Evolution of leaf shape: A pattern emerges.

Leaf shape is a highly variable trait. Ancestrally, all leaves are proposed to have derived from modifications of branched shoot systems. The formation of blade, smooth margins or serrations on the blade, or distinct leaflets that are the characteristic features of some leaves provides an opportunity to study the generation of morphogenesis in organs that are evolutionarily homologous and yet developmentally distinct in patterning. Intense research in several model species with distinct leaf morphologies has revealed a complex network of genes that interact to pattern the leaf. Several parallels between leaf patterning and shoot patterning exist. The plant growth hormone auxin is emerging as a key player in the specification of both shoot and leaf patterning. The outcome of this underlying auxin pattern may be determined by variation in the two opposing developmental forces of differentiation and indeterminancy. The expanded suite of genetic and physiological factors regulating leaf shape has provided interesting insight into the mechanisms by which morphological innovation is accomplished.