Functional Analysis Of PtrMYB48 In Wood Formation Of Populus Trichocarpa

MYB family genes are widely distributed in higher plants,especially R2R3-MYB subfamily.The R2R3-MYB subfamily has a variety of functions and is involved in many biological processes,including plant morphogenesis,biological and abiotic stress,primary and secondary growth,etc.Many members of R2R3-MYB gene are involved in secondary growth and wood transcriptional regulation in forest trees,but R2R3-MYB gene involved in both phloem and xylem growth and development is rarely reported.In this study,a PtrMYB48 gene of R2R3-MYB type was identified from Populus trichocarpa,which is involved in the growth and development of phloem and secondary xylem.The results are as follows:Gene expression analysis by Aspwood tool showed that PtrMYB48 gene was highly expressed in phloem and mature xylem.The CDS sequence length of the gene is 726 bp,encoding a protein composed of 241 amino acid residues.Bioinformatics was used to analyze the structure of its encoded protein.The results showed that PtrMYB48 belonged to the R2R3-MYB subfamily.In order to identify the tissue expression characteristics of PtrMYB48 promoter,four ProPtrMYB48::GUS transgenic plants were constructed and genetically transformed.Histochemical staining results showed that GUS activity was mainly in phloem and developmental xylem.Therefore,PtrMYB48 promoter activity was mainly expressed in phloem and developmental xylem.In order to identify the function of PtrMYB48 gene,pCAMBIA1300-35S::PtrMYB48-3×Flag overexpression vector was constructed,and 15 overexpressed poplar lines were obtained through genetic transformation and molecular identification.Compared with the wild type,overexpressed plants had lower plant height and thinner stem diameter.Microscopic observation of stem tissue staining showed that the xylem width of overexpressed plants was narrower than that of wild-type plants,the layers of xylem fiber cells were less,and the secondary phloem was thicker.Phloroglucinol lignin staining showed that the red staining of stem tissues of overexpressed plants was deeper than that of the wild type.It is suggested that the overexpression of PtrMYB48 gene not only affects the lignin deposition of poplar,but also affects the growth and development of poplar.In order to understand the changes of PtrMYB48 gene overexpression on phloem and xylem,the fiber cell walls of the developing and mature xylem of the overexpressed plants were thinner and the secondary phloem was wider than that of the wild type.In addition,ABSL and Anthrone methods were used to detect the chemical composition of wood.The lignin content of overexpressed sapling’s wood was higher than that of the wild type and the cellulose content was lower.In conclusion,the expression activity of PtrMYB48 promoter was mainly expressed in phloem and xylem.Overexpression of PtrMYB48 gene resulted in xylem narrowing,fiber cell wall thinning,secondary phloem thickening,lignin content increasing and cellulose content decreasing.Therefore,PtrMYB48 gene plays a role in regulating the growth and development of phloem and xylem.