The Mocular Mechanism Of AtABCG14 Regulating Drought Resistance In Arabidopsis

Drought is one of the common abiotic stresses for plants,which can affect plant growth or development and then reduce biomass.In the process of adapting to drought stress,plants gradually evolved well-run drought-resistant protection system,which enabled them to quickly accept environmental signals and make physiological and biochemical responses.Plant hormones,as intracellular signaling molecules,play important roles in regulating plant responses to abiotic stresses.Endogenous cytokinin(CK)is one of the critical signaling molecule participated in stress responses.It is reported that cytokinins are involved in regulating stomatal movement,reducing transpiration and improving plant drought tolerance under physiological water shortage.tZ(trans-zeatin)plays a major role in these processes.tZ-type cytokinins are mainly synthesized in root and transported over long distances,which act on target cells.AtABCG14 is a key factor of tZ transport,and its absence will affect the acropetal translocation of tZ,resulting in changes in mutant morphology.tZ signaling is a primary response for drought stress.How it regulate the transcription level of stress,developmental genes or stomatal movement in plants remains an significant problem to be solved urgently.To further study the mechanism of cytokinin transported to the apex by AtABCG14 in drought resistance,we combined the genetic,cytologic,hormone metabolism analysis and transcriptomic approaches.The molecular mechanism of AtABCG14 in drought resistance was preliminarily elucidated.The major results are as follows:(1)The expression of AtABCG14 and the content of tZ-type CKs were bothdecreased under drought stress.The GUS staining and qRT-PCR analysis showed that the expression of AtABCG14 was significantly decreased in Arabidopsis seedlings after treated with mannitol or abscisic acid.Hormone quantification showed that most types of the CKs were decreased in the seedlings after treated with mannitol or abscisic acid.(2)The transpiration of atabcg14 mutant was reduced due to disabled stomatal opening.Compared with wild type,the transpiration intensity,water loss were reduced and the drought tolerance was improved in the atabcg14 mutant.The stomatal density was normal but the stomatal diameter was smaller in the mutant,indicating that AtABCG14 participated in regulating stomatal movement and improved water-holding capacity of plants.(3)tZ-type CKs partially rescued the stomatal opening phenotype of atabcg14 to wild-type.Treatment of atabcg14 mutant with low concentration tZ-type CKs can regulate stomatal movement,partly restore stomatal diameter and transpiration of the mutant,indicating that tZ-type CKs transported by AtABCG14 are key factors mediating stomatal movement.(4)Salicylic acid(SA)is located downstream of tZ in the drought resistance pathway mediated by AtABCG14.LC-MS/MS analysis showed that the contents of tZ-type CKs decreased and SA increased respectively in Arabidopsis thaliana wild-type after drought treatment,while the contents of tZ-type CKs and SA-type hormones did not change significantly in atabcg14 mutant.The trends of the change of other hormones were same in wild-type and mutant,suggesting that both tZ-type CKs and SA participated in the drought resistance pathway mediated by AtABCG14.(5)Active oxygen species(ROS)accumulation was regulated by CK and SA signaling in atabcg14 mutant.Dichlorofluorescein staining analysis and real-time fluorescence quantitative PCR analysis showed that the expression of CK responsive genes ARRs,SA synthesizing genes ICS1,ICS2 and ROS synthesizing genes PRX33,PRX34 in atabcg14 mutant was down-regulated compared with wild type.The content of ROS in atabcg14 mutant guard cells was lower than that in wild type,showing that both CK and SA signalings can regulate stomatal closure by loweringthe level of reactive oxygen species.The above results showed that AtABCG14 mediates cytokinin transport and regulates drought resistance in Arabidopsis thaliana under drought conditions.In the above-ground tissues,tZ-type CKs decreased,and the expression of cytokinins response genes A-type ARRs was down-regulated.At the same time,the synthesis of SA also was decreased and it weakened SA signal transduction,resulting in the decrease of active oxygen species content,inhibition of stomatal opening,reduction of transpiration,and improvement of plant drought resistance.