The secondary cell wall (SCW) is essential for plant growth and development in vascular plants, and its biosynthesis is mainly controlled by a complex hierarchical regulatory network involving multiple transcription factors (TFs) at the transcription level. However, TFs that specifically regulate secondary xylem have not been widely reported. In this study, we described a poplar KNOTTED1-like homeobox (KNOX) TF PtoKNAT3a1, which was mainly expressed in the expanding xylem cells of stems. PtoKNAT3a1 overexpression caused fiber SCW thickening and increased all measured SCW compositions by upregulating the expression of SCW -biosynthetic genes and -associated TFs, but had no effect on the vessels of SCW. The opposite phenotype was observed in the PtoKNAT3a1-knockout lines. Hence, we further demonstrated that PtoKNAT3a1 could physically interact with the NAC master switches PtoWND2A/3A to enhance the expression of downstream MYB TFs and SCW biosynthetic genes (including PtoMYB20, PtoMYB21, PtoMYB90, PtoCoMT2, PtoGT43B and PtoCesA8). Meanwhile, the studies also demonstrate that the KNAT3 has functional differentiation in xylem development. Taken together, these data suggest that the KNAT3a1-WND2A/3A module positively regulates fiber development of the secondary xylem in poplar via the WND2A/3A-mediated hierarchical regulatory network, and supplies useful information for fiber SCW formation. The research not only deepens the understanding of the hierarchical regulatory network affecting SCW formation but also supplies genetic resources and molecular targets for plant fiber utilization.

中文翻译：

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KNAT3a1-WND2A/3A 模块正向调节毛白杨纤维次生细胞壁生物合成


次生细胞壁 （SCW） 对维管植物植物的生长发育至关重要，其生物合成主要由涉及转录水平多个转录因子 （TFs） 的复杂分层调控网络控制。然而，专门调节次生木质部的 TFs 尚未得到广泛报道。在本研究中，我们描述了一种杨树KNOTTED1样同源框 （KNOX） TF PtoKNAT3a1，它主要在茎的扩增木质部细胞中表达。PtoKNAT3a1 过表达通过上调 SCW 生物合成基因和相关 TFs 的表达导致纤维 SCW 增厚并增加所有测量的 SCW 组成，但对 SCW 的血管没有影响。在 PtoKNAT3a1 敲除细胞系中观察到相反的表型。因此，我们进一步证明 PtoKNAT3a1 可以与 NAC 主开关 PtoWND2A/3A 物理相互作用，以增强下游 MYB TFs 和 SCW 生物合成基因（包括 PtoMYB20、PtoMYB21、PtoMYB90、PtoCoMT2、PtoGT43B 和 PtoCesA8）的表达。同时，研究还表明 KNAT3 在木质部发育中具有功能分化。综上所述，这些数据表明 KNAT3a1-WND2A/3A 模块通过 WND2A/3A 介导的分层调控网络正向调节杨树次生木质部的纤维发育，并为纤维 SCW 的形成提供有用的信息。该研究不仅加深了对影响 SCW 形成的分层调控网络的理解，还为植物纤维利用提供了遗传资源和分子靶点。