Symplasmic fields in the tunica of the shoot apical meristem coordinate morphogenetic events

In plants, complex cellular interactions, which require the exchange of morphogenetic signals, underlie morphogenesis at the shoot apical meristem. Since all apical meristem cells are interconnected by plasmodesmata, we have investigated if symplasmic paths are available which may preferentially channel metabolites and potential morphogens in the apical meristem, and whether they could support both the formation of determinate appendages and the sustainment of an undifferentiated centre. Experiments in which the permeability of the symplasm was probed with fluorescent dye revealed that the tunica of the apical meristem of birch seedlings (Betula pubescence Ehrh.) is symplasmically compartmentalized into two concentric fields, which restrict the symplasmic diffusion of small potential morphogens to the cells inside their boundaries. A transient connection between the two fields was established early in a plastochron, potentiating the radial exchange of symplasmically diffusing signalling molecules. We suggest that the symplasmic subdivision of the tunica offers a means to unite cells into communication compartments, invoke boundary interactions between them, and shield the distal meristem cells from organogenesis. Electrophysiological measurements indicate that, in addition, the cells of these fields constitute metabolic working units. The relevance of these symplasmic fields for morphogenesis was established experimentally by treatment with short photoperiod, which induced breakdown of the fields into symplasmically isolated cells. Tannic acid staining and in situ immunolocalisation revealed that cell isolation was due to the activation of glucan synthase complexes intrinsic to sphincters. As a result callose plugs were formed on all plasmodesmata leading to morphogenetic deactivation.