背景回顾：Tension wood (TW) is a specialized xylem tissue formed in angiosperm trees under gravitational stimulus or mechanical stresses (e.g., bending). 

提出问题：Knowledge of underlying genetic regulations of this important mechanism remains limited. 

结果1-激光显微切割+RNA-seq鉴定关键转录因子PtrLBD39：Here, we used laser capture microdissection of stem xylem cells coupled with full transcriptome RNA-sequencing to analyze TW formation inPopulus trichocarpa. We revealed that after bending the trees PtrLBD39 is the most significantly induced transcription factor gene, which has a phylogenetically paired homolog, PtrLBD22. 

结果2-基因敲除：CRISPR-based knockout of PtrLBD39/22 led to a severely inhibited TW formation with low cellulose and high lignin. 

结果3-双突RNA-seq：Transcriptomic analyses of CRISPR-based double mutants of PtrLBD39/22 showed that these two genes regulate a set of TW-related genes. 

结果4-PtrLBD39下游靶基因：Moreover, chromatin immunoprecipitation sequencing (ChIP-seq) was used to identify PtrLBD39’s direct targets. 

结果5-转录调控网络：We integrated transcriptomic analyses and ChIP-seq assays to construct a transcriptional regulatory network (TRN) mediated by PtrLBD39. In this TRN, PtrLBD39 directly regulates 26 novel TW responsive transcription factor genes. 

结论：Our work suggested that PtrLBD39 and PtrLBD22 specifically control TW formation by mediating a TW-specific TRN in Populus.

 摘 要 

应拉木是被子植物树木在重力刺激或弯曲等机械压力下形成的一种特化木质部组织。有关树木应拉木形成的分子机制还所知甚少。本文中，作者通过结合激光捕获显微切割木质部细胞和全转录组RNA-seq的方法，解析了毛果杨应拉木的形成。作者发现在毛果杨受到弯曲后，PtrLBD39是最显著被诱导的转录因子，该基因还有一个最近缘的旁系同源基因PtrLBD22。基于CRISPR的方法敲除PtrLBD39/22会抑制应拉木的形成，纤维含量较低，木质素含量较高。PtrLBD39/22功能缺失双突的RNA-seq分析显示，这两个基因调控了一系列的与应拉木相关的基因。此外，作者通过染色质免疫共沉淀分析鉴定了PtrLBD39的直接靶标。作者整合了RNA-seq和ChIP-seq的结果，构建了一个由PtrLBD39介导的转录调控网络。在该网络中，PtrLBD39直接调控26个新的应拉木响应转录因子基因。本文的工作揭示了毛果杨中PtrLBD39和PtrLBD22通过介导一个应拉木特异的转录调控网络来特异性控制应拉木的形成。