Movement through plasmodesmata: an evaluation of the physical basis and available experimental methods

The movement of molecules through plasmodesmata (PD) is a fundamentally important process in plant tissues. Several decades of investigations have provided deeper insights into the basics of diffusive and advective flow through PD. However, PD still represent one of the least understood plant structures of major importance. This is based on their complex variable shape-composed of proteins, membranes, and wall material at nanoscale. We critically discuss approaches to study movement through PD, including microinjection/microdiffusion, expression of fluorescent proteins and photoactivatable probes. In addition, we highlight physical principles that form the basis for movement through PD.