Tobacco TGA7 mediates gene expression dependent and independent of salicylic acid

ABSTRACTBasic region leucine zipper (bZIP) transcription factors of the TGA family control gene expression in response to diverse stimuli. Arabidopsis clade II and clade III TGA factors mediate salicylic acid (SA)-induced expression of PATHOGENESIS-RELATED GENE1 (PR-1) via interplay with NONEXPRESSOR OF PR GENES1 (NPR1, a.k.a. NIM1). Interaction with TGA factors occurs through the central ankyrin repeat domain of NPR1. In a yeast two-hybrid screen with the NPR1 bait, we identified TGA7, a novel member of the tobacco (Nt) TGA family grouping to clade III. TGA7 is most similar to NtTGA1a, and, like NtTGA1a, TGA7 displays transcription activity in yeast. Unexpectedly, TGA7 preferentially and uniquely interacts with the SA-sensitive C-terminal region of NtNPR1, demonstrating that NtNPR1 harbors multiple distinct TGA factor binding sites. Interaction with NPR1 impairs TGA7 transcription activity in yeast. Furthermore, TGA7 binding to the NtNPR1 C-terminus is outcompeted by SA-induced type 2 NIM1-INTERACTING (NIMIN) proteins. In tobacco plants, a TGA7–Gal4 DNA-binding domain chimeric protein (TGA7GBD) mediates SA-responsive reporter gene expression in young leaf tissue and spontaneous reporter activation in older leaves displaying PR-1 gene expression. Astonishingly, TGA7GBD is also able to activate the reporter independent from PR-1 gene expression in noninduced cotyledons of tobacco seedlings. Together, our findings support a model in which TGA7 mediates both SA-dependent and SA-independent gene activation controlled by the plant’s developmental stage and by the C-terminal region of constitutively accumulating NtNPR1.

From Pioneers to Team Players: TGA Transcription Factors Provide a Molecular Link Between Different Stress Pathways

The plant immune system encompasses an arsenal of defense genes that is activated upon recognition of a pathogen. Appropriate adjustment of gene expression is mediated by multiple interconnected signal transduction cascades that finally control the activity of transcription factors. These sequence-specific DNA-binding proteins act at the interface between the DNA and the regulatory protein network. In 1989, tobacco TGA1a was cloned as the first plant transcription factor. Since then, multiple studies have shown that members of the TGA family play important roles in defense responses against biotrophic and necrotrophic pathogens and against chemical stress. Here, we review 22 years of research on TGA factors which have yielded both consistent and conflicting results.

Arabidopsis Clade I TGA Transcription Factors Regulate Plant Defenses in an NPR1-Independent Fashion

Transcriptional reprogramming during induction of salicylic acid (SA)-mediated defenses is regulated primarily by NPR1 (NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1), likely through interactions with TGA bZIP transcription factors. To ascertain the contributions of clade I TGA factors (TGA1 and TGA4) to defense responses, a tga1-1 tga4-1 double mutant was constructed and challenged with Pseudomonas syringae and Hyaloperonospora arabidopsidis. Although the mutant displayed enhanced susceptibility to virulent P. syringae, it was not compromised in systemic acquired resistance against this pathogen or resistance against avirulent H. arabidopsidis. Microarray analysis of nonelicited and SA-treated plants indicated that clade I TGA factors regulate fewer genes than NPR1. Approximately half of TGA-dependent genes were regulated by NPR1 but, in all cases, the direction of change was opposite in the two mutants. In support of the microarray data, the NPR1-independent disease resistance observed in the autoimmune resistance (R) gene mutant snc1 is partly compromised by tga1-1 tga4-1 mutations, and a triple mutant of clade I TGA factors with npr1-1 is more susceptible than either parent. These results suggest that clade I TGA factors are required for resistance against virulent pathogens and avirulent pathogens mediated by at least some R gene specificities, acting substantially through NPR1-independent pathways.

NPR1 enhances the DNA binding activity of the Arabidopsis bZIP transcription factor TGA7This paper is one of a selection of papers published in a Special Issue from the National Research Council of Canada – Plant Biotechnology Institute.

Pathogen-induced transcriptional reprogramming of the plant genome is mediated predominantly by the cofactor NPR1 (NON-EXPRESSOR OF PATHOGENESIS-RELATED GENES1). NPR1 lacks any known DNA-binding domain and is proposed to regulate transcription through interactions with TGA transcription factors that bind to as-1-like promoter elements. Previous studies have focused on the interaction of NPR1 with subgroup I (TGA1, TGA4) or subgroup II (TGA2, TGA5, TGA6) factors. Using the yeast two-hybrid system, we showed that a member of subgroup III (TGA7) interacts with wild-type NPR1 but not with mutants in the ankyrin repeats that are important for disease resistance. Mutations in the NPR1 BTB/POZ domain also greatly reduced interaction with TGA7. NPR1 substantially increased the binding of TGA7 to cognate promoter elements in vitro, including a salicylic-acid-inducible element of the PR-1 promoter. While TGA7 interacted with all TGA factors tested, interactions were not observed between TGA2 and subgroup I factors, indicating that cross-clade interaction is not a general property of the family. Transcripts from subgroup III TGA factors were weakly inducible by salicylic acid and pathogens, but only TGA3 expression was dependent on NPR1. These results suggest that NPR1-mediated DNA binding of TGA7 could regulate the activation of defense genes.