Study On Salt Tolerance And Molecular Regulation Mechanism Of Allotriploid Populus Cathayana

Polyploid plants are generally considered to have strong resistance to stress.The study of salt tolerance mechanism of allopolyploid provides an important reference for analyzing the theory of polyploid resistance breeding.This study uesd two triploid Populus populations with different kinds of 2n gamete source and one diploid population as experimental materials.These three populations were generated by the same female（P.pseudo–simonii×P.nigra var.italica）and male（P.×beijingensis）parent.Under the same culture environment and Na Cl solution treatment,the growth state and salt resistance related indexes of its leaves were measured,and the RNA-Seq、Small RNA-Seq high-throughput sequencing methods were used.The characteristic of salt responsive genes expression changes and regulation of salt responsive gene expression by miRNA in allotriploid Populus were analyzed.The main results were as follows:（1）The salt tolerance of allotriploid Populus population was generally stronger than that of diploid Populus population,and there was no significant difference between FDR（the first-division restitution）and SDR（the second-division restitution）allotriploid.The results showed that salt stress inhibited the growth and photosynthesis of triploid and diploid,but the triploid produced relatively more antioxidant enzymes and osmotic regulators.Compared with diploid,triploid leaves showed significant differences in chlorophyll content,relative membrane permeability,POD activity and Pro content.Salt stress had a certain inhibitory effect on the growth and photosynthesis of triploid and diploid,but allotriploid Populus would produce more antioxidant enzymes and osmotic regulators.At the same time,it was found that there were some differences among genotypes in each population,indicating that there were genotypes with weak salt tolerance in allotriploid Populus population.But the genotype with strong salt tolerance in triploid Populus population were much better than those with strong salt tolerance in diploid Populus population.（2）Under short-term salt stress,there were a small number of unique transient salt responsive differentially expressed genes in allotriploid Populus.The specific DEGs of FDR and SDR triploid were 426 and 644 respectively.Through functional analysis,four salt tolerance related genes with significantly up-regulated expression were screened from FDR triploid poplar,which were JAZ1、PRX25、RAP2.1and WR3.37 key salt response genes were screened from SDR triploid poplar,which were mainly related to hormone mediated signal pathway and antioxidant enzyme activity.（3）Under continuous salt stress,the allotriploid Populus mainly regulated differentially expressed genes in ion balance regulation,antioxidant enzyme activity and signal transduction.Compared with the diploid,the allotriploid Populus DEGs were mainly related to redox reaction,ion transmembrane transport,catalytic activity,and cell homeostasis,and participated in dehydration stress,salt tolerance and growth-related pathway.Subsequently,we screened 35 key salt tolerance genes of allotriploid Populus participated in the hormone biosynthesis and signal pathway,such as ABA,JA and GA3,ion transmembrane transport and the antioxidant enzyme activity.In addition,from the physiological indexes of Populus leaves,allotriploid Populus had better sodium and potassium ion regulation ability,higher hormone regulation levels such as ABA and JA,and produces more antioxidant enzymes and osmoregulation substances.（4）Under continuous salt stress,many significantly differentially expressed transcription factors related to stress response were found.Among them,NAC,MYB、MYB＿related and AP2/ERF were accounted for a high proportion,followed by b HLH,WRKY and b ZIP.The total amount of related transcription factors was the largest in FDR triploid.In addition,the study found that FDR and SDR triploids were likely to have similar regulation patterns of salt responsive genes overall,and some salt responsive DEGs were up-regulated in diploid Populus,which jointly participated in the salt tolerance process.（5）Different miRNAs in salt stress response of allotriploid.Populus had a dose effect.The study found the salt response related miRNA-m RNA regulatory network relationship between 22pairs of target genes compared with diploid,focusing on the target genes of mi R7835 and ptc-mi R6462 were CRK and P5CR,respectively,which were significantly up-regulated in triploid poplar,and the target genes of mi R169 and mi R6425 were YDA and JAZ,respectively,which were significantly down regulated in triploid poplar.The regulation of chemical signals to hormone signal transduction played a key role in the enhancement of salt tolerance of allotriploid Populus.The down-regulated miRNAs such as mi R399,mi R477 and mi R6462,which negative regulation target genes SDIR1,Ann AT,P5CR,played an important role in sensing stress signals and activating downstream genes of regulatory network.In conclusion,the molecular regulation mechanism model of salt response of allotriploid Populus was proposed.Under continuous salt stress,the negative regulation of the differential expression of mi R477,mi R7835,mi R169,mi R 6425,in the triploid leaves of Populus under corresponding salt stress strengthens or weakens its inhibitory effect on the target genes Ann AT,CRK,YDA,JAZ,resulting in the significant up-regulation of DEGs positively related to salt tolerance,such as CRK,MAPKKK,MPK,NCED,Sn RK2,ABF,JAR and other genes,and the significant down-regulation of DEGs negatively related to salt tolerance,such as PP2C,JAZ and other genes.That resulted the gene expression and regulation ability of MAPK cascade mediated by Ca²⁺signal,ABA and JA signal response,ion balance regulation and ROS clearance in triploid leaves were enhanced,which improved the salt tolerance of allotriploid Populus.It laid a theoretical foundation for salt resistance breeding of Populus.