To identify genes involved in vascular patterning in Arabidopsis (Arabidopsis thaliana), we screened for abnormal venation patterns in a large collection of leaf shape mutants isolated in our laboratory. The rotunda1-1 (ron1-1) mutant, initially isolated because of its rounded leaves, exhibited an open venation pattern, which resulted from an increased number of free-ending veins. We positionally cloned the RON1 gene and found it to be identical to FRY1/SAL1, which encodes an enzyme with inositol polyphosphate 1-phosphatase and 3'(2'),5'-bisphosphate nucleotidase activities and has not, to our knowledge, previously been related to venation patterning. The ron1-1 mutant and mutants affected in auxin homeostasis share perturbations in venation patterning, lateral root formation, root hair length, shoot branching, and apical dominance. These similarities prompted us to monitor the auxin response using a DR5-GUS auxin-responsive reporter transgene, the expression levels of which were increased in roots and reduced in leaves in the ron1-1 background. To gain insight into the function of RON1/FRY1/SAL1 during vascular development, we generated double mutants for genes involved in vein patterning and found that ron1 synergistically interacts with auxin resistant1 and hemivenata-1 but not with cotyledon vascular pattern1 (cvp1) and cvp2. These results suggest a role for inositol metabolism in the regulation of auxin responses. Microarray analysis of gene expression revealed that several hundred genes are misexpressed in ron1-1, which may explain the pleiotropic phenotype of this mutant. Metabolomic profiling of the ron1-1 mutant revealed changes in the levels of 38 metabolites, including myoinositol and indole-3-acetonitrile, a precursor of auxin.