Early primordium morphogenesis during lateral root initiation in Arabidopsis thaliana

The first morphogenetic events of lateral root primordium (LRP) formation i n the Arabidopsis thaliana (L.) Heynh. pericycle occur soon after cells of the primary root complete elongation. Pericycle cells in direct contact wit h underlying protoxylem cells participate in LRP formation. Two types of LR P initiation were found. longitudinal uni- and bi-cellular. These occur whe n a single or two pericycle cells within a file. respectively. become found er cells for the entire longitudinal extent of the LRP. Histochemical and c ytological analysis suggests that three is the minimum number of cells requ ired 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 deve lopment, cell growth is limited and therefore cytokinesis leads to a reduct ion of cell volume, similar to cleavage division cycles during animal and p lant embryogenesis . The striking coordination of proliferation between per icycle cells in adjacent riles in direct contact with the underlying protox ylem implies that intercellular signaling mechanisms act in the root apical meristem or later in development.