Genome-wide And Transcriptome Sequencing Analysis Of The Causes Of Leaf Sunburn In Sorbus Pohuashanensis （Hance） Hedl.

Sorbus pohuashanensis（Hance）Hedl.),is a native landscape tree of Sorbus genus with leaves,flowers and fruits as a whole.It has high garden and ecological value and is distributed in the vast areas of northwest,North and northeast China.Due to insufficient excavations,the evolutionary relationships between species and populations of Sorbus have not been revealed,and the causes of "sunburn" in leaves of S.pohuashanensis introduced to low altitude areas have not been explained.In this study,we assembled the genome of S.pohuashanensis at chromosomal level based on whole genome sequencing technology,and used resequencing technology to explore the genetic and evolutionary relationships among different species of Sorbus and different populations of sorbus.Based on the physiological and biochemical data of S.pohuashanensis under natural heat stress,Transcriptome technology was used to explore the key genes and core pathways involved in the regulation of "sunburn" in S.pohuashanensis leaves.The main results are as follows:（1）Reference genome using Pac Bio long reads.Based on the new reference genome,we resequenced a core collection of 22 Sorbus spp.samples,which were divided into 2 groups（G1and G2）based on phylogenetic and PCA analyses.These phylogenetic clusters were highly consistent with their classification based on leaf shape.Natural hybridization between the G1 and G2 groups was evidenced by a sample（R21）with a highly heterozygous genotype.Nucleotide diversity（π）analysis showed that G1 had a higher diversity than G2 and that G2 originated from G1.During the evolution process,the gene families involved in photosynthesis pathways expanded and the gene families involved in energy consumption contracted.（2）The heat in the summer,respectively for open and shade condition growth S.pohuashanensis,outdoor conditions’ helios’ S.pohuashanensis mensurate the photosynthetic parameters,comparing the diurnal change of photosynthetic characteristics,found S.aucuparia‘Tiatan’ has higher net photosynthetic rate than S.pohuashanensis tree,but they are daily change curve of bimodal type;The overall variation trend of intercellular CO2 concentration was slightly different,but it was higher in the morning and evening and lower in the noon.The diurnal variation trend of transpiration rate was the same as that of net photosynthetic rate.Stomatal conductance is one of the main factors affecting photosynthetic rate of Sorbus ropa.It is greatly affected by temperature and diurnal variation is bimodal.（3）The physiological and biochemical indexes of the leaves of S.pohuashanensis after natural high temperature stress were measured.The content of malondialdehyde increased with the increase of stress time,indicating that the cell membrane was damaged to some extent.There was no significant difference in soluble protein content,while soluble sugar showed an increasing trend,indicating that osmotic regulatory substances play a role in maintaining osmotic balance when cell membranes are damaged.The activities of SOD,POD and CAT decreased significantly at the later stage of stress,indicating that reactive oxygen species had accumulated.However,except CAT,POD and SOD activities showed an increasing trend in the early stage of stress,which played a certain role in scavenging ROS.（4）Transcriptomic sequencing was performed on the leaves of S.pohuashanensis under natural heat stress,and 182 differentially expressed genes were identified.It was found that flavonoids biosynthesis and HSP-HSF pathway play an important role in preventing sunburn in S.pohuashanensis leaves.（5）Bioinformatics-based methods,identified 33 Hsfs in S.pohuashanensis.Phylogenetic analysis of Hsfs from S.pohuashanensis and other species revealed that they were more closely related to apples and white pears,followed by Populus trichocarpa,and most distantly related to Arabidopsis.Moreover,the Hsfs were clustered into three major groups: A,B,and C.Gene structure and conserved motif analysis revealed a high degree of conservation among members of the same class.Collinearity analysis revealed that segmental duplication played an essential role in increasing the size of the Sp Hsfs gene family in S.pohuashanensis.Additionally,several cis-acting elements associated with growth and development,hormone response,and stress were found in the promoter region of Sp Hsfs genes.Furthermore,expression analysis in various tissues of S.pohuashanensis showed that the genes were closely associated with heat,drought,salt stress,growth,and developmental processes.This study first used Pac Bio three generation sequencing and Hi-C technology to assemble the chromosome level of S.pohuashanensis genome,and use the heavy sequencing technology reveals the S.pohuashanensis belong to different species as well as genetic and evolutionary relationship of sorbus species,combined the technology of the transcriptome explores the participation the leaves of S.pohuashanensis of key genes regulated by "sunburn" phenomenon and core pathways,It provides a new insight into the evolution of the genome of S.pohuashanensis.In addition,genomic resources and identified genetic information,especially those related to stress,will contribute to future studies on breeding and introduction and domestication of S.pohuashanensis.