Trans-dominant suppression of plant TGA factors reveals their negative andpositive roles in plant defense responses

Salicylic acid (SA) is a key regulator for the induction of systemic acquir ed resistance (SAR), and NPR1 is a critical mediator for the biological eff ects of SA. Physical interactions between NPR1 and TGA factors, a conserved family of basic-leucine-zipper (blip) proteins in plants, have suggested a role for these transcription factors in mediating SAR induction via the re gulation of defense genes. To elucidate this function, we constructed a tra ns-dominant mutant that specifically eliminates DNA-binding activities of t his class of bZip proteins in transgenic tobacco plants. Our results demons trate that the loss of TGA DNA-binding activities is correlated with suppre ssion of two xenobiotic-responsive genes, GNT35 and STR246, and enhanced in duction of pathogenesis-related (PR) genes by SA. In addition, these TGA-su ppressed plants exhibited higher levels of PR gene induction by pathogen ch allenge and an enhanced SAR. These results suggest that TGA transcription f actors serve both negative and positive regulatory roles in mediating plant defense responses.