Integrative analysis of the transcriptome, targeted metabolome, and anatomical observation provides insights into the brassinosteroids-mediated seasonal variation of cambial activity in Chinese fir

Chinese fir (Cunninghamia lanceolata) is widely planted in southern China due to its strong adaptability and fastgrowing. Wood formation is derived from the periodic changes of cambial activity that is regulated by environmental cues and phytohormone signals. Brassinosteroids (BRs), a class of plant-specific steroid hormones, play central roles in modulating cambial activity and xylem development. To decipher the regulatory role of BR in the dynamic change of Chinese fir cambium, a comprehensive analysis of the transcriptome, targeted metabolome, and anatomical observation was conducted. The application of 24-epibrassinolide (EBR, 10 mu M) significantly increased the cambial cell layers, resulting in the thickening of the secondary xylem. Conversely, brassinazole (Brz) treatment strikingly inhibited the xylem development by attenuating cambial activity. Moreover, 1583 up-regulated differentially expressed genes (DEGs) were shared in stages A (reactivating), C (active), and E (dormant), which were annotated into the xylem formation and BR signaling pathway. Metabolomic profiling of phytohormone indicated that the contents of brassinolide (BL, the most active BR), castasterone (CS), and typhasterol (TY) were distinctly increased in stage C. Furthermore, ClBES1/BZR1, a hub gene in the co-expression network, dramatically activated the expression of ClPAL2 and ClMYB4 by binding to their promotors. Collectively, these results elucidate the potential role of BR in dynamic changes in the cambial zone and provide insight into the regulatory mechanism of wood formation in conifers.