Clonal analysis of the Arabidopsis root confirms that position, not lineage, determines cell fate

The cellular organization of the Arabidopsis thaliana (L.) Heynh. root meri stem suggests that a regular pattern of cell divisions occurs in the root t ip. Deviations from this pattern of division might be expected to disrupt t he organization of cells and tissues in the root. A clonal analysis of the 3-d-old primary root meristem was carried out to determine if there is vari ability in division patterns, and if so to discover their effect on cellula r organization in the root. Clones induced in the seedling meristem largely confirmed the predicted pattern of cell divisions. However, the cellular i nitials that normally give rise to the different cell files in the root wer e shown to exhibit some instability. For example, it was calculated that a lateral root cap/epidermal initial is displaced every 13 d. Furthermore, th e existence of large marked clones that included more than two adjacent cel l layers suggests that intrusive growth followed by cell division may occur at low frequency, perhaps in response to local cell deaths in the meristem . These findings support the view that even in plant organs with stereotypi cal cell division patterns, positional information is still the key determi nant of cell fate.