Auxin is a plant hormone involved in regulating growth and development throughout the life cycle of the plant. Auxin plays an essential role in a diverse range of processes, including pattern formation in embryogenesis, organ development, apical dominance (inhibition of lateral bud growth by the apical bud of a shoot), shoot and root growth, lateral root initiation, leaf formation, and tropisms (directional growth in response to environmental stimuli). Auxin itself is a very simple molecule; yet despite over 100 years of research, the mechanisms of auxin biosynthesis, signaling, and action are not fully understood.