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.