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

In protoplast-derived Solanum nigrum micro-calluses, plasmodesmal connectiv ity and cell division behaviour of the sister cells were examined by repeat ed pressure-injection experiments with the fluorescent dye Lucifer Yellow ( LYCH; M-r 457) and concomitant light-microscopical long-term live observati ons. The studies revealed that the plasmodesmal permeability of the culture d cells differs in the distinct stages of microcallus development. There wa s a correlation between the symplasmic connectivity of the cells and the sy nchronousness of their mitotic activity. Sister cells which were symplasmic ally interconnected by functional plasmodesmata, permitting the diffusion o f LYCH, were always found to divide synchronously. However, asynchronous mi totic divisions were exclusively observed in those sister cells whose plasm odesmata were closed to LYCH. The temporary symplasmic isolation is presuma bly performed by reversible gating of plasmodesmata. Repeated dye-coupling experiments on the same microcalluses showed that symplasmically interconne cted sister cells may become uncoupled and vice versa, according to their d ivision behaviour. These findings on cultured cells indicate that modulatio n of the symplasmic connectivity determines the synchronization of mitotic activity. Yet it remains to be proven whether this is true in planta as wel l. The results are discussed with respect to the possible role of plasmodes mata in exerting "supracellular control" over mitotic activity by trafficki ng mitosis-regulating signals.