Study On Regulation Of Latewood Formation In Larix Kaempferi

Larch(Larix Spp.)is one of the main afforestation species in northern China.Its cambium activity shows obvious seasonal dynamics.The cells formed in the late growth stage are larger with thikcer cell wall than the cells formed in the early stage.The boundary between earlywood and latewood changes rapidly,and the late wood percentage can reach 60%.Latewood percentage is one of the important factors affecting wood properties,which is regulated by strict genetic mechanism during the development of wood(secondary xylem).However,little work has been carried out about the mechanism of latewood formation.In order to explore the developmental characteristics and genetic information related to the latewood percentage of Larch,by comparing growth traits and wood property traits over 12 years,the High Latewood percentage Clone(HLC)and Low Latewood percentage Clone(LLC)of Japanese larch(Larix kaempferi)with the same growth were selected as materials.We analysized the chemical composition of them,and compared the seasonal growth characteristics of vascular tissues between clones by phenological and anatomical observation.The gene expression patterns of immature xylem in HLC during a seasonal growth were studied,and the key genes regulating the transformation of earlylood formation towards latewood formation were found via transcriptome sequencing and bioinformatics analysis.The gene expression patterns in immature latewood cells of HLC and LLC were compared by using the differentially expression profile sequencing(DGE).The regulation mechanism of latewood formation was preliminarily explained.The main findings are as follows:(1)By comparing the growth traits of 78 clones and the latewood percentage of 20 clones of 16-year-old Japanese larch clone test forest for 12 years,two groups of clones showed significant difference in latewood percentage but the same growth traits were obtained.One group is High Latewood percentage Clone 1(HLC-1)and Low Latewood percentage Clone 2(LLC-2),and the other are HLC-7 and LLC-11.The latewood percentage of HLC-1,LLC-2,HLC-7,LLC-11,were 52.33%,34.76%,51.84% and 40.01%,respectively.(2)According to the anatomical observation of vascular tissues,seven phases of secondary xylem developmental process were identified: reactivation,early and late stages of earlywood formation,early and late stages of latewood formation,cessation and dormancy.When late stages of earlywood formation turned to the early stages of latewood formation,the number of cambium zoon cells and expansion cells decreased and the secondary wall synthesis increased.(3)Analysis of transcriptomes of immature xylem at key developmental stages showed that the transcriptomes changed largly between reactivation,active growth period and cessation but only 11 genes differentially expressed between late stages of earlywood formation and early stages of latewood formation.Hormonal-related genes encoding PIN protein and ABC transporters,gibberellin 3-beta-dioxygenase 1(GA3ox1)and 1-aminocyclopropane-1-carboxylate oxidase(ACO)were up-regulated in reactivation phase and active growth period;transcription factor genes WRKYs,NACs,ERFs and DOFs involving in regulation of cellular processes predominantly expressed at the reactivation phase;transcription factors genes MYB26,MYB46,and MYB8,and genes involving in secondary wall components biosynthesis all showed high transcript levels at active growth period but unrderepresented at ceassation phase;when latewood formation started,the expression level of HB-8,which regulates the activity of the cambial cells,was down-regulated,and the expression level of MYB46,which regulates the synthesis of the secondary cell wall,was up-regulated.(4)Significant differences in the secondary xylem developmental sequence,chemical contents and gene expression level were observed between clones with different latewood percentage.The HLC began latewood formation earlier and ended later.It also presented higher ratio of ?-cellulose / lignin.The content of ?-cellulose in HLC-1 and LLC-2 was 42.16% and 38.01%,respectively,and the lignin content was 29.84% and 31.34%,respectively.The expression level of several genes involved in auxin transport and response(ABCB4,auxin inducing protein,auxin response factor 23),transcription factor genes(NAC66,NAC7,MYB46,MYB1,MYB3,MYB8 and MYB39)and secondary wall synthetic enzymes(cellulose synthase,sucrose synthase,cellulose-like enzyme,laccase,etc.)were up-regulated in high latewood percentage.The results above suggest that HB-8,MYB46 transcription factors may be involved in the regulation of the transformation from earlywood to latewood formation;NAC7?NAC66?MYB46 are key regulators of secondary cell wall biosynthesis;The prolonged latewood formation,hormonal signal maintenances and active expresstion of secondary cell wall biosynthetic genes could induce a high latewood percentage.The found that the high latewood percentage clone has higher cellulose/lignin value is of great significance to the cultivation of pulpwood.This study lays a foundation for further study on genetic regulation mechanism of latewood percentage of Japanese larch.