BETA-GLUCURONIDASE AS A MARKER FOR CLONAL ANALYSIS OF TOMATO LATERAL ROOTS

In higher plants, the root-shoot axis established during embryogenesis is extended and modified by the development of primary and lateral ap ical meristems. While the structure of several shoot apical meristems has been deduced by combining histological studies with clonal analysi s, the application of this approach to root apical meristems has been limited by a lack of visible genetic markers. We have tested the feasi bility of using a synthetic gene consisting of the maize transposable element Activator(Ac) inserted between a 35S CaMV promoter and the cod ing region of a beta-glucuronidase (GUS) reporter gene as a means of m arking cell lineages in roots. The GUS gene was activated in individua l cells by Ac excision, and the resulting sectors of GUS-expressing ce lls were detected with the histochemical stain X-Gluc. Sectors in late ral roots originated from both Ac excision in meristematic cells and f rom parent root sectors that bisect the founder cell population for th e lateral root initial. Analysis of root tip sectors confirmed that th e root cap, and root proper have separate initials. Large sectors in t he body of the lateral root encompassed both cortex and vascular tissu es. The number of primary initial cells predicted from the size and ar rangement of the sectors observed ranged from two to four and appeared to vary between roots. We conclude that transposon-based clonal analy sis using GUS expression as a genetic marker is an effective approach for deducing the functional organization of root apical meristems.