| Poplar is the main plantation forest species in China.It has the characteristics of wide adaptability and early rapid growth,and widely used in pulp timber forests.Populus×euramericana has become the main force in the construction of pulp forests in various regions.The ideal plant type is the goal and law for selection breeding and molecular breeding,however,the progress of related researches in forest trees is slow.Forest trees have a long growth cycle,early selection has always been an important issue for breeders.Based on the ideal plant type of high-yield P.×euramericana in high density planting during short rotation of adult trees obtained by our research group in early stage,the retrospective study was carried out in this research under more controllable and consistent conditions.The characteristics of plant growth,branch,leaf,stomatal,photosynthetic physiology and phenophase of P.×euramericana grown under different planting densities were analyzed,three-dimensional laser scanning technique was used to obtain the parameters of leaves that from simulated density test?i.e.shading treatment?,and the phenotype and gene expression were also jointly analyzed.Meanwhile,the gene expression differences and joint analysis at different ages of ideal plant type of high-yield P.×euramericana were carried out.The growth traits related to ideal plant type with high-yield was analyzed and explored from multiple levels of growth,photosynthetic physiology,phenotypic and transcriptome,which provided important support for further understanding of ideal plant type of poplar as early and more reliable as possible.It has a great reference significance to poplar breeding with high quality and high yield.The main conclusions are as follows:?1?The growth,phenotypic traits and photosynthetic physiology of P.×euramericana clones under different planting densities were significantly different.Plant height,DBH,number of branches,angle of branch,number of leaves,leaf length,leaf width,petiole length,single leaf area,chlorophyll relative content,stomatal length,stomatal width,stomatal conductance,intercellular CO2concentration and transpiration rate of plants at high density were lower than the values of low density,and density ultimately influenced the yield by affecting various traits.Based on the comprehensive analysis of early growth model,seven indexes including plant height,DBH,number of leaves,angle of branch,single leaf area,leaf mass per area and water use efficiency,were selected as the early indirect selection indexes for high-yield P.×euramericana.Ideal plant type 97,111 and 171 clones with high-yield,which have followed relative characteristics:rapid growth of plant height,DBH and ground diameter,more leaves and branches,larger single leaf area,longer petiole length and total retention period of the leaves.?2?The plants showed a slender growth state under the shading treatment of density simulation experiment,the relative chlorophyll content of leaves increased under shading treatment.Based on the growth traits and relative content of chlorophyll,the principal component analysis combining with subordinate function method,the clones 171 and 97 with strong shade tolerance were screened out.The analysis of leaf related characters measured by three-dimensional scanning imaging showed that lower shading moderately increased leaf length,leaf width and single leaf area,but with the increasing of shading,the corresponding index value decreased.The petiole length,internode length and zenith angle increased with the increasing of shading degree.50,268 and 124 co-existed differentially expressed genes?DEGs?were found in the two clones in three shading treatments compared with the control group,respectively.GO analysis showed that these differential genes were mainly involved in transcription,DNA template,flavonoid biosynthesis process,nucleus,ATP binding and DNA binding,metal ion binding etc.20 DEGs involved in shading response were found by transcriptome sequencing.The weighted gene co-expression network was constructed by the joint analysis of leaf phenotypic traits from three-dimensional scanning and gene expression from transcriptome sequencing.It was found that the midnightblue module had a high correlation with zenith angle and internode length,and genes with top 10 connectivity were screened out as the core genes in the module.It is speculated that these genes may be important candidate genes involved in the regulation of related traits,which provides a new clue and reference for the analysis of complex regulatory networks related to agronomic traits.?3?Transcriptome sequencing of juvenile two P.×euramericana clones of ideal plant type with high-yield showed that a large number of DEGs were identified from different density and part comparisons,which played an important role in the early formation of high yield ideal plant type.6547 and 7471 co-existed DEGs were found in the comparisons between different parts in high and low densities.27 and 21 co-existed DEGs were found in the comparisons between different densities in upper and bottom parts.GO analysis showed that the differential genes in the comparison between upper and bottom parts in same density were mainly related to the internal composition of membrane,plasma membrane,response to abiotic stimulate,small molecular metabolic process,catalytic activity,transferase activity etc.The comparison between high and low density in the same part mostly involved in transcription DNA templates,nuclear and transcription factor activity sequence-specific DNA binding etc.KEGG analysis showed that fatty acid,amino sugar,nucleoside sugar,starch and sucrose metabolism were significantly affected by different parts under two densities.The identification of 12 DEGs,related to leaf senescence from transcriptome sequencing can provide an important basis for explaining the senescence mechanism of early leaf shedding in the bottom part of plant grown under high density.In addition,five co-existing light-harvesting complexes I and II chlorophyll a/b binding proteins were screened out in the comparison between upper and bottom parts of different densities in two clones.These genes can be used as important reference genes for screening high light efficiency.?4?A large number of DEGs were found between different density and part comparisons by transcriptome sequencing in the mature P.×euramericana of ideal plant type with high-yield,which acted an important part in regulating the high yield ideal plant type.There were 4929 up-regulated genes and 4082 down-regulated genes in comparison of different parts among the high density.1488 up-regulated genes and 2199 down-regulated genes existed in comparison of different parts of low density.There were 1885 up-regulated genes and 2119down-regulated genes in the different densities comparison in the same part.GO analysis showed that most of the DEGs in different parts in the same density were related to cell wall,while the DEGs from upper part between different densities were mostly related to the process of material metabolism,the DEGs from middle part were mostly related to the biological process,and the DEGs from bottom part were mostly related to the material reaction.KEGG analysis showed that flavonoids biosynthesis was a significant pathway affected by different parts at both two densities,while for the comparison between different densities in same part,DEGs were mainly enriched in plant hormone signal transduction pathway.Five light-harvesting chlorophyll a/b binding protein genes,a series of DEGs that response to light stimulus,were found in the sequencing results,which could be used as reference genes for screening high light efficiency of ideal plant type.?5?A total of 14,987 DEGs were found in a joint analysis of gene expression in ideal plant type with high-yield between different age comparisons of density and part,which played an important role in each development stage of high yield ideal plant type.GO analysis showed that the DEGs of upper and bottom parts under high density were mainly related to biosynthesis and metabolic processes.The DEGs of upper part in low density were mainly related to enzyme activity and intracellular substances,while the DEGs of bottom part were mainly related to external response and metabolic process.KEGG analysis showed that the most significant enrichment was plant hormone signal transduction pathway.A series of DEGs related to ideal plant type branch growth and development were found in the transcriptome joint analysis between different ages,including genes related to auxin,cytokinin and strigolactone biosynthesis and signal transduction.Finally,the gene network of branch growth and development regulated by plant hormone in P.×euramericana was constructed,which provided an important reference for the subsequent study on the establishment of ideal plant type of poplar.A number of DEGs enriched in photosynthesis and photosynthesis-antenna protein pathways were identified from transcriptome analysis between different ages.The identified photosynthesis-related genes can provide important reference for the follow-up study of high light efficiency research in poplar.The ideal plant type of high-yield P.×euramericana of different ages were studied in this research,and the early indirect evaluation index of high yield ideal plant type were screened out through the retrospective study.Under the shading treatment of density simulation experiment,the character parameters of leaves were obtained by three-dimensional scanning technique,and the genes related to shade tolerance were found form transcriptome sequencing.A gene co-expression network was established to excavate the key candidate genes involved in the regulation of growth specific traits.A series of important functional genes related to growth of high yield ideal plant type,were excavated from sequencing and joint analysis of transcriptome at different ages.The exploratory study on growth-related traits of high yield ideal plant type of P.×euramericana was carried out from multi-omics level,which provided an important reference and basis for poplar breeding with high quality and high yield. |
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