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TCV isolate, TCV-M, supports a family of satellite (sat-) RNAs. The virulent sat-RNA C intensifies TCV symptoms on turnip cultivar Just Right. In this thesis, I report that sat-RNA C (356 b) exacerbates symptoms on all hosts where TCV produces visible symptoms including cultivars of Brassica rapa and Arabidopsis thaliana. This finding has led to studies of TCV-resistance using A. thaliana, a small plant with a well characterized genome, as a host. After screening over 6,000 M2 A. thaliana plants from EMS treated Columbia cultivar seeds and 22 ecotypes of A. thaliana, ecotype Dijon was found to be resistant to TCV. A second isolate, TCV-B, supports an RNA species with a size similar to that of sat-RNA C. Northern hybridization and cDNA cloning and sequencing demonstrate that this RNA is actually a defective interfering (DI) RNA, denoted DI RNA G. Infection of turnip with virus derived from cloned transcripts of TCV-B resulted in de novo generation of a DI RNA, DI1 RNA. Unlike DI RNAs associated with other plant viruses (or animal viruses), TCV DI RNAs intensify TCV symptoms. To understand sequences required for DI RNA infectivity, a series of mutation have been generated in a full length cDNA copy of DI RNA G. Stepwise deletions at base 98 of DI RNA G, at which an Apa I linker had been inserted, has shown that DI RNA GA (DI RNA G with a 8-base insertion at base 98) harboring deletions of bases 74-98 or less are infectious. However, deletion of bases 73-98 or more abolishes RNA infectivity. DI RNA GA with a deletion of bases 107-124 is infectious. However, DI RNA GA harboring a deletion of bases 107-138 is not infectious. I have found that infectivity of RNA harboring 31- or 32-base deletions can be restored by inserting foreign sequences into deletion sites. This implies that at least 71 bases (including the 8-base insertion) in DI RNA GA near the 5$\sp\prime$ end are not specifically required for RNA infectivity. By combining deletions of infectious clones to generate larger deletions (40 or 43 bases deleted), I demonstrate that the infectivity of the RNA is abolished. This result suggests that size of the DI RNA is important in maintaining RNA infectivity.