Functional analysis of plant TGA transcription factors

To understand further plant regulatory mechanisms underlying gene networks, I investigated the in vivo contributions of individual TGA factors and their cognate as-1-type cis-elements to the expression of plant genes involved in protective mechanisms such as detoxification and host-defense. I showed that TGA factors contribute directly to the expression of these protective genes, perhaps through a conserved regulatory mechanism. These studies resulted in elucidation of several regulatory circuits, from activation of TGA factors to their binding of cognate promoters and the up-regulation of target genes.; Prior findings indicated that a tobacco TGA factor termed TGA1a is involved in xenobiotic (chemical) detoxification. I showed that within the context of the native factor its regulatory carboxyl-terminal domain inhibits trans-activation by the amino-terminal domain under non-stimulated conditions, whereas exposure to xenobiotic stress relieves this repression. This activity is likely to be mediated by a nuclear co-repressor protein known as p120, given that the timing of its binding to TGA1a is inversely proportional to the xenobiotic stress-induced expression of a target glutathione S-transferase (GST) detoxification gene. I further showed that the stimulus-induced binding in vivo of TGA1a occurs at a subset of as-1 -containing GST promoters that are responsive to xenobiotic stress.; TGA factors are also implicated in host-defense. I confirmed this point by showing that two Arabidopsis TGA factors, namely TGA2 and TGA3, are the principal contributors among nuclear proteins that bind in vitro to the activating as-1-type element in the promoter of the defense gene PR-1. The defense hormone salicylic acid (SA) and the mediator NPR1 enhance this binding activity. The biological significance of this binding activity was inferred from the fact that TGA2 and TGA3 are specifically recruited in vivo to the PR-1 promoter in a SA- and NPR1-dependent manner, with kinetics that parallel the enhanced transcription of the gene. I showed that TGA3 is a transcriptional activator, as previously noted for TGA2, which is consistent with the notion that their recruitment enhances transcription. Findings from these studies are also consistent with a role for a repressor in regulating TGA factor activity in the host-defense pathway.