| As the major organ of plants to absorb water and nutrient,root plays a critical role in plant growth and development and adaption to stresses.Plants could change root architecture or growth directions in order to adapt to various environmental stresses,The root growth plasticity was mainly reflected by remolding root architecture in response to the changing environmental cues.So it has great scientific meaning to deeply study the growth plasticity of root,which can also provide a basis for genetic improvement and crop breeding based on root architecture in the future Root in nature can be divided into two very different types,and one is taproot system represented by the model plant Arabidopsis thaliana and the other one is fibrous root system like rice and maize,the important cereal crops.Though the research of root growth plasticity gets great advances using the model plant Arabidopsis thaliana,the molecular regulation mechanism of growth plasticity in fibrous root system,including maize,is largely unknown.Maize has a typical fibrous root system,which is comprised of embryonic primary and seminal roots,shoot-borne crown(underground)and brace(aboveground)roots,the lateral roots initiated in the pericycle of the roots mentioned above and the root hairs.It is worth mentioning that adventitious roots are also generated from mesocotyl in some inbred line,like W22.Different types of maize roots are gradually evolved in order to adapt to various environmental cues.However,whether different root types express differential growth responses to various stresses has not been reproted before.Previously maize root system has been largely studied as a whole But whether the different root types have differential growth responses to various environmental stress and what's the molecular regulation mechanism leading to this difference?This is an important research aspect for this article.Aluminum(Al)stress is the major limiting factor which affects plant growth and development in acid soil.In order to clarify whether different root types have differential growth response to Al stress,we firstly checked difference in the elongation rate of each root type under Al stress.Our data showed that,compared to wild-type control(without Al),50 ?M Al3+inhibited CR,PR and SR by 42.4%,52.1%and 76.5%respectively,indicating that CR is more resistant to Al stress compared with PR and SR,and that SR is more sensitive to Al stress than CR and PR.and the differentially expressed miRNAs are likely to play significant roles to control these differential growth responses.The results showed that,on the one hand,the expression patterns of miRNAs are highly diversified in different maize roots;on the other hand,the same kind of miRNAs has differential expression level in different root types in response to Al stress.To identify whether or not the differential growth response of different maize root types under Al stress be universal for plant in response to stresses,we treated the three root types with salt stress aiming at this problem.As we all know,soil salinity is a major constraint to crop growth and yield.Previous study showed that the primary and lateral roots of Arabidopsis thaliana are known to respond differentially to a number of environmental stresses,including salinity.Although the maize root system as a whole is known to be sensitive to salinity,whether or not different structural root systems show differential growth responses to salinity stress has not yet been investigated.The maize primary root(PR)was more tolerant of salinity stress than either the crown root(CR)or the seminal root(SR).To understand the molecular mechanism of these differential growth responses,RNA-Seq analysis was conducted on cDNA prepared fi om the PR,CR and SR of plants either non-stressed or exposed to 100 mM NaCl for 24 h.Then the differential expression genes in the three root types under salt stress were detected through RNA sequencing.After comparative analysis of the data,we find that 444 genes were shown to be regulated by salinity stress in PR,SR and CR.The transcription pattern and the number of genes associated with the plant salinity stress response differed markedly between the various types of root,and for example,PR expressed obviously difference from CR and SR.Function analysis showed that these salt-induced differential expressed genes oxidoreductase,glycosyl hydrolysis,receptor-assotiated kinase and ABA signaling?As mentioned above,the reduction of root elongation under Al stress is an important reflection of growth plasticity in plant.Auxin has been shown to enhance root-growth inhibition under Al stress in Arabidopsis.However,in maize,it has been suggested that auxin alleviates the aluminum(Al)-induced inhibition of root growth.But there was no genetic and cellular evidence to support this hypothesis..This issue was systematically and deeply investigated in this study using three mutants of auxin efflux transporter gene ZmPGP1.Three mutants from the ethyl methanesulfonate(EMS)induced mutant population of maize inbred line B73 were identified,which displays semi-dwarf stalks phenotype and the absence of aboveground brace roots.We mapped the mutant site using EcMutMap method and the result showed that the three mutants were caused by a mutation in the same gene ZmPGP1.ZmPGP1 is an auxin efflux carrier and maize pgp1 mutants were employed to define the contribution of the auxin efflux carrier ZmPGP1 to Al-induced root growth inhibition.Root growth in the zmpgp1 mutant,which forms foreshortened roots and is hyposensitive to auxin,was less inhibited by Al stress than that in the inbred line B73.In order to monitor the changes of auxin signal during A1 stress,we conducted the marker line DR5rev:RFP into zmpgpl.After quantifying free IAA(Indole-3-Acetic Acid)and detecting the Fluorescence detection of RFP,we found that the auxin level and the fluorescence intensity were reduced obviously under Al stress,but in zmpgpl,the mutant root tips displayed higher IAA accumulation and enhanced RFP signaling compared with that in wild type B73,indicating that the efflux rate of auxin was markedly reduced by Al treatment due to the mutantion of auxin efflux transporter ZmPGP1,which then made zmpgp1 be more resistant to Al stress than B73.So these results illustrated that AI stress reduced the auxin level in the root tip of maize leading to the inhibition of root elongation which was the opposite situation of Arabidopsis thaliana:Al stress increased IAA accumulation,inhibiting root growth.In addition,the auxin efflux vector ZmPGP1 was involved in auxin efflux in root tip of maize under Al stress.What's more,the expression of ZmPGP1 was upregulated by Al stress;on the contrary ethylene downregulated ZmPGP1 and enhanced auxin signaling in root tip,suggesting that ethylene might involved in Al-induced regulation of ZmPGP1.To sum up,this study indicated that the different root types of maize expressed obviously differential growth response to abiotic stress such as Al and salt.The differential expressed miRNA in different root types and their differential response to Al stress participated in differential growth response of different root types under Al stress.The differential transcription of these genes such as transcription factors,and the accumulation of compatible solutes such as soluble sugars probably underlie the differential growth responses to salinity stress of the three types of roots in maize.Furthermore,through genetic,cytological and molecular methods,we investigted the role of auxin and polar auxin transport in Al-induced inhibition of root growth in maize.Al stress depleted auxin from root tips,and thus caused the inhibition of root growth.On the contrary of the tendency in Arabidopsis thaliana,in maize,auxin alleviated the inhibition of root growth under Al stress.The auxin transporter ZmPGP1 depletes auxin from root tips thus mediates the Al-induced auxin efflux in the root tip of maize. |
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