Genetic Analysis Of Maize Seedling Traits And Root 3D Architecture Response Characteristics Of Typical Inbred Lines Under Low Phosphorus Stress

Root plays a vital role in nutrient and water uptake,which affects plant growth and yield formation.It is of great significance to achieve high resources efficiency of the crop by genetic improvement of root traits.However,natural variation and underlying genetic mechanism of maize root traits in response to low phosphorus(P)stress are poorly understood at the seedling stage,the critical period of maize.In addition,root system architecture at the three-dimentional(3D)levels responses to low P stress has also not yet been reported.In this study,the aboveground biomass,leaf area,the symptom of phosphorus deficiency,root number and length of axial root were evaluated using an association population of 480 inbred lines under two contrasted P levels in the field.Then,a genome-wide association analysis(GWAS)was conducted to dissect genetic mechanism and detect the elite allele variation of the aboveground and root traits of maize seedling under high(HP)and low(LP)conditions.In addition,using the established root 3D phenotypic analysis platform,root achitecture of selected 11 inbred lines with different phosphorus efficiency was reconstructed,and their responses to LP stress were further analyzed.The main results are presented as follows:1.The inbred maize lines from China showed higher plant height(PH),leaf area(LA),SPAD value and crown root length(CRL)compared with US and the International Maize and Wheat Improvement Center(CIMMYT)germplasm,and these traits were decreased of 18%,37.2%,7.1%and 9.3%,respectively under LP stress conditions.The average P deficiency grade of Chinese mazie inbred lines was 1.18.The lines from the US had higher aboveground biomass(SDW),root dry weight(RDW)and crown root number(CRN)in the HP conditions.However,SDW and RDW showed remarkably decrease of 41.5%and 16%under LP stress conditions.The average P deficiency grade of the US inbred lines was 1.37.The CIMMYT germplasm showed the largest seminal root length(SRL)at both P conditions.The average P deficiency grade of the CIMMYT inbred lines was 1.29.2.In LP condition,CRN and CRL were significant positive correlation with PH and LA(r=0.28-0.33).By cluster analysis,the maize lines were classified into three groups,high P efficient genotypes in HP condition(Group 1),double P inefficient genotype(Group2)and double efficient genotype(Group3)The lines from Groupl and Group3 showed higher SDW than Group2 under the HP condition,and different Groups(1,3 and 2)decreased by 46.7%,37.1%and 34.5%under the low P,respectively.For the root traits,CRL and CRN of the Groupl showed significantly decreased by 10.2%and 16.7%,and PRL significantly increased by 7.2%under the LP stress condition.CRL and CRN of the Group2 were the significant decreases of 9.5%and 15.0%,respectively under the LP condition.CRL and CRN of the Group3 showed significantly decreased by 7.3%and 16.0%,and SRL,PRL significantly increased by 5.8%and 6.1%under the LP condition.3.Extensive genetic variations wer observed for the root traits with association panel,which the coefficiency of variation ranged from 6.52-16.07%in the LP condition and ranged from 7.31-18.09%in the HP condition.High broad-sense heritability was also observed at both P levels,which ranged from 31.9-64.9%under LP and 36.6-72.3%under HP.A total of 97 significant SNP loci were detected by GWAS.The highest contribution to phenotypic variance in single SNP was 5.89%,and the phenotypic contribution to phenotypic variance of all significant SNPs of seminal root number(SRN)was 44.16%.In total,35 candidate genes were identified,including 6 novel genes with unknown function.On chorosome 2,the SNPs associated candidate gene GRMZM2G015610 represented a member of the PP2C family of protein phosphate,which was suggested to be involved in the regulation of ABA synthesis for stress resistance.4.Using the high-throughput root 3D phenotypic platform,the selected 11 genotypes with different phosphorus efficiency were divided into 2 groups according to root 3D architecture response to LP stress.Under LP stress,the Group 1 had 106.4%greater the root volume,15.8%wider root Max Width,23%larger MaxWidth/MaxDepth,221.1%greater Maximumroots number,160.5%larger the root surface area(TRSSA),64.9%bigger Convexhull,8.59%bigger Volume Distribution,13.4%larger the total root length to volume ratio(SRL),shallower centroid,84%larger SDW and 68.2%larger RDW than Group2.The results indicated that P efficient genotypes may modify the root 3D architecture to enhance P acquisition under LP stress.Taken together,the results showed the presence of large natural variation of root traits in association panel at the seedling stage.A close genetic relationship was also detrermined between root traits and SDW,PH and LA under LP condition.Under low P stress,the high P efficiency genotypes increased PRL and SRL,no change of SRN,and slight decreased of CRN and CRL compared with HP condition.Many underlying candidate genes,expecially for SRN trait,were identified by GWAS,which contribute to the root-based approached to genetically improving phosphorus use efficiency.In addition,the maize 3D platform was established,and it was suggested that changing maize 3D architecture improved plant P acquisition unde LP condition.The root 3D architecture traits merits consideration as a selection target to improve P efficiency.