Movement proteins (MPs) encoded by plant viruses interact with components of the plant transport system in which plasmodesmata (PDs) as cytoplasmic cell-to-cell-connections resume a central role in the symplastic transport.Constitutive expression of the Potato leafroll virus (PLRV) 17-kDa MP (MP17) fused to green fluorescence protein GFP in Arabidopsis thaliana was accompanied by the localisation of MP17 at PDs of several cell types. It resulted in a dramatic change of carbohydrate status, biomass allocation and seed yield. These effects were independent from ecotype but strictly bound to MP17:GFP protein amount. Low MP17:GFP expression level led to an increased vegetative biomass production paralleled by smaller carbohydrate contents and coexisting increased sucrose transport rates. Converse effects in terms of a strong growth retardation, massive carbohydrate accumulation in source leaves and delayed virus spreading occurred after high MP17:GFP protein contents. However, during the reproductive stage, the inhibition of the assimilate export block in high MP17:GFP expressing Arabidopsis plants was abolished and led to an enhanced seed yield and harvest index. The reversion of the symplastic transport block in later developmental stages suggests alterations of PD structure and function during progressive leave senescence.The observed effects were also achieved after expression of MP17 without GFP fusion. Though, considerable higher protein levels were necessary for the development of comparable, phenotypic changes compared to fusion protein expressing plants. These phenotypic alterations are probably not ascribed to callose as no significant increased callose contents were detectable. Hence, the effects seem to be rather induced by MP17.By means of cell specific MP17:GFP expression the protein dose-dependent development of the MP17 mediated phenotype was narrowed down to plasmodesmatal connections in chloroplast containing cells.For the genetic identification of the underlying mechanisms and plant interacting factor(s), two suppressor mutants were obtained. Because of their recessive, non-allelic mutation they can be applied for the map-based cloning. In parallel, a crossing partner was generated by crossing of MP17:GFP from the Columbia parental line in the Landsberg erecta background. Consequently, the crossing partner contains the MP17 insertion at the same place in the genome as the parental line and shows comparable phenotypic characteristics.