disease susceptibility, or disease resistance. It was demonstrated for the first time that one of the earliest plant responses is the induction of bioactive redox sensitive metabolites from the symplast which accumulate in the apoplast. These metabolites were found to be capable of affecting the redox environment, inhibiting microbial growth, and having bioactive effects on the interactions. Preliminary evidence indicates that this same response occurs in other plants although the specific metabolites may vary. The results provide new insight into designing both biotechnical and non- -toxic chemical control of plant disease.


Accomplishments
1. A simpler and more sensitive technique to detect changes in chlorogenic acid (CGA) in plants, a metabolite found in most plants and has been linked to disease resistance. Chlorogenic acid (CGA) is a major metabolite found in most plants and is stored within the plant cell. ARS scientists at Beltsville, Maryland, developed a leaf tissue assay to determine the concentration of CGA. The results of this research demonstrated that the CGA concentration of the tissue did not change due to bacterial infection but relocated from the plant cell into the apoplast, where the bacteria were located. The increase of CGA could protect bacteria from the high oxidative environment, which is the normal host response to infection.


Review Publications
Aver'Yanov, A.A., Pasechnik, T.D., Lapikova, V.P., Romanova, T.S., Baker, C.J. 2017. Systemic reduction of rice blast by means of photosensitizers. Frontiers in Plant Science. 64:543-552. 10.1134/s1021443717030037.
Baker, C.J., Mock, N.M., Smith, J.M., Aver'Yanov, A.A. 2017. A simplified technique to detect variations of leaf chlorogenic acid levels between and within plants caused by maturation or biological stress. Physiological and Molecular Plant Pathology. 98:97-103. doi: 10/1016/j.pmpp.2017.04.003.