AtPIN1 regulates polar auxin transport in Arabidopsis vascular tissue. Mutations in the AtPIN1 gene reduce polar auxin transport, leading to phenotypic defects such as naked inflorescences and abnormal vascular development. AtPIN1 encodes a 67-kDa protein with similarity to bacterial and eukaryotic carrier proteins, localized at the basal end of auxin transport-competent cells. It is proposed to function as a transmembrane component of the auxin efflux carrier. The AtPIN1 gene was identified through transposon insertion mutagenesis, and its structure was analyzed. The gene consists of five exons and is expressed in various organs, with reduced expression in mutants. AtPIN1 is localized in vascular tissues, specifically in cambial and xylem cells, and its polar localization supports the role in basipetal auxin transport. Genetic and biochemical analyses confirm that AtPIN1 is essential for polar auxin transport, and its disruption leads to vascular patterning defects. The study shows that AtPIN1 functions as a catalytic auxin efflux carrier, contributing to the directional transport of auxin in vascular tissues. The findings highlight the importance of AtPIN1 in plant development and auxin transport regulation.AtPIN1 regulates polar auxin transport in Arabidopsis vascular tissue. Mutations in the AtPIN1 gene reduce polar auxin transport, leading to phenotypic defects such as naked inflorescences and abnormal vascular development. AtPIN1 encodes a 67-kDa protein with similarity to bacterial and eukaryotic carrier proteins, localized at the basal end of auxin transport-competent cells. It is proposed to function as a transmembrane component of the auxin efflux carrier. The AtPIN1 gene was identified through transposon insertion mutagenesis, and its structure was analyzed. The gene consists of five exons and is expressed in various organs, with reduced expression in mutants. AtPIN1 is localized in vascular tissues, specifically in cambial and xylem cells, and its polar localization supports the role in basipetal auxin transport. Genetic and biochemical analyses confirm that AtPIN1 is essential for polar auxin transport, and its disruption leads to vascular patterning defects. The study shows that AtPIN1 functions as a catalytic auxin efflux carrier, contributing to the directional transport of auxin in vascular tissues. The findings highlight the importance of AtPIN1 in plant development and auxin transport regulation.