Abstract: The PIN proteins contain multiple transmembrane domains, and are involved in the efflux of auxin, affecting many growth and development-related processes in higher plants. Trichoderma is a biocontrol agent that promotes the growth and improves the defense mechanisms of plants against various diseases. Therefore, it is important to study the effects of Trichoderma on the distribution of auxin in the woody species Populus davidiana×P.alba var. pyramidlis(Shanxin poplar). In this study, the gene PodaPIN9 was cloned from Shanxin poplar, and its nucleotide and protein sequences were analyzed. The constructed phylogenetic tree showed that PodaPIN9 has a high identity with 9 PIN genes from 6 species(>80%). By the qRT-PCR analysis, PodaPIN9 was expressed in the stem tips, mature leaves, and roots of the plant. The expression of PodaPIN9 was significantly low in the roots, but was significantly high in the stem tips and leaves(503 and 346 times higher than that in the roots, respectively). Following inoculation with Trichoderma, the expression of PodaPIN9 was significantly downregulated in the stem tips and leaves, whereas its expression was significantly upregulated in the roots. The expression of PodaPIN9 in the roots was 32.01 times higher than the control group. Additionally, we found that the levels of auxin in the stem tips, leaves, and roots of the plant were decreased at 48 h after inoculating the Trichoderma. It demonstrated that Trichoderma can affect auxin levels and the expression of PodaPIN9 in the stem tips, leaves, and roots of poplar. Furthermore, by Pearson correlation analysis, the correlation between the expression of PodaPIN9 and auxin levels differed in the stem tips, leaves, and roots.