The study identifies NAC1, a new member of the NAC-domain gene family, as a key regulator of lateral root development in Arabidopsis. NAC1 is induced by auxin and acts as a transcriptional activator, activating the expression of two auxin-responsive genes, DBP and AIR3. Transgenic plants expressing sense or antisense NAC1 cDNA show increased or reduced lateral root formation, respectively. The auxin response mutant tir1 exhibits reduced lateral root development, which can be rescued by overexpression of NAC1. These findings indicate that NAC1 acts downstream of TIR1 in the auxin signaling pathway, promoting lateral root initiation and development.The study identifies NAC1, a new member of the NAC-domain gene family, as a key regulator of lateral root development in Arabidopsis. NAC1 is induced by auxin and acts as a transcriptional activator, activating the expression of two auxin-responsive genes, DBP and AIR3. Transgenic plants expressing sense or antisense NAC1 cDNA show increased or reduced lateral root formation, respectively. The auxin response mutant tir1 exhibits reduced lateral root development, which can be rescued by overexpression of NAC1. These findings indicate that NAC1 acts downstream of TIR1 in the auxin signaling pathway, promoting lateral root initiation and development.