EMBRYONIC ORIGIN OF THE ARABIDOPSIS PRIMARY ROOT AND ROOT-MERISTEM INITIALS

The embryonic origin of the Arabidopsis root and hypocotyl region has been investigated using histological techniques and clonal analysis. O ur data reveal the pattern of cell division in the embryo giving rise to the various initials within the root promeristem. A small region of the root at its connection with the hypocotyl appears not to be deriv ed from the promeristem initials. This region contains two cortical ce ll layer and [H-3]thymidine incorporation data suggest that it lacks p ostembryonic cell divisions. Sectors marked by transposon excision fro m the beta-glucuronidase marker gene are used to investigate cell line ages giving rise to root and hypocotyl. The position of end points fro m sectors with embryonic origin show little variation and hence reveal preferred positions in the seedling for cells derived from different regions of the embryo. The radial extent of complete root sectors is c onsistent with the radial arrangement of root meristem initials at the heart stage of embryogenesis inferred from histological analysis. Usi ng the clonal data, a fate map is constructed depicting the destiny of heart stage embryonic cell tiers, in the seedling root and hypocotyl. The variability in the sector end points indicates that distinct cell lineages are not restricted for root or hypocotyl fate. In contrast, derivatives of the hypophyseal cell do appear to be restricted to the columella and central cell region of the root.