CKXs Integrate Auxin And Ethylene Signal To Regulate Root Growth And Development In Arabidopsis

In recent years,it is one of the hotspots that how these hormones interact with each other to promote and maintain plant growth and development.Auxin,cytokinin and ethylene are the main plant hormones and play an important role on the patterning of the root.Studies identified a hormonal interaction network in which auxin controls local activation of cytokinin.In this network,the auxin activates the biosynthesis of cytokinin to control vascular tissue patterning.Cytokinin can regulate the transportation of auxin and the formation of lateral roots.A complex regulatory network is formed by different plant hormones regulating the synthesis,metabolism,signal transduction and polar transport of each other,and involves in plant growth and development and environmental adaptation.Cytokinin is one of the main plant hormones that affect many aspects of plant growth and development,including cell division,shoot initiation and growth,leaf senescence,apical dominance,sink/source relationships,nutrient uptake,phyllotaxis,and vascular,gametophyte,and embryonic development,as well as the response to biotic and abiotic factors.Cytokinins are N6 substituted adenine derivatives.CKX(Cytokinin Oxidase)encodes cytokinin oxidase degrades cytokinins irreversibly,and maintain the homeostasis of cytokinin.There are seven CKXs genes in Arabidopsis rhaliana.The spatio temporal expression patterns and molecular regulation mechanisms of them in plants are not well understood.Overexpression of CKX caused abnonmal growth of Arabidopsis thahana root suggested CKX were involved in plant root development.In this paper,we conduct the prehmitary study of signalling intergration between auxin,ethylene and cytokinin in regulating primary root and lateral root development and get following conclusions:1.Phenotypic analysis indicated that the primary root is shorter in ckx2-1,ckx6-1,ckx6-2 and ckx5/6 mutants,and the density of lateral root decreased in ckx5-1,ckx5-2,ckx6-1 and ckx6-2 mutants when compared to wild type.These studies can indicate that CKXs are involved in the growth and development of Arabidopsis zhaliana's PR and LR.2.Expression of CKXs in lateral root and primary root was induced by auxin in CKXp::GUS line.The CKX5 and CKX6 are expressed in the PR and LR,and auxin can increase the expression of GUS in CKX5p::GUS and CKX6p::GFP-GUS.3.With the NAA treatment,the lateral root density of the ckxs mutants increases significantly,compared with wild type.It can indicate that CKXs may mediate auxin signaling and regulate the growth and development of lateral roots.The expression of CKX is significantly up-regulated under NAA treatment while NAA can not induce the upregulation of CKX in arf7/arf19 double mutant.Protoplast transient expression experiments also demonstrated that ARF7 is a transcriptional activator of the CKX.In conclusion,auxin can up-induce the expression of CKX,and then affect the formation of Arabidopsis thaliana by regulating the content of intracellular cytokinin.4.Ethylene is an crucial plant hormone.It is necessary for growth and development of roots to keep moderate concentration.Treating ckxs mutants with ethylene demonstrate that the is sensitive to ethylene and show a shorter root under the exogenous ACC(1-aminocyclopropane-1-carboxylic acid),compared with wild type.And this phenotype only occurs in the ckx2-1 mutant;the other ckx mutant has no obvious phenotype.These results suggested that regutaltion of Cytokine signaling pathway by ethelyne is especially by CKX2.We examine the expression of CKXs in 35S::EIL1,35S::EIN3 and double mutant ein3/eill and find the expression of CKX2 and CKX4 obviously up-regulated in ein3eill double mutant.Phylogenetic tree analysis show that CKX2 and CKX4 are close to each other.It is not confirmed by the transient expression of protoplasts that 35S::EIL1 and 35S::EIN3 induce the expression of CKXs.However,in the ein3eill double mutant,TCSn::GFP signal is significantly weaker than that of wild type.It indicates that EIN3 and EILI inhibit the expression of the CKX2 and increase the content of endogenous cytokinin,thereby inhibit the cytokinin signaling pathway.This study indicated that the transcription factors in auxin signaling pathway,ARF7 and ARF19,regulate the expression of CKXI,CKX3,CKX5,and CKX6 while EIN3/EIL1 regulate the expression of CKX2 and CKX4,to influence the internal content of cytokinin.In summary,our study showed that auxin and ethylene regulate the expression of CKXs and influence the internal stability of cytokinin,thereby regulate the growth and development of Arabidopsis thaliana root.The complex relationship in the three hormones needs to be further studied.