Host regulation of the cauliflower mosaic virus multiplication cycle.

The DNA genome of cauliflower mosaic virus (CaMV) replicates in the cytoplasm of infected plant cells by reverse transcription of an RNA template. Viral RNA is generated in the nucleus by transcription of an episomal minichromosome containing supercoiled DNA. We have assessed the relative activities of the nuclear and cytoplasmic phases of the CaMV multiplication cycle by monitoring unencapsidated viral DNA forms and polyadenylylated RNAs in different organs of one host plant and in different host species. Systemically infected leaves of a highly susceptible host, turnip (Brassica rapa), contained abundant 35S RNA and 19S RNA transcripts and unencapsidated reverse transcription products but relatively little supercoiled DNA. In contrast, supercoiled DNA accumulated in roots and other tissues of turnip plants but without significant amounts of steady-state viral RNA. Infected but asymptomatic leaves of a less susceptible CaMV host, kohlrabi (Brassica oleracea), contained supercoiled DNA almost exclusively but negligible viral RNA and DNA products of reverse transcription. An allotetraploid species, rape (Brassica napus), exhibited infection characteristics and minichromosome expression levels intermediate between the other two species from which it was derived. We conclude that expression of the CaMV minichromosome is a key phase of the virus multiplication cycle, which is regulated differentially in organs of a highly susceptible host species. Furthermore, this regulation exhibits genetic variation among different Brassica species and controls host susceptibility to CaMV infection.