Auxin-induced Stress Potentiates trans-activation by a Conserved Plant Basic/Leucine-zipper Factor

Abstract

The promoter element activation sequence-1 (as-1) confers tissue-specific and signal-responsive transcription in plants. Hormone and chemical stress cues are thought to activate as-1-dependent transcription through specific basic/leucine-zipper proteins, termed TGA factors, that bind this element. We report here that a highly conserved TGA factor of tobacco, TGA1a, can selectively activate transcription in response to micromolar concentrations of auxin hormones or their analogs. This induction is chemically specific, as a range of other compounds tested at similar concentrations had little or no effect. Auxin was found to augment the transactivation potential of TGA1a through carboxyl-terminal residues. The amino-terminal domain of TGA1a, by gain-of-function assays, was found to both constitutively activate transcription and maximize the response to auxin. Further evidence indicates that the trans-activation potential of this domain in TGA1a is repressed, under basal conditions, by carboxyl-terminal residues. Because TGA1a and endogenous TGA factors are stimulated by auxin only at concentrations that inhibited cell growth, this response is likely to involve chemical stress.The expression of nuclear genes in response to cellular and environmental cues is largely regulated by trans-acting factors and their cognate cis-elements. In plants, auxin hormones (e.g., indole-3-acetic acid) promote growth and affect a number of cellular processes. Auxins are believed to mediate at least some of their effect by enhancing the expression of specific genes through target cis-elements (1, 2). The prototype for one class of auxin-responsive element is activation sequence-1 (as-1), 1 which was first identified by Lam et al. (3) in the cauliflower mosaic virus (CaMV) 35 S promoter. Homologs of as-1 (e.g., ocs and nos), moreover, occur in the promoters of plant-transforming T-DNA genes of agrobacteria (4 -6). Apart from regulating these genes in plants, a broader role for as-1 elements has been suggested by their functional presence in plant glutathione S-transferase genes (7-9). For example, auxin-responsive expression of at least one member of the tobacco GST gene family, GNT35, is mediated by as-1 (8, 9). Unlike other auxin-responsive elements, as-1 requires physiologically high (i.e. micromolar) concentrations of auxins to activate transcription, suggesting that this response may involve chemical stress. This view is further supported by the observation that biologically inactive analogs of auxins are also strong inducers of as-1-dependent activity.Multiple TGA factors that bind selectively to as-1 and related motifs have been cloned from several plant species, and some or all of these factors are likely to mediate as-1-dependent transcription. Based on their amino acid homology, at least three subclasses of TGA factors occur in plants, with multiple members in each class. Heterodimerization between these factors may generate new combinations with potentially distinct functional properties (10). Posttranscriptional regulation ...

In Vivo Interaction between NPR1 and Transcription Factor TGA2 Leads to Salicylic Acid–Mediated Gene Activation in Arabidopsis

“…However, the target genes for TGA2 have yet to be identified. The binding sites for the TGA factors exist in many genes, responding to diverse signals in pathogen defense, wounding, and xenobiotic stresses (Xiang et al, 1996;Pascuzzi et al, 1998;Chen and Singh, 1999). In tobacco, TGA1a is involved in regulating several auxin-induced glutathione S-transferase genes such as GNT1 and GNT35 (Pascuzzi et al, 1998;Johnson et al, 2001).…”

“…In this study, we focus on understanding the function of TGA2. As shown in Figure 1A, the TGA2 transcription factor consists of three distinct domains: the N-terminal domain (NT; amino acids 1 to 46), which affects the stability (Zhou et al, 2000) and possibly the transactivation activity of the protein (Pascuzzi et al, 1998); the bZIP domain (bZIP; amino acids 47 to 94), which is involved in DNA binding and dimerization (Katagiri et al, 1992); and the C-terminal domain (CT; amino acids 95 to 330), which is sufficient for interaction with NPR1 in yeast Zhou et al, 2000). Crystal structure studies of other bZIP transcription factors suggest that each of these domains is independent in folding and function (O'Shea et al, 1991;Ellenberger et al, 1992).…”

Salicylate Accumulation Inhibits Growth at Chilling Temperature in Arabidopsis

“…The apparent capacity of NPR1 to function in diverse responses may reflect the versatility of the TGA transcription factors with which it may interact. Seven TGA factors with differing affinities for NPR1 are known in Arabidopsis (Després et al, 2000;Zhou et al, 2000), and TGA factor binding sites occur in promoters responding to diverse stresses (Pascuzzi et al, 1998;Chen and Singh, 1999). Clarke et al (2000) noted that the npr1 mutation caused an increase in endogenous SA and proposed NPR1 may not only transduce the SA signal, but also reduce SA accumulation.…”

Functional Analysis of Regulatory Elements in the Gene Promoter for an Abscission-Specific Cellulase from Bean and Isolation, Expression, and Binding Affinity of Three TGA-Type Basic Leucine Zipper Transcription Factors

“…The involvement of TGA-type bZIP transcription factors in plant development is well documented (Izawa et al, 1993) and, although there have been descriptions of bZIP transcription factors involved in auxin-inducible (Pascuzzi et al, 1998), pathogeninduced (PR), and salicylic acid-regulated gene expression (Despres et al, 2000;Zhou et al, 2000), we are unaware of any reports of bZIP factors acting independently to direct ethylene-induced gene expression. However, the Arabidopsis ocs-binding factor (OBF4), a bZIP protein, and an ethyleneresponsive element-binding protein (AtEBP) form a complex that activates the ethylene-inducible PR promoter from the tobacco PRB-1b gene (Buttner and Singh, 1997).…”

“…The reverse orientation of the Z-BAC element is the same orientation as the TGA motifs in the as-1 element. In this context, it is interesting that auxin has been demonstrated to enhance as-1-dependent transcription (Pascuzzi et al, 1998).…”