Directional cell-to-cell communication in theArabidopsis root apical meristem II. Dynamics of plasmodesmatal formation

Abstract

Cell development in the root apical meristem is thought to be regulated by position-dependent information, but as yet, the underlying mechanism for this remains unknown. In order to examine the potential involvement of the symplasmic transmission of positional signals, plasmodesmatal frequency and distribution was quantitatively analyzed in root apical meristem cell walls ofArabidopsis thaliana during root development. A consistent distribution pattern of plasmodesmata was observed in the root apex over four weeks. While cells within initial tiers were uniformly interconnected, more symplasmic connections between the initial tiers and their immature-cell (primary-meristem) derivatives were observed than within the initial tiers. Immature cells were connected across transverse walls by primary plasmodesmata according to a tissue-specific pattern. Cells of the immature vascular tissue and cortex had the highest plasmodesmatal frequencies, followed by the immature epidermis and root cap. Although the numbers of plasmodesmata in transverse walls (primary plasmodesmata) was reduced in all tissues as the root aged, the tissue-specific distribution remained constant. The extent of symplasmic coupling across the boundaries of each tissue appeared to be limited by fewer secondary plasmodesmata in longitudinal walls. The frequency of all plasmodesmata decreased as the root aged. The primary plasmodesmata within each tissue increased at one week and then dramatically decreased with root age; the frequency of secondary plasmodesmata in longitudinal walls also decreased, but more gradually. These findings are discussed with respect to the roles likely played by plasmodesmata in facilitating transport of position-dependent information during root development.