The Molecular Mechanism Of IAR4 Modulation Of Primary Root Growth Under Salt Stress In Arabidopsis

Soil salinization is a common environmental stress that severely affects the plant development.With the increasing area of salinization,high salinity is one of the major abiotic stresses that plants encounter,and influences crop production worldwide and food security.Plants have evolved several regulation mechanisms to adapt the environment of high salinity.Roots are the initial and direct organs to perceive the signal,which are heavily inhibited under salt stress.Therefore,identification of key regulators under salt stress and illustration of their mechanisms may aid in the understanding how plants cope with salt stress and will lead to new approaches for improving growth performace of crop plants under salinity conditions.In this work,we screened the growth performace of T-DNA insertion lines of Arabidopsis under salt stress by reverse genetics,in each of which one salt-stress inducible gene was mutated.IAA-CONJUGATE-RESISTANT 4(IAR4),encoding a pyruvate dehydrogenase E1?homolog,was screened out,which played a key role in primary root growth under salinity stress conditions.Mutation of IAR4 led to increased sensitivity to salt stress,with strongly inhibited primary root growth and reduced survival rate in iar4 mutants.iar4 mutants accumulated greater Na~+and exhibited a greater Na~+/K~+ratio under NaCl treatment by reducing SOS1 and SOS3 expression,which disturbed the homeostasis of Na~+/K~+,and inhibited the plant growth.Through determining the physiological mechanism that IAR4 regulates primary root growth under salt stress,we found the retarded root growth of iar4 mutants under salinity was due to the increased ROS level.Under salt stress,the activation of ROS scavenging genes was inhibited,which subsequently accumulated more ROS in the iar4 mutants and broke the balance of ROS production and ROS scavenging,leading to heavily inhibited plant growth.Mutation of IAR4 slightly decreased the meristem activity,root number and cell length under normal growth condition.iar4 mutants showed increased sensitivity to salt stress,by dramatically decreasing the root meristem activity and cell division.This salt hypersensitivity could be partially reversed by GSH supplement,which scavenged ROS in iar4.Root growth and meristem activity maintenance are modulated by auxin transport and distribution.Further physiological,molecular and cell biological experiments showed NaCl treatment greatly suppressed the expression levels of auxin polar transporters(ProPIN1:PIN1-GFP,ProPIN2:PIN2-GFP,ProPIN3:PIN3-GFP),and auxin response reporter(ProDR5:GFP),which reduced auxin level in root tips and reduced meristem activity,leading to short primary roots in iar4 mutants.GSH or auxin treatment greatly recovered the PIN expressions,auxin distribution and primary root growth in the iar4mutants,suggesting ROS is a vital mediator between salt stress and auxin response in primary root growth.A new salt-regulated gene,IAR4,was found in our data through reverse genetics,which support a model in which IAR4 integrated ROS and auxin pathways to modulate primary root growth under salinity,by regulation of PIN-mediated auxin transport.The potential biological role of IAR4 was illustrated in salt-mediated root growth via the ROS-auxin crosstalk.