Isolate-specific synergy in disease symptoms between cauliflower mosaic and turnip vein-clearing viruses

Simultaneous infection of a plant by two viruses can cause more severe disease than is caused by infection with either virus alone. Such synergy may be due to effects on the replication of one virus by the second virus or to other causes. The tobamovirus turnip vein-clearing virus (TVCV), itself causing almost imperceptible symptoms in infected turnips, exacerbated symptoms of infection of turnip by the Cabbage S isolate of the caulimovirus cauliflower mosaic virus (CaMV). The synergy in symptom production was most evident in a reduced size of leaves, providing an objective measure of synergy. In contrast, synergy did not occur when the CM4-184 isolate of CaMV was used in combination with TVCV. Both isolates of CaMV increased the level of TVCV accumulated in leaves. TVCV did not increase the level of the Cabbage S CaMV isolate. The use of Cabbage S-CM4-184 chimeras revealed that a region critical for isolate synergy in stunting was within the coat protein gene and/or the 5' one third of the reverse transcriptase gene. We conclude that the disease symptom synergy between TVCV and Cabbage S CaMV is not caused by altered levels of accumulation of the viruses, but instead reflects subtle genetic interactions mapping to the ORF IV-ORF V region of CaMV DNA.