Molecular Mechanism Of Arabidopsis TGA7 Transcription Factor In Responses To Drought Stress

Drought is one of the major environmental stress factors for plant growth and development.Cellular ABA content is rapidly accumulated under drought stress and leads to a series of stress responses.The increase of ABA level in plants is mainly derived from the release of conjugated ABA and de novo synthesis.Thus,studing the regulatory mechanism of enzymes and regulatory factors involved in this process is important for understanding how plants respond to drought stress.The transcription factor TGA7 belongs to the D group of the bZIP transcription factor family.The previously reported studies have shown that these transcription factors are mainly involved in plant defense against pathogens and development.The preliminary experiments showed that TGA7 null mutant(tga7)displayed drought tolerant phenotype comapared to wild-type plants.This dissertation focused on the regulation mechanism of Arabidopsis response to drought stress by transcription factor TGA7.TGA7 was localized in the nucleus,which had the transcriptional activation activity and the DNAbinding activity.TGA7 was mainly expressed in the leave guard cells.Compared with wild-type plants,tga7 mutant was more tolerant to drought stress and lost less water,and the complementation line could restore this phenotype.In contrast,TGA7-overexpressing lines were much more sensitive to drought stress and lost more water.These results demonstrated that TGA7 involved in plants response to drought stress.Induction of stomatal closure and inhibition of stomatal opening by abscisic acid(ABA)were aggravated in the tga7 mutant and impaired in the TGA7-overexpressing lines.Furthermore the ABA levels in leaves of TGA7-overexpressing lines were more lower than wild-type plants under drought stress,indicating that TGA7 may involve in drought stress response mediated by ABA-regulation of stomatal movement.The Arabidopsis ?-glucosidase BG1 is an enzyme responsible for hydrolyzing conjugated,biologically inactive ABA to produce active ABA.Transcription of BG1 is up-regulated under drought stress and bgl mutant is drought-sensitive.The qRT-PCR results showed that BG1 transcription in TGA7-overexpressing lines was significantly decreased and no significant difference in the tga7 mutant compared to wild-type plants under normal irrigation,while it was significantly enhanced in the tga7 mutant compared to wild-type plants under drought stress.The transient expression assays in Nicotiana benthamiana showed that TGA7 inhibited the expression of BG1.EMSA and ChIP assays showed that TGA7 bound to the BG1 promoter in vitro and in vivo,respectively.ChIP and Western blot results showed that TGA7 bound to the BG1 promoter under the control condition;TGA7 protein was reduced,and the binding of BG1 promoter was abolished under drought stress.These results demonstrated that TGA7 functions as a negative regulator of BG1 transcription in plant response to drought stress.The further experiments showed that the tga7 bgl double mutant was drought sensitive and ABA insensitive in regulation of stomatal movements as the bgl sigle mutant,and the TGA70E BG1OE-12 double overexpressing line was drought tolerant and ABA sensitive in regulation of stomatal movements as the BG1OE13 line.These genetic evidences demonstrated that BG1 acts as a downstream target of TGA7.In summary,the experimental results presented in this dissertation demonstrated that the Arabidopsis transcription factor TGA7 negatively regulated the BG1 transcription for maintaining a basal ABA level under the control condition.Under drought stress,TGA7 protein was reduced,and the inhibition of BG1 was released for increasing the ABA content to resisit drought stress.TGA7 plays an important role in plant response to drought stress by regulating ABA content.