A PtrLBD39-Mediated Transcriptional Network Regulates Tension Wood Formation in Populus trichocarpa

Abstract Tension wood (TW) is a specialized xylem tissue formed in angiosperm trees under gravitational stimulus or mechanical stresses (e.g., bending). Knowledge of underlying genetic regulations of this important mechanism remains limited. Here, we used laser capture microdissection of stem xylem cells coupled with full transcriptome RNA-sequencing to analyze TW formation in Populus trichocarpa. We revealed that after bending the trees PtrLBD39 is the most significantly induced transcription factor gene, which has a phylogenetically paired homolog, PtrLBD22. CRISPR-based knockout of PtrLBD39/22 led to a severely inhibited TW formation with low cellulose and high lignin. Transcriptomic analyses of CRISPR-based double mutants of PtrLBD39/22 showed that these two genes regulate a set of TW-related genes. Moreover, chromatin immunoprecipitation sequencing (ChIP-seq) was used to identify PtrLBD39’s direct targets. 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. Our work suggested that PtrLBD39 and PtrLBD22 specifically control TW formation by mediating a TW-specific TRN in Populus.