The formation of symplasmic domains by plugging of plasmodesmata: a general event in plant morphogenesis?

The plasmodesmal network was examined in multicellular protoplast-derived c alluses of the dicotyledon Solanum nigrum which had not yet formed any visi ble adventitious organs and in globular proembryogenic structures developed from scutellar calluses of the monocotyledon Molinia caerulea. Electron mi croscopical analyses revealed that both calluses and proembryos consisted o f small, undifferentiated cells. The interconnecting plasmodesmata at many cell interfaces were structurally inconspicuous in both systems; in particu lar cell walls, however, all plasmodesmata were occluded with an osmiophili c, dense material. As the blocking material was obviously located in the mi crochannels of the plasmodesmal cytoplasmic sleeves; the plugged plasmodesm ata can be assumed to be nonfunctional. Thus, selective occlusion of all th e plasmodesmata in specific cell walls resulted in the symplasmic disconnec tion of particular adjacent cells. Complex patterns of symplasmic continuit y and discontinuity were established within the developing tissues. Some ce lls or groups of cells were entirely symplasmically disconnected from the s urrounding cells by plugged plasmodesmata and might function as independent domains. However, blockage of plasmodesmata was achieved by the surroundin g cells rather than by those cells belonging to the isolated domains. The d emarcation of symplasmic domains might be a general prerequisite for differ ential morphogenesis, since they were found to be established very early in the course of morphogenetic processes.