In Arabidopsis, the two-dimensional expansion of leaves is regulated via the polarized elongation of cells. The ROTUNDIFOLIA3 (ROT3) protein, a member of the family of cytochromes P450, is involved in this process and regulates leaf length. Transgenic plants that overexpressed a wild-type ROT3 gene had longer leaves than parent plants, without any changes in leaf width. The shapes of floral organs were also altered, but elongation of the stem, roots, and hypocotyls was unaffected. To our knowledge, no similar specific regulation of leaf length has been reported previously. Transgenic plants overexpressing the rot3-2 gene had enlarged leaf blades but leaf petioles of normal length. Morphological alterations in such transgenic plants were associated with changes in shape of leaf cells. The ROT3 gene seems to play an important role in the polar elongation of leafy organs and should be a useful tool for the biodesign of plant organs.