Light-derived signals from the shoot tissue promote root growth. Brassinosteroids (BRs), which are phytohormones, are crucial for root growth and leaf expansion. We previously revealed that photoreceptor-dependent signals promote the expression of DWARF4 ( DWF4), which encodes a key BR biosynthesis-related enzyme, in the root tips. Furthermore, auxin is indispensable for root growth. In this study, we focused on the relationships among light-derived signals, auxin, and BR in the root tips. Polar auxin transport (PAT) in the root tips was undetectable in plants grown on medium lacking sucrose for 24 h in darkness; however, a BR treatment restored PAT in darkness. We also observed that PAT was non-existent in the cell division zones under BR-deficient conditions, even under light. On the basis of these results, we propose that the light-dependent cooperative effects of auxin and BR in the root tip cell division zone are necessary for sustaining root growth.