Transcription activator involved in the control of organ size (PubMed:24486766). Acts as an auxin-dependent regulator for cell expansion during organ size control (PubMed:24486766). Binds to the promoter of PHI-1, a gene that regulates cell expansion, and positively regulates its expression (PubMed:24486766). Cooperatively functions in a transactivating complex with DLT to enhance the transcription of the SMOS1 target PHI-1, and to regulate plant organ size (PubMed:28069545). Interaction between SMOS1 and DLT is a crosstalk point for auxin and brassinosteroid (BR) signaling (PubMed:28069545). Acts as positive regulator of BR signaling (PubMed:28100707). Cooperatively functions in a transactivating complex with BZR1 to enhance the transcription of BR biosynthetic genes (PubMed:28100707). Interacts (via C-terminus) with DLT (PubMed:28069545). Interacts with NSP2 and SHR1 (PubMed:28069545). Interacts with BZR1 and GSK2 (PubMed:28100707). Localizes mostly in the nucleus. Expressed in the apical region of leaf blades, young leaf sheaths, a broad area below the shoot apical mersistem (SAM) and near the tip of primordial leaves (PubMed:24486766). In developing roots, expressed at the tip of seminal roots and lateral root tips (PubMed:24486766). Expressed in developing nodes and basal region of internodes (PubMed:24486766). In flowers, expressed in lemma, palea and anthers (PubMed:24486766). Induced by auxin (PubMed:24486766). Induced by brassinosteroid (at protein level), but repressed by brassinosteroid at mRNA level probably via a negative regulatory feedback loop (PubMed:28100707). Phosproylated by GSK2 (PubMed:28100707). Phosphorylation of SMOS1 leads to its degradation by an unknown mechanism (PubMed:28100707). Reduced size of various organs due to decreased cell size, increased cell number, and abnormal cell arrangement and microtubule orientation (PubMed:24486766). Brassinosteroid (BR) loss-of-function phenotype with semidwarf plants, reduced length of internodes, erect leaves, up-regulation of BR biosynthetic genes and decreased sensitivity to BR (PubMed:28100707). Belongs to the AP2/ERF transcription factor family. AP2 subfamily. Truncated N-terminus.