Probing Plasmodesmal Transport with Plant Viruses

Plant intercellular connections, the plasmodesmata, effectively link individual cells into one symplastic continuum and function as conduits for transport and cell-to-cell communication processes. Although plasmodesmal ultrastructure has been extensively studied (see below), very little is known about the biochemistry, molecular biology, and regulation of these intercellular channels. This lack of knowledge stems mainly from technical difficulties in isolation and purification of plasmodesmata structures that are embedded in the plant cell-wall matrix. Thus, plasmodesmata remain essentially a biological "black box." Recently, however, a potentially powerful approach with which to study plasmodesmal function has been developed. This approach is based on the general observation that complex biological pathways can be functionally dissected using specific biologically active compounds (e.g. inhibitors or inducers) or mutations that perturb the pathway. In the case of plasmodesmata, a unique class of biological molecules is known to specifically and dramatically alter plasmodesmal function: the cell-to-cell movement proteins of plant viruses. In this review, I focus on the use of plant virus cell-to-cell movement proteins as molecular tools to study plasmodesmata. For detailed descriptions of the role of movement proteins in cell-to-cell spread of plant viruses, the reader is referred to recent comprehensive reviews (Atabekov and Taliansky, 1990; Citovsky and Zambryski, 1991; Hull, 1991; Maule, 1991; Deom et al., 1992; Citovsky and Zambryski, 1993; McLean et al., 1993).