Viroid replication and pathogenesis provide model systems in which molecular mechanisms controlling host-pathogen interaction and gene expression in uninfected plant cells are accessible to experimental manipulation. My laboratory is using site-directed mutagenesis and a variety of other recombinant DNA-based techniques to explore the relationship between the biological properties of viroids and their unusual structures. Areas of current interest include (i) identification of structural features controlling host range, (ii) characterization of pathways used by viroids to enter and exit the nucleus, move from cell to cell, and transit the vascular system, and (iii) development of improved viroid-based citrus dwarfing agents.