Synchronization of mitotic activity in protoplast-derived Solanum nigrum L. microcalluses is correlated with plasmodesmal connectivity.

In protoplast-derived Solanum nigrum microcalluses, plasmodesmal connectivity and cell division behaviour of the sister cells were examined by repeated pressure-injection experiments with the fluorescent dye Lucifer Yellow (LYCH; M(r) 457) and concomitant light-microscopical long-term live observations. The studies revealed that the plasmodesmal permeability of the cultured cells differs in the distinct stages of microcallus development. There was a correlation between the symplasmic connectivity of the cells and the synchronousness of their mitotic activity. Sister cells which were symplasmically interconnected by functional plasmodesmata, permitting the diffusion of LYCH, were always found to divide synchronously. However, asynchronous mitotic divisions were exclusively observed in those sister cells whose plasmodesmata were closed to LYCH. The temporary symplasmic isolation is presumably performed by reversible gating of plasmodesmata. Repeated dye-coupling experiments on the same microcalluses showed that symplasmically interconnected sister cells may become uncoupled and vice versa, according to their division behaviour. These findings on cultured cells indicate that modulation of the symplasmic connectivity determines the synchronization of mitotic activity. Yet it remains to be proven whether this is true in planta as well. The results are discussed with respect to the possible role of plasmodesmata in exerting "supracellular control" over mitotic activity by trafficking mitosis-regulating signals.