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Auxin response in Arabidopsis thaliana requires the SCF^TIR1 ubiquitin ligase. In response to the hormone, SCF^TIR1 targets members of the auxin/indoleacetic acid (Aux/IAA) family of transcriptional regulators for ubiquitin-mediated proteolysis. To identify additional regulators of SCF^TIR1 activity, we conducted a genetic screen to isolate enhancers of the tir1-1 auxin response defect. Here, we report our analysis of the eta2 mutant. Mutations in ETA2 confer several phenotypes consistent with reduced auxin response. ETA2 encodes the Arabidopsis ortholog of human Cullin Associated and Neddylation-Dissociated (CAND1)/TIP120A, a protein recently identified as a cullin-interacting factor. Previous biochemical studies of CAND1 have suggested that it specifically binds to unmodified CUL1 to negatively regulate SCF assembly. By contrast, we find that ETA2 positively regulates SCF^TIR1 because Aux/IAA …