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. Interacts (via the DUF647 domain) with RUS2 (via the DUF647 domain). Expressed in cotyledons, roots, hypocotyls, leaf veins, root vascular tissues and root tips. Expressed near the root apical meristem, in the cortex region of the root elongation zone, in lateral roots and emerging lateral roots. Not detected in extreme root apical meristem or root cap. No visible phenotype under normal growth conditions. Extremely stunted growth, failure to develop true postembryonic leaves and arrested primary root elongation, when grown in vitro. Several mutations in ASP2 (AC P46645), but not all, and any of the non-phosphorylated B6 vitamers can suppress the rus phenotype. Belongs to the RUS1 family.