E3 Ubiquitin Ligase PdSINA2D in Populus deltoides Positively Regulates ABA-Dependent Stress Tolerance through Ubiquitination of FREE1 in Poplar.

Populus deltoides "Shalinyang" (PdS) displays reduced tolerance to abiotic stresses. SINA (seven in absentia) proteins, a subtype of E3 ubiquitin ligases, play crucial roles in regulating plant growth and responses to abiotic stresses. However, the characteristics and potential functions of SINA proteins in PdS remain unclear. We identified 12 SINA genes via comprehensive bioinformatic characterization. Overexpression of PdSINA2D improved drought and saline-alkali tolerance in PdS, whereas gene silencing exacerbated plant sensitivity to these stresses. We also found that the abscisic acid (ABA) sensitivity was increased in transgenic Arabidopsis seeds germination. In addition, we identified an interaction between PdSINA2D and PdFREE1, a negative regulator of the ABA signaling pathway, wherein PdSINA2D ubiquitinates PdFREE1 and promotes its degradation. The results demonstrate that PdSINA2D in PdS positively regulates ABA-dependent stress tolerance by degrading PdFREE1. These findings define the mechanism of PdSINA2D in ABA-dependent poplar stress tolerance and provide genetic resources for breeding stress-resistant woody plants.