) were employed t o further explore the mode by which this viral protein interacts with cellular metabolism t o change carbohydrate allocation. Dye-coupling experiments established that expression of the TMV-MP alters plasmodesmal function in both potato leaves and tubers when expressed in the respective tissues. However, whereas the sizeexclusion limit of mesophyll plasmodesmata was increased to a value greater than 9.4 kD, this size limit was smaller for plasmodesmata interconnecting tuber parenchyma cells. Starch and sugars accumulated in potato leaves t o significantly lower levels in plants expressing the TMV-MP under the ST-LS1 promoter, and rate of sucrose efflux from petioles of the latter was higher compared to controls. It is interesting that this effect was expressed only in mature plants after tuber initiation. No effect on carbohydrate levels was found in plants expressing this protein under the 833 promoter. These results are discussed in terms of the mode by which the TMV-MP exerts its influence over carbon metabolism and photoassimilate translocation, and the possible role of plasmodesmal function i n controlling these processes.It is generally accepted that most if not a11 viruses move from cell to cell via plasmodesmata. This process requires an interaction between a specific virally encoded protein, termed the MP, and proteins within the plasmodesmata of the host plant . Expression of the TMV-MP in transgenic tobacco plants provided the first evidence that this protein potentiates the short-distance transport of viral infectious material (Deom et al., 1987 Immunolocalization studies indicated that the TMV-MP was localized mainly in secondary plasmodesmata connecting mesophyll cells and bundle-sheath cells to phloem parenchyma cells (Ding et al., 1992). Dye-coupling studies established that the SEL of plasmodesmata interconnecting the mesophyll cells of these transgenic plants was greater than 9.4 kD, as compared to 800 D in control plants, indicating that the TMV-MP has a direct effect on plasmodesmal function (Wolf et al., 1989). Based on plasmodesmal frequencies and dye-coupling experiments, it has been assumed that plasmodesmata play an important role in regulating symplasmic transport (Robards and Lucas, 1990). Pursuant to this assumption, transgenic plants in which plasmodesmata are modified provide an elegant system in which to further test the hypothesis that diffusion of SUC through plasmodesmata acts as a limiting step to symplasmic transport. Our comparative analysis of leaf photosynthetic performance, carbohydrate level, and carbon export in TMV-MP transgenic and control tobacco plants revealed a complex influence of the MP over these parameters (Lucas et al., 1993b;Olesinski et al., 1995). Fully expanded leaves of transgenic tobacco plants expressing the TMV-MP accumulated much higher levels of SUC, Glc, Fru, and starch during the day than did those of control plants. Direct measurements of 14C-photosynthate translocation from source leaves indicated that export was lower in plan...