In a screen for leaf developmental mutants we have isolated an activator T-DNA-tagged mutant that produces leaves without a petiole. In addition to that leafy petiole phenotype this lettuce (let) mutant shows aberrant inflorescence branching and silique shape. The LEAFY PETIOLE (LEP) gene is located close to the right border of the T-DNA insert linked with these dominant phenotypes and encodes a protein with a domain with similarity to the DNA binding domain of members of the AP2/EREBP family of transcription factors. Introduction of the activation-tagged LEP gene in wild-type plants conferred all the phenotypic aberrations mentioned above. The leafy petiole phenotype consists of a conversion of the proximal part of the leaf from petiole into leaf blade, which means that leaf development in let is disturbed along the proximodistal axis. Therefore, LEP is involved in either cell division activity in the marginal meristem or patterning along the proximodistal axis.