Microinjection reveals cell-to-cell movement of green fluorescent protein in cells of maize coleoptiles.

During the evaluation of dual-purpose plant/fungal expression systems, we found that green fluorescent protein (GFP) has the ability to move from cell to cell in the epidermis of Zea mays L. cv. Mutator coleoptiles as well as into underlying cortical cells. Movement of GFP was observed both when DNA encoding GFP and bacterially expressed GFP were microinjected into epidermal cells. This suggests that GFP is capable of cell-to-cell movement. From experiments using dextrans of known molecular weight linked to fluorescein isothiocyanate and tetramethylrhodamine isothiocyanate, we estimate that the plasmodesmata of these cells have a size exclusion limit < 4.4 kDa. Cell-to-cell GFP movement did not occur when GFP was altered to include a nucleus- or endoplasmic reticulum-retention sequence. The fact that these transcripts differ from that of cytoplasmic GFP by a small number of nucleotides suggests that the transcripts are not capable of movement, but movement of nucleic acid cannot be excluded. Since GFP is widely used to study cell-to-cell movement and to localize the expression of transgenes, caution should be exercised when interpreting results where GFP expression is used for localization.