Polychlorinated biphenyls (PCBs) pose serious harm to humans and the environment. PCBs have a hormesis effect, but the inner mechanism still remains unknown. In this paper, we used Populous tomentosa seedlings to investigate the effect of treatment with a PCB compound, Aroclor1254 (3 mg·L-1), on the differentiation of adventitious roots, phytohormone content and the expression of P009g125900, P006g142600, and P002g222700 genes related to auxin expression as well as P005g2489 and P005g2376 genes related to cytokinin expression. Aroclor1254 could promote the differentiation of adventitious roots, shorten the initial root formation time of adventitious roots, and enhance the adventitious root number. During adventitious root differentiation, Aroclor1254 treatment alone had a similar effect as IBA alone on IAA/(ZR+dhZR) content and the gene expression of P006g142600, P002g222700, P009g125900, P005g2489, and P005g2376. To further verify the auxin effect of Aroclor1254, we used Zea mays and transgenic Arabidopsis thaliana modified with a DR5::GUS auxin reporter gene to investigate the growth of coleoptiles or the response pattern of DR5::GUS to Aroclor1254 exposure. Aroclor1254 at a suitable concentration range could induce DR5::GUS gene expression but had no effect on the growth of coleoptiles. Thus, Aroclor1254 has a biological effect of auxin and can positively affect the differentiation of adventitious roots of P. tomentosa seedlings; a phenomenon called hormesis, but is not a plant growth regulator.