Role of Viral Movement and Coat Proteins and RNA in Phloem‐dependent Movement and Phloem Unloading of Tobamoviruses

Abstract

Abstract The role of virus‐encoded proteins and viral RNA in phloem‐dependent movement and phloem unloading was investigated using temperature‐sensitive (ts) tobamoviruses. Phloem‐dependent movement of ts‐tobamoviruses was not prevented by the non‐permissive temperature; only a slight, temporal effect on systemic movement of Tobacco mosaic virus Ni2519 (TMV‐Ni2519), which encodes a ts‐movement protein (ts‐MP) and ts‐origin‐of‐assembly (ts‐OAS), was detected. Intact viral coat protein (CP) was not essential in the phloem‐unloading step using a modified differential temperature treatment (mDTT), in which the lower inoculated part of a plant was maintained at the permissive temperature and the upper part at the non‐permissive temperature. Grafted plants with wild‐type rootstocks and MP transgenic scions (MP expressed under transcriptional control of the CaMV 35S promoter) supported phloem unloading of Tomato mosaic virus Ls1 (encoding ts‐MP) but not that of TMV‐Ni2519 when subjected to mDTT. However, TMV‐Ni2519 was still capable of being translocated within the vascular system at the temperature at which the MP was non‐functional but could not unload from the vascular tissue. These data suggest that functionality of MP and CP is not essential for phloem‐dependent movement and phloem unloading of tobamoviruses, and also indicate that viral RNA participates in the subsequent (cell–cell) movement of tobamoviruses after phloem unloading.