The regulation of polar auxin transport in Arabidopsis vascular tissue is crucial for various developmental processes, including the formation of vascular tissue. The *PIN-FORMED* (*PIN1*) gene, which encodes a 67-kilodalton protein, plays a key role in this process. Mutations affecting *PIN1* result in diminished polar auxin transport, leading to abnormal inflorescence development and vascular tissue patterning. Southern blot analysis and genetic studies identified the En-1 transposon insertion responsible for the mutant phenotype, confirming that the Atpin1::En134 mutant is allelic to the pin-formed mutant. Northern blot analysis showed that *AtPIN1* gene expression is absent in homozygous transposon insertional mutants, while heterozygous plants exhibit wild-type expression. The AtPIN1 protein is localized to the basal end of auxin transport-competent cells in vascular tissue, consistent with its role as a catalytic auxin efflux carrier. Chemical inhibition of polar auxin transport in wild-type plants also leads to similar alterations in vascular development, further supporting the importance of AtPIN1 in polar auxin transport. These findings highlight the significance of AtPIN1 in maintaining proper vascular tissue formation and function in Arabidopsis.The regulation of polar auxin transport in Arabidopsis vascular tissue is crucial for various developmental processes, including the formation of vascular tissue. The *PIN-FORMED* (*PIN1*) gene, which encodes a 67-kilodalton protein, plays a key role in this process. Mutations affecting *PIN1* result in diminished polar auxin transport, leading to abnormal inflorescence development and vascular tissue patterning. Southern blot analysis and genetic studies identified the En-1 transposon insertion responsible for the mutant phenotype, confirming that the Atpin1::En134 mutant is allelic to the pin-formed mutant. Northern blot analysis showed that *AtPIN1* gene expression is absent in homozygous transposon insertional mutants, while heterozygous plants exhibit wild-type expression. The AtPIN1 protein is localized to the basal end of auxin transport-competent cells in vascular tissue, consistent with its role as a catalytic auxin efflux carrier. Chemical inhibition of polar auxin transport in wild-type plants also leads to similar alterations in vascular development, further supporting the importance of AtPIN1 in polar auxin transport. These findings highlight the significance of AtPIN1 in maintaining proper vascular tissue formation and function in Arabidopsis.