ClBES1/BZR1-1, a brassinosteroid-responsive transcription factor, negatively regulates lignin deposition during secondary xylem formation in Cunninghamia lanceolata

Abstract

Cunninghamia lanceolata (Chinese fir) is an important industrial coniferous timber tree in southern China with high yield and excellent wood traits. Brassinosteroids (BRs), a class of polyhydroxylated steroidal hormones, play critical roles in the formation of wood, which is the most abundant biomass produced by plants. However, the molecular regulatory network of BR-mediated wood formation remains largely uncharacterized in coniferous trees, especially in Chinese fir. To this end, systematic identification and characterization of the BES1/BZR1 transcription factor (TF), the specific BR-responsive TF, were performed. Seven ClBES1/BZR1 members were screened and distributed on four chromosomes. ClBES1/BZR1s showed various expression profiles in different tissues and growth stages. ClBES1/BZR1-1, a hub gene in BR-mediated xylem development, repressed the transcription activity of ClCCoAOMT and ClMYB3 by directly binding to their promotors. Furthermore, ClBES1/BZR1-1 overexpression significantly decreased the lignin biosynthesis and xylem differentiation compared to wild-type. In contrast, ClBES1/BZR1-1 dominant repression greatly promoted lignin deposition during secondary cell wall (SCW) development. Consequently, by repressing the lignin-related genes’ expression, ClBES1/BZR1-1 negatively regulated the SCW deposition and xylem development in Chinese fir. These findings shed light on the regulatory role of BR modulating lignin accumulation during wood formation.