AITRs,a Novel Family Of Transcription Repressors Regulate ABA Signalling And Abiotic Stress Responses In Arabidopsis

The plant hormone abscisic acid(ABA)plays an important role in regulating many aspects of plant growth and development,including seed development,seed germination,bud dormancy,and stomatal movement.However,ABA is best known as a key stress hormone regulating plant responses to multiple environmental stresses such as drought,cold,heat,and high salinity.ABA regulates plant growth and development and plant responds to the environmental cues in different ways.For example,ABA promotes root stem cell maintenance through ABFs and WOX5,and maintains primary root growth by restricting the ethylene production through ACS2/5.ABA limits water loss through promoting wax biosynthesis and stomatal closure.ABA promotes bud dormancy by coordinating leaf senescence,starch degradation and carbon translocation from source to sink via transcriptional reprogramming.ABA protects plants from drought stress by promoting the accumulation of LEAs and prolines.But all these functions of ABA are achieved via signal transduction.ABA signaling is controlled by interplay of several key regulators including receptors PYR1(PYRABACTIN RESISTANCE 1)/PYL(PYR1?like)/ RCAR(REGULATORY COMPO-NENT of ABA RECEPTOR),A-group PP2Cs(A?group PROTEIN PHOSPHATASE 2Cs),SnRKs(SUCROSE NONFERMENTING 1(SNF1)?RELATED PROTEIN KINASES),and ABF/AREB/ABI5?type bZIP(BASIC REGION LEUCINE ZIPPER)transcription factors.PP2 Cs are negative regulators of ABA signaling,whereas SnRK2 s are positive regulators of ABA signaling.ABA signaling is initiated by the perception of ABA molecules by the receptors.Based on genetic and biochemical evidence,it is thought that PP2 Cs interact with SnRK2 s at the absence of ABA.Binding of ABA promotes the interaction of PYR/PYL/RCAR receptors with PP2 Cs,leading to the derepression of SnRK2 kinases.SnRK2 s are then able to phosphorylate and activate their targets such as ABF/AREB/ABI5?type bZIP transcription factors,thus eliciting ABA responses.We report here the identification of AITRs(ABA?INDUCED TRANSCRIPTION REPRESSORS)as a family of novel transcription factors.We found that the expression of all the 6 Arabidopsis AITR genes was induced by exogenously ABA,and their expression levels were decreased in ABA biosynthesis mutant aba1?5,suggesting that AITRs are ABA response genes.When recruited to the promoter region of the reporter gene by a fused DNA binding domain,all AITRs inhibited reporter gene expression in transfected protoplasts,indicating that AITRs function as transcription repressors.The aitr Arabidopsis mutants showed reduced sensitivities to ABA and to stresses such as salt and drought,indicating that AITRs positively regulate plant response to ABA and abiotic stresses.No obviously different was observed between the mutants and the Col-0 wild type plant at all the growth stages during their whole life cycle,including vegetative growth,flowering and seed producing stages,and plant response to pathogen infection was also not affected in the mutants.These observation suggest that knockout of the AITR family genes in Arabidopsis enhanced abiotic stress tolerance without fitness costs.Quantitative RT?PCR analysis demonstrated that the ABA response of some PP2 C and PYR/PYL/RCAR genes was reduced in AITR5 transgenic plants but increased in the aitr2 aitr5 aitr6 triple mutant.Ch IP?PCR results showed that AITR5 was directly associated with the promoter region of ABI2.These results indicate that AITRs positively regulate ABA signalling by regulating the expression of key regulatory factor genes.To summarize,our results showed that AITRs are ABA-induced transcription repressors that act as positive feedback regulators in ABA signaling.The Arabidopsis aitr mutants showed reduced sensitivity to ABA,and enhanced tolerance to abiotic stresses without fitness costs.