Polar auxin transport (PAT) plays a crucial role in the regulation of many aspects of plant growth and development. We report the characterization of a semidominant Arabidopsis thaliana bushy and dwarf1 (bud1) mutant. Molecular genetic analysis indicated that the bud1 phenotype is a result of increased expression of Arabidopsis MAP KINASE KINASE7 (MKK7), a member of plant mitogen-activated protein kinase kinase group D. We showed that BUD1/MKK7 is a functional kinase and that the kinase activity is essential for its biological functions. Compared with the wild type, the bud1 plants develop significantly fewer lateral roots, simpler venation patterns, and a quicker and greater curvature in the gravitropism assay. In addition, the bud1 plants have shorter hypocotyls at high temperature (29 degrees C) under light, which is a characteristic feature of defective auxin action. Determination of tritium-labeled indole-3-acetic acid transport showed that the increased expression of MKK7 in bud1 or the repressed expression in MKK7 antisense transgenic plants causes deficiency or enhancement in auxin transport, indicating that MKK7 negatively regulates PAT. This conclusion was further substantiated by genetic and phenotypic analyses of double mutants generated from crosses between bud1 and the auxin-related mutants axr3-3, tir1-1, doc1-1, and atmdr1-1.