Construction Of High-density Genetic Map And Analysis Of Genetic Basis Of High Temperature Tolerance At Flowering In Maize

Maize（Zea mays L.）is the highest-yielding crop of all crops in the world and plays an important role in maintaining world food security and sustainable agricultural development in the future.However,the increasing frequency of extreme high temperature has brought a great threat to the development of maize throughout its life cycle,especially during the flowering phase.Maize is one of the most vulnerable crops to climate change.From 1980to 2010,the global maize production volatility was close to 18%due to climate change.Therefore,it is particularly important to study the genetic basis of high temperature tolerance at flowering in maize as the maize at flowering is most vulnerable to temperature change.Understanding the molecular mechanism of regulation of high temperature response has become a very urgent and important research topic.However,the current understanding of the genetic basis of high temperature tolerance at flowering is limited,and the molecular mechanism regulating the high temperature response at flowering in maize is still unclear.Leaf scorching damage is one of the major phenotypes under high temperatures at flowering in maize.This study systematically analyzed the phenotypic characteristics of leaf scorching traits and its genetic basis.The main research work and results are summarized as follows:1)A new maize inbred line L403 with leaf scorch-free damage under high temperatures at flowering was identified,and we dissected its genetic basis using a 264 F_2:8 recombinant inbred line（RIL）population generated from a cross between L403 and B72.This population is a permanent mapping population for the mapping analysis of quantitative trait loci（QTLs）.It has the characteristics of genotype homozygosity,stable inheritance and easy reuse.2)The whole genome resequencing combining with QTL-seq was used to identify the genomic region controlling high temperatures at maize flowering stage.A total of10,316,744 SNPs and 1,488,302 small In Dels sites were excavated between the parents;a total of 2,693,054 SNPs and 313,757 small In Dels sites were excavated between the two DNA pools.These SNP and In Del sites lay an important foundation for future molecular marker development and assisted selection breeding.Finally,through the Euclidean Distance（ED）algorithm and the SNP-index method,a 7.41Mb genomic region controlling high temperatures at flowering was obtained.Seven candidate genes related to high temperature response were identified by bioinformatics analysis.A candidate gene Zm00001d033339 was further annotated to regulate of thermotolerance at flowering by involving in stomatal movement（GO:0010119）via Abscisic acid（ABA）pathway（KO04075）.This work could provide an opportunity for gene cloning and pyramiding breeding to improve thermotolerance at flowering in maize.3)Using a simplified genome sequencing method,Specific-Locus Amplified Fragment Sequencing（SLAF-seq）,a high-density and high-quality molecular genetic map of the F_2:8RIL population was constructed.The original sequencing data of 272.03 Gb was obtained in this study.In total,this study developed 10,144 polymorphic SLAF markers for the construction of genetic maps with a total genetic map distance of 1,657.6 c M,an average genetic map distance of 0.16 c M,and a topographic marker integrity of 99.83%.This study laid an important foundation for the subsequent QTLs analysis of high temperatures at flowering and other QTLs mapping of agronomic traits.4)We systematically analyzed the leaf scorching traits of high temperatures at flowering in maize,and firstly divided the leaf scorching traits into leaf scorching damage（LS）,leaf scorching degree（LSD）and leaf scorching ratio（LSR）.Using the constructed genetic map,a total of 16 QTLs associated with leaf scorching traits were identified and distributed on all chromosomes except chromosomes 7 and 10.By combining with the QTL-seq result,six candidate genes related to leaf scorching traits of high temperatures at flowering were identified.5)340 maize inbred line populations from around the world and 264 F_2:8 RIL populations were planted in 3 years at 2 locations.Based on correlation analysis between genotype and flowering phenotype,a total of 225 significant SNP loci and 105 QTLs loci associated with flowering were detected.Among them,the SNP marker（chr3＿32750576）and the q HFA3 locus associated with flowering on chromosome 3 coincide with the q LS3locus associated with leaf scorching,and the SNP marker（chr6＿168706460）and q HNS6locus on the chromosome 6 and the q LSD6 locus associated with leaf scorching coincide.These cross-genetic loci may be important candidate regions for participation in high-temperature regulation at flowering.In this study,three genes that may be involved in the regulation of maize flowering,Zm APG,Zm RCD and Zm BHLH,were also identified.These work provided an important basis for the molecular mechanism analysis of future maize flowering genes.