The ubiquitin-mediated 26S proteasome pathway (UPS) is of great importance to plant growth and development. Previously research showed that a subunit of the 26S proteasome, named RPN1a, was involved in trichome's branching in Arabidopsis. Mutation in RPN1a give rise to more trichome branches on leaves. Here, we found that T-DNA insertion mutation in RPN1a resulted in increased trichome branches on main stem, and trichome number on rosette leaves and the main stem compared with the wild type plant. Expression analysis results showed that the transcription levels of ZFP6, ZFP5, GIS, GL1, GL2, GL3, TTG1 and MYB23, which promote trichome initiation, were up-regulated in the rpn1a mutant, and expression of FRC4, which is responsible for increased trichome branching, was also increased in the rpn1a mutant. Moreover, the mRNA expression level of RPN1a was significantly repressed by GA (gibberellin) and CK (cytokinin) treatment, which are two important phytohormones that play essential roles in trichome development. These results demonstrate that RPN1a may be involved in trichome development through the GA and CK signaling pathways.