TOBACCO MOSAIC-VIRUS MOVEMENT PROTEIN-MEDIATED PROTEIN-TRANSPORT BETWEEN TRICHOME CELLS

Tobacco mosaic virus movement protein (TMV MP) is required to mediate viral spread between plant cells via plasmodesmata. Plasmodesmata are cytoplasmic bridges that connect individual plant cells and ordinarily limit molecular diffusion to small molecules and metabolites with a m olecular mass up to 1 kD. Here, we characterize functional properties of Nicotiana clevelandii trichome plasmodesmata and analyze their inte raction with TMV MP. Trichomes constitute a linear cellular system and provide a predictable pathway of movement. Their plasmodesmata are fu nctionally distinct from plasmodesmata in other plant cell types; they allow cell-to-cell diffusion of dextrans with a molecular mass up to 7 kD, and TMV MP does not increase this size exclusion limit for dextr ans. In contrast, the 30-kD TMV MP itself moves between trichome cells and specifically mediates the translocation of a 90-kD beta-glucuroni dase (GUS) reporter protein as a GUS::TMV MP fusion. Neither GUS by it self nor GUS in the presence of TMV MP moves between cells. These data imply that a plasmodesmal transport signal resides within TMV MP and is essential for movement. This signal confers selectivity to the tran slocated protein and cannot function in trans to support movement of o ther molecules.