C2H2-type zinc finger protein (ZFP) transcription factors influence root growth and development. However, their potential roles in inhibiting adventitious root (AR) and lateral root (LR) formation in trees remain unclear. Here, we report that the ABA-responsive C2H2-type zinc finger protein transcription factor (PuZFP1) regulates Populus ussuriensis root development to enhance drought tolerance. PuZFP1 negatively regulates LR development by binding to the PuWRKY46 promoter and inhibiting its expression. At the same time, PuZFP1 promotes AR elongation by repressing Clade E Growth-Regulating (EGR) Type 2C protein phosphatases (PuEGR1). In PuZFP1-overexpressing lines, a higher ABA/IAA ratio in the differentiated zone (DZ) drives PuWRKY46-mediated LR inhibition. Conversely, a lower ABA/IAA ratio is associated with AR elongation and the expression of the downstream target gene PuEGR1 in the elongation zone (EZ). Notably, PuZFP1 physically interacts with Ubiquitin-like protein 5 (PuUBL5) and undergoes 26S proteasome-mediated degradation. Taken together, our findings shed light on the role of the PuUBL5–PuZFP1 module in mediating the crosstalk between LR emergence and AR elongation via ABA/auxin signaling in drought-stressed P. ussuriensis, and provide insights into the regulatory network underlying PuZFP1-mediated root growth in poplar.

Proteins containing the homologous E6-AP carboxyl-terminal (HECT) domain are a class of E3 ubiquitin ligases involved in the ubiquitin–proteasome pathway, which plays an irreplaceable role in plant growth, development, and stress resistance. However, a phylogenetic analysis and expression profile of the HECT gene (PtrHECT) in the model plant Populus trichocarpa (Torr. & Gray) have not been reported. In this study, we identified 13 PtrHECT genes using genome-wide analysis, and then these were divided into four groups. The protein interaction networks showed that the PtrHECT protein may interact with PTR6 and participate in ABA signal regulation. Abiotic stress is the main environmental factor limiting plant growth and development. The qRT-PCR results showed that PtrHECT1, 4, 7, 8, and 9 were significantly up-regulated in leaves at each time point under drought stress, and most PtrHECT genes responded to both drought and high salt stress, consistent with their promoter sequence analysis, revealing the presence of an important number of phytohormone-responsive and stress-related cis-regulatory elements. This study provides useful information for further analysis of the functions of the HECT gene family in P. trichocarpa.