DISTRIBUTION OF TOBAMOVIRUS MOVEMENT PROTEIN IN INFECTED-CELLS AND IMPLICATIONS FOR CELL-TO-CELL SPREAD OF INFECTION

The intercellular and intracellular distribution of the movement prote in (MP) of the Ob tobamovirus was examined in infected leaf tissues us ing an infectious clone of Ob in which the MP gene was translationally fused to the gene encoding the green fluorescent protein (GFP) of Aeq uorea victoria. In leaves of Nicotiana tabacum and N. benthamiana, the modified virus caused fluorescent infection sites that were visible a s expanding rings. Microscopy of epidermal cells revealed subcellular patterns of accumulation of the MP:GFP fusion protein which differed d epending upon the radial position of the cells within the fluorescent ring. Punctate, highly localized fluorescence was associated with cell walls of all of the epidermal cells within the infection site, and ap parently represents association of the fusion protein with plasmodesma ta; furthermore, fluorescence was retained in cell walls purified from infected leaves. Within the brightest region of the fluorescent ring, the MP:GFP was observed in irregularly shaped inclusions in the corti cal regions of infected cells. Fluorescent filamentous structures pres umed to represent association of MP:GFP with microtubules were observe d, but were distributed differently within the infection sites on the two hosts. Within cells containing filaments, a number of fluorescent bodies, some apparently streaming in cytoplasmic strands, were also ob served. The significance of these observations is discussed in relatio n to MP accumulation, targeting to plasmodesmata, and degradation.