GROOT, E.P., K. CHAPMAN*, and T.L. ROST. Section of Plant Biology, University of California, Davis, CA 95616. - Root apical meristem organization is a dynamic character in dicot roots.
The root apical meristem (RAM) is the source of the cells making up
the root cap and body. Initial cells in the RAM divide, producing
derivative cells that elongate, mature and differentiate into specific
cell types. In closed RAM organization these initials are arranged in
specific tiers (histogen layers); in open organization cell files
terminate in a zone of initials lacking apparent organization. Despite
this obvious difference, both open and closed roots form protoderm and
peripheral root cap through a periclinal T-division of a root cap
protoderm initial. It has been commonly assumed that RAM organization
remains constant over the developmental life of the root. The primary
roots of Arabidopsis (a closed species) became determinate in
3-4 weeks, this process was correlated with a loss of closed RAM
organization. In addition, the RAM of Pisum, an open species,
changed in shape and size during the lifetime of the root. Studies of
other dicot species reinforced these findings. T-divisions appeared to
be a constant feature among dicots because it was present regardless
of root age and organization. We will propose that RAM organization in
dicot roots undergoes certain changes through ontogeny. 1. All
the radicles of early embryos have closed organization. 2. The
organization changes by the time germination occurs in open species,
but remains the same in closed species. 3. By the time a root
has reached its determinate stage, all RAMs have open organization.
The RAM of dicots is a dynamic structure; therefore, it is important
to consider the developmental state of the root when classifying
apical organization types.
Key words: closed organization, determinacy, ontogeny, open organization, root apical meristem, t-division