We report the identification and functional analysis of TRANSPARENT LEAF AREA1 (TLA1), a maize (Zea mays) gene representing a novel class of secreted, extremely hydrophobic peptides (proteolipids) with a C-terminal Caax box-like motif. ZmTLA1 encodes 27 amino acid residues and is most strongly expressed in the egg cell and microspores. Lower transcript amounts were detected during vegetative development. Transgenic maize expressing an antisense transcript displayed a variety of phenotypes. The most visible phenotypes were dwarfism and transparent leaf areas resulting from defective morphogenesis of mesophyll, bundle sheath, stomatal, and epidermal cells during leaf development. Incomplete cell walls were observed, indicating a defect of cytokinesis. The accumulation of gerontoplasts was probably a secondary effect caused by defects of leaf cell morphogenesis. A defect of anther maturation was observed in 30% of the plants displaying the tla phenotype. Male sterility was mainly caused by incomplete disintegration of the tapetal cell layers and tetrad callose as 90% of the microspores developed into functional pollen. Overexpression of ZmTLA1 seemed to have a lethal effect both in maize and Arabidopsis thaliana. Development of primary roots, root hairs, primary leaves, and chloroplasts was suppressed in Arabidopsis seedlings expressing an inducible ZmTLA1-green fluorescent protein (GFP) fusion protein. GFP signals were exclusively detected in cell walls. Based on our observations, we suggest that the ZmTLA1 peptide represents a class of novel plant morphogens required for the development and maturation of leaf and reproductive tissues.