Abstract
Lignin is a complex polymer in plant cell walls whose proportion is second only to that of cellulose and plays an important role in the mechanical properties of wood and stress resistance of plants. Here, we induced tension wood (TW) formation in Catalpa bungei by artificial bending and analyzed the lignin metabolism of the TW. LC-MS analysis showed that a significantly higher content of coniferyl aldehyde was observed in the TW cell wall than in the opposite wood (OW) and normal wood (NW) cell walls. TW had significantly lower contents of coniferyl alcohol than OW and NW. Raman spectroscopy results indicated that TW had lower total lignin than OW and NW. The transcription and translation levels of most of the differentially expressed genes (DEGs) involved in lignin monomer biosynthesis indicated upregulation in TW/OW and TW/NW. We found no significant difference in the transcription levels of three collision gases (CADs) between TW and OW or between NW, but their translation levels were significantly downregulated in TW, suggesting post-transcriptional control for CAD. We predicted and analyzed transcription factors that could target DEGs involved in lignin monomer biosynthesis in TW. Based on the analysis of the relationships of targeting and coexpression, we found that NAC (evm.model.group1.695) could potentially target 4CLs and CCoAOMT, that HD-Zip (evm.model.group7.1157) had potential targeting relationships with CCoAOMT, F5H, and CCR, and that their expression levels were significantly positive. It is speculated that the upregulation of NAC and HD-ZIP transcription factors activates the expression of downstream target genes, which leads to a significant increase in coniferyl aldehyde in TW. However, the decrease in total lignin in TW may be caused by the significant downregulation of CAD translation and the significant decrease in precursors (coniferyl alcohol). Whether the expression of CAD genes is regulated by post-transcriptional control and affects TW lignin metabolism needs further study.
Highlights
Wood, the secondary xylem of trees, is a multipurpose renewable material and is commonly used in papermaking, architecture, furniture manufacturing, and energy production due to its varied properties (Nakahama et al, 2018; Papadopoulos et al, 2019)
tension wood (TW) had a thinner cell wall than opposite wood (OW) and normal wood (NW) according to the ultrastructure of the cell wall and the statistical results for cell wall thickness in our previous study (Xiao et al, 2020)
These results suggested that the formation of the TW cell wall was inhibited and that its chemical composition might be greatly changed
Summary
The secondary xylem of trees, is a multipurpose renewable material and is commonly used in papermaking, architecture, furniture manufacturing, and energy production due to its varied properties (Nakahama et al, 2018; Papadopoulos et al, 2019). Wood formation is considered secondary growth and involves the formation of vascular tissue, secondary cell wall (SCW), lignification, programmed cell death, and heartwood formation (Tian et al, 2007). Wood quality is mainly determined by cellulose, hemicellulose, and lignin contents (Nakahama et al, 2018). Lignification can increase the hardness and mechanical supporting force of the secondary xylem cell wall and enhance the ability of plants to transport water over long distances and resist biological and abiotic stresses (Vance et al, 1980; Zhao, 2016; Hu et al, 2019). It is of great significance to study the distribution, synthesis and regulation of lignin in xylem for the genetic improvement of wood properties
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