Susceptibility of Brassica species to cauliflower mosaic virus infection is related to a specific stage in the virus multiplication cycle

The relative susceptibilities and symptom responses of different Brassica species to infection by cauliflower mosaic virus (CaMV) have been compared and related to molecular events of the virus multiplication cycle. Variants ofB. rapa (genome descriptor aa) were highly susceptible to infection by CaMV strain Cabb B-JI and contained relatively large amounts of virus; B. oleracea (cc) variants howed low susceptibility and contained small amounts of virus. B. nigra (bb) and allotetraploid species, B. juncea (aabb), B. napus (aacc) and B. carinata (bbcc), showed moderate responses to CaMV. CaMV unencapsidated DNA forms were isolated from different Brassica plants and examined by two-dimen-sional gel electrophoresis and blot hybridization. Viral RNA was estimated by dot blot analysis. These analyses howed differences in accumulation of key viral replication cycle intermediates within the broad range of host plants studied. The most susceptible species contained relatively small amounts of super-coiled (SC) DNA, a component of the CaMV mini-chromosome, but abundant viral transcripts and re-verse transcription replication products. Tolerant plant hosts contained high levels of SC DNA but low levels of viral transcripts and reverse transcription DNA pro-ducts. Allotetraploids contained SC DNA, RNA transcripts and replication product levels which were generally intermediate between those of their respec-tive progenitor species. Evidence is presented that accumulation fCaMV SC DNA in the less susceptible host species is probably not due to autonomous DNA replication or tissue-specific expression. We conclude that a major component of the susceptibility of Brassica plants (and probably all CaMV host species) to CaMV infection is the level of viral minichromo-some expression, influenced directly by the host genotype