| Phosphorus(P)is one of the essential macronutrients for plant growth and metabolism.Oilseed rape(Brassica napus L.)is an important oil crop in China as well as in the world,which needs more phosphorus(P)than other crops and shows sensitive to phosphorus deficiency.P efficiency is a complex quantitative trait.Root system architecture traits,especially lateral root length and root dry weight,play a key role in P absorption,and thus for the seed and oil yield production.In this study,a total of 405 Brassica napus cultivars and inbred lines were collected and root system architecture traits were investigated at low P and high P by hydroponics(HS I and HS II)and at low P by ?pouch and wick‘ system.The genetic variations associated with root system architecture traits investigated under different P levels were identified by genome-wide associated study(GWAS).Moreover,the breeding history of a Pinefficient cultivar ?Tapidor‘ and a P-efficient cultivar ?Ningyou7‘ were studied and the difference in genetic transfer and selection breeding on the important agromomic traits,e.g.root morphology traits were revealed.The main results are as follow.1.Genetic variants associated with the root system architecture of oilseed rape(Brassica napus L.)under contrasting phosphate supplyThe 60 K Brassica Infinium SNP(single-nucleotide polymorphisms)array contains 52,157 single nucleotide polymorphisms(SNPs).A total of 30,976 SNPs matched to a unique location in the reference genome of cultivar darmor-bzh and 19,397 highquality SNPs with MAF > 0.05 and call frequencies of SNPs < 0.8 were selected to assess the population structure.The population could be divided into two clusters;Subgroup1(66 cultivars and inbred lines)and Subgroup2(339 cultivars and inbred lines).Six root system architecture traits under contrasting phosphate supply by hydroponic experiment and nine root system architeture traits under low phosphate supply by ?pouch and wick‘ system were investigated.A total of 285 SNPs associated with the root system architecture traits at varying phosphorus levels were detected,including 50 at low P and 9 at normal P in HS I and HS II;31 at low P and 58 at normal P;and 165 at low P.Nine of 285 SNPs were co-located the quantitative trait loci(QTLs)for root system architecture traits identified by Bna TNDH population.Four differential expression genes(DEGs,Bna A03g05800 D,Bna A03g05820 D,Bna A03g05830 D and Bna A03g05850D)identified in the transcriptome profile of ?Eyou changjia? at LP and HP,were located in the interval of QTL and significant SNP by GWAS for root system architecture on A03.RT-q PCR results showed that the expression of the four candidate genes showed higher in P-efficient varieties than in P-inefficient varieties.Four DEGs were located in the LD decay region of the peak SNP and formed the haplotype-Bn A3 Hap.The peak SNP could explain 9.2%,6.0%,12.4%,6.3%,13.8%,14.5%,5.9% phenotypic variations(PVE)of primary root length(PRL),lateral root number(LRN),lateral root length(LRL),mean lateral root length(MLRL),total root length(TRL),root dry weight(RDW)and shoot dry weight(SDW)at LP,respectively.The varieties with haplotype 6(GAAAAAGG)had higher average values in all the investigated RSA traits than those with the other haplotypes.Whereas,the varieties with haplotype 2(AGCCGAGG)had lower average values in all of the investigated RSA traits than those with the other haplotypes.The RSA parameters of Haplotype 7(GAAAACTA)varieties were between that of haplotypes 6 and 2 varieties,respectively.In addition,the Tajima‘s D values of Bn A3 Hap haplotypes in B.napus varieties were strongly associated with the distribution of the agronomic P surpluses in soil.2.Breeding histories and selection criteria for a P-efficient and a P-inefficient cultivar identified by genome wide pedigree dissectionWe genotyped the P-inefficient cultivar Tapidor(European winter type oilseed rape),the P-efficient cultivar Ningyou7(Chinese semi-winter oilseed rape)and their progenitors(Shengliyoucai,Chengduai,Chuanyou2,Ningyou1 and Ningyou7 in Ningyou7 pedigree;Liho,Bronowski,Regent,Bienvenu and Tapidor in Tapidor pedigree)with the Brassica 60 K Illumina Infinium SNP array and mapped a total of 29,347 SNP markers onto the reference genome of Darmor-bzh.A large number of consensus QTL in Bna TNDH and genes were identified in the IBD block regions.Two primary artificial selection processes are apparent in the breeding history of Tapidor and Ningyou7.A total of 42 genes,15 within the Tapidor and 27 within the Ningyou7 pedigrees,were detected where genetic transfer was associated within the earlier breeding selection process.Among these genes,two for flowering time andone for seed erucic acid content in the Tapidor pedigree appeared to have been subject to breeding selection.In the Ningyou7 pedigree,12 genes,including 5 for flowering time,2 for seed erucic acid content,3 for seed glucosinolate content and 2 for root morphology and P uptake,involved in breeding selection In the later breeding selection process,only two genes in Tapidor,but 116 genes in Ningyou7 were detected as being associated with genetic transfer.Of these,only 1 from the Tapidor pedigree and 65 from the Ningyou7 pedigree appeared to have been subject to breeding selection.A total of 24 favorable gene clusters,10 in the A subgenome and 14 in the C subgenome were identified from progenitors to offspring in Tapidor and Ningyou7 pedigrees.Genetic transfer was detected within the Ningyou7 pedigree(18 clusters),the Tapidor pedigree(2 clusters)and within both pedigrees(4 clusters).In addition,12 gene clusters were associated with combinations of two or more important agronomic traits,such as flowering time,root system architecture traits(No 6,A6);flowering time and seed oil content(No.14,C2;No.20,C5);flowering time,seed protein and seed oil content(No.22,C8);and flowering time,seed protein and root system architecture traits(No.24,C9).These 24 genes clusters were formed by pyramid breeding. |
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