Genetic Analysis Of Root Traits And The Relationship With Nitrogen Use Efficiency In Maize (Zea Mays L.)

Roots play essential roles in the acquisition of water and nutrients from soils.Genetic improvement of root traits has been proposed as a key approach for increasing crop yield and natural resources use efficiency simultaneously.However,the genetic basis of root system architecture(RSA)as well as the genetic relationship between RSA and nutrient use efficiency remains to be elucidated.In this thesis,the genetic architectures of RSA and nitrogen use efficiency(NUE)traits in maize were investigated using a recombination inbred line population that was derived from two lines contrasted for both traits.Moreover,a major QTL for effective brace root number(qEBRN2)was fine mapped.Additionally,combining a Random-Open-Parents Association Mapping(ROAM)and an association mapping(AM)strategies,the genetic architecture of trait of nodal root number was dissected and the underlying candidate genes were further identified.The major results are as follows:1.In the field experiments across four environments we evaluated ten NUE-related traits including plant biomass(GY,SY and HI),plant N concentration(GNC,SNC,and NHI),and N efficiency(NUE,Nup,NupE,and NutE).By three independent hydroponic experiments we evaluated nine root traits including root biomass(RDW),root number(RN)of seminal,crown and lateral roots(SRN,CRN,LRN),and root length(RL)of primary,seminal,crown roots(PRL,SRL,CRL).In contrast to N utilization efficiency(NutE),N uptake efficiency(NupE)had significant phenotypic correlations with RSA,particularly traits of seminal roots(r = 0.15-0.31)and crown roots(r = 0.15-0.18).A total of 331 quantitative trait loci(QTLs)were detected including 184 and 147 QTLs for NUE-and RSA-related traits,respectively.These QTLs were assigned into 64 distinct QTL clusters,and?70%of QTLs for N-efficiency(NUE,NupE,and NutE)coincided in the clusters with those for RSA.Five important QTLs clusters at the chromosomal region bin 1.04,2.04,3.04,3.05/3.06,and 6.07/6.08 were determined in which QTLs for both traits had favorite effect of alleles coming from large-rooted and high-NupE parent Ye478.Introgression of these QTL clusters in the advanced backcross-derived lines conferred average increases of grain yield 13.8%(HN)and 15.9%(LN)line per se and 11.0%(HN)and 20.8%(LN)in testcross.2.Under two years field condition we validated the effect of a QTL on chromosomal region bin2.04,QTL-qEBRN2,controlling the effective brace root number in advanced F2:3 population.This QTL had LOD value at 18 and explained 42%of total phenotypic variation.By QTL fine-mapping strategy using the recombinant-derived progenies,qEBRN2 was further narrowed down to a 61.4kb physical region between makers SSR7r and indel233 containing four predicted genes.3.Under field condition we evaluated 10 traits for nodal root number in ROAM population(-800 lines)across 3 environments,and AM population(-500 lines)across 8 environments at the maturation stage.The abundant variations were detected for the investigated traits of nodal root number with high broad-sense heritability,which ranged from 54.1%to 79.9%with four RIL populations,and from 82.3%to 91.7%within AM population.The phenptypic correlation analysis,PCA(Principal Component Analysis),HCA(Hierarchical Cluster Analysis)and QTL overlap analysis all revealed a distinct genetic architeture between nodal roots belowground in the case of crown roots and those aboverground for brace roots.Through a genome-wide association study(GWAS)in ROAM and AM population,we showed that the traits of nodal root were controlled by numerous QTLs with the small-effects.Many of these QTL regions co-localized with priori candidate genes for various phytohormones transport and metabolim that were essential in root development and grwoth.Importantly,a gene network pathway for auxin(TIR1/ABF2-AUX/IAA-ARF-LBD)was also identifed for the nodal root number in maize,suggesting the conserved mechnism across different plant species.In addtion,53 gene regions were further identifed without any co-localized priori gene,implicating the involvement of novel genes and even regulatory pathways for nodal root development.Taken together,our findings revealed a significant genetic relationship of RSA to NUE traits,and uncover the genetic basis of the trait of nodal root number.Many important RSA-QTLs and underlying candiate genes were identified,which contribute to the root-based approaches to genetically improving maize yield and resource use efficiency.