This study is concerned with the analysis of the AtEMl0l promoter trap line of Arabidopsis thaliana. AtEMl0l seedlings show GUS expression in the tips of both primary and lateral roots, and more weakly in the hypocotyl and cotyledons. GUS activity in mature plants is found variably in both rosette and cauline leaves, stem nodes and also siliques but not other floral organs. Active auxins rapidly upregulate whilst cytokinins downregulate GUS transcript levels. AtEMl0l roots are shorter than those of the wild-type, a phenotype which is putatively linked to elevated ethylene levels. AtEMl0l roots were also found to be hypersensitive to exogenous cytokinins. Root patterning is not affected, but cells distal to the elongation zone are shorter in the AtEMl0l line than the wild-type. The T-DNA in line AtEMl0l was found to have inserted in a small, low abundance gene named POLARIS, which encodes a putative 36 amino acid polypeptide, which does not share homology to any known genes. POLARIS shows unusual genome organisation, with its 5' end overlapping with the 3' end of an upstream gene. Upstream sequence, embedded within the upstream gene, when fused to GUS were able to direct expression in root tips whilst a longer fragment mimics the GUS expression of the AtEMl0l line. Retransformation of the AtEMl0l line with a wild-type allele of POLARIS was able to complement the mutant phenotype indicating that the T-DNA insertion into POLARIS is responsible for the AtEMlOl phenotype. Overexpression of POLRIS resulted in transgenic plants with reduced sensitivity to both cytokinins and ACC. The structure of the POLARIS locus and the potential role of POLARIS in regulating cytokinin-induced ethylene levels, with regards to the control of root growth, are discussed.