Light-dependent systemic infection of solanaceous species by cauliflower mosaic virus can Be conditioned by a viral gene encoding an aphid transmission factor.

Gene II of cauliflower mosaic virus (CaMV), which encodes an 18-kDa protein originally identified as an aphid transmission factor (ATF), influences host specificity in a light-dependent manner. A point mutation within the ATF gene that occurs in several CaMV strains was responsible for conditioning light-dependent systemic infections. A point mutant of CaMV strain W260 that carried the single mutation within the ATF gene was able to systemically infect Nicotiana bigelovii at low light intensity (100-180 micromol m-2 sec-1), but not at a higher light intensity level (350-450 micromol m-2 sec-1), while the wild-type W260 virus could systemically infect N. bigelovii under both light conditions. The same point mutation also affected the stability of the amorphous CaMV inclusions and previous studies have shown that it abolishes transmission of CaMV by aphids. The point mutation within the ATF gene that mediated light-dependent infections was complemented by transgenic N. bigelovii plants that express the CaMV gene VI product, a viral protein that has been identified as a translational transactivator. The complementation studies indicated that the ATF gene may influence systemic infections through an interaction with the CaMV gene VI product. The ATF gene of CaMV may contribute to viral infections by regulating expression of downstream genes or by influencing cell-to-cell or long distance movement within the plant.