Study Of The Organellar Genomes And Drought Tolerance Mechanisms In Hemiptelea Davidii

Hemiptelea davidii(Hance)Planch is recognized as a valuable forest tree resource for arid and semi-arid sandy environments in china.So far,the research on the molecular biology of H.davidii is very little,and the origin and evolution of H.davidii,and the mechanism of stress response are still unclear.In this study,the complete chloroplast and mitochondrial genome of H.davidii were successfully sequenced and assembled by high-throughput sequencing technology.The genetic information of organelle genomes and phylogenetic relationship of Hemiptelea were analyzed.Based on the transcriptome analysis,the molecular mechanisms of nuclear and organelle genomes against drought stress were further investigated.The main results are as follows:(1)The chloroplast and mitochondrial genome sequences of Hemiptelea are firstly assembled in this research.The complete chloroplast genome of H.davidii is 160195 bp in length.The GC content of the whole chloroplast genome is 35.28%.It contains 132 genes,and 6543 bp repeat sequences.The complete mitochondrial genome of H.davidii is 460941 bp in length,containing 59 genes.The GC content of the whole mitochondrial genome is 44.84%.The genome has 10299 bp repeat sequences.Sequence migration analysis revealed that nine chloroplast-derived fragments were identified in the mitochondrial genome of H.davidii in a total length of 8014 bp,which accounted for 1.74%of the total mitochondrial genome and 5.00%of the total chloroplast genome in length,respectively.(2)The codon usage by organelle genome is bias in H.davidii.GC₁ shows the highest GC content,followed by GC₂and GC₃,which indicates the preferential usage of A or T at the end of a codon in organelle genes of H.davidii.There is a weak codon usage bias in organelle genome level,while the chloroplast genes have stronger codon usage bias than mitochondria genes.Neutrality plot,ENC-plot and PR2-plot analyses showed that the codon usage bias of chloroplast genes and mitochondrial genes of H.davidii are greatly influenced by natural selection pressures rather than mutation pressure.(3)Phylogenetic analysis of Hemiptelea was firstly carried out based on chloroplast genome sequence,mitochondrial genome sequence and codon usage frequency of organelle genes,respectively.They all support the APG IV classification for Ulmaceae plants.The Hemiptelea has a close relationship with the genera Zelkova and Ulmus,and suggest that Ulmaceae is monophyly.While the other eight genera,such as Aphananthe,Celtis and Chaetachme supported the classification of Cannabaceae.(4)Transcriptome analysis of H.davidii under drought stress reveals the regulation mechanisms of nuclear and organelle genomes against drought stress.A total of 2860 differentially expressed genes(DEGs)were identified,including 1333 up-regulated and 1527 down-regulated.KEGG enrichment analysis showed that photosynthesis-related pathways,plant hormone signal transduction pathways,and secondary metabolite synthesis pathways were significantly enriched under drought stress.97transcription factors were identified in DEGs,of which 69 transcription factors were up-regulated under drought stress.Transcription analysis of organelle genes showed that 92.11%of chloroplast genes and94.44%of mitochondrial genes had reduced expression under drought stress.Among them,the expression level of rps18,rpl32,and mat K gene reduced by more than 5 times.The reduced expression of these genes directly or indirectly affects the energy metabolism and the production of reactive oxygen species during the electron transfer chain.(5)Functional analysis of a chromatin-remodeling factor gene Trx G-like was carried out.Over-expression of Trx G-like gene could effectively improve the drought resistance of transgenic tobacco.The chlorophyll content and free proline content in the transgenic lines displayed higher levels than the wild-type lines,while the relative conductivity and MDA content showed a lower level.They all determined that the Trx G-like gene is involved in the drought response of H.davidii and improves the drought resistance via regulating the expression of the down-stream drought resistance genes.