Genetic Mapping Of QTL QPRL-C06 For Phosphorus Efficiency And Response Of Ionome To Phosphorus Deficiency In Brassica Napus

Phosphorus(P)is an essential macronutrient for plant growth and development,which is a component of biomacromolecule and is widely involved in multiple biological processes.Brassica napus is an important oil crop,which has a high demand for P and shows very sensitive to P deficiency.Most of agricultural soils used for B.napus production are deficient in plant-available P in China,leading to a severe decline in the seed yield.P efficiency is a complex quantitative trait,and the root traits related to root length and root dry weight play a key role in the efficient uptake of P and shoot growth.In addition,interactions between different mineral elements and P can also affect plant growth and development.Thus,mapping and cloning of quantitative trait loci(QTLs)for root traits and ionomic traits under P deficient condition,will be helpful for understanding of the molecular mechanisms of P efficiency,and uptake and partitioning of mineral elements in B.napus.In the present study,the responses of root traits of the Bna TNDH population to P deficiency were comparably analysed in an‘agar'system and a‘pouch and wick'system,and QTLs for P efficiency were mapped.In addition,a major QTL q PRL-C06 for primary root length(PRL)associated with P efficiency was identified by QTL-seq,classical QTL mapping and regional association mapping,which was further validated by an advanced backcross population.This study investigated the physiological responses of shoot and root ionomes of B.napus to P deficiency,and detected the QTLs and epistatic QTLs associated with these traits.The main results are as follow.(1)Identification of QTLs affecting relative root traits associated with P efficiency in an‘agar'system and a‘pouch and wick'systemAt a low P supply(LP),more double haploid(DH)lines of the Bna TNDH population had greater total root length,PRL,total lateral root length and mean lateral root length in the‘pouch and wick'system than the‘agar'system.Correlations between three pairs of relative root traits(relative PRL and relative lateral root number,relative PRL and relative lateral root density,relative total lateral root length and relative lateral root density)were not stable across the two culture systems.In the‘agar'system,P deficiency significantly inhibited the elongation of the primary root of Arabidopsis and B.napus cultivars when roots were exposed to light,but had no effect when roots grew in darkness,suggesting that shielding roots from light reduced the sensitivity of root system elongation to P deficiency in plants.A total of ten QTLs impacting relative root traits were identified in the‘agar'system,which were distributed on chromosome A04,A07,A08,A09,C04 and C08.Two QTLs affecting relative root traits were identified in the‘pouch and wick'system,which were located on chromosome A03 and A04.However,the QTL for the same trait in the‘agar'system did not overlap with that in the‘pouch and wick'system.Two and one QTL clusters identified in the‘agar'system were located on chromosome A09(Cluster1and Cluster2)and C04(Cluster3),respectively.RLRN?A04b,RSDW?A09a and Cluster1 were found to affect the seed yield and/or yield-related traits in two field trials.(2)Identification of a major QTL q PRL-C06 regulating P efficiency and genetical analysis of it in an advanced backcross populationTwo extreme bulks,L-PRL bulk and S-PRL bulk were constructed.Each bulk consisted of 20 DH lines with the extreme long or short PRL in two independent experiments in the‘agar'system at LP.Whole-genome resequencing was conducted for Tapidor,Ningyou 7 and two bulks.A total of 1,667,403 SNPs and 426,681 In Dels were identified between cultivars Tapidor and Ningyou 7,which were non-evenly distributed along the 19 chromosomes of B.napus,and the DNA polymorphisms in the A subgenome were more abundant than those of the C subgenome.A total of filtered 1,232,804 SNPs and 285,383 In Dels were employed for the QTL-seq analysis,and 20 QTLs were mapped on chromosome A01,A02,A06,A07,A08,A09,A10,C01,C04,C06 and C09.Among these QTLs,the alleles with positive effects in ten QTLs were contributed by Tapidor,while the alleles with positive effects in the remaining ten QTLs were contributed by Ningyou 7.For PRL at LP,80%of the QTLs identified by the original molecular markers could be detected by QTL-seq.A major QTL q PRL-C06 controlling PRL could be also identified by two independent experiments at LP and one experiment at an optimal P supply(OP)of the Bna TNDH population in the‘agar'system.In addition,q PRL-C06 could be detected by a regional association analysis using the PRL of an B.napus association panel at LP in the‘pouch and wick'system,which explained for 5.7%of the phenotypic variation.The line 1067-1 in which the genotype of q PRL-C06 was homozygous from Tapidor,had a long primary root,were identified from a previously established T-BC₄F₃population.An advanced backcross population that segregated in the target region was generated from cross between 1067-1 and Ningyou 7 twice,and q PRL-C06 was then validated and the interval was refined to be 0.37 Mb.In this region,although there was no genes that annotated to be associated with P uptake and utilization,three genes,such as Bna C06g34980D,Bna C06g35430D and Bna C06g35530D,were involved in the root initation and development.Nine SNPs and five In Dels were found in the promoter of Bna C06g35430D using the requencing data of cultivars Tapidor and Ningyou 7.There were 18 SNPs and 9 In Dels,and 2 non-synonymous SNPs identified in the promoter and coding sequence of Bna C06g35530D,respectively.(3)The responses of shoot and root ionomes of B.napus to P deficiency and detection of QTLs associated with themShoot and root concentrations of eleven mineral elements were investigated using inductively coupled plasma optical emission spectrometry in the Bna TNDH population and two parental lines grown at OP and LP in an‘agar'system.The plant contents of B,Ca,Cu,K,Mg,Mn,Na,P and Zn were reduced by P starvation,while the plant content of Fe was increased by P starvation in B.napus.The partitioning to shoot of Ca,Cu,K and P was reduced by P starvation,while the partitioning to shoot of B,Fe,Na and Zn was increased by P starvation.Shoot B,Ca,Mg,Mn,Na and P concentrations in B.napus were greater than those in the root,while root Fe,S and Zn concentrations were greater than those in the shoot at both OP and LP,indicating that mineral element composition is organ specific in plants.A total of 133 and 123 QTLs for shoot and root ionomic traits were identified at OP and LP,respectively.However,most of the QTLs identified at OP differed from the QTLs detected at LP.Six QTL clusters were associated with more than three traits impacting mineral element composition.A major QTL cluster on chromosome C07 affecting shoot Mg and S concentrations at LP was confirmed and narrowed down to a 28.5 Mb–30.6 Mb region using an advanced backcross population.In addition,a total of 54 epistatic interactions were identified for various ionomic traits in the Bna TNDH population at OP and LP,but most of these epistatic interactions did not involve any additive QTLs,indicating that the B.napus ionome was controlled by a complex genetic network.