) 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...

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Abstract. Transgenic tobacco (Nicotiana tabacum L.)plants expressing the 30-kDa movement protein of tobacco mosaic virus (TMV-MP) were employed to investigate the influence of a localized change in mesophyllbundle sheath plasmodesmal size exclusion limit on photosynthetic performance and on carbon metabolism and allocation. Under conditions of saturating irradiance, tobacco plants expressing the TMV-MP were found to have higher photosynthetic CO2-response curves compared with vector control plants. However, this difference was significant only in the presence of elevated CO2 levels. Photosynthetic measurements made in the greenhouse, under endogenous growth conditions, revealed that there was little difference between TMV-MP-expressing and control tobacco plants. However, analysis of carbon metabolites within source leaves where a TMV-MP-induced increase in plasmodesmal size exclusion limit had recently taken place established that the levels of sucrose, glucose, fructose and starch were considerably elevated above those present in equivalent control leaves. Although expression of the TMV-MP did not alter total plant biomass, it reduced carbon allocation to the lower region of the stem and roots. This difference in biomass distribution was clearly evident in the lower root-to-shoot ratios for the TMV-MP transgenic plants. Microinjection (dye-coupling) studies established that the TMV-MP-associated reduction in photosynthate delivery (allocation) to the roots was not due to a direct effect on root cortical plasmodesmata. Rather, this change appeared to result from an alteration in phloem transport from young source leaves in which the TMV-MP had yet to exert its influence over plasmodesmal size exclusion limits. These results are discussed in terms of the rate-limiting steps involved in sucrose movement into the phloem.

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