In plants, as-1-type cis elements and their trans-acting factors confer tissue-specific and signal-responsive activities to the promoters of several glutathione S-transferase (GST) genes. Regulation of as-1 is widely thought to involve trans-acting factors that belong to a family of basic/leucine-zipper 'TGA factors' that selectively bind this element. We have previously shown that TGA 1a, a highly conserved TGA factor of tobacco, enhances transcription through as-1 in response to xenobiotic-stress cues. To better understand the functional contribution of this transcription factor to the expression of as-1-regulated genes, we have studied its tissue- and cell-specific localization in tobacco seedlings. We show here that the relative amount of TGA1a transcripts expressed in roots and shoots correlate with the as-1-regulated, basal-level expression of a GUS transgene and two putative target GST genes. In situ hybridization of intact seedlings demonstrated that TGA1a and these GST genes are preferentially expressed in root tip meristems. Similar findings were made with a gene-specific probe for PG13, a homologue of TGA1a, demonstrating that both factors are likely to be present in the same root meristem cells. Furthermore, TGA1a protein was immunologically detected exclusively in the primary root and its meristem. Collectively, these studies suggest that TGA1a, and perhaps PG13, may contribute to the expression of GST isoenzymes, especially in root tip meristems. The biological significance of these observations is discussed.