Wood formation in trees responding to nitrogen availability.

The development of wood (i.e. secondary xylem) follows a sequential process that determines its anatomical and chemical properties, which is significantly influenced by environmental factors such as nitrogen (N). Timber plantations are frequently planted on marginal lands with severely constrained soil N levels, leading to restrictions in wood production. N fertilization is commonly employed to ensure the high productivity of these timber plantations, where N availability becomes a crucial factor influencing trees' wood formation in the field. Much progress has been made to understand the growth and development of secondary xylem in response to N availability, and to elucidate the underlying transcriptomic mechanisms. Limiting N supplies often result in decreased xylem width, cell layers and lumina diameters of vessel and fiber cells, and increased thickness of fiber cell walls, as well as longer fiber cells. Conversely, high N-treated woody plants often exhibit opposite growth and anatomical effects. Low N conditions typically lead to increased hemicellulose and lignin contents, while N fertilization often results in increased cellulose and hemicellulose deposition and lower lignin concentration. The global transcriptomic reprogramming underlying N-driven wood formation is associated with N metabolism, carbohydrate metabolism, water transport, phytohormone metabolism, cell wall biosynthesis and modification, and stress response. Transcriptional networks, including circRNA/lncRNA-miRNA-mRNA networks, that underlie wood formation in response to N nutrition have been constructed. Several key structural genes, transcription factors, and non-coding RNAs have been identified within these networks. This review highlights the crucial roles played by N availability in wood formation in trees and serves as a foundation for optimizing N fertilizer utilization to enhance wood yield and quality. • Wood growth was highly altered by N level in greenhouse experiments, but not in field trails. • N availability affected wood anatomical and chemical properties. • Low N-induced circRNA/lncRNA-miRNA-mRNA networks underlying wood formation were dissected. • The crucial role played by N availability in wood formation were discussed. [ABSTRACT FROM AUTHOR]