Transgenic tobacco (Nicotiana tabacum L. cv. Xanthi) plants expressing wild-type or mutant forms of the 30-kDa movement protein of tobacco mosaic virus (TMV-MP) were employed to study the effects of the TMV-MP on carbon metabolism in source leaves. Fully expanded source leaves of transgenic plants expressing the TMV-MP were found to retain more newly fixed 14C compared with control plants. Analysis of 14C-export from young leaves of TMV-MP plants, where the MP is yet to influence plasmodesmal size exclusion limit, indicated a similar pattern, in that daytime 14C export was slower in TMV-MP plants as compared to equivalent-aged leaves on control plants. Pulse-chase experiments were used to monitor radioactivity present in the different carbohydrate fractions, at specified intervals following 14CO2 labeling. These studies established that the-TMV-MP can cause a significant adjustment in short-term 14-C-photosynthate storage and export. That these effects of the TMV-MP on carbon metabolism and phloem function were not attributable to the effect of this protein on plasmodesmal size exclusion limits, per se, was established using transgenic tobacco plants expressing temperature-sensitive and C-terminal deletion mutant forms of the TMV-MP. Collectively, these studies establish the pleiotropic nature of the TMV-MP in transgenic tobacco, and the results are discussed in terms of potential sites of interaction between the TMV-MP and endogenous processes involved in regulating carbon metabolism and export.