QTL Mapping And Epistasis Effect Analysis On Dynamic Root Growth Of Populus Euphratica Under Salt Stress

Roots are the belowground resource-acquiring organ of plants and understanding mechanisms regulating root growth is critically important in developmental biology.Although Populus euphratica is an important ecological tree species in northwest China,studies on the genetic mechanism underlying resistance of this tree species to stress are still scarce.Through studies on genetic mapping and gene analysis of salt-tolerant root traits in P.euphratica,we can not only reveal the salt-tolerant mechanism,but also find excellent germplasm resources with strong salt-tolerant ability and stable root development,which has important practical significance for the screening of salt-tolerant varieties of P.euphratica.To this end,we cloned the constructed P.euphratica F1 mapping population and conducted a salt treatment experiment.We acquired root phenotypic dynamic data of P.euphratica on 14 time points through 360°photo,analyzed significant QTL loci of root growth dynamic traits in control and salt-stress environments using function mapping model,unraveled the salt-tolerant genetic mechanism of phenotypic variation of P.euphratica from dynamic and developmental perspectives.Further,we used the 2HIGWAS mathematical model to locate the relevant genetic loci,and systematically described the effects of gene interactions on root growth of P.euphratica.Our main results and conclusions are as follows:1.During the whole observation period,salt stress inhibited the elongation of the adventitious root of P.euphratica,while the inhibition of lateral root mainly occurred in the middle and late growth period Salt stress inhibited the surface area and volume of the main and lateral roots,and the effect was more significant in the middle and late stage.Salt stress had a weak inhibitory effect on the number of the main roots.The inhibitory effect on the number of lateral roots was significant and mainly occurred in the middle and late stage of growth.The results of residual analysis and goodness of fit showed that logistic equation could fit individual phenotypes of roots well under two environmental conditions,and the diversity between individuals was strong.The average population growth curve showed that the starting time of growth and development and the duration of exponential growth period of different root phenotypes were root-dependent.2.According to the functional mapping,there were 113,78 and 96 significant loci for total,lateral and adventitious roots respectively under the control condition,and 5 significant loci were identified at the three root levels.Under salt stress,there were 91,89 and 95 significant loci for total,lateral and adventitious roots respectively.After the removal of the repeat markers,238 and 248 significant QTLs for all root phenotypes respectively under the control and salt stress conditions,while 14 identical QTL loci were located under the two conditions.Functional annotation of significant loci detected by blast2go and String platform revealed that 48 and 49 loci were annotated respectively under control and salt stress,and these loci were involved in a series of biological processes related to cellular components,molecular functions and responses to external stimuli.Besides,the same QTL affects the heterochronic variation of the root growth trajectories between two environments,describing by a set of key parameters including the timing of inflection point on growth rate curve,the timing of maximum acceleration,the timing of maximum deceleration,and the duration of linear growth.3.Based on 2HIGWAS,main-effect QTL mapping was performed on adventitious screening SNPs and root traits of P.euphratica under the two growth environments,and 229 and 184 significant loci were detected for the control and salt stress conditions,respectively.Under the control condition,the two analysis methods did not locate the same main-effect loci,while the two analysis methods located a total of 5 identical significant loci under salt stress condition.Functional annotation was carried out on the main-effect loci discovered under 2HIGWAS method.It was found that 27 pieces of information were successfully annotated under control condition,and 10 pieces of information were successfully annotated under salt stress condition.4.Using 2HIGWAS to analyze the gene interaction between significant QTL and main-effect loci,it was found that 158 pairs of QTL-QTL interaction were detected under the control condition,among which 134 pairs were AA interaction,24 pairs were AD interaction,and 3 pairs were DD interaction.A total of 118 pairs of QTL-QTL interactions were detected under salt stress,including 93 pairs of AA interaction,24 pairs of AD interaction,and 1 pair of DD interaction.QTLs of each pair showed different effect directions(positive and negative)and effect intensity over time.The genetic strength of QTLs in the 14 root phenotypes changed with time.Under the two environmental conditions,the heritability of single interaction QTL for lateral root length,number of lateral roots,total root length and total root number was more than 5%,and other root traits were less than 5%.Different from that under salt stress,the heritability of root branching ratio under control condition was more than 5%than that under single interaction QTL,and the heritability of lateral root length was significantly stronger than that under salt stress.Our study obtained the QTL loci and main-effect QTL loci related to growth and salt-tolerance of P.euphratica roots dynamic growth trajectory,analysed the biological function of key genes and the gene interaction effects of significant QTLs.These findings have important theoretical and practical guidance to reveal the saltstress genetic mechanism and breeding of salt-tolerant varieties of P.euphratica.