Directional cell-to-cell communication in the Arabidopsis root apical meristem - I. An ultrastructural and functional analysis

As a foundation for studies on directional intercellular communication and its regulation in apical development, the network of plasmodesmata in Arabidopsis root apical meristems was characterized by quantitative electron microscopy and dye-coupling analysis, using symplasmic probes, and real-time imaging in confocal laser scanning microscopy. A tissue-specific plasmodesmatal network, which interconnected the cells in the root apical meristem, was characterized by the following features. (a) Plasmodesmatal distribution and density were found to be tissue-specific. (b) Primary and secondary plasmodesmata were differentially grouped and regulated. Primary plasmodesmata were formed in large numbers in the transverse walls of each tissue, and were subject to deletion during cell differentiation. Secondary plasmodesmata were mostly distributed in longitudinal walls between cell files and common walls between neighboring tissues; they also provided a symplasmic path between different initial tiers in the meristem. Small fluorescent tracers moved through the plasmodesmatal network of the root apical meristem in two distinct phases. At low concentrations molecules trafficked in a non-tissue-specific manner, whereas at higher concentrations, their distribution reflected the presence of tissue-specific movement consistent with plasmodesmatal distribution. These findings are discussed in terms of the role of tissue-specific plasmodesmatal domains in the control of root development.