Cell division patterns of the protoderm and root cap in the "closed" root apical meristem of Arabidopsis thaliana

The peripheral root cap and protoderm in Arabidopsis thaliana are organized into modular packets of cells derived from formative T-divisions of the ro ot cap/protoderm (RCP) initials and subsequent proliferative divisions of t heir daughter cells. Each module consists of protoderm and peripheral root cap packets derived from the same periclinal T-division event of an RCP ini tial. Anatomical analyses are used to interpret the history of extensively coordinated cell divisions producing this modular construction. Within a gi ven layer of root cap, the columella and RCP initials divided in a centrifu gal sequence from the innermost columella initials toward the RCP initials. All RCP initials in the lineages around the circumference of the root divi ded nearly simultaneously in "waves" to form one module prior to the next w ave of initial divisions forming a younger module. The peripheral root cap and protoderm packets within each module completed four rounds of prolifera tive divisions in the axial plane to produce, on average, 16 cells per pack et in the basalmost modules in axial view. Peripheral root cap and protoder m cells predominantly in the T-type (trichoblast) lineages also underwent r adial divisions as they were displaced basipetally. The regularity in the c ellular pattern within the modules suggests a timing mechanism controlling highly coordinated cell division in the initials and their daughter cells.