Antagonistic Regulation By Cytokinin And ABA Signaling In The Stress Responses In Arabidopsis

Sessile plants are suffered from various environmental stresses,and have evolved sophisticate mechanisms to cope with adverse conditions.Drought stress affects plant growth and development,as well as crop production.Abscisic acid(ABA)is identified as a stress hormone which is important for plant development and responses to stress.Drought stress dramatically induces ABA accumulation to induce stomata closure and inhibit transpiration,leading to enhanced drought tolerance of plants.Recent studies have shown that cytokinin(CK)and ABA antagonistically regulate many developmental processes and environmental stress responses in plants.However the underlying molecular mechanism remains unclear.Here,we show that type-B ARRs antagonistically affect ABA signaling pathway on post-germination and dehydration stress response.Biochemical studies showed that ARR1,11,and 12 physically interact with SnRK2.6/OST1 and inhibit its kinase activity.Consistent with this result,the arrl,11,12triple mutant showed hypersensitive to ABA during seed germination and drought tolerance.Genetic analysis demonstrated that SnRK2s act downstream of ARR1,11,12 in mediating ABA response.The ABA sensitivity and drought tolerance of arr1,11,12 was partially rescued by snr2.2,2.3 and snrk2.6 respectively.These data suggest that type-B ARRs redundantly repress ABA signalling,which is dependent on SnRK2s.type-A ARRs are negative regulators of cytokinin signaling pathway via feedback loop.Conversely,ABA-activated SnRK2.6 phosphorylate type-A ARR5 at residues Ser21,Ser33,Ser72,and Ser117 to enhance its stability.Plants overexpressing ARR5 but not the non-phosphorylated ARR54A exhibited hypersensitivity to ABA.Therefore,we propose that SnRK2s-ARRs module modulates the antagonistic relationship underlying ABA-CK signalling in the response of dehydration stress in plants.In addition,we found exogenous ABA inhibits the cytokinin signal activity.The transcription factor ABI4(abscisc acid insensitive4)promotes ARR6,ARR7,and ARR15 expression by directly binding to the CACCG motif of their promoters during seed germination.The single and multiple mutants of ARR6,7,15 showed ABA hypersensitivity during seed germination.Genetic analysis showed that arr7 arr15 can partially rescue the ABA insensitive phenotype of abi4-1.These data indicate that ARR6,ARR7,and ARR15 act as direct target genes of ABI4 to regulate the ABA response in seed germination.Taken together,this study reveals the antagonistic mechanism between ABA and cytokinin signaling at both the transcriptional and the post-translational level in seed germination and stress response.This study will shed more light on the molecular mechanism underlying interactions of plant hormones in plant responses to stress.