CHANGING PATTERNS OF LOCALIZATION OF THE TOBACCO MOSAIC-VIRUS MOVEMENT PROTEIN AND REPLICASE TO THE ENDOPLASMIC-RETICULUM AND MICROTUBULES DURING INFECTION

Tobacco mosaic virus (TMV) derivatives that encode movement protein (M P) as a fusion to the green fluorescent protein (MP:GFP) were used in combination with antibody staining to identify host cell components to which MP and replicase accumulate in cells of infected Nicotiana bent hamiana leaves and in infected BY-2 protoplasts. MP:GFP and replicase colocalized to the endoplasmic reticulum (ER; especially the cortical ER) and were present in large, irregularly shaped, ER-derived structur es that may represent ''viral factories.'' The ER-derived structures r equired an intact cytoskeleton, and microtubules appeared to redistrib ute MP:GFP from these sites during late stages of infection. In leaves , MP:GFP accumulated in plasmodesmata, whereas in protoplasts, the MP: GFP was targeted to distinct, punctate sites near the plasma membrane. Treating protoplasts with cytochalasin D and brefeldin A at the time of inoculation prevented the accumulation of MP:GFP at these sites. It is proposed that the punctate sites anchor the cortical ER to plasma membrane and are related to sites at which plasmodesmata form in walle d cells. Hairlike structures containing MP:GFP appeared on the surface of some of the infected protoplasts and are reminiscent of similar st ructures induced by other plant viruses. We present a model that postu lates the role of the ER and cytoskeleton in targeting the MP and vira l ribonucleoprotein from sites of virus synthesis to the plasmodesmata through which infection is spread.