| The high application and low efficiency of nitrogen fertilizer have had an important impact on the environment and production.Balancing the economic and ecological benefits of crops has become a difficult problem for modern agriculture to respond to green development.Therefore,improving the nitrogen use efficiency and yield of crops while protecting the environment has become a current trend of nitrogen nutrient research.Sweetpotato is the seventh largest food crop in the world with high and stable yield and high nutritional value.As a root nutrient crop,the nitrogen fertilizer environment has an important impact on the balance and distribution of photosynthetic products during its growth period.The nitrogen fertilizer application and utilization efficiency affect the plant morphogenesis,root growth and development,yield,and physiological traits.It is of great significance to improve the nitrogen use efficiency and maintain the yield while reducing the nitrogen application rate for efficient production of sweetpotato,reducing resource consumption and protecting the ecological environment.Methods to improve nitrogen use efficiency in sweetpotato include selecting low nitrogen tolerant sweetpotato genotypes and mining nitrogen efficient candidate genes to guide molecular breeding.At present,there are relatively few studies on the screening of sweetpotato low-nitrogen-tolerant varieties,and the complexity of the genome,the mining and identification of nitrogen-efficient genes is still in its infancy,and the related nitrogen-efficient genes that can be used to guiding study that sweetpotato molecular-assisted breeding are lacking.Based on this,the experiment used 129 sweetpotato materials collected and preserved in the"National Sweetpotato Germplasm Bank"of Xuzhou Sweetpotato Research Center,Jiangsu,and set two nitrogen treatment levels(0mmol·l-1and 14mmol·l-1pure nitrogen).Hydroponics experiments were conducted by investigating shoot biomass,shoot biomass increase,root biomass increase,plant biomass increase ratio,root shoot ratio,vine length,root length,leaf number,chlorophyll content index,nitrogen accumulation and nitrogen physiological utilization efficiency.11 agronomic trait values were used to screen low-nitrogen-tolerant sweetpotato cultivars by statistical principles;Based on the resequencing genome data of 129 sweetpotato cultivars,18 agronomic trait values related to nitrogen uptake and utilization were analyzed by GWAS to identify candidate genes screening range,functional annotation of candidate genes infer nitrogen-efficient candidate genes.The main findings are as follows:1.Based on principal component analysis,correlation analysis and regressionanalysis of low nitrogen tolerance index of 11 traits,an evaluation system for low nitrogen tolerance in sweetpotato seedling stage was established.Shoot biomass increase,root biomass increase,shoot biomass,leaf number,vine length,root length,nitrogen accumulation and nitrogen physiological utilization efficiency;2.Using the 8 agronomic indicators in the low nitrogen tolerance evaluation systemto conduct cluster analysis,126 sweetpotato varieties were divided into 7 low nitrogen tolerant type,12 intermediate type and 107 low nitrogen tolerance type.Sweetpotato varieties including:Yuzi 3,Yuzi 6,Zhezishu 2,13104-2/Zishu 1,Yuzixiang 10,Luozishu 1and Yibin Sweet Potato;Xinxiang,Xuzi Shu No.3,Jizishu No.3 and Hongweishu belong to the low nitrogen intolerance type;3.The variance analysis of three types of sweetpotato varieties with differentlow-nitrogen tolerance and low-nitrogen tolerance index of each agronomic trait showed that under low nitrogen stress,the low-nitrogen tolerant genotypes performed better than the intermediate type and the low-nitrogen-intolerant cultivar.,and the type of sweetpotato with strong low-nitrogen tolerance was significantly higher than the low-nitrogen-intolerant type,and the increase of root biomass increase,shoot biomass increase and shoot biomass were important to the differences in the low-nitrogen tolerance of varieties;4.Based on the trifida reference genome,a genome-wide scan of 129 cultivars wasperformed using the MLM model and molecular markers,and the correlations were related to shoot biomass increase,root biomass increase,root nitrogen accumulation and shoot physiological use efficiency of nitrogen,a total of 134 QTL segments and 888 SNP markers were associated with 4 traits.Based on multiple traits stable expression and Manhattan plots,10 QTLs with high reliability and 93 SNP candidate loci within their confidence intervals were screened from shoot nitrogen use efficiency traits;5.Amplify 10kb of the physical location of the screened SNP site upstream and downstream as the candidate gene screening region,and functionally annotate it in the trifida reference genome bank,and obtain 92 candidate genes.Based on functional annotation,six candidate genes related to nitrogen utilization efficient were predicted,itf01g08120.t1,itf01g22030.t1,itf01g22080.t2,itf01g22100.t2,itf01g22400.t1 and itf01g25900.t1.The focus was on the enzyme regulation genes itf01g08120.t1 and itf01g22400.t1,which encode glutamate dehydrogenase and nitrite reductase,respectively,which are metabolic regulatory enzymes in the process of ammonium assimilation.itf01g22030.t1 and itf01g25900.t1 are presumed to be transcriptional regulator genes cascaded with upstream and downstreamnitrogen metabolism-related genes in the process of plant hormone signal transduction.itf01g22080.t2 and itf01g22100.t2 encode structural proteins.To sum up,by screening low-nitrogen-tolerant varieties,the characteristics of thevarieties themselves can be used to adapt to different fertility soils,and the current situation of low nitrogen use efficiency and unreasonable distribution of photosynthetic products can be improved.It provides convenience,and the prediction of nitrogen-efficient candidate genes provides a reference for the genetic improvement of sweetpotato from the aspect of molecular breeding. |
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