Clonal Analysis Reveals Gradual Recruitment of Lateral Root Founder Cells and a Link between Root Initiation and Cambium Formation in Arabidopsis thaliana

The pericycle gives rise to lateral roots (LRs) and lateral meristems (LMs; cambium and phellogen), however, a thorough clonal analysis of pericycle cell lineage has not been investigated. This study fills in this gap and addresses pericycle impact in LR and LM development. Heath-shock inducible DS1 transposition in 35S-DS1-H2B:YFP HS-Ac seedlings results in production of YFP-labelled cell clones (Kurup et al., 2005, Plant J 42: 444-453). These clones in pericycle cell derivatives were identified with a confocal microscopy and subjected to 3D reconstructions and analysis. It was found that participation of pericycle founder cells (FC) in LR formation is more variable than previously considered. LR initiation was found most commonly involved the specification of just one FC in the longitudinal and one or two cells in transverse direction. The data suggest existence of a stage 0 of LR primordium formation. This stage starts when a single FC in a longitudinal direction undergoes an anticlinal division and gives rise to roughly equal pericycle cells whose nuclei migrate to the recently formed cell wall. After LR initiation, new FCs continue to be recruited in both directions from pre-existing cells. Anticlinal divisions in the pericycle resulting in LMs start already in the young differentiation zone where only the protoxylem is differentiated. The clonal analysis demonstrated that pericycle cell activity related to LR formation is not separated in time and space from that related to LM formation and that LR FC recruitment is a gradual process. All pericycle descendants become LRs or LMs and no self-renewal property is maintained in this primary plant tissue. The work was funded by DGAPA-PASPA-UNAM, DGAPA-PAPIIT-UNAM (IN200818), and CONACyT (237430).