Genetic Dissection Of Branch Angle In Oilseed Rape (Brassica Napus L.)

Branch angle affects light capture efficiency,canopy light transmittance,planting density,and crop yield,and is an essential target trait for plant architecture improvement.Oilseed rape is an important oilseed crop grown around the world,and the branch angle has a significant impact on planting density,resistance to stem lodging and disease,as well as crop yield.Although studies on branch angle in oilseed rape have been reported,the underlying genes and regulatory mechanisms are still largely unknown.In this study,the regulatory mechanisms of branch angle in oilseed rape were more systematically dissected at the physiological,cytological,genetic,and molecular levels using the constructed association population and extreme accessions.The main results were as follows:1.Branch angle exhibited a dynamic change from an increase in the early stage to a gradual decrease until reaching a stable state during the whole developmental process.Cytological observation showed that branch angle variation was mainly due to xylem size difference in the vascular bundle of the branch junction.2.Hypocotyls from five compact accessions and five loose accessions were subjected to gravitropism experiments.The results revealed that compact accessions responded more sensitively to gravitropism as evidenced by the average bending angle of compact accessions’ hypocotyls being greater than that of loose accessions during the six hours of observation.3.Branch angle(BA)and branch dispersion(BD)were used to evaluate the branch architecture of oilseed rape.A pair of representative parents(8S084 and 20P19)were selected for joint analysis of six-generation population(P1,P2,F1,F2,BCP1,and BCP2),the result of the genetic model analysis showed that the optimal model for BA and BD was two pairs of additive major genes plus additive-dominant polygenes model(MX2-A-AD),indicating that BA and BD were mainly regulated by two pairs of major genes and minor-effect polygenes.The average heritability of the major genes in the BA and BD was 17.32% and 17.95%,respectively.In addition,the effects of polygenes on BA and BD were additive and dominant,respectively.4.The phenotypic analysis of 327 natural accessions showed that BA in six environments ranged from 24.3° to 67.9°,with a broad-sense heritability(H2)of 88.18%.BD in three environments varied from 4.20 cm to 21.4 cm,with a broad-sense heritability(H2)of 88.54%.A total of 115 significant loci were detected through genome-wide association study(GWAS)in three models(MLM,mr MLM,and Farm CPU),which explained 0.53%-19.4% of the phenotypic variation.Of them,10 loci were repeatedly detected in different environments and models,one of which q BAD.A03-2 was verified as a stable QTL using a secondary segregation population derived from the remaining heterozygous line(8S128).5.A total of 1066 differentially expressed genes(DEGs)were identified at the early flowering stage in four extremely large or small BA/BD accessions through comparative transcriptome sequencing.These DEGs were significantly enriched in the pathways related to auxin biosynthesis and transport as well as cell expansion such as indole alkaloid biosynthesis,other glycan degradation,and fatty acid elongation.6.Four BA candidate genes Bna A02g16500D(PIN1),Bna A03g10430D(PIN2),Bna C03g06250D(LAZY1),and Bna C06g20640D(ARF17)were identified through integrating GWAS and RNA-seq results,whose homologues have been reported to involve in regulating the asymmetric distribution of auxin.