Mining For Favorable Loci Of Root Traits At Seedling Stage In Wheat(triticum Aestivum L.)and These Effects On Nitrogen Use Efficiency Of Plants

Wheat（Triticum aestivum L.）,as the third main food crop in China,is of great significance to ensure food security in our country,however,in recent years,due to the high incidence of extreme weather problems,especially drought abiotic stress,wheat production of China is confronted with severe challenges.Root architecture systems and anatomical traits are highly important for wheat to cope with drought and other abiotic stresses,which play an significant role in wheat stress resistance improvement.Therefore,dissecting the regulation mechanism of wheat root traits,exploring root candidate genes,and developing root molecular markers will lay a solid foundation for root improvement and stress-resistant wheat varieties selection.In this study,genome-wide association studies of wheat root traits at seedling stage were performed to screen significant SNPs and associated root candidate genes.At the same time,we analyzed the correlations of root traits at seedling stage with root and shoot related traits at adult stages,and the effects of wheat roots on nitrogen use efficiency of plants at seedling stage.The key results were as follows:1.Genome-wide association studies of root traits at seedling stage in wheatIn this study,189 diverse wheat cultivars were assembled as an association-mapping panel,five root traits including total root length（TRL）,total root area（TRA）,total root volume（TRV）,average root diameter（ARD）and root dry weight（RDW）were investigated by growing in two culture conditions（Hoagland nutrient solution and pure water）.Combined with wheat 660K SNP array,a total of 95 QTLs significantly associated with root traits cultured in two conditions(P<10^-3)were identified by genome-wide association studies with four models of BLINK（bayesian-information and linkage-disequilibrium iteratively nested keyway）,CMLM（compressed mixed linear model）,Farm CPU（fixed and random model circulating probability unification）and MLM（mixed linear model）.Among them,18 QTLs were detected in both culture conditions and distributed on chromosomes of 7A,1B,2B,3B,7B,1D,2D,and 3D,which explained8.68%-14.07%of phenotypic variation.Based on these SNPs,KASP markers XNR7143and XNR3707 were developed for total root volume and root dry weight,respectively.In addition,12 candidate genes possibly regulating root development were found by mining the genes within the interval of co-localization significant SNPs.Of them,Traes CS7A02G160600,encoding 3-oxoacyl-[acyl-carrier-protein]synthase,is involved in the synthesis of root fatty acids;Traes CS2B02G117900,encoding WUSCHEL-related homeobox,is a key regulator for adventitious root growth and development;Traes CS2B02G119200,encoding apyrase-like protein,contributes to elongation of root hairs.2.Correlations of root traits at seedling stage with root and shoot traits at adult stages in the fieldWe measured shoot traits of 189 wheat varieties in Yang Ling and He Yang,and found that agronomic and physiological traits were correlated with root traits at seedling stage.The results showed that plant height（PH）,peduncle length（PL）and flag leaf width（FLW）in Yang Ling were significantly positively correlated with ARD at seedling stage,fraction vegetation coverage（FVC）was extremely significantly positively correlated with TRL（R²=0.22）,and significantly positively correlated with TRA（R²=0.17）,TRV（R²=0.16）,and RDW（R²=0.16）,canopy temperature（CT）was significantly negatively correlated with TRA（R²=-0.18）and RDW（R²=-0.19）;In He Yang,FLW was significantly positively correlated with ARD at seedling stage,while CT was extremely significantly and significantly negatively correlated with RDW（R²=-0.29）and ARD（R²=-0.16）.Meanwhile,root traits at seedling stage were also correlated with yield traits:ARD was extremely significantly positively correlated with grain number per spike（GNPS）（R²=0.24）of wheat in Yang Ling;In He Yang,TRV was significantly positively correlated with spike length（SL）（R²=0.19）and thousand grain weight（TGW）（R²=0.16）,RDW was significantly positively correlated with TGW（R²=0.16）and yield per plant（YPP）（R²=0.15）.In addition,5 extreme meterials with developed root system and 5 accessions with poor root systems were selected based on 189 wheat accessions,the correlation analyses of root traits of 10 extreme root materials at seedling stage,jointing stage and grain-filling stage showed that there was a certain correlation between root traits at seedling stage and those at adult stages in the field.However,these correlations and differences with two kinds of root extreme materials were weaken along with the wheat growth process.3.Effects of wheat roots at seedling stage on nitrogen use efficiency of plants10 root extreme varieties were used to investigate effects of wheat root systems on nitrogen use efficiency（NUE）of plants under two treatments of normal(5 mmol·L^-1)and high(45 mmol·L^-1)nitrogen.It was found that root nitrogen accumulation（RNA）,total plant nitrogen accumulation（TNA）,and P_Nunder normal nitrogen treatment were significantly different between two kinds of root extreme materials;under high nitrogen condition,root nitrogen content（RNC）,shoot nitrogen accumulation（SNA）,RNA,TNA and P_Nof wheat materials with better root systems were significantly higher than those of materials with poorer root systems,while nitrogen of dry matter productivity ratio（NDMPR）of materials with better root systems was significantly lower than that of materials with poorer root systems.Moreover,the correlation analyses of root morphological traits with nitrogen efficiency at seedling stage showed that TRL,TRA,TRV,RDW,and root tips number（RTN）were positively correlated with SNA,RNA,TNA,and P_Nunder two nitrogen treatments.However,correlations between root anatomical structure and nitrogen efficiency at seedling stage were different under two nitrogen treatments:Under normal nitrogen treatment,cortex:stele ratio（Ratio Cto S）was significantly positively and negatively correlated with NDMPR（R²=0.64）and RNA（R²=-0.64）,while there was no significant correlation between root anatomical traits and nitrogen efficiency at seedling stage under high nitrogen treatment.Comparing root anatomy between the N-efficient variety（Huaimai 25）and the N-inefficient variety（Tianmin 369）,we found that the number of xylem in Huaimai 25 was greater than that in Tianmin 369,and the cells of root cortex were more closely arranged than that in Tianmin 369.