CLONAL RELATIONSHIPS AND CELL PATTERNING IN THE ROOT EPIDERMIS OF ARABIDOPSIS

The development of the post-embryonic root epidermis of Arabidopsis th aliana is described. Clonal analysis has identified three sets of init ials that give rise to the columella root cap cells, epidermis and lat eral root-cap cells, and the cells of the cortex and endodermis respec tively. The mature epidermis is composed of two cell types, root hair cells (derived from trichoblasts) and non-hair cells (derived from atr ichoblasts). These cells are arranged in sixteen or more discrete file s. Each hair cell file overlies the anticlinal (radial) wall of the un derlying cortical cells and is separated from the next by one or two n on-hair files. The root hair forms as a tip-growing projection from th e basal end of the trichoblast i.e. the end nearest the root meristem. The non-hair epidermal cells are significantly longer than the hair f orming cells and are located over the outer periclinal (tangential) wa ll of the underlying cortical cells. The size difference between the t wo cell types is apparent in the cell division zone before hairs form. This suggests that the signals required for the differentiation of th e root epidermis function in the meristem itself. Ectopic hairs are pr esent in the ctr1 root epidermis suggesting that a Raf protein kinase may play a role in pattern formation/differentiation in the root epide rmis and that ethylene may be a diffusible signal involved in specifyi ng pattern in the root epidermis.