A Systematic Dissection Of The Mechanisms Underlying The Natural Variation Of Silique Number In Rapeseed Germplasm

Among the three components(silique number,seed number per silique and seed weight)of yield in rapeseed,silique number is highly correlated with yield and contributes the most to yield(Shi et al 2015).This trait is complex in genetic basis and susceptible to environment.In recent years,the research progress of silique number is few.In rapeseed germplasm resources,silique number shows extensive variation,which is invaluable for its study and improvement.However,the comprehensive and systematic study on this kind of natural variation is poor,which is a constraint to our in-depth understanding and genetic improvement of this important trait.In the current study,we aimed to identify the key mechanism responsible for silique number variation in rapeseed(Brassica napus L.)germplasm through a systematic study.We investigated the genetic,physiological and molecular causes of silique number variation,the main results were as follows:1.Our study explicitly proposed the academic idea of decomposing the silique number into two sub-traits(flower organ number and silique setting rate).We found that the difference of flower organ number is the main reason for the natural variation of silique number(relative contribution rate is 75.2%).So we performed a continuous microscopic observation on the whole process of flower bud differentiation that determines the number of floral organs.Microscopic analysis showed that the difference among extremely lines normally occurred at the amount of flower organ but not morphology.Furthermore,we measured and compared many physiological indicators that may determine the silique/flower number.The result showed that the silique number was highly associated with the total photosynthesis and biomass.2.A panel of 331 core lines was employed to genome wide association study(GWAS)by high-density SNP typing and accurate phenotypic identification in multi-environment.A total of 27 loci(including 20 novel loci)were identified and 12 loci could be repeatedly identified.In addition,based on the physical map of rapeseed,all of the currently and previously reported silique number QTLs in rapeseed were integrated,about 120 loci were successfully mapped in the physical map which represented the mostcomprehensive genetic architecture of silique number in rapeseed until now.This study established the basis for gene cloning of silique number in Brassica napus L.and marker-assisted selection of this trait in breeding programme.3.A total of 2746 differentially expressed genes(DEGs)were produced by transcriptome analysis of shoot apical meristem(SAM)from few-silique and more-silique lines.Most of enriched groups were the auxin biosynthesis/metabolism,photoperiod,circadian rhythm and stress which were closely related vegetative growth and nutrition/energy accumulation.Comparative transcriptomic analysis proved the differences of silique number between more-and fewer-silique lines were mainly due to biomass/nutrition accumulation and auxin-related regulation at the molecular level.4.By integrating GWAS and RNA-seq results,six novel candidate genes were identified,and two other candidate genes were identified in former research.They were Bna A02g08800 D,Bna A02g08900 D,Bna A03g09920 D,Bna A05g01050 D,Bna A09g05170 D,Bna A10g00780 D,Bna A10g14470 D and Bna C02g02900 D.All of them were used for overexpression vectors construction and functional verification in Arabidopsis.Among them,Bna A03g09920 D were initially confirmed to be the functional genes of silique number.The results of the above-mentioned genetic,physiological,microscopic and molecular studies were highly consistent.The total leaf area and nutrient accumulation of rapeseed before flowering are the key factors determining the flower/silique number.Our work provides theoretical guidance and technical support for the genetic improvement of silique number.