SYMPLASMIC FIELDS IN THE TUNICA OF THE SHOOT APICAL MERISTEM COORDINATE MORPHOGENETIC EVENTS

In plants, complex cellular interactions, which require the exchange o f morphogenetic signals, underlie morphogenesis at the shoot apical me ristem, Since all apical meristem cells are interconnected by plasmode smata, we have investigated if symplasmic paths are available which ma y preferentially channel metabolites and potential morphogens in the a pical meristem, and whether they could support both the formation of d eterminate appendages and the sustainment of an undifferentiated centr e, Experiments in which the permeability of the symplasm was probed wi th fluorescent dye revealed that the tunica of the apical meristem of birch seedlings (Betula pubescence Ehrh,) is sym]plasmically compartme ntalized into two concentric fields, which restrict the symplasmic dif fusion of small potential morphogens to the cells inside their boundar ies. A transient connection between the two fields was established ear ly in a plastochron, potentiating the radial exchange of symplasmicall y diffusing signalling; molecules. We suggest that the symplasmic subd ivision of the tunica offers a means to unite cells into communication compartments, invoke boundary interactions between them, and shield t he distal meristem cells from organogenesis. Electrophysiological meas urements indicate that, in addition, the cells of these fields constit ute metabolic working units. The relevance of these symplasmic fields for morphogenesis was established experimentally by treatment with sho rt photoperiod, which induced breakdown of the fields into symplasmica lly isolated cells. Tannic acid staining and in situ immunolocalisatio n revealed that cell isolation was due to the activation of glucan syn thase complexes intrinsic to sphincters. As a result callose plugs wer e formed on all plasmodesmata leading to morphogenetic deactivation.