Auxin and brassinosteroid regulate transcription mediating cell expansion by both independent and interconnected mechanisms. Only components known to act in transcriptional regulation of auxin- and brassinosteroid-response pathways are shown. Dashed lines indicate speculative relationships in the model. Auxin promotes the degradation of (AUX/IAA) proteins, which negatively regulate auxin response factor (ARF) function. ARFs are implicated in the regulation of gene expression downstream of both auxin and brassinosteroid. Other regulators of transcription that may bind directly to promoters are proposed (proteins X and Y). The bri1-EMS-Suppressor 1 (BES1) and Brassinazole resistant 1 (BZR1) proteins regulate brassinosteroid-induced transcription by unknown mechanisms, possibly involving higher-order transcriptional complexes that include ARFs or other factors. Gene expression regulated by auxin and brassinosteroid coordinates expansion growth of cells and promotes the elongation of hypocotyls in seedlings. Modified from .