High-level constitutive expression of the cell-to-cell movement protein from the phloem-restricted potato leafroll virus (PLRV-MP17) in transgenic tobacco plants leads to growth retardation and severe phenotypic changes of source leaves paralleled by a drastic accumulation of soluble sugars and starch (Herbers et al., 1997). To investigate whether the MP17-induced alteration in carbon metabolism is related to the targeting and modification of specific plasmodesmata (Pd) or is rather due to pleiotropic effects caused by high MP17 protein amounts, non-phenotypic tobacco plants expressing a MP17:GFP fusion protein were obtained and compared with previously described MP17 transgenic lines. Confocal laser scanning microscopy and immunogold labelling studies revealed an overall affinity of MP17 to Pd in vascular and non-vascular tissue of source leaves, whereas in sink leaves GFP fluorescence was restricted to Pd of trichomes. In source leaves, plasmodesmal size exclusion limits of mesophyll cells were likewise increased by MP17 and MP17:GFP independent from steady-state levels of the protein amount and phenotypic alteration. Conversely, carbohydrate contents in source leaves strictly correlated with quantified MP17 protein levels. Low expression of MP17 and MP17:GFP decreased soluble sugars and starch contents in leaves possibly due to changes in plasmodesmal permeability while increasing MP17 protein levels led to carbohydrate accumulation and a stunted growth. Infection of transgenic lines with the unrelated potato virus Y (PVY)N revealed an expression level-dependent mode of MP17-mediated resistance. Phenotypic changes and carbohydrate-mediated defence responses as indicated by elevated levels of PR-protein transcripts were crucial for increased viral resistance, whereas plasmodesmal targeting and modification by MP17 per se had either no effect or even increased susceptibility to PVY. Thus, our results implicate that the absolute level of expression needs to be critically considered when elucidating the effect of MPs on carbon metabolism, biomass allocation and virus resistance.