Early primordium morphogenesis during lateral root initiation in Arabidopsis thaliana.

The first morphogenetic events of lateral root primordium (LRP) formation in the Arabidopsis thaliana (L.) Heynh. pericycle occur soon after cells of the primary root complete elongation. Pericycle cells in direct contact with underlying protoxylem cells participate in LRP formation. Two types of LRP initiation were found, longitudinal uni- and bi-cellular. These occur when a single or two pericycle cells within a file, respectively, become founder cells for the entire longitudinal extent of the LRP. Histochemical and cytological analysis suggests that three is the minimum number of cells required to initiate an LRP. In young primordia comprising less than 32 cells, the average cell-doubling time was 3.7 h, indicating a drastic acceleration of cell cycle progression after lateral root initiation. Early in LRP development, cell growth is limited and therefore cytokinesis leads to a reduction of cell volume, similar to cleavage division cycles during animal and plant embryogenesis. The striking coordination of proliferation between pericycle cells in adjacent files in direct contact with the underlying protoxylem implies that intercellular signaling mechanisms act in the root apical meristem or later in development.