Alteration in carbon partitioning induced by the movement protein of tobacco mosaic virus originates in the mesophyll and is independent of change in the plasmodesmal size exclusion limit

The influence of the 30 kDa movement protein of tobacco mosaic virus (TMV‐MP) on carbon partitioning in trans‐genie tobacco plants (Nicotiana tabacum cv. Xanthi) expressing the TMV‐MP was investigated. Using reciprocal grafting of transgenic tobacco plants expressing this movement protein and vector control plants, as well as transgenic tobacco plants expressing the TMV‐MP in phloem cells only, we showed that the interactive site involved in carbon allocation to roots is localized to the mesophyll tissue. Biomass partitioning experiments conducted on transgenic plants, in which various deletion mutant forms of the TMV‐MP (two of which included deletions in the domain responsible for increasing the size exclusion limit) were expressed, revealed that the TMV‐MP exerts its influence on carbon allocation via a mechanism that is completely independent of the TMV‐MP‐induced increase in the plasmodesmal size exclusion limit. Furthermore, small N‐ and C‐terminal deletions in the MP revealed the complexity of the interactions likely to be involved between the MP and an endogenous regulatory mechanism. We propose that the TMV‐MP interferes with an endogenous signal transduction pathway that involves macromolecular trafficking through plasmodesmata to regulate biomass partitioning between the source and various sink tissues.