Lateral root formation and root system architecture are critical for soil exploration by plant roots, allowing for uptake of both water and nutrients. Conversion of auxin precursor indole-3-butyric acid (IBA) to active auxin (indole-3-acetic acid; IAA) modulates lateral root formation. However, mechanisms regulating IBA-to-IAA conversion are not well understood. We identified TRANSPORTER OF IBA1 (TOB1) as a vacuolar IBA transporter that acts to limit lateral root formation, likely by sequestering IBA in the vacuole to prevent its contribution to the active auxin pool in lateral root formation. Moreover, TOB1 is upregulated by cytokinin, which inhibits lateral root formation. The increased production of lateral roots in tob1 mutants, TOB1 transport of IBA into the vacuole, and cytokinin-regulated TOB1 transcript levels provide a mechanism linking cytokinin response to IBA contributions to the auxin pool to modulate root system architecture.
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