No abstract

In higher plants, the root-shoot axis established during embryogenesis is extended and modified by the development of primary and lateral apical meristems. While the structure of several shoot apical meristems has been deduced by combining histological studies with clonal analysis, the application of this approach to root apical meristems has been limited by a lack of visible genetic markers. We have tested the feasibility of using a synthetic gene consisting of the maize transposable element Activator (Ac) inserted between a 35S CaMV promoter and the coding region of a [3-glucuronidase (GUS) reporter gene as a means of marking cell lineages in roots. The GUS gene was activated in individual cells by Ac excision, and the resulting sectors of GUS-expressing cells were detected with the histochemical stain X-Gluc. Sectors in lateral roots originated from both Ac excision in meristematic cells and from parent root sectors that bisect the founder cell population for the lateral root initial. Analysis of root tip sectors confirmed that the root cap, and root proper have separate initials. Large sectors in the body of the lateral root encompassed both cortex and vascular tissues. The number of primary initial cells predicted from the size and arrangement of the sectors observed ranged from two to four and appeared to vary between roots. We conclude that transposon-based clonal analysis using GUS expression as a genetic marker is an effective approach for deducing the functional organization of root apical meristems.