PROMOTER TRAP MARKERS DIFFERENTIATE STRUCTURAL AND POSITIONAL COMPONENTS OF POLAR DEVELOPMENT IN ARABIDOPSIS

To investigate mechanisms involved in establishing polar organization in Arabidopsis embryos and seedlings, we used promoter trapping to ide ntify molecular markers (beta-glucuronidase fusion genes) expressed in spatially restricted patterns along the apical-basal axis, Three mark ers were identified that are expressed, respectively, in the embryonic and seedling root tip (POLARIS), cotyledons and shoot and root apices (EXORDIUM), and root cap (COLUMELLA). Each marker was crossed into th e mutants hydra and emb30, which are defective in embryonic and seedli ng morphogenesis, All three markers were expressed in hydra mutants in patterns similar to those observed in phenotypically wild-type embryo s and seedlings. In emb30 mutants, the EXORDIUM marker was expressed i n cotyledons but not in the expected position of shoot and root merist ems, and the marker COLUMELLA was not expressed at all, which is consi stent with the view that the emb30 mutant, but not hydra, lacks shoot and root meristems. However, POLARIS was expressed in the basal part o f hydra embryos lacking an embryonic root and in the basal parts of bo th hydra and emb30 seedlings. Expression of POLARIS is inducible by ex ogenous auxin and suppressed by cytokinin but is unaffected by inhibit ors of polar auxin transport or cell division. We conclude that POLARI S differentiates positional aspects of polar development from structur al aspects.