| Peach originated from China and is one of the important economic fruit tree.By 2020,China’s peach cultivation area accounts for 55.06% of the world’s total cultivation area.The growth of peach trees is vigorous,it is necessary to carry out repeated pruning in the process of production and cultivation,which greatly increases costly in terms of time and labour.In addition,improper management can lead to the formation of a large number of useless thick branches,which will not only increase the consumption of tree nutrients,but also increase the difficulty of shaping and pruning,and disrupt the original tree shape.In order to analyze the mechanism of branch thickening of peach tree,In this study,“Qiumihong” peach was used as research material,the thickness of xylem and internode thickness of branches of “Qiumihong” peach were analyzed by paraffin section,and the relationship between xylem thickening and branch thickness was clarified.Transcriptome data of different internodes of branches were analyzed to screen out transcription factors that may be involved in regulating the growth of peach branches,and their functions were verified.At the same time,the effect of exogenous GA3 on the diameter of peach branches was analyzed.The interaction between DELLA and NAC transcription factors was analyzed by yeast two-hybrid and BiFC.The main results are as follows:1.Paraffin sectioning of the 1 to 6 internodes of “Qiumihong” branches showed that from the apex to bottom,branches gradually became thicker and the degree of xylem region gradually increased,but there was no significant difference in the width of pith region.It is suggested that the thickening of xylem region inside branches may promote the thickening of peach branches.2.Two candidate NAC transcription factors PpNAC1 and PpNAC2 were screened out based on the RNA-sequencing analyses of different internodes of “Qiumihong”branches.The CDS length of PpNAC1 and PpNAC2 was 1017 bp and 1155 bp,and encodes 400 and 436 amino acids,respectively.PpNAC1 was located on chromosome2 and PpNAC2 on chromosome 5.The results of multiple sequence alignment showed that PpNAC1 and PpNAC2 had a high amino acid sequence similarity with the NAC family transcription factors from other species,and had a conserved NAC domain.In addition,phylogenetic analysis of NAC protein showed that PpNAC1 and PpNAC2 were found to be close to the NAC related to secondary wall synthesis of other species,suggesting that PpNAC1 and PpNAC2 may have biological functions in regulating xylem formation of peach branches.3.In order to verify the function of the PpNAC1/2 genes,the over-expression vectors of p SAK277-PpNAC1/2 were constructed and transformed into Arabidopsis and poplar.The results showed that the rosette leaf of transgenic PpNAC1/2 appeared different degrees crimpiness in the early stage of vegetative growth.And the leaf area and petiole length were also smaller than the WT control.The plant height and stem diameter of the PpNAC1/2 transgenic poplar was lower than that of the GFP control.The leaves of PpNAC1 transgenic poplar showed significant crimping and wrinkling,which was similar to relevant reports.The leaf area and the petiole length of PpNAC2 poplar was significantly smaller than that of the control.In addition,the expression of genes related to secondary wall synthesis was significantly up-regulated in all three lines of PpNAC1/2 transgenic poplar.These results suggest that PpNAC1/2 may be involved in regulating xylem secondary wall synthesis in peach branches.4.The exogenous GA3 treatment of peach seedling showed that GA could promote the stem thickness.In addition,paraffin sectioning of the treated materials showed that the xylem was significantly thicker than that of the control,which indicted that GA can promote the xylem differentiation of peach stem.Therefore,it is speculated that GA3 can promote the branches thickening by regulating xylem thickening.5.The protein interaction between GA signaling inhibitor DELLA and PpNAC1/2 was analyzed using Yeast two-hybrid and BiFC.The results showed that PpNAC1/2interact with PpDELLA2,not interact with PpDELLA1.In conclusion,PpNAC1 and PpNAC2 transcription factor may be involved in the formation of xylem secondary wall and regulateing stem thickness.Gibberellin signal could promote xylem thickening through the interaction between PpDELLA2 and PpNAC1/2,then regulate the branches thickening.This study provides a new idea for further understanding the mechanism of peach branch thickening,and provides a theoretical basis for the reasonable regulation of peach branches thickness. |
