Potato spindle tuber viroid: Investigation of the long-distance, intra-plant transport route.

Potato spindle tuber viroid (PSTV) could be detected in tomato plants by dot-blot hybridization as early as 6-7 days postinoculation (p.i.), approximately 1 week before the appearance of symptoms. The viroid was always first detectable in the shoot tip of the plant and in leaves that were very close to the tip, followed temporally by detection in leaves further from the tip down to and eventually including the inoculated leaf. Although PSTV also moved down to the roots, it did not move into leaves below the inoculated leaf, unless such leaves were shaded to reverse the normal direction of movement of photosynthetic materials from these leaves. Shading the inoculated leaf had the effect of delaying the movement of PSTV out of the inoculated leaf. These results are all consistent with the rapid systemic movement of PSTV from the inoculated leaf to actively growing tissue via the phloem, a route also utilized by most plant viruses.