Most plant tissues are symplasms in which cells are connected by plasmodesmata, membrane-lined cytosplasmic bridges that enable diffusive and/or advective cell-to-cell movements of components of the cytosol. Current models assume that hydrodynamic radius alone governs the mobility of molecules through plasmodesmata. In contrast, physical theory predicts that nm-sized pores with electrically charged walls are selective for counterions. Narrow plasmodesmal pores lined by membranes―which carry negative surface charges―therefore should be permselective for cations. Quantifying cell-to-cell movements of fluorophores varying in molecular mass and electrical charge in cell types with differently sized plasmodesmata, we confirmed the applicability of physical theory to plasmodesmata. Surprisingly, narrow plasmodesmata were permselective for anions rather than cations, suggesting a major flaw in our current understanding of plasmodesma structure. We hypothesize that structural proteins known to exist in plasmodesmata may establish the cationic electrostatic environment required to explain our findings.
Howell et al. (Mon,) studied this question.