Poplar PdRabG3f inhibits root elongation and increases salt tolerance by enhancing endogenous abscisic acid synthesis

Proteins belonging to the small GTPases family, particularly Rab proteins, are involved in plant root development. The RabG subfamily proteins, in particular, play a role in regulating plant stress responses. This study revealed that the poplar RabG protein PdRabG3f exhibited tissue-specific activity in root epidermis, root hairs, and vascular stele. Functional characterization further demonstrated that overexpression of PdRabG3f in transgenic poplar significantly inhibited root elongation, reducing maximum root length, and fresh root weight, whereas suppression of PdRabG3f enhanced root growth - a phenotype consistent with its spatial expression pattern. In PdRabG3f overexpression lines, elevated endogenous abscisic acid (ABA) levels were observed, which correlated with enhanced ABA sensitivity and further inhibition of root elongation under exogenous ABA treatment. As a result, these lines exhibited improved resilience under salt stress, including higher peroxidase activity, reduced malondialdehyde content, and maintained relative water content. Transcriptome analysis further supported their enhanced salt tolerance, revealing significant upregulation of stress-responsive genes and enrichment in stress response pathways. These findings establish PdRabG3f as a negative regulator of root growth via ABA biosynthesis and highlight its role in enhancing salt tolerance through ABA-dependent stress signaling. This study provides novel insights into Rab protein-mediated hormonal regulation in woody plants, offering potential targets for improving stress resilience in poplar.