Clonal relationships and cell patterning in the root epidermis of Arabidopsis

The development of the post-embryonic root epidermis of Arabidopsis thaliana is described. Clonal analysis has identified three sets of initials that give rise to the columella root cap cells, epidermis and lateral root-cap cells, and the cells of the cortex and endodermis respectively. The mature epidermis is composed of two cell types, root hair cells (derived from trichoblasts) and non-hair cells (derived from atrichoblasts). These cells are arranged in sixteen or more discrete files. Each hair cell file overlies the anticlinal (radial) wall of the underlying cortical cells and is separated from the next by one or two non-hair files. The root hair forms as a tip-growing projection from the basal end of the trichoblast i.e. the end nearest the root meristem. The non-hair epidermal cells are significantly longer than the hair forming cells and are located over the outer periclinal (tangential) wall of the underlying cortical cells. The size difference between the two cell types is apparent in the cell division zone before hairs form. This suggests that the signals required for the differentiation of the root epidermis function in the meristem itself. Ectopic hairs are present in the ctr1 root epidermis suggesting that a Raf protein kinase may play a role in pattern formation/differentiation in the root epidermis and that ethylene may be a diffusible signal involved in specifying pattern in the root epidermis.