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SUMMARY Cell differentiation and morphogenesis are crucial for the establishment of diverse cell types and organs in multicellular organisms. Trichome cells offer an excellent paradigm for dissecting the regulatory mechanisms of plant cell differentiation and morphogenesis due to their unique growth characteristics. Here, we report the isolation of an Arabidopsis mutant,aberrantlybranchedtrichome 3–1(abt3‐1), with a reduced trichome branching phenotype. Positional cloning and molecular complementation experiments confirmed thatabt3‐1is a new mutant allele ofAuxin resistant 1(AXR1), which encodes the N‐terminal half of ubiquitin‐activating enzyme E1 and functions in auxin signaling pathway. Meanwhile, we found that transgenic plants expressing constitutively active version ofROP2(CA‐ROP2) caused a reduction of trichome branches, resembling that ofabt3‐1. ROP2 is a member of Rho GTPase of plants (ROP) family, serving as versatile signaling switches involved in a range of cellular and developmental processes. Our genetic and biochemical analyses showedAXR1genetically interacted withROP2and mediated ROP2 protein stability. The loss ofAXR1aggravated the trichome defects ofCA‐ROP2and induced the accumulation of steady‐state ROP2. Consistently, elevatedAXR1expression levels suppressedROP2expression and partially rescued trichome branching defects inCA‐ROP2plants. Together, our results presented a new mutant allele ofAXR1, uncovered the effects ofAXR1andROP2during trichome development, and revealed a pathway ofROP2‐mediated regulation of plant cell morphogenesis in Arabidopsis.