QTL Effects And Analysis Of Flowering Time And Important Agronomic Traits In Soybean[Glycine Max(L.)Merr]

Soybean,as a short day crop,is photoperiodic sensitive which can determine growth duration.Growth period is related to the yield and the adaptability of a soybean cultivar.Flowering is a typical characteristic that represents the transition from the vegetative to the reproductive phase in plants.Flowering time is controlled by environmental factors and internal physiological and molecular process in plants.Branching is also an important agronomic trait related to yield,which often affects canopy architecture,light reception,lodging and density of plant.So,the identification and utilization of genes underlying flowering time and branching will help us to understand more about the molecular mechanism and to give technical advice for molecular breeding.To date,a series of E loci associated with flowering time and maturity have been identified in soybean,including E1-E9 and J.Among them,El,E2,E3 and E4 play important roles in flowering time.However,the ranges of time to flowering and maturity were quite large in some cultivar soybean with different genetic background,implying the presence of some other unknown QTL or genes that are related to control flowering time or maturity.In this study,we developed eight F2 populations,eight F2.3 populations,and eight specific sub-populations from a cross between a Japanese cultivar Toyomusume?el-nl,e2,E3-Mi,E4?and a Chinese cultivar Suinong 10?E1,e2,e3-T,E4?for identifying novel QTL controlling flowering time and studying the relationship between flowering time genes and branching numbers.The genetic linkage map was constructed by SSR markers in 141 plants of F3 population between Toyomusume and Suinong 10.A total of 671 pairs of SSR markers covering 20 chromosomes of soybean were screened.Of them,174 SSR primer pairs showed polymorphisms between the parents and among the F3 population.Apart from 16 unlinked markers,157 SSR markers were successfully mapped to 27 groups.SNP genotyping for parents?Toyomusume × Suinong 10?and F2 population?N=100?were performed using the Illumina Iscan platform?Illumina,Inc.San Diego,CA?.Illumina SoySNP8k iSelect BeadChip with 7189 SNPs was specifically manufactured at Infinium HD Ultra.The 2363 SNP polymorphic markers were analyzed using QTL IciMapping₄.0 software.After discarding the redundant or unmapped markers,1306 unique loci SNP markers were retained and mapped leading to the construction of 20 linkage groups corresponding to the 20 chromosomes of soybean.The typical S type curve represents the consistency between genetic map and the physical position in the chromosome?Current version Gmax₂75-Wm82.a2.v1?,which is a basis for identifying QTL accurately.Three QTL for flowering time were located on Chr 02,Chr 06,Chr 16 in both the F2 and F3 populations using SNPs and SSRs,and we verified the results.The results showed that qFT6 was a major QTL for delaying flowering time mapped onto Chr 06 nearby El gene in the F2 and F3 populations,which has the same effective of El.Judging from different El loci between parents,and genotyping F2 and F3 populations,we firmly conclude QTL of qFT6 was corresponding to E1.Similar to the qFT6 result,we conclude QTL of qFT19 and qFT16 were corresponding to E3 and E9 respectively.A novel QTL?qFT₂,new QTL,no reported?for flowering time was located on Chr 02 in both the F2 and F3 populations using SNPs and SSRs.We genotyped each individual in specific population using markers located in qFT₂ region,and made correlation analysis.We concluded that qFT₂ locus was real existence.At the same time,we also detect a novel minor QTL?qFT₃?in F3 population.In order to study the relationship among the QTL identified in the population,we genotyped each individual in eight F2 and F3 populations and eight specific-populations at El,E3,E9,qFT₂ locus and made correlation analysis with flowering time.The results indicated that El alleles have a major effect on flowering and maturity,E9 has a relative large effect compared to E3 in this population,and qFT₂ has a minor effect.Flowering variation regularity of eight type combinations in different El,E3 and E9 allelic has consistent performance in different environment,that is E1/E3-Mi/e9>E1/e3-T/e9>E1/E3-Mi/E9>E1/e3-T/E9>e1-nl/E3-Mi/e9>e1-n1/e3-T/e9>e1-nl/E3-Mi/E9>e1-nl/e3-T/E9.The QTL for branching number identified in both the F2 and F3 populations using SNPs and SSRs were all nearby QTL for flowering time.In order to study the relationship between E loci and branching number,we made correlation analysis between E1,E3 and branching in eight F2 and F3 populations and eight specific sub-populations.We also made correlation analysis between discrepant site nearby E9 and branching in eight F2 and F3 populations and eight specific sub-populations.The results showed that branching number was significantly positive correlation with E1 genotype,branching number links to the discrepant site nearby E9,and there is no significant correlation between E3 genotype and branching number.We speculated that the genes for flowering time have a pleiotropic effect on branching at the genetic level.Overexpression of E1 significantly enhanced branching number in transgenic lines,which further suggest E1 have a pleiotropic effect on branching,but this needs to be further confirmed by genetic studies and at the molecular level.QTL for other agronomic traits were also mapped nearby the flowering time genes?E1,E3,E9?except the QTL for branching number.Correlation analysis of E1,E3,E9 genotype with phenotype of plant height,nodes on the stem,pods on the stem,pods on the branching,total pods showed that E1 have a significant effect on plant height,nodes on the stem,pods on the branching,seeds on the branching,and had no significant effect on pods on the stem,seeds on the stem.E3 have certain influence on plant height,nodes,pods on the stem,and seeds on the stem.Because it is an early flowering gene,E9 has no significant effect on plant height and nodes.We used four soybean varieties?super high yield soybean cultivars Shennong 12,Zhonghuang 35,Liaodou 14 and common soybean cultivar Liaodou 11?with similar flowering time period and the same flowering time genes?E1,E2,E3 and E9 genes?which were identified for cloned flowering time genes.We studied the relationship among soybean yield and photosynthetic system,antioxidant enzyme activity via field experiment and physiological test.The results showed that the cultivars with the same flowering time genes did not show "noontime snooze" at R6 growth stage,and the photosynthesis of super-high-yield cultivars was higher than that of common cultivar after 14.O'clock.These indicated that super-high-yield cultivars had greater photosynthetic assimilation capacity than that of common cultivar at R6 growth stage,which was one of the important physiological reasons for super-high-yield cultivars greater yield excluding the influence of flowering time genes.In present study,the SOD activity of the four cultivars with the same flowering time genes declined with the developmental process and the content of MDA increased.At the same time,SOD activity and MDA content of the super-high-yield soybean varieties were higher and lower than that of common soybean variety at R1,R6,and R7 growth stage,respectively.The results also showed that POD activity of the super-high-yield soybean are higher than that of common soybean at R6,R7 stage under 225 kg/hm,300 kg/hm2,indicating that the super-high-yield soybean cultivars had the stronger active oxygen clearing ability.In this study,the nitrogen fertilizer could not only enhance the SOD and POD activities,but also decreased the MDA content,indicating that fertilizer treatment could delay leaves senescence effectively.In order to map novel QTL for flowering time and branching,we developed the population from Japan soybean cultivar?Toyomusume:e1-nl,e2,E3-Mi,E4,and e9?and China soybean cultivar?Suinong 10:E1,e2,e3-T,E4,and E9?,which have big genetic background differences between parents,and the population can provide good materials for studying interaction among flowering time genes.In this study,we revealed that flowering time genes have a pleiotropic effect on branching at the genetic level.Furthermore,the population in this study will be very useful for the fine-mapping of minor QTL on Chr 02 or Chr 03 for flowering time,and for future study of interactions among E1,E3,E9,and other QTL.Our understanding of the regulatory networks for flowering time and other agronomic traits will be extended and highly beneficial for molecular designer breeding.