Auxin Mediates The Regulation Of Weak Alkaline Condition On Arabidopsis Root Development

Soil is an indispensable and limited resource for human beings and a basic condition for agricultural production.As a result of artificial irrational farming practices,soil degradation is severe and soil alkalization becomes one of the important manifestations of soil degradation.A large number of studies have shown that alkaline environment can inhibit the growth of plant roots,and the specific mechanism of action has long been highly concerned,and a large number of studies have been carried out regarding this topic.However,little is known about the effects of weak alkaline and neutral environments on plant growth.We selected dicotyledonous plant Arabidopsis and preliminarily explored the root responses under different pH conditions.We found that the neutral and weak alkaline growth conditions(pH 7.0-pH 8.0)promoted the growth of Arabidopsis roots,as the primary root(PR)elongation and lateral root(LR)formation were both increased.Then,to explore the mechanism of promoting root growth in Arabidopsis under weak alkaline condition,we determined a series of root growth parameters,including the number of prebranch sites and lateral root primordium,the activity of meristem activity,the changes of auxin response and cell cycle expression,the oscillation frequency and amplitude of DR5 signal in the oscillation zone under weak alkaline growth conditions;the root phenotype of PLT(PLETHORA)mutants was determined;the root phenotype and the localization of auxin transporter proteins of auxin related mutants were also determined.We found that the weak alkaline condition promoted root growth of Arabidopsis by stimulating polar auxin transport and auxin signaling pathway.The main results obtained are as follows:1.Weak alkaline condition promoted primary root elongation and increased lateral root number in Arabidopsis.The weak alkaline condition up-regulated auxin signal,and thus induced cell cycle activity and cell division to promote PR elongation;The weak alkaline condition accelerated the oscillation frequency by inducing auxin signals in the oscillation zone,thereby promoting the establishment of prebranch site and ultimately increasing the number of lateral roots.2.PLT is a key transcription factor that regulates root elongation and lateral root formation in Arabidopsis.PLT1 and PLT2 encode the AP2(APETALA 2)protein,a transcription factor that regulates the formation of root tip stem cells.Under weak alkaline condition,the primary root length and lateral root number of PLT mutants plt14,plt2-2 and plt1-4plt2-2 were increased significantly compared with that under control condition(pH 5.8),indicating that weak alkaline condition-promoted PR elongation and lateral root formation is not depend on PLTs-mediated signaling pathway.3.Auxin is the core regulatory factor of root growth.Compared with the control condition(pH 5.8),Arabidopsis root under weak alkaline condition accumulated high auxin response.TIR1(TRANSPORT INHIBITOR RESPONSE 1),AFB2(AUXINBINDING F-BOX PROTEIN 2)and SHY2(SHORT HYPOCOTYL 2)are auxin receptors in the auxin signaling pathway,ARF7(AUXIN RESPONSE FACTORS 7)and ARF19 are auxin responses factor,and their mutants tir1afb2,arf7arf19,and pCASP1:shy2-2 were defect on the auxin signaling pathway.Weak alkaline conditions could still promote PR elongation of tir1afb2 and afr7arf19 mutants,but had no significant effect on lateral root number of the mutants.Weak alkaline condition was not able to increase the number of lateral root primordium and prebranch siteof arf7arf19 and pCASP1:shy2-2 mutants.These results indicate that the promotion of lateral root development under weak alkaline condition depends on the auxin signal,while PR elongation was associated with SHY2 signal.4.IBR1((INDOLE-3-BUTYRIC ACID RESPONSE1),IBR3,IBR10 and ECH2(EnoyI-CoA hydratase 2)are involved in the IBA-to-IAA conversion pathway.YUCCA also plays a very important role in the pathway of auxin synthesis of IPA,which is responsible for converting IPA into IAA.Under weak alkaline condition,the primary root length and the lateral root number of the auxin biosynthesis mutants ibrlibr3ibr10,ech2ibr1ibr3ibr10 and yucl-D significantly increased compared with those under control condition.Loss of auxin biosynthesis did not affect PR elongation and lateral root formation under weak alkaline growth condition,indicating that weak alkaline growth condition-promoted PR elongation and lateral root formation is independent of auxin synthesis.5.AUXI(AUXIN-RESISTANT1)is auxin influx carrier,PIN(PIN-FORMED is polar auxin efflux carrier,and PGP(ATP-BINDING CASSETTE GROUP B)is auxin efflux carrier.Auxin transport inhibitors(NPA,BUM)inhibit the growth of Arabidopsis roots under weak alkaline condition.The inhibition of auxin polar transport also blocked the promotion of primary root length and lateral roots number of Arabidopsis by weak alkaline growth condition,indicating that the promotion of weak alkaline conditions on root growth is related to polar auxin transport.Mutations of auxin transporters(aux1-21,pin2pgplpgp19,pin2pin3pin7)suppressed the promotion of weak alkaline growth conditions on PR elongation and lateral root formation;weak alkaline condition also promoted the expression of PGP1,PGP 19,PIN2 at the root tip,indicating that polar transport of auxin mediates the effects of weak alkaline condition on root development.Collectively,the weak alkaline condition induced the accumulation of auxin in the root apical meristem by affecting auxin polar transport,in turn accumulated cell cycle gene expression and enhanced the cell division activity at the meristem,leading to the promotion of PR elongation.Meanwhile,auxin signling is accumulated in the oscillation zone,and thus induced the oscillation frequency and prebranch site formation.Weak alkaline condition regulated the expression of auxin downstream genes and root growth by affecting auxin signaling pathway.This study revealed a molecular mechanism underlying the root growth under weak alkaline condition.