SMALL ORGAN SIZE 1 and SMALL ORGAN SIZE 2/DWARF AND LOW-TILLERING Form aComplex to Integrate Auxin and Brassinosteroid Signaling in Rice

Although auxin and brassinosteroid （BR） synergistically control various plant responses, the molecular mechanism underlying the auxin-BR crosstalk is not wen understood. We previously identified SMOS1, an auxin-regulated APETALA2-type transcription factor, as the causal gene of the small organ size 1 （smosl） 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 smosl. SMOS2 was identical to the previously reported DWARF AND LOWoTILLERING （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 ceils. Taken together, our results suggest that SMOS1 and SMOS2/DLT form a keystone complex on auxin-BR signaling crosstalk in rice.