Involved in a root UV-B sensing pathway and in the protection against the hypersensitivity to very low-fluence-rate (VLF) UV-B. RSU1 and RUS2 are probably both negative modulators of the same UV-B perception pathway, which when overstimulated in the roots causes a block to postgermination development. Required for polar auxin transport by maintaining the proper levels of auxin transporters AUX1 (AC Q96247) and PIN proteins on the plasma membrane