1.Secondary vascular tissue pattern formation and its regulatory network

We use thebark girdling and regeneration experimental system of poplarsin vivoandin vitroto study the secondary vascular tissue generation and the interaction of plant hormones, transcriptional regulation and epigenetic regulation in the process, and to establish the regulatory network of secondary vascular tissue pattern (Chen et al., New Phytol., 2019; Hou et al., New Phytol., 2020). At present, the laboratory has obtained a number of poplar lines with overexpression and gene knockout of a variety of vascular tissue development regulators, such as plant hormones, transcription factors and miRNAs. For example, we analyzed the expression ofmiR319aandPtoTCP20in the vascular tissue development ofPopulus tomentosa, confirmed the targeted cleavage effect ofmiR319aon PtoTCP20, and proposed that PtoTCP20 promotes cambium cell proliferation by interacting with PtoWOX4a and promotes secondary xylem development by activating the expression ofPtoWND6A/B. Thie research will provide an experimental basis for revealing the mechanisms of the secondary vascular development in trees, and provide a theoretical basis for the rapid growth of trees and wood quality improvement.

2. Early signals and epigenetic regulation of xylem cell differentiation

Cotyledone-induced vascular tissue differentiation experimental system is used to study the early signal, transcriptional and epigenetic regulation of xylem cell differentiation. (Lin et al. Sci. Adv., 2020). At present, the laboratory has performed transcriptomic, proteomic and DNA methylation analyses at genome-wide level during xylem tracheary element (TE) differentiation inArabidopsis thaliana, and found passive CHH demethylation in the centromere region, while some specific regions have active demethylation of CG and CHG dependent on DNA demethylases (ROS1, DML2 and DML3). A group of genes regulated by active DNA demethylation, such asXCP2, MYB20, etc., are identified based on the combined methylation and transcriptome analysis of the TE differentiation of the wild-type and triple mutant of DNA demethylase. We find a new function of active DNA demethylation in plant development, and revealed an epigenetic mechanism of xylem TE differentiation regulation. The research will provide now experimental data for a comprehensive understanding of plant vascular development.

3. Programmed cell death during xylem development

Xylem development is a process of xylem cell terminal differentiation that includes initial cell pision, cell expansion, secondary cell wall formation and programmed cell death (PCD). To study the cytological process and regulation of programmed cell death during xylem cell differentiation, the experimental system of vascular tissue differentiation induction using roots and cotyledons ofArabidopsisand Poplar is established. We found that the 20S proteasome is responsible for plant caspase-3-like activity in xylem development, and is involved in PCD during TE differentiation. (Han et al., Plant J., 2012). And Ca2+-dependent nucleases that play a role in the programmed death of secondary xylem cells are also identified (Chen et al., J. Integr. Plant Biol., 2012). Further research will focus on the dynamic changes of vacuoles during programmed cell death of xylem TE. This research will provide new information for comprehensively understanding the plant vascular development and programmed cell death in plants.

Chunhao Liu, Runzhou Huang, Yufei Zhang, Zihao Wang, Yuexin Wu, An Li