LONG-DISTANCE MOVEMENT OF CAULIFLOWER MOSAIC-VIRUS IN INFECTED TURNIP PLANTS

The long distance movement of cauliflower mosaic virus (CaMV) in systemically infected turnip plants was visualized by leaf blot hybridization techniques. Girdling experiments demonstrated that CaMV moved systemically in turnip plants through phloem channels. In time course experiments, CaMV exited the inoculated leaf and invaded the vasculature five days after inoculation. At this time, foci of viral DNA, as detected by hybridization, first appeared on the inoculated leaf. The movement of CaMV was compared to the translocation of photoassimilates labeled with (CO2)-C-14. The patterns were similar and influenced by both plant phyllotaxis and leaf developmental stage. Both virus and photoassimilate generally accumulated in younger (sink) leaves on the side of the leaf nearest the Point of insertion of the mature inoculated (source) leaf. As leaves underwent the sink-to-source transition in Photoassimilate import, there was a progressive basipetal (leaf tip to base) decline in the amount of photoassimilate and the number of virus particles entering the lamina so that, in more mature sink leaves, only the base of the leaf became infected. Therefore, Phyllotaxis determines what side of a leaf the virus will invade, and the leaf developmental stage determines how far toward the apex the virus will progress.