Although auxin and brassinosteroid (BR) synergistically control various plant responses, the molecular mechanism underlying the auxin–BR crosstalk is not well understood. We previously identified SMOS1 , an auxin-regulated APETALA2-type transcription factor, as the causal gene of the small organ size 1 ( smos1 ) mutant that is characterized by a decreased final size of various organs in rice. In this study, we identified another smos mutant, smos2 , which shows the phenotype indistinguishable from smos1 . SMOS2 was identical to the previously reported DWARF AND LOW-TILLERING ( DLT ), which encodes a GRAS protein involved in BR signaling. SMOS1 and SMOS2/DLT physically interact to cooperatively enhance transcriptional transactivation activity in yeast and in rice nuclei. Consistently, the expression of OsPHI-1 , a direct target of SMOS1, is upregulated only when SMOS1 and SMOS2/DLT proteins are both present in rice cells. Taken together, our results suggest that SMOS1 and SMOS2/DLT form a keystone complex on auxin–BR signaling crosstalk in rice. [ABSTRACT FROM AUTHOR]