CELLULAR-ORGANIZATION OF THE ARABIDOPSIS-THALIANA ROOT

The anatomy of the developing root of Arabidopsis is described using c onventional histological techniques, scanning and transmission electro n microscopy. The root meristem is derived from cells of the hypophysi s and adjacent cells of the embryo proper. The postembryonic organizat ion of the root is apparent in the mature embryo and is maintained in the growing primary root after germination. Cell number and location i s relatively invariant in the primary root, with 8 cortical and endode rmal cell files but more variable numbers of pericycle and epidermal c ells. The organisation of cells in lateral roots is similar to that of the primary root but with more variability in the numbers of cell fil es in each layer. [H-3]thymidine labeling of actively growing roots in dicates that a quiescent centre of four central cells (derived from th e hypophysis) is located between the root cap columella and the stele. This plate of four cells is surrounded by three groups of cells in, p roximal, distal and lateral positions. The labeling patterns of these cells suggest that they are the initials for the files of cells that c omprise the root. They give rise to four sets of cell files: the stele , the cortex and endodermis, the epidermis and lateral root-cap and th e columella. A model of meristem activity is proposed based on these d ata. This description of Arabidopsis root structure underpins future w ork on the developmental genetics of root morphogenesis.