Changes in macromolecular movement accompany organogenesis in thin cell layers of Torenia fournieri

A range of fluorescently labelled probes of increasing molecular weight was used to monitor diffusion via the symplast in regenerating thin cell layer (TCL) explants of Torenia fournieri. An increase in intercellular movement of these molecules was associated with the earliest stages of vegetative shoot regeneration, with the movement of a 10 kDa dextran (FD 10000) observed between epidermal cells prior to the appearance of the first cell divisions. A low frequency of dextran movement in thin cell layers maintained under non-regenerating conditions was also observed, indicating a possible wound induced increase in intercellular movement. Dextran movement between epidermal cells reached a peak by day 4 of culture and then declined as cell division centres (CDCs) formed, became meristematic regions and finally emerged as adventitious shoots. Within CDCs, testing with small fluorescent probes (CF: carboxyfluorescein, mw 376 Da and F(Glu)3: fluorescein-triglutamic acid, mw 799 Da) revealed a mosaic of cell isolation and regions of maintained symplastic linkage. Within shoots, surface cells of the presumptive apical meristem permitted the intercellular movement of 10 kDa dextrans but epidermal cells of the surrounding leaf primordia did not permit dextran movement. In some cases, intercellular movement of CF was maintained within leaf primordia. Symplastic movement of labelled dextrans during regeneration in Torenia thin cell layers represents a significant increase in the basal size exclusion limit (SEL) of this tissue and reveals the potential for intercellular trafficking of developmentally related endogenous macromolecules.