Comparative mutant analyses reveal a novel mechanism of ARF regulation in land plants.

The plant hormone auxin regulates a wide variety of transcriptional responses depending on the cell type, environment and species. How this diversity is achieved may be related to the specific complement of auxin-signalling components in each cell. The levels of activators (class-A AUXIN RESPONSE FACTORS) and repressors (class-B ARFs) are particularly important. Tight regulation of ARF protein levels is probably key in determining this balance. Through comparative analysis of novel, dominant mutants in maize and the moss Physcomitrium patens, we have discovered a ~500-million-year-old mechanism of class-B ARF protein-level regulation mediated by proteasome degradation, important in determining cell fate decisions across land plants. Thus, our results add a key piece to the puzzle of how auxin regulates plant development.