Transcriptional regulatory framework for vascular cambium development in Arabidopsis roots

Vascular cambium, a lateral plant meristem is a central producer of woody biomass. Although a few transcription factors (TFs) have been shown to regulate cambial activity1, the phenotypes of the corresponding loss-of-function mutants are relatively modest, highlighting our limited understanding of the underlying transcriptional regulation. Here, we utilize cambium cell-specific transcript profiling followed by a combination of TF network and genetic analyses to identify 62 novel TF genotypes displaying an array of cambial phenotypes. This approach culminated in virtual loss of cambial activity when both WUSCHEL-RELATED HOMEOBOX 4 (WOX4) and KNOTTED-like from Arabidopsis thaliana 1 (KNAT1; also known as BREVIPEDICELLUS (BP) were mutated, thereby unlocking the genetic redundancy in the regulation of cambium development. We also identified TFs with dual functions in cambial cell proliferation and xylem differentiation, including WOX4, SHORT VEGETATIVE PHASE (SVP) and PETAL LOSS (PTL). Using the TF network information, we combined overexpression of the cambial activator WOX4 and removal of the putative inhibitor PTL to engineer Arabidopsis for enhanced radial growth. This line also showed ectopic cambial activity, thus further highlighting the central roles of WOX4 and PTL in cambium development.This work was supported by Finnish Centre of Excellence in Molecular Biology of Primary Producers (Academy of Finland CoE program 2014-2019) decision #271832, the Gatsby Foundation (GAT3395/PR3)), the University of Helsinki (award 799992091) and the European Research Council Advanced Investigator Grant SYMDEV (No. 323052) to Y.H.; Academy of Finland (grants #132376, #266431, #271832), University of Helsinki HiLIFE fellowship to A.P.M.; National Research Foundation of Korea (2018R1A5A1023599 and 2016R1A2B2015258) to J-Y. L