Project Number: 1275-22000-193-00
Project Type: Appropriated

Start Date: Jul 15, 2002
End Date: Jul 14, 2007

Objective:
Viroids are parasites of higher plants that are far smaller and simpler than conventional plant viruses. The existence of these small, highly structured, circular RNA molecules lacking the protective protein capsid and mRNA activity characteristic of viral RNAs.raises several fundamental questions: What molecular signals do viroids possess that induce host enzymes to accept them as templates and permit their autonomous replication? At the molecular level, how do viroids interact with their hosts to induce disease? How do these small unencapsidated RNAs move from cell to cell? Previous mutational analyses of potato spindle tuber and related viroids have allowed us to genetically isolate and dissect the normally interrelated processes of replication, cell-to-cell movement, and disease induction. During the next five years, we will use this information to: 1. Identify molecular interactions mediating viroid movement in the host vascular system; 2. Determine the role of a viroid-induced protein kinase in the disease process; 3. Identify structural features responsible viroid entry into the chloroplast; and 4. Develop and test new strategies to increase citrus production efficiency.

Approach:
We will use two complementary approaches to examine the role of the phloem lectin in mediating long distance viroid movement: first, UV- and/or formaldehyde-mediated cross-linking to detect putative viroid-lectin complexes in vivo; second, a functional assay which will test the lectin's ability to restore systemic movement to a TMV mutant lacking a coat protein. To examine the role of PKV and other protein kinases in the host response to viroid infection, the yeast two-hybrid interaction and complementation assays will be used to identify host regulatory proteins involved in viroid pathogenesis. Once putative regulatory proteins are identified, physiological factors controlling their expression will be determined. To identify the sequence and/or structural elements that direct avocado sunblotch viroid to the chloroplast, we will test their ability to restore gene expression from tobacco mosaic and potato virus X gene vectors in which gene expression is dependent upon RNA editing and/or mRNA processing. Translation of cellular mRNAs in the chloroplast has been shown to greatly increase protein accumulation. To test the ability of citrus viroid III (CVd-III) to effectively dwarf citrus growing under subtropical conditions, two lines of experimentation are planned. In Spring 2002, field trials comparing the dwarfing effects of CVd-IIIa and CVd-IIIb will be initiated at three Florida locations. While these field trials are underway, we will screen a population of CVd-III variants generated by combinatorial cassette mutagenesis or hypermutagenic PCR for sequences whose dwarfing effects are not limited to trifoliate orange and related hybrid rootstocks. BSL-1; Certified April 9, 2002.

Project Team

Owens, Robert Hammond, Rosemarie

Project Annual Reports

FY 2003

Related National Programs

Plant Diseases (303)

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