Plant hormone and peptide signaling converge in the genetic network regulating cambium activation in Arabidopsis roots

Plant secondary growth is driven by 2 concentric meristems: the inner vascular cambium and outer cork cambium. The periclinal cell divisions of both meristems, providing thickness and protection to plant organs, are activated with a delay after the primary development. Cytokinins and a set of downstream transcription factors are key players in promoting transition from primary to secondary development; however, it is unknown whether other factors play a role in this transition. Here, using time-course transcriptome analysis of cytokinin-treated, cytokinin-deficient isopentenyltransferase1,3,5,7 (ipt1,3,5,7) mutant, we show that during cambium activation cytokinins positively regulate auxin and TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF) peptide signaling in Arabidopsis (Arabidopsis thaliana) root. Correspondingly, mutants defective in TDIF peptide signaling displayed reduced cytokinin-induced secondary growth, and auxin signaling was found to be required for proper cytokinin response. Additionally, auxin and cytokinin signaling transiently overlapped in activating procambial cells and acted additively in promoting secondary development. Network analysis revealed that transcription factors belonging to the DNA-BINDING WITH ONE FINGER (DOF) and ETHYLENE RESPONSE FACTOR (ERF) gene families, are regulated by cytokinin during cambium activation and mutant analysis demonstrated delayed cambium activation and xylem formation phenotypes. Overall, we find that cytokinin, auxin, and TDIF form a tightly intertwined network of positive regulators for activation of secondary growth in the Arabidopsis root, indicating extensive redundancy in this process.