In Arabidopsis, Arabidillo-1 and Arabidillo-2 have great sequence homology to Dictyostelium and metazoan $eta$-catenin/Armadillo, which are important to animal and Dictyostelium development. Arabidillo-1 and Arabidillo-2 promote lateral root formation redundantly in Arabidopsis. Here, we showed that gibberellins ($GA_3$) has a greater inhibitory effect on lateral root growth from the mutant arabidillo-1 than from the wild type, suggesting that the mechanism for Arabidillo-1-regulated modulation of lateral root proliferation is associated with $GA_3$-metabolic or signaling pathways. Our yeast two-hybrid analysis demonstrated that Arabidillo-1 interacts with ASK2 and ASK11, and that ASK2 can bind with the F-box domain of Arabidillo-1. Therefore, Arabidillo-1 is involved in the ubiquitin/26S proteasome-mediated proteolytic pathway. Based on these results, we conclude that Arabidillo-1 can degrade some positive regulator of the $GA_3$ signaling pathway through selective protein degradation of ubiquitin/26S. Moreover, that process is believed to be the mechanism for Arabidillo-1 promotion of lateral root development in Arabidopsis.