Endogenous hormonal dynamics and transcriptional regulations in internodal growth in Cunninghamia lanceolata: based on comparative analysis of two clones with different internode growing patterns
Background Internodal characteristics are critical for cultivating knot-free Cunninghamia lanceolata, but limited knowledge of internode growing mechanisms has hindered breeding efforts for longer-internode types of C. lanceolata. In this study, we selected two clones with distinct internodal traits (C1 clone exhibited a 25.03% longer internodal length than C11) as materials. Enzyme-linked immunosorbent assay (ELISA) method and RNA-seq were used to investigate the changes in endogenous hormones and their associated transcriptional regulation during internodal growth and lateral bud formation. Result Our findings suggest that the differences in the dynamic rhythm of Indole-3-Acetic Acid (IAA) in apical buds are a key factor in C1's longer internodal growth. Specifically, IAA levels in C1 showed a significant decline at the S4 stage, which occurred later than in C11. Higher levels of IAA and cytokinins in the apical buds of C1 clone likely supported sustained internodal growth. The greater number of upregulated IAA-related genes in the upper phloem, including PIN1 and SAURs, which involved in polar transport and signal response, indicates a stronger capacity to establish apical dominance. The higher strigolactones (SLs) content, combined with IAA-driven apical dominance, delayed lateral bud emergence. Meanwhile, the lower brassinosteroids content and the high expression of brassinosteroid-activity-regulating genes such as UGT85A1 and BAS1 suggest that brassinosteroids (BRs) may serve as significant secondary hormonal signals promoting internodal growth. Weighted Gene Co-expression Network Analysis (WGCNA) further indicated that hormones transport may regulated by very-long-chain fatty acids (VLCFAs), coordinating both lateral bud emergence and internodal growth. Conclusion This study offers a preliminary understanding of the mechanisms behind internodal growth in C. lanceolata, providing insights for genetic control and germplasm selection in breeding, and serves as a guide for using exogenous hormone interventions to regulater internodal growth for cultivate of knot-free timber. Overall design: RNA-seq profiling of the Apical Bud (AB), Upper Phloem (UP) and Lateral Bud (LB) tissues of two Chinese fir clones (C1 and C11) with obvious differences in growth and branching characteristics.