Cell geometry and nuclear organization are associated with founder-cell competence for lateral root initiation in the...

Lateral root initiation in Arabidopsis thaliana occurs in xylem pole pericycle (Px) cells, yet the structural features associated with founder-cell competence in differentiated root tissues remain insufficiently defined. Here, we combined spatial cell biology approaches, including three-dimensional analysis of cell geometry, nuclear morphometry, EdU labeling, and auxin-induction experiments, to determine whether Px cells exhibit a structural prepattern before visible organogenesis. Px cells were already distinguishable in the root apical meristem (RAM) by their shorter length along the longitudinal root axis and larger volume relative to the remaining non-xylem-pole pericycle cells (Pr), and this geometrical distinction persisted into the differentiation zone. In the same zone, Px nuclei were less elongated and more spherical than Pr nuclei, indicating a characteristic nuclear organization. In the absence of exogenous auxin, DNA replication activity was confined mainly to inner tissues, particularly the pericycle, vasculature, and developing lateral root primordia. Upon auxin treatment, PIN1 accumulation and EdU incorporation appeared in discrete domains rather than uniformly along the root axis, indicating localized auxin responsiveness and localized cell-cycle re-entry. Continuous auxin treatment for 36h induced formative divisions in the Px domain of the early differentiation zone, where lateral root initiation occurs. Early primordia further displayed internal geometric zonation and pronounced deformation of the overlying endodermis, indicating that internal organization and tissue accommodation arise early during primordium development. Together, these observations support a model in which Px-associated cell geometry and nuclear organization represent structural correlates of founder-cell competence, while emphasizing that functional tests in signaling mutants are required to determine causality.