Plant Molecular Biology 42: 679–688, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 679 A xenobiotic-stress-activated transcription factor and its cognate target genes are preferentially expressed in root tip meristems Susan Klinedinst, Pete Pascuzzi, Julia Redman, Mihir Desai and Jonathan Arias ∗ Center for Agricultural Biotechnology, University of Maryland Biotechnology Institute and Dept. of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA ( ∗ author for correspondence) Received 11 May 1999; accepted in revised form 15 December 1999 Key words: as-1, auxin, glutathione S-transferase, PG13, root meristem, TGA1a, xenobiotic stress Abstract 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 TGA1a, 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. Introduction As-1-type cis elements were first identified in the 35S promoter of cauliflower mosaic virus (CaMV), and later in the promoters of Agrobacterium T-DNA and plant glutathione S-transferase (GST) genes (Bouchez et al., 1989; Lam et al., 1989; An et al., 1990; Benfey et al., 1990; van der Zaal et al., 1991; Kim et al., 1994; Liu and Lam, 1994; Ulmasov et al., 1994; Droog et al., 1995; van der Kop et al., 1996; Xiang et al., 1996). In plants, these cis elements confer root- specific gene expression and enhance transcription in response to hormone and xenobiotic-stress cues (An et al., 1990; Liu and Lam, 1994; Ulmasov et al., 1994; Xiang et al., 1996). Multiple genes that code for a number of related but distinct as-1-binding proteins, termed ‘TGA factors’, have been cloned from a num- ber of higher-plant species. Based upon their predicted amino acid sequences, these plant TGA factors be- long to a large eukaryotic family of signal-responsive basic/leucine-zipper (bZIP) proteins. Among the TGA factors, only TGA1a of tobacco (Katagiri et al., 1989) has yet been shown to mediate basal and stimulus- responsive transcription in vivo (Neuhaus et al., 1994; Pascuzzi et al., 1998). Based on their expression patterns in plants, TGA factors are likely to have tissue-specific contributions to gene expression. For example, transcripts encoding for TGA1a and PG13, two homologous TGA factors of tobacco, are highly expressed in roots (Katagiri et al., 1989; Fromm et al., 1991), whereas the six known TGA factors of Arabidopsis appear to be dif- ferentially expressed in roots, shoots, flowers, and siliques (Schindler et al., 1992; Miao et al., 1994; de Pater et al., 1996; Xiang et al., 1997). In some cases, multiple TGA factors are expressed in the same