Plant intercellular communication via plasmodesmata.

The development and function of multicellular organisms rely on cell-to-cell communication. Detailed studies of animal cells show that this communication can occur by secretion of chemical signals, such as hormones and neurotransmitters, and by contact-dependent signals transmitted through extracellular matrixand membrane-anchored molecules. Plants use similar modes of communication, although they are not as well characterized as those in animal systems. The transport of small signal molecules, such as hormones, regulates the proper growth of plant cells (see Creelman and Mullet, 1997; Kende and Zeevaart, 1997, in this issue), and cell-to-cell contact via an extracellular matrix-located glycoprotein and a receptor kinase plays a role in the self-incompatibility reaction between pollen and stigma (Stein et al., 1991). Plant cells, however, have an additional and unique mode of cell-to-cell communication derived from two of their characteristic features: the deposition of cell wall material, and the incomplete separation of the cytoplasm during cytokinesis. Plasmodesmata (PD) are structurally complex channels that span the cell wall and connect the cytoplasm of one plant cell with that of its neighbors, consequently facilitating communication between cells. In higher plant embtyos, initially all cells are interconnected by PD (Figure 1A; Schulz and Jensen, 1968; Mansfield and Briarty, 1991; see below) and integrated into a single symplast, the domain of common cytoplasm that is bounded by the plasma membranes of connected cells (Munch, 1930). As the plant differentiates, individual cells or groups of cells become isolated, possibly by loss of functional PD (Carr, 1976; Bergmans et al., 1993; Duckett et al., 1994). This symplastic isolation allows subsets of cells to function as distinct compartments within the plant. Indeed, mature flowering plants could best be described as mosaics of symplastic domains. Because PD provide the symplastic connections between cells, communication and transport within and between these symplastic domains are intimately linked to the frequency, distribution, and function of PD. Plasmodesmal frequencies among cells have been used to characterize symplastic transport pathways, but this approach assumes that PD are both structurally and functionally uniform, which, as described below, does not appear to be the case (van Bel and Oparka, 1995). Severa1 techniques have been used