Study On The Regulation Of PtrMYB92 Gene In Xylem Development And Drought Stress Response In Populus Trichocarpa

Wood is the most widely used renewable raw material for industrial and biomass energy production.The secondary xylem of trees is not only lignified to form wood,but also responsible for transporting water and ions absorbed by roots to other tissues.Therefore,by studying the response of xylem to drought stress,we can learn more about the mechanism of drought resistance and the effect of drought stress on the production of woody biomass.MYB family transcription factors not only play a key transcriptional regulatory role in wood formation,but also play an important regulatory role in plant response to drought stress.The results of previous studies in our laboratory showed that PtrMYB92 gene was specifically expressed in the xylem of poplar and was regulated by drought stress,indicating that PtrMYB92 gene may be involved in the regulation of xylem development and drought stress response in Populus trichocarpa.In this study,we firstly generated the overexpression and CRISPR mutation vectors of PtrMYB92 gene.Genetic transformation of P.trichocarpa was carried out by using a rapid genetic transformation method mediated by Agrobacterium tumefaciens.The transgenic plants were tested at the DNA and RNA levels and were confirmed by sequencing.Thirteen overexpressing transgenic lines and four mutant lines of PtrMYB92 gene were obtained.Phenotypic analysis of the transgenic and mutant plants revealed that the PtrMYB92 overexpressing transgenic plants had fewer roots,thinner stems,smaller leaves and shorter plant height and the mutants had no obvious morphological differences,comparing to wild-type plants.Histological analysis showed that the xylem development in PtrMYB92 overexpressing transgenic plants had significant changes.The number of vessel cells was increased significantly,and the,size of vessel cells was decreased,and the area of vessels(void area)in the transverse section of the woody stem was increased in the PtrMYB92 mutant.It was observed by scanning electron microscope that the cell wall of the PtrMYB92 overexpressing transgenic plants was thickened,while the cell wall of the PtrMYB92 mutant was not significantly changed.The results suggest that PtrMYB92 and its orthologs may have functional redundancy in regulation of cell wall thickening.Further studies showed that the expression of lignin biosynthesis genes in PtrMYB92 overexpressing transgenic plants was up-regulated and the lignin content was significantly increased.In the PtrMYB92 mutant,the expression of lignin biosynthesis genes was down-regulated and the content of lignin was decreased.These results suggest that PtrMYB92 gene was involved in the regulation of lignin biosynthesis.Moreover,drought stress treatment was applied to the PtrMYB92 overexpressing transgenic plants.It was found that,comparing to wild-type plants,the PtrMYB92 overexpressing transgenic plants were more sensitive to drought.In summary,our study revealed the function of PtrMYB92 gene in xylem development and response to drought stress in P.trichocarpa,which provides new clues for the use of genome editing technology to create new forest germplasm with high quality and high resistance.