Unraveling The Genetic Basis Of Harvest Index And Seedling Hypocotyl Elongation In Brassica Napus L.

Rapeseed is one of the most important oilseed crops in the world,but seed yield per unit is so low for many years.The balance among photosynthetic “ source ”,“ flow ” and“ sink ” is critical for plant yield improvement.This balance can be evaluated using harvest index(HI)as the criterion.There is no any knowlegy about the genetic basis of HI in rapeseed.Additionally,well-developed seedling traits can contribute directly to an increase in and stabilization of yield and its heterosis.Hypocotyl elongation is considered a typical seedling trait,but less attention has been focused on it in B.napus.Thus,a better understanding of the genetic mechanism of HI or hypocotyl elongation is crucial for improving yield in B.napus.In the study,GWAS performed with 210 accessions was conducted to detect the quantitative trait loci(QTL)associated to HI and hypocotyl elongation in rapeseed.Transcriptome sequencing was carried out in two groups with extreme hypocotyl elongation in DH-6004.The main results are as following:1.Phenotypic variations of HI showed that HI of accessions ranged with an average of 0.20 to 0.27 among four envrionments.However,soybean,one of the most important oil crops,has a HI ranging from 0.4 to 0.6.Thus,there is still great potential for HI improvement in B.napus.2.The correlations among HI and other traits indicated that HI was negatively correlated with plant height(PH)in E3,and strongly significantly correlated with seed yield per plant(SY)in four environments.3.Total nine SNPs were detected to significantly associate with HI,and could explain 3.42% of the observed variation.Five SNPs were detected to significantly associate with both HI and SY.These SNPs were in high linkage disequilibrium with each other.4.Total of 63.33% of hypocotyl elongation values were between 2.00 and 3.00.In DH-6004,the lines with extremely early-flowering was significant higher than the lines with extremely late-flowering for hypocotyl elongation.5.Correlation analysis showed that hypocotyl length positively correlates with seed yield per plant,biomass yield per plant and plant height,but not correlates with HI.6.Five SNPs on C07 were highly significantly associated with hypocotyl elongation at P < 2.13 E-06,and could explain 4.82% of the total phenotypic variance.Five genes(Bna C07g46740 D,Bna C07g46760 D,Bna C07g46770 D,Bna C07g46780 D,and Bna C07g46800D)were detected in the candidate region.Total of 196 genes were obtained in the enlarged candidate region between LD upstream and downstream of the associated peak.7.Joint GWAS and transcriptome sequence analysis detected 29 significant DEGs.Among them,Bna C07g46770 D previously found to directly relate to flower time in B.napus.8.GO analysis,KEGG pathway analysis and homologous comparison analysis confirmed that the genes controlling hypocotyl elongation are mainly related to circadian clock,flowering morphogenesis,hormone biosynthesis,or important metabolic signaling pathways in B.napus.