Root architecture differences have been linked to the survival of plants on phosphate (P)-deficient soils, as well as to the improved yields of P-efficient crop cultivars. To understand how these differences arise, we have studied the root architectures of P-deficient Arabidopsis (Arabidopsis thaliana Columbia-0) plants. A striking aspect of the root architecture of these plants is that their primary root elongation is inhibited when grown on P-deficient medium. Here, we present evidence suggesting that this inhibition is a result of iron (Fe) toxicity. When the Fe concentration in P-deficient medium is reduced, we observe elongation of the primary root without an increase in P availability or a corresponding change in the expression of P deficiency-regulated genes. Recovery of the primary root elongation is associated with larger plant weights, improved ability to take up P from the medium, and increased tissue P content. This suggests that manipulating Fe availability to a plant could be a valuable strategy for improving a plant's ability to tolerate P deficiency.